Filter system for vehicle

Abstract

An air filter system for a vehicle, particularly for use as part of a downforce system, is provided. The system has a separation chamber 102 with an inlet 104, primary and secondary air outlets 108, 1

Description

FIELD OF THE INVENTION The present invention relates to an air filter system for a vehicle, the air filter system comprising a separation chamber for removing debris from an airflow through the air filter system. The air filter system is particularly suited to use in a downforce system for a vehicle. BACKGROUND In order to improve a vehicle’s grip (or traction) on a ground surface on which it is traveling, such as a road, a downforce can be generated which acts on the vehicle and causes the vehicle to be pressed towards the ground surface. Such a downforce may increase tyre adhesion with the ground surface, which may enable the vehicle to travel around bends at greater speeds without losing grip as well as enhance its ability to accelerate and decelerate. One technique for generating downforce involves using aerodynamic characteristics of the vehicle. The vehicle may include one or more surfaces which are configured to generate a downforce when air flows over those surfaces as the vehicle moves. As an example, the vehicle may include an aerofoil (or wing) which is configured to generate a downforce that presses the vehicle towards the ground surface as the vehicle drives over the ground surface. Another technique for generating downforce involves using a powered downforce system. Such a downforce system is configured to create a low-pressure region under the vehicle, which results in a downforce pressing the vehicle towards the ground surface. This can be achieved by evacuating air from under the vehicle using a fan or other suitable air flow source, with the air being expelled via an exhaust (or outlet) of the downforce system that is typically located at a rear of the vehicle. A difficulty associated with this arrangement is that any dust or debris located on the ground surface under the vehicle may be entrained in the air flow and expelled via the exhaust, which may be hazardous for other nearby vehicles and bystanders as well as cause damage to the downforce system itself. To mitigate issues caused by dust and debris on the ground surface, it is known to incorporate a filter in the downforce system, to remove particles from air flowing through the downforce system. For example, the applicant’s earlier application GB 2588394 A, which is incorporated herein by reference, discloses a downforce system including a cyclonic filtration apparatus for removing dust and debris from air flowing through the downforce system. SUMMARY OF THE INVENTION At its most general, the present invention provides an air filter system for a vehicle, the air filter system being configured such that there is a minimal air pressure drop across the air filter system. This is achieved by providing the air filter system with a separation chamber connected to a primary flow path and a secondary flow path, the separation chamber containing a deflector arranged to deflect debris from air flowing through the separation chamber towards the secondary flow path where the debris can be captured. In this manner, a majority of the air flow may pass unimpeded through the primary flow path resulting a low pressure drop across the air filter system, whilst debris is efficiently removed via the secondary flow path. The inventors have found that filters typically used in a downforce system may cause large pressure drops which can negatively impact performance of the downforce system. For example, although a cyclonic filtration apparatus may provide high filtering effectiveness, there is also a large pressure drop across the cyclonic filtration apparatus. This may limit the amount of air which can be evacuated from the low-pressure region under the vehicle, which in turn limits a pressure differential between the low-pressure region and the surrounding atmosphere. Accordingly, the generated downforce acting on the vehicle may be reduced when a cyclonic filtration apparatus is used. Other known types of filters such as mesh filters or cartridge filters may rapidly become clogged, resulting in an increased pressure drop and therefore a reduced downforce. In contrast, the air filter system of the invention may be particularly suited to use in a downforce system for a vehicle, e.g. to remove debris from an air flow through the downforce system. In particular, the low pressure drop across the air filter system may enable the pressure differential between the low-pressure region under the vehicle and the surrounding atmosphere to be increased, resulting in an increased downforce acting on the vehicle. The air filter system of the invention may also contribute to improved energy efficiency of the downforce system, as the air flow source of the downforce system may require less power to generate a given downforce. Moreover, as debris is deflected towards the secondary flow path, the primary flow path may remain unimpeded during use, which may avoid increases in pressure drop across the air filter system over time. In contrast, in many conventional filters a build-up of debris in the filter may cause an increase in the pressure drop across the filter over time. Although the air filter system of the invention is discussed above in the context of a downforce system, it can be used in other vehicle systems where it is desirable to remove debris from an air flow. For example, the air filter system may be used as an air filter for an engine of the vehicle. Other uses of the air filter system include filtration of air for interior cooling systems, such as battery cooling. According to a first aspect of the invention, there is provided an air filter system for a vehicle, the air filter system comprising: a separation chamber comprising an air inlet, a primary air outlet connected to a primary flow path and a secondary air outlet connected to a secondary flow path, the separation chamber containing a deflector; an air flow source connected to the primary air outlet via the primary flow path, wherein the air flow source is configured to generate an air flow through the separation chamber from the air inlet towards the primary air outlet; wherein the deflector is configured to deflect debris from the air flow towards the secondary air outlet; and wherein the secondary flow path comprises a debris trap for capturing debris from air flowing along the secondary flow path. The separation chamber may be a substantially hollow body through which air can flow, with the deflector being located within the hollow body. The air inlet, primary air outlet and secondary air outlet may be formed by respective openings or apertures in the separation chamber. The primary flow path is connected to the primary air outlet, such that air flowing out of the separation chamber via the primary air outlet travels along the primary flow path. For example, the primary flow path may be defined by a passageway (e.g. a tube, pipe or channel) that is connected to primary air outlet. The secondary flow path is connected to the secondary air outlet, such that air flowing out of the separation chamber via the secondary outlet travels along the secondary flow path. For example, the secondary flow path may be defined by a passageway (e.g. a tube, pipe or channel) that is connected to secondary air outlet. The air flow source is connected to the primary flow path. In other words, the primary flow path is connected between the primary air outlet of the separation chamber and the air flow source. In this manner, the air flow source can generate an air flow through the separation chamber from the air inlet towards the primary air outlet. In particular, the air flow source can generate an air flow along the primary flow path, e.g. by drawing air from the separation chamber along the primary flow path, which in turn causes the air flow in the separation chamber from the air inlet towards the primary air outlet. The air flow source may comprise any suitable apparatus for generating an air flow. For example, the air flow source may comprise a fan, a blower, an air pump or the like. In some cases, the air flow source may comprise a centrifugal fan. The air flow source may be active, i.e. powered. The deflector is configured to deflect debris from the air flow towards the secondary air outlet. In this manner, debris carried by the air flow may enter the secondary flow path, where it can be captured in the debris trap. In particular, the deflector may comprise a structure that is positioned and shaped so that debris carried by the air flow is deflected by the structure towards the secondary air outlet. For example, the deflector may comprise a surface configured to deflect debris from the air flow towards the secondary air outlet. In some embodiments, the deflector may be formed of a hollow body. This may serve to reduce a mass of the air filter system. The secondary air outlet may be positioned so that it is located on a deflection trajectory followed by debris deflected by the deflector, to ensure that deflected debris exits the separation chamber via the secondary air outlet. On the other hand, the primary air outlet may be positioned away from the deflection trajectory, to avoid debris exiting the separation chamber via the primary air outlet. The primary and secondary air outlets may be shaped and positioned to ensure a high level of air flow from the separation chamber through the primary air outlet, whilst minimising an amount of debris reaching the primary air outlet. Herein, the term ‘debris’ may generally refer to any dust, particles, dirt, water droplets, or debris that may be entrained in an air flow. Debris sizes may, for example, range from 0.05 mm to 5 mm, although the air filter system may be adapted to handle debris of different sizes. The secondary flow path comprises (or is connected to) the debris trap, which is configured to capture debris from air flowing along the secondary flow path. In this manner, debris that was deflected by the deflector towards the secondary air outlet may be captured in the debris trap in the secondary flow path. This may ensure that debris is captured and retained by the air filter system, and is not blown out of an exhaust of the air filter system. The secondary flow path may pass through (at least part of) the debris trap. Any suitable device for capturing debris from an air flow may be used as the debris trap. As an example, the debris trap may comprise a receptacle (or debris collector) for receiving debris from the air flowing along the secondary flow path. The debris trap may be arranged such that debris carried by air flowing along the secondary flow path drops into the receptacle. Other types of debris trap such as baffles, a panel filter, a cartridge filter, a cone filter, and / or a cyclonic filtration apparatus may be used. In some cases, the secondary flow path may comprise two or more debris traps connected in series, each debris trap being configured to capture debris from air flowing along the secondary flow path. This may serve to maximise a proportion of the debris carried along the secondary flow path which is captured. The two or more debris traps may include different types of debris traps, which may facilitate capturing different types of debris (e.g. debris of different sizes). In use, the air flow source may be activated to generate the air flow through the separation chamber from the air inlet towards the primary air outlet. Thus, air may flow into the separation chamber via the air inlet, and flow generally towards the primary air outlet. After entering the separation chamber, the airflow encounters the deflector which causes debris carried by the air flow to be deflected towards the secondary air outlet. The deflector may also deflect the air flow in the separation chamber, e.g. the air flow may pass along a surface of the deflector. Due to its inertia, the deflected debris may exit the separation chamber via the secondary air outlet, whilst a majority of the air flow may be pulled towards the primary air outlet under action of the air flow source. In this manner, a majority of the air flow may exit the separation chamber via the primary air outlet with minimal impedance, whilst the deflected debris and a small portion of the air flow may exit the separation chamber via the secondary air outlet. The deflected debris may pass into the secondary flow path, where it is captured and retained by the debris trap. As a result, air flowing along the primary flow path may be substantially free of debris. The air flowing along the primary flow path may subsequently exit the air filter system via an exhaust or outlet of the air filter system, such that air exiting the system is substantially free of debris. As the debris trap is located on the secondary flow path, air flowing through the primary flow path will see very little impedance on its journey through the air filter system, which may result in a very low pressure drop for air flowing along this path. Moreover, as the air flow source is connected to the primary flow path, a majority of the air flow in the system can be made to flow along the primary air flow path, resulting in a low overall pressure drop across the air filter system. In other words, only a small portion of the total air flow through the separation chamber may pass through the secondary flow path, such that the overall pressure drop across the air filter system may be low. As the air flow source is connected to the primary flow path, this may avoid debris coming into contact with the air flow source, thus reducing a risk of damage and wear to the air flow source. Further, as the debris trap is located in the secondary flow path, build-up of debris in the debris trap may not have any substantial effect on air flowing through the primary flow path, which may enable reliable performance of the air filter system over extended periods of time. The air flow source may further be configured to generate an air flow along the secondary flow path. This may ensure that there is an air flow along the secondary flow path, to entrain debris along the secondary flow path towards the debris trap. Additionally, using the same air flow source to generate the air flow along both the primary and the secondary flow paths may serve to simplify the air filter system. An air outlet of the secondary flow path may be connected to the primary flow path. This may enable the air flow source to generate the air flow along the secondary flow path. In particular, when the air flow source is activated, it generates an air flow along the primary flow path which in turn may cause air flow along the secondary flow path towards the air outlet of the secondary flow path. Thus, with this arrangement, the air flow source can generate air flow along both the primary and secondary flow paths, whilst only having to connect the primary flow path to the air flow source, which may simplify a configuration of the air flow source. The secondary flow path may extend between the secondary air outlet in the separation chamber and its air outlet which is connected to the primary flow path. The debris trap may then be located between the secondary air outlet and the air outlet of the secondary flow path, i.e. the debris trap may be located on the secondary flow path upstream of the air outlet of the secondary flow path. In this manner, the air flow along the secondary flow path may pass through the debris trap, so that debris carried by the air flow may be captured in the debris trap. Alternatively, a separate (e.g. second) air flow source may be connected to the secondary flow path to generate an air flow along the secondary flow path. In other words, a first air flow source may be connected to the primary flow path, and a second air flow source may be connected to the secondary flow path. This may facilitate control of relative flow rates through the primary and secondary flow paths. The deflector and primary air outlet may be arranged such that a direction of the air flow incident on the deflector is substantially aligned with a direction of the air flow through the primary air outlet. This may serve to enhance an air flow rate and air flow speed through the separation chamber from the air inlet to the primary air outlet, thus reducing a pressure drop across the separation chamber. In particular, as the direction of the air flow incident on the deflector is substantially aligned with the direction of air flow through the primary outlet, air flow through the separation chamber to the primary air outlet may be along a same general direction. In other words, the air flow incident on the deflector may pass around the deflector, to exit the separation chamber along the same direction. This arrangement may avoid sharp bends or changes of direction in the air flow through the separation chamber, which might cause a slowing of the air flow and an increase in the pressure drop across the separation chamber. The deflector may comprise a deflection surface located at an upstream end of the deflector, the deflection surface being configured to deflect the debris from the air flow towards the secondary outlet. In this manner, the air flow through the separation chamber may be incident on the deflection surface of the deflector, which acts to deflect debris from the air flow towards the secondary air outlet. The upstream end of the deflector may be an end of the deflector that faces in an upstream direction of the air flow, i.e. towards the oncoming air flow. A shape of the deflection surface may be arranged to deflect the debris towards the secondary air outlet. For example, at least part of the deflection surface may have a non-zero angle relative to a direction of the airflow incident on the deflection surface, which may cause the debris to be deflected away from the direction of the incident air flow and towards the secondary air outlet. The deflection surface may have a shape that bulges outwards from the upstream end of the deflector. In other words, a cross-sectional area of the deflector may increase away from its upstream end. This may enable effective deflection of debris outwards from the air flow and towards the secondary air outlet. As an example, the deflection surface may have a conical shape, with a tip of the conical shape facing towards the upstream direction of the air flow. The tip of the conical shape may be pointed or rounded. In this manner, the deflection surface may effectively deflect the debris towards the secondary air outlet, whilst reducing a drag and turbulence of the air flow through the separation chamber. This may contribute to enhancing air flow rate and air flow speed through the separation chamber towards the primary air outlet, thus reducing a pressure drop across the separation chamber. An upstream end of the deflector may be reinforced. This may serve to protect the upstream end from debris impacts, reducing a risk of damage to the upstream enddue to debris impacts. As a result, a durability and reliability of the air filter system may be improved. For example, the upstream endmay comprise a reinforcing element (e.g. plate), e.g. a plate made of metal (e.g. steel), a composite material or some other impact-resistant material. As an example, composite materials containing aramid fibres such as Kevlar or polymer composite materials such as Dyneema ® can be used. The reinforcing element may be disposed on an outside of the deflector, at the upstream end. Additionally or alternatively, a thickness of a material forming the deflector may be increased in a region forming the upstream end, in order to reinforce the upstream end. The deflector may comprise a guide surface configured to guide the air flow in the separation chamber towards the primary air outlet. In this manner, the guide surface of the deflector may guide the air flow towards the primary air outlet, so that a majority of the air flow exits the separation chamber via the primary air outlet, whilst the inertia of the deflected debris causes it to continue on its trajectory towards the secondary air outlet. In particular, the inventors have found that using a surface of the deflector to guide the air flow towards the primary air outlet may improve air flow through the separation chamber and increase a proportion of the airflow that exits the separation chamber via the primary air outlet. As a result, this arrangement of the deflector may contribute to a reduced pressure drop across the separation chamber. The deflector may thus serve the dual purposes of deflecting debris towards the secondary air outlet, and guiding air flow towards the primary air outlet. The guide surface may have any suitable shape for guiding the air flow towards the primary air outlet. For example, at least part of the guide surface may extend towards the primary air outlet, so that air can flow along the guide surface towards the primary air outlet. The guide surface may be shaped so that the air flow attaches itself to the guide surface, so that the air flow follows the guide surface to reach the primary air outlet. The guide surface may be located at (towards) a downstream end of the deflector. In other words, the guide surface may be located at an end of the deflector facing towards the primary air outlet. This may enable the guide surface to effectively guide the air flow towards the primary air outlet. The guide surface of the deflector may be adjacent to the deflection surface mentioned above, and may act to guide the air flow back towards the primary air outlet following deflection caused by the deflection surface. In particular, the deflection surface may be located at the upstream end of the deflector, whilst the guide surface may be located at the downstream end of the deflector. The deflector may comprise a curved side surface. This may facilitate guiding the air flow towards the primary air outlet. In particular, following deflection caused by the deflection surface of the deflector, the curved side surface may act to direct the air flow back towards the primary air outlet. The curved side surface may be shaped so that the airflow attaches itself (i.e. sticks) to the curved side surface, so that the air flow can be efficiently guided towards the primary air outlet. Providing the deflector with a curved side surface may further contribute to reducing drag through the separation chamber and reducing turbulence in the separation chamber, thus enhancing flow rate and air flow speed through the separation chamber. The deflection surface and the guide surface may join together at the curved side surface, i.e. the curved side surface may be located at an interface between the deflection surface and the guide surface. In this manner, the curved side surface may provide a smooth transition between the deflection surface to the guide surface, to promote efficient air flow along an outside of the deflector towards the primary air outlet. An air flow passage in the separation chamber may be defined between a sidewall of the separation chamber and the deflector, and a portion of the air flow passage may progressively narrow to accelerate the air flow. This may cause an acceleration of debris carried by the air flow, thus increasing its inertia and making it less likely to change direction and return towards the primary air outlet following its deflection towards the secondary air outlet. In this manner, a possibility of debris passing through the primary air outlet may be reduced, thus improving a filtering efficiency of the air filter system. The progressive narrowing of the air flow passage may be caused by a shape of the sidewall of the separation chamber and / or the deflector. For example, a distance between a surface of the deflector and the sidewall of the separation chamber may become progressively smaller along a direction of the air flow through the separation chamber. For instance, a cross-sectional area of the deflector may increase along the direction of the air flow through the separation chamber, causing the air flow passage to progressively narrow. In some cases, the air flow passage may be defined between the deflection surface of the deflector mentioned above and the sidewall of the separation chamber. The outwards bulging of the deflection surface may cause the progressive narrowing of the air flow passage. The air flow passage may progressively narrow towards a narrowest point (or region) of the air flow passage. The air flow passage may then widen following the narrowest point, for example in a vicinity of the primary air outlet. In other words, the air flow passage may progressively narrow and then progressively widen along a direction of air flow through the separation chamber. Such a widening of the air flow passage may result in a pressure decrease in the vicinity of the primary air outlet, which may encourage the air flow towards the primary air outlet. In line with the discussion above, the subsequent widening of the air flow passage may result from the distance between the surface of the deflector and the sidewall of the separation chamber becoming progressively larger in the direction of air flow through the separation chamber, following the narrowest point. For example, the cross-sectional area of the deflector may decrease along the direction of the air flow through the separation chamber following the narrowest point of the air flow passage, causing the air flow passage to progressively widen. In this way, the airflow passage defined between the sidewall of the separation chamber and the deflector may be configured as a Venturi passage. The primary flow path may comprise a Venturi passage, to accelerate the air flow along the primary flow path. The Venturi passage may be located in an upstream portion of the primary flow path, e.g. adjacent to the primary air outlet, which may promote rapid air flow through the primary air outlet into the primary flow path. The Venturi passage may be formed by a narrowing and subsequent widening along a portion of the primary flow path. The secondary air outlet may be arranged around the primary air outlet. In other words, the secondary air outlet may at least partially extend around the primary air outlet. In some cases, the secondary air outlet may surround the primary air outlet. In this manner, a majority of the air flow through the separation chamber may be directed towards the primary air outlet, whilst the debris may be deflected radially outwards to the surrounding secondary air outlet. This arrangement of the primary and secondary air outlets may serve to improve an efficiency with which debris can be deflected into the secondary flow path. In particular, with the secondary air outlet arranged around the primary outlet, the secondary air outlet can receive debris deflected in different directions around the primary air outlet. As an example, the secondary air outlet may comprise an annular (e.g. ring-shaped) outlet arranged around the primary air outlet. The secondary air outlet may be concentric with the primary air outlet The secondary air outlet may comprise two or more secondary air outlets arranged around the primary air outlet. In other words, there may be multiple secondary air outlets arranged around the primary air outlet. The two or more secondary air outlets may be arranged in line with the above discussion, e.g. in an annular or ring-shaped arrangement around the primary air outlet. Providing two or more secondary air outlets around the primary air outlet may increase the likelihood of debris being deflected into one of the secondary air outlets. Moreover, using two or more secondary air outlets may facilitate covering a wide range of deflection angles around the primary air outlet, which may further increase the likelihood of capturing deflected debris. The two or more secondary air outlets may each be connected to the secondary flow path, so that debris deflected into any of the secondary air outlets can be captured in the debris trap as discussed above. The secondary flow path may comprise two or more secondary outlet passages, each of which is connected to a respective one of the secondary air outlets, and wherein the two or more secondary outlet passages are joined together by a manifold upstream of the debris trap. This enables each of the two or more secondary air outlets to be joined together upstream of the debris trap, so that debris deflected into any of the secondary air outlets can be captured in the debris trap. Using respective secondary outlet passages which are connected together by a manifold may facilitate capturing debris, as the same debris trap can be used to capture debris exiting the separation chamber by any of the two or more secondary air outlets. The manifold may have an upstream end which is connected to each of the secondary outlet passages. The secondary flow path may then comprise a passage that connects a downstream end of the manifold to the debris trap. In other words, the manifold may combine the two or more secondary outlet passages into one passage that is connected to the debris trap. At least one of the two or more secondary outlet passages may curve at least partially around an outside of the primary flow path. This may facilitate connecting each of the two or more secondary outlet passages to the manifold. In particular, with the two or more secondary air outlets arranged around the primary air outlet, curving at least one of the two or more secondary outlet passages around an outside of the primary flow path may enable each of the two or more secondary outlet passages to be joined together at the manifold. Any bends in the two or more secondary outlet passages may have a radius of curvature equal to or greater than four times a width (e.g. diameter) of one of the two or more secondary outlet passages. The inventor has found that keeping the radius of curvature of bends in the secondary outlet passages in this range may avoid excessive slowing down of air flow through the secondary outlet passages, which might cause debris to drop out of the air flow and fall back into the separation chamber. In other words, keeping the radius of curvature of bends in the secondary outlet passages in this range may increase the likelihood of the debris reaching the debris trap. Accordingly, keeping the radius of curvature of bends in the secondary outlet passages in this range may improve a filtering efficiency of the air filter system. Each secondary outlet passage may be arranged such that a minimum cross-sectional area of the secondary outlet passage is not less than a cross sectional area of the passage connecting the manifold to the debris trap. The inventors have found that this may ensure adequate air flow for carrying the debris to the debris trap. The deflector may be centred relative to the primary air outlet. For example, a longitudinal axis of the deflector may be substantially aligned with a centre of the primary air outlet. This may reduce a disruption to the air flow through the separation chamber caused by the deflector, and serve to guide the air flow towards the primary air outlet, e.g. via the guide surface of the deflector discussed above. The deflector may comprise a tip located at a downstream end of the deflector, the tip being centred relative to the primary air outlet. This may serve to guide the air flow towards the primary air outlet, with the tip acting to direct the air flow towards the primary air outlet. The tip may represent a narrowing (or tapering) of the deflector towards its downstream end, giving it a streamlined shape which may enhance its aerodynamic characteristics and promote efficient air flow through the separation chamber. In some cases, the tip may protrude into the primary air outlet. This may further improve the deflector’s ability to guide the air flow towards the primary air outlet, as the air flow can follow the surface of the deflector directly into the primary air outlet. The tip may be located at an upstream end of the guide surface of the deflector discussed above. In other words, the guide surface may terminate at the tip. The tip located at the downstream end of the deflector may be pointed. This may further serve to enhance the aerodynamic characteristics of the deflector. Alternatively, the tip may be rounded. The deflector may be suspended inside the separation chamber, such that the air flow passes through an annular passage around the deflector. In this manner, air may flow around a periphery of the deflector, which may enable an increased flow rate through the separation chamber. Additionally, this may contribute to making the air flow more uniform within the separation chamber, which may reduce turbulence and promote higher air flow speeds. Suspending the deflector inside the separation chamber may also facilitate centring the deflector relative to the primary air outlet. For example, the deflector and the primary air outlet may be arranged such that they are substantially centred about a longitudinal axis of the separation chamber, which may contribute to efficient air flow through the separation chamber to the primary air outlet. The annular passage may be defined between a sidewall of the separation chamber and an outer surface of the deflector. In particular, the annular passage may correspond to the air flow passage defined between the sidewall of the separation chamber and the deflector, which progressively narrows and subsequently widens, as discussed above. The deflector may be suspended inside the separation chamber via one or more support elements. The one or more support elements may, for example, extend between the sidewall of the separation chamber and the deflector. The one or more support elements may be aerodynamically shaped, to minimise a disruption to the air flow in the separation chamber. For example, the one or more support elements may be in the form of vanes or blades. The deflector and a surrounding portion of the separation chamber may be integrally formed as one part. This may facilitate assembly of the air filter system, and improve integrity of the separation chamber, as the separation chamber may not need to be openable in order to mount the deflector. This may also improve aerodynamic characteristics of the separation chamber and deflector, as it may avoid using connectors or fasteners for mounting the deflector in the separation chamber. The deflector and the surrounding portion of the separation chamber may be made via additive manufacturing, e.g. using selective laser sintering or multi-jet fusion. The primary air outlet may comprise a lip that protrudes into the separation chamber, and the lip may flare outwards from the primary air outlet. The lip may act to inhibit debris from entering the primary air outlet, as debris located near an edge of the primary air outlet may bounce off of the lip away from the primary air outlet. Moreover, the outward flaring of the lip may encourage air flow into the primary air outlet, whilst deflecting debris away from the primary air outlet. The lip may also help prevent mixing of air and debris in the vicinity of the primary air outlet, acting as a barrier to the debris. The lip may extend around an edge of the primary air outlet, and protrude into the separation chamber. Thus, the lip may form a barrier between the primary air outlet and the secondary air outlet. This may be particularly beneficial in embodiments where the secondary air outlet is disposed around the primary air outlet, as the primary and secondary air outlets may be in close proximity. The secondary flow path may comprise a restrictor located downstream of the debris trap, the restrictor being configured to restrict air flow along the secondary flow path. This may enable air flow along the secondary flow path to be reduced, to enable efficient capture of debris by the debris trap. In particular, the inventors have found that by slowing down the air flow in the secondary flow path, an effectiveness of the debris trap may be improved. Locating the restrictor downstream of the debris trap may serve to ensure that debris reaches the debris trap and does not drop out of the air flow before the debris trap. Additionally, by adjusting the air flow along the secondary flow path, it is possible to control a proportion of the air flow from the separation chamber which enters the secondary flow path. Thus, restricting the air flow along the secondary flow path may reduce a proportion of the air flow exiting the separation chamber through the secondary flow path and increase a proportion of the air flow exiting the separation chamber through the primary flow path, in turn reducing a total pressure drop across the air filter system. It should be noted that in some embodiments the restrictor may be located upstream of the debris trap, rather than downstream. The restrictor may correspond to a constriction or impedance in the secondary flow path. For example, the restrictor may be implemented as an orifice plate or a washer disposed in the secondary flow path. The restrictor may be adjustable, to adjust the air flow along the secondary flow path. For example, adjusting the restrictor may enable a speed of the air flow along the secondary flow path to be controlled. This may enable debris capture by the debris trap to be adjusted or optimised. For example, reducing the restriction in the secondary flow path may increase the air flow speed along the secondary flow path, which may increase an amount of debris carried along the secondary flow path to the debris trap. However, faster moving debris may be less likely to be captured by the debris trap. On the other hand, increasing the restriction in the secondary flow path may reduce the air flow speed along the secondary flow path, which may reduce an amount of debris carried along the secondary flow path to the debris trap. However, slower moving debris may be more likely to be captured by the debris trap. Adjusting the restrictor may also enable the relative proportions of air flow from the separation chamber entering the primary and secondary flow paths to be adjusted. For instance, adjusting the restrictor to reduce air flow through the secondary flow path may increase a proportion of the air flow exiting the separation chamber via the primary air outlet and reduce a proportion of the air flow exiting the separation chamber via the secondary air outlet, and vice versa. The adjustable restrictor may be implemented using any suitable mechanism. For example, the adjustable restrictor may be implemented as an adjustable valve or the like. The debris trap may be arranged at a height below the secondary air outlet. This may facilitate capturing debris with the debris trap, as gravity may cause debris to fall out of the air flow into the debris trap. The secondary flow path may comprise a portion in which the air flow along the secondary flow path is in a downwards direction, and wherein the debris trap is connected at an end of the portion of the secondary flow path. In this manner, the debris may travel downwards towards the debris trap, so that gravity may cause the debris to fall out of the air flow into the debris trap. A portion of the secondary flow path downstream of the debris trap may extend in an upwards direction, which may further contribute to debris dropping down into the debris trap. The debris trap may be arranged at a height below the air flow source. Similarly to the above, this may encourage debris to drop out of the air flow into the debris trap, under the action of gravity. The debris trap may comprise a plurality of baffles for slowing air flow through the debris trap. In this manner, air flowing through the debris trap may be slowed down, which may cause debris to fall out of the air flow into the debris trap. The plurality of baffles may be arranged to cause the air flow to travel along a convoluted path, e.g. such that the air flow must change direction and / or undergo sharp bends, thus causing the air flow to slow down. Accordingly, an efficiency of the debris trap may be improved. The debris trap may comprise a cyclone separator configured to separate debris from the air flowing along the secondary flow path. The cyclone separator allows for efficient separation of the debris from the air flowing along the secondary flow path, facilitating capture of the debris by the debris trap and cleaning of the air flowing along the secondary flow path. An air inlet of the cyclone separator may be connected to the secondary flow path to receive the (debriscarrying) air flowing along the secondary flow path. The cyclone separator may further comprise an air outlet via which air cleaned by the cyclone separator flows out of the cyclone separator. The air outlet of the cyclone separator may, for example, be connected to the primary flow path, e.g. so that the air flowing along the secondary flow path can rejoin the primary flow path. Alternatively, in line with the above, the air outlet of the cyclone separator may be connected to a second air flow source which is configured to generate the air flow along the secondary flow path. The cyclone separator may comprise a cyclone chamber arranged to form a cyclone with the air flow received from the secondary flow path. Cyclonic motion of the air flow in the cyclone chamber causes debris carried by the air flow to separate from the air flow. The cyclone separator may also be referred to as cyclonic filtration apparatus, or a cyclonic filter. The debris trap may comprise a debris receptacle for receiving debris from the air flowing along the secondary flow path, the debris receptacle being removable or emptiable. In this manner, the debris receptacle can be emptied when it becomes full of debris, to avoid the secondary flow path becoming clogged with debris. Providing the debris trap with the debris receptacle may facilitate removing debris from the debris trap, as the debris receptacle may be removed or emptied without dismounting other parts of the system. The debris receptacle may, for example, be in the form of a tray or container arranged to receive the debris from the second air flow. Where the debris trap comprises a cyclone separator, the debris receptacle may be connected to (or otherwise integrated with) the cyclone separator, so as to receive debris separated from the airflow by the cyclone separator. For example, the debris receptacle may be connected to a debris outlet of the cyclone separator, to receive debris from the cyclone separator. The debris receptacle may comprise an outlet valve that is openable to empty the debris receptacle. This may facilitate emptying of the debris receptacle, as the outlet valve can be controlled in order to empty the debris receptacle. Moreover, providing the debris receptacle with an outlet valve can reduce pressure losses via the outlet of the debris receptacle, by closing the outlet valve when the debris receptacle is not being emptied. This in turn contributes to reduction of the pressure drop across the air filter system. The outlet valve may be located at an outlet of the debris receptacle, such that debris can be evacuated from the debris receptacle when the outlet valve is opened. The outlet valve may comprise any suitable type of valve. The outlet valve may comprise an open state for emptying debris from the debris receptacle, and a closed state in which air flow through the outlet valve is prevented. The outlet valve may be controllable (operable) to move it between the open and closed states. In some cases, the outlet valve may be configured to be in the closed state when a pressure in the air filter system is different from (e.g. lower than) a surrounding atmospheric pressure. Typically, the air pressure in the air filter system will be lower than the atmospheric pressure during operation due to the generated air flows in the system. Accordingly, such an arrangement may prevent pressure losses of the system via the debris receptacle during operation of the system. The valve may be arranged to return to the open state when there is no pressure difference, or the pressure difference is below a predetermined threshold. In this manner, the debris receptacle may then be emptied when the system is inactive. As an example, the outlet valve may comprise a passive valve which is configured to be in the closed state when there is a pressure difference between an interior of the air filter system and the surrounding atmosphere. The passive valve may be configured to be in the open state when there is no pressure difference between an interior of the air filter system and the surrounding atmosphere, or when the pressure difference is below a predetermined threshold. Such a valve may be referred to as a suction valve. The suction valve is passive as a negative pressure in the air filter system causes it to seal. Thus, no active control of the suction valve may be needed, such that the valve can automatically open and close in response to changes in pressure in the air filter system. By way of example, an outlet of the suction valve may be formed of a flexible material (e.g. rubber), which is arranged to deform in response to a negative pressure in the interior of the air filter system, so as to seal the outlet of the suction valve. In some cases, the outlet valve may comprise a pinch valve. Advantageously, a pinch valve may be less likely to become obstructed (clogged) by debris compared to other types of valves such as solenoid valves. Pinch valves also enable tight seals to be formed, even when debris is present in the valve. A pinch valve may comprise a flexible membrane which is pinched together when the valve is closed. The air filter system may be configured to control opening and closing of the outlet valve of the debris receptacle. For example, the air filter system may comprise a controller configured to control (operate) the outlet valve. The air filter system may implement automated control of the outlet valve, to perform automated emptying of the debris receptacle, as discussed further below. The air filter system may further comprise an inlet valve at an inlet of the debris receptacle, the inlet valve being closable to prevent the air flow along the secondary flow path from entering the debris receptacle. Thus, respective valves may be provided both at the inlet and the outlet of the debris receptacle. This allows the inlet and outlet valves to be operated as an airlock, to avoid pressure loss via the debris receptacle during emptying of the debris receptacle. For example, the inlet valve can be closed prior to opening of the outlet valve for emptying of the debris receptacle. This prevents air flowing along the secondary flow path from exiting the system via the debris receptacle when the outlet valve of the debris receptacle is opened. Then, after emptying of the debris receptacle, the outlet valve can be closed, following which the inlet valve can be re-opened, to allow debris to enter the debris receptacle. Accordingly, the air filter system may be configured to put the inlet valve in a closed position when the outlet valve is in an open position. The air filter system may be configured to put the inlet valve in the closed position prior to putting the outlet valve in the open position. The controller mentioned above may be configured to control both the inlet valve and the outlet valve, e.g. to enable automated emptying of the debris receptacle. The inlet of the debris receptacle may be arranged such that debris carried by the air flow in the secondary flow path enter the debris receptacle via the inlet. Where the debris trap includes a cyclone separator, a debris outlet of the cyclone separator may be connected to the inlet of the debris receptacle. The inlet valve may comprise an open position for receiving debris in the debris receptacle, and a closed position which prevents air (and debris) flowing along the secondary flow path from entering the debris receptacle. The inlet valve may comprise any suitable type of valve. In some cases, the inlet valve may comprise a pinch valve, e.g. to provide the advantages mentioned above. The air filter system may further comprise an actuator configured to convey debris received in the debris receptacle towards an outlet of the debris receptacle. This may facilitate emptying of the debris receptacle, and avoid unwanted build-up of debris in the debris receptacle over time. Any suitable type of actuator capable of conveying debris towards the outlet of the debris receptacle may be used. As an example, the actuator may comprise a screw (e.g. an Archimedes screw), which is rotatable to convey debris along its axis with one or more threads. The axis of the screw may extend towards the outlet of the debris receptacle, so as to convey debris towards the outlet when the screw is rotated. As another example, the actuator may comprise an air flow source (e.g. blower, fan, compressed air source) arranged to blow the debris towards the outlet. As a further example, the actuator may comprise a movable member (e.g. a spatula or brush) arranged to push the debris towards the outlet. The actuator may be arranged to convey debris from the inlet of the debris receptacle towards the outlet of the debris receptacle. Where the debris receptacle comprises an outlet valve, the actuator may be configured to convey the debris towards the outlet valve. In this manner, when the outlet valve is opened, the actuator can be operated to evacuate the debris from the debris receptacle. The air filter system may be configured to control operation of the actuator. For example, the air filter system may be configured to operate the actuator to convey debris towards the outlet when the outlet valve is opened, in order to evacuate debris from the debris receptacle. The air filter system may be configured to control emptying of the debris receptacle, e.g. the air filter system may have a controller for controlling emptying of the debris receptacle. This allows emptying of the debris receptacle to be automated, thus facilitating use of the air filter system. The steps performed by the air filter system for emptying the debris receptacle may vary, depending on the configuration of the debris receptacle and debris trap. For instance, where the debris receptacle has an outlet valve, the air filter system may open the outlet valve to empty the debris receptacle. Where there is also an inlet valve, the air filter system may further close the inlet valve during emptying of the debris receptacle. Where there is an actuator, the air filter system may control the actuator to convey debris towards the outlet to empty the debris receptacle. As an example, the air filter system (or controller) may be configured to control emptying of the debris receptacle over time. For instance, the debris receptacle may be emptied at regular time intervals. This may avoid excessive build-up of debris in the debris receptacle. This also allows emptying of the debris receptacle to be dosed over time, which may avoid making large deposits of debris. As another example, the air filter system (or controller) may be configured to empty the debris receptacle when a (predetermined) filling threshold of the debris receptacle is reached. For example, the debris receptacle may comprise a sensor for detecting a filling level of the debris receptacle, so that the debris receptacle can be emptied when the filling level reaches the filling threshold. As a further example, the air filter system (or controller) may be configured to empty the debris receptacle based on one or more driving conditions of the vehicle. For instance, driving conditions on which emptying of the debris receptacle can be based include vehicle speed, vehicle location (e.g. GPS coordinates, and / or location on a race track), proximity to other vehicles, and whether the vehicle is travelling in a straight line or going around a bend. In this manner, it can be ensured that the debris receptacle is only emptied when suitable predetermined driving conditions are met. The air filter system may thus receive as inputs from one or more vehicle systems, to enable the air filter system to determine the one or more driving conditions and control emptying of the debris receptacle accordingly. The air filter system may be configured to control an amount of debris from the debris receptacle. This allows deposits of debris to be spread out and avoids making a large deposit of debris. For example, the air filter system may set a time interval over which the outlet valve is opened, to control the amount of debris emptied from the debris receptacle. The air filter system may be configured such that a majority of the air flow in the separation chamber exits the separation chamber via the primary air outlet. As noted above, this may result in a low overall pressure drop across the air filter system, as air flow exiting via the primary air outlet will have seen minimal impedance during its journey through the system (as it will not have passed through the debris trap). In contrast, air flowing through the secondary flow path may see a higher impedance, due to the debris trap. However, as only a relatively small proportion of the total air flow passes through the secondary flow path, the overall pressure drop across the air filter system may remain low. The relative proportions of the air flow exiting the separation chamber via the primary air outlet and the secondary air outlet may be determined by various parameters of the air filter system, such as relative dimensions (e.g. cross-sectional areas) of the primary flow path and the secondary flow path, and / or the restrictor in the secondary flow path discussed above. For example, the primary flow path may have a larger cross-sectional area than the secondary flow path, which may contribute to a larger proportion of the air flow exiting the separation chamber to pass via the primary air outlet than via the secondary air outlet. In some embodiments, the primary flow path and the secondary flow path may be arranged such that 80%-98% of the air flow through the separation chamber exits the separation chamber via the primary air outlet, and 2%-20% of the air flow through the separation chamber exits the separation chamber via the secondary air outlet. This may ensure that there is a low pressure drop across the air filter system, as most of the air flow will pass through the primary outlet and therefore will see minimal impedance on its way through the air filter system. In line with the discussion above, these relative flow proportions may be achieved based on relative dimensions (e.g. cross-sectional areas) of the primary flow path and the secondary flow path, and / or by adjusting the restrictor in the secondary flow path. In some cases, 80-95% of the air flow through the separation chamber may exits the separation chamber via the primary air outlet, and 5%-20% of the air flow through the separation chamber may exit the separation chamber via the secondary air outlet. The deflector may comprise a body having an adjustable shape. This may enable the shape of the deflector to be adjusted to compensate for ambient air pressure changes, air flow speed changes, and / or changes in average debris mass. In this manner, the air filter system may provide effective air filtering under a variety of different conditions. As an example, the deflector may have a deflection surface as discussed above, and an angle of the deflection surface relative to the direction of incident air flow on the deflector may be adjustable. For instance, the deflection surface may be cone-shaped (e.g. with a rounded end), with the cone-shaped deflection surface being adjustable. In particular, a radius of the cone may be adjustable, so that an angle of the deflection surface relative to the incident air flow can be adjusted. The air filter system may further comprise: a debris sensor for detecting debris in the primary flow path; and a controller configured to control one or more operating parameters of the air filter system based on an output signal received from the debris sensor. This may enable the air filter system to be tuned based on the debris, to provide more effective filtering. For example, if debris is detected by the debris sensor in the primary flow path, the controller may adjust the one or more operating parameters in order to reduce a volume or mass of debris entering the primary flow path. Additionally or alternatively, a debris sensor may be arranged in a different location in the air filter system. In other words, the debris sensor need not necessarily be in the primary flow path. For example, a debris sensor may be located in the separation and / or in the secondary flow path. The one or more operating parameters may include any adjustable parameters of the air filter system. For instance, the one or more operating parameters may include a restriction in the secondary flow path (e.g. the controller may control the adjustable restrictor discussed above), a shape of the deflector (e.g. the controller may control the adjustable shape of the deflector discussed above), and / or a speed of the air flow (e.g. the controller may control the air flow source to adjust a speed of the air flow). The one or more operating parameters may also include a speed of any further air flow sources used in the air filter system, such as the second air flow source mentioned above (for generating an air flow along the secondary flow path) or the additional air flow source mentioned below. The air filter system may be used in various different environments, and so may encounter different types of debris. For example, common debris types may include dust, grass, sand, rubber particles, pebbles, etc. Different types of debris may have different shapes and masses, meaning that they will have different inertias as they travel through the separation chamber. Accordingly, by adjusting operating parameters of the air filter system based on the debris detected by the debris sensor, it is possible to improve the air filter system’s ability to capture the type of debris currently passing through the air filter system. As an example, if, based on the output signal from the debris sensor, the controller determines that an average debris size is large (e.g. an average size or mass of detected debris exceeds a predetermined threshold), then the controller may be configured to control the air flow source to generate a lower air flow speed. On the other hand, if, based on the output signal from the debris sensor, the controller determines that an average debris size is small (e.g. an average size or mass of detected debris is less than the predetermined threshold), then the controller may be configured to control the air flow source to generate a higher air flow speed. The debris sensor may comprise any suitable type of sensor for detecting debris flowing along the primary flow path. The debris sensor may be capable of detecting an existence, size and / or mass of the debris. The controller may comprise any suitable processor or computing system for performing the described control steps. By way of example, the debris sensor may include an optical sensor (e.g. cameras, lasers, light gates). In some cases, the debris sensor may detect debris by detecting impact on a surface in the air filter system. For instance, the debris sensor may include one or more strain gauges for detecting impacts of debris on a surface. As another example, an air quality sensos (e.g. laser particle counter, light scattering detector, or beta attenuation monitor) may be used to observe debris exiting an outlet of the air filter system. A mass of debris collecting in the debris trap may also be measured for obtaining information about debris passing through the air flow system. For example, the debris trap may include a sensor for detecting a mass of debris contained in the debris trap. The air filter system may further comprise an additional air flow source connected to the separation chamber, and configured to urge debris in the air flow towards the secondary air outlet. For example, the additional air flow source may be arranged to blow debris carried by the air flow in the separation chamber towards the secondary air outlet. Thus, the additional air flow source may cooperate with the deflector, to increase a proportion of debris exiting the separation chamber via the secondary air outlet. In some cases, the additional air flow source may be configured to generate an air flow to accelerate entrained debris in the air flow passage defined between the sidewall of the separation chamber and the deflector (e.g. the curved side surface of the deflector). In such a case, the air flow generated by the additional air flow source may not necessarily be directed towards the secondary air outlet, but may act to accelerate the debris to encourage it towards the secondary air outlet. The additional air flow source may include any suitable air flow source, such as a fan, blower, or compressed air source. In some embodiments, the air filter system may comprise two or more separation chambers connected in series, which may improve a filtering efficiency of the system. For example, an air inlet of a second separation chamber may be connected to the primary flow path of a first separation chamber, so that air flowing out of the first separation chamber via the primary flow path then passes through the second separation chamber. Each separation chamber may have a respective deflector and be connected to a respective secondary flow path, which may function as described above. In this manner, the air flow may be progressively filtered as it passes through each separation chamber. In some embodiments, an apparatus may be formed by connecting two or more air filter systems of the invention in parallel to the same air flow source. In other words, the primary flow path of each of the two or more air filter systems may be connected to a same air flow source, such that the air flow source is configured to generate an air flow through the separation chamber of each air flow system, as discussed above. Connecting multiple air filter systems in parallel in this manner may serve to increase a filtration capacity, without a significant increase to power requirements as a common air flow source is used. The filter system of the first aspect of the invention may be used as part of a downforce system for a vehicle. Thus, according to a second aspect of the invention, there is provided a downforce system for a vehicle, the downforce system comprising: a barrier configured to restrict a flow of air into a region under the vehicle; and an air filter system according to the first aspect of the invention; wherein an air outlet of the barrier is connected to the air inlet of the separation chamber, such that the air flow source is configured to generate a pressure differential across the barrier. In this manner, when the air flow source is activated, air may flow from the region under the vehicle, and into the separation chamber of the air filter system. Debris entrained by the air flow may then be deflected towards the secondary air outlet and captured by the debris trap, as discussed in relation to the first aspect of the invention. Air flowing along the primary flow path will therefore have a reduced debris content, and can be expelled from an exhaust or outlet of the air filter system. Thus, debris picked up in the air flow by the downforce system may be captured, so that it is not blown out from the exhaust of the system. As a result, a safety and performance of the downforce system may be improved. Any of the features discussed above in relation to the first aspect of the invention may be shared with the second aspect of the invention. The barrier is configured to restrict a flow of air into the region under the vehicle. In this manner, when air is evacuated from the region due to activation of the air flow source, a pressure differential may be generated between the region under the vehicle and the surrounding atmosphere, resulting in a downforce acting on the vehicle. In other words, activation of the air flow source may result in a lowering of the pressure in the region under the vehicle relative to atmospheric pressure, resulting in the downforce. The barrier may be any suitable barrier capable of restricting air flow into the region under the vehicle. For example, the barrier may be arranged to extend downwards from an underside of the vehicle towards the ground surface, to thereby restrict air flow into the region under the vehicle. The barrier may comprise a rim disposed at a lower end of the barrier. The rim may be configured to form an at least partial seal with a ground surface on which the vehicle is disposed, to further restrict air flow into the region. The downforce system may comprise an upper surface, from which the barrier is arranged to extend downwards towards the ground surface. The upper surface may be configured to be disposed on an underside of the vehicle. For example, the upper surface may be mountable on an underside of the vehicle. Alternatively, the upper surface may be provided by a part of the underside of the vehicle, i.e. the upper surface of the barrier may correspond to part of the underside of the vehicle. The upper surface may be defined by a plate, e.g. which is mountable on the underside of the vehicle, or which is provided on an underside of the vehicle. Herein, an underside of a vehicle may refer to an undercarriage of the vehicle. The underside of the vehicle may be a lower part of a chassis of the vehicle which faces towards the ground surface. Thus, in use, the upper surface may face towards the ground surface. The air outlet of the barrier may be formed in the upper surface, so that air from inside the region under the vehicle can be evacuated via the air outlet. Other locations of the air outlet of the barrier are also possible. The air outlet of the barrier may be connected to the air inlet of the separation chamber via any suitable air flow passage (e.g. tube, pipe or channel). The barrier may be configured to allow relative movement between the rim and the upper surface. For example, the barrier may comprise a flexible or supple material, such that the barrier may deform (e.g. bend, flex or stretch) in response to relative movement between the rim and the upper surface. For instance, the flexible material may comprise a textile material (e.g. Nylon), rubber and / or a suitable composite material. In some cases, the barrier may be in the form of flexible bellows. However, the barrier need not necessarily include a flexible material. For example, the barrier may comprise a telescopic coupling, e.g. made of two or more telescopic sections, which are movable relative to one another so as to enable relative movement between the rim and the upper surface. The downforce system may further comprise an actuation system configured to control a height of the rim above the ground surface. In this manner, for example, the rim can be lifted away from the ground surface when the downforce system is not in use. The air filter system of the first aspect of the invention may be incorporated into a vehicle. Thus, according to a third aspect of the invention, there is provided a vehicle comprising an air filter system according to the first aspect of the invention. The vehicle may be any type of vehicle such as a road car or race car. The vehicle may be an electric vehicle, a hybrid vehicle, a vehicle with an internal combustion engine, a fuel-cell-powered vehicle, or any other type of powered vehicle. The air filter system may be configured to filter air for use in one or more vehicle systems. According to a fourth aspect of the invention, there is provided a vehicle comprising a downforce system according to the second aspect of the invention. The barrier of the downforce system may be arranged to restrict a flow of air into a region under the vehicle. For example, the barrier may be mounted on an underside of the vehicle, and arranged to extend downwards from the underside of the vehicle. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are discussed below with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of an air filter system according to an embodiment of the invention; Fig. 2 is a schematic diagram of an air filter system according to an embodiment of the invention; Fig. 3a is a schematic diagram showing a cutaway view of part of the air filter system of Fig. 2; Fig. 3b is a schematic diagram showing a cross-sectional view of a deflector of the air filter system of Fig. 2; Figs. 4a and 4b are schematic diagrams showing perspective views of a portion of the air filter system of Fig. 2; Fig. 5 is a schematic diagram showing a cross-sectional view of part of the air filter system of Fig. 2; Fig. 6 is a schematic diagram showing a cross-sectional view of a debris trap of the air filter system of Fig. 2; Fig. 7 is a schematic diagram showing a perspective view of the air filter system of Fig. 2; Fig. 8 is a schematic diagram showing a side view of a debris trap that may be used in an air filter system according to an embodiment of the invention; Fig. 9 is a schematic sectional side view of a cyclone separator of the debris trap of Fig. 8; Fig. 10 is a schematic sectional side view of a debris receptacle of the debris trap of Fig. 8; Fig. 11 is a schematic sectional top view of the debris receptacle of Fig. 8; and Fig. 12 is a schematic diagram of a vehicle according to an embodiment of the invention. DETAILED DESCRIPTION; FURTHER OPTIONAL FEATURES Fig. 1 shows a schematic diagram of an air filter system 100 according to an embodiment of the invention. The air filter system 100 comprises a separation chamber 102 having an air inlet 104 via which air can flow into the separation chamber 102, as indicated by arrows 106. The separation chamber 102 includes a primary air outlet 108 connected to a primary flow path 110, and a secondary air outlet 112 connected to a secondary flow path 114. The primary air outlet 108 may be arranged at an opposite end of the separation chamber 102 relative to the air inlet 104. Additionally, a deflector 116 is disposed within the separation chamber 102. An air flow source 118 is connected to the primary air outlet 108 via the primary flow path 110, which may be implemented via any suitable duct, pipe or tube. In other words, the primary flow path 110 is connected between the primary air outlet 108 and the air flow source 118. The secondary flow path 114 includes a debris trap 120, so that air flowing out of the separation chamber 102 via the secondary air outlet 112 passes through the debris trap 120. An outlet of the secondary flow path 114 is connected to the primary flow path 110, so that air can flow out of the secondary flow path 114 into the primary flow path 110. Thus, the debris trap 120 is connected between an upstream portion of the secondary flow path 114 which is connected to the secondary air outlet 112, and a downstream portion of the secondary flow path 114 which is connected to the primary flow path 110. The air flow source 118 may comprise any suitable device for generating an air flow, such as a fan, a blower, or an air pump. The air flow source 118 is configured to cause an air flow along the primary flow path 110, from the primary air outlet 108 towards the air flow source 118. This in turn causes an air flow through the separation chamber 102, from the air inlet 104 towards the primary air outlet 108. Thus, when the air flow source 118 is activated, this generates an airflow into the separation chamber 102 (see arrows 106) towards the primary air outlet 108. The deflector 116 is configured to deflect debris carried by the air flow in the separation chamber 102 towards the secondary air outlet 112, as indicated by arrow 122. Thus, debris and a small portion of the air flow may exit the separation chamber 102 via the secondary air outlet 112 to pass along the secondary flow path 114. In contrast, a majority of the air flow in the separation chamber 102 may pass around the deflector 116, and exit the separation chamber 102 via the primary air outlet 108, as indicated by arrows 124. An inertia of the deflected debris may cause it to continue on its trajectory towards the secondary air outlet 112 after deflection, whereas a majority of the air flow may be more readily pulled back towards the primary air outlet 108 under action of the air flow source 118 after deflection. In this manner, debris entering the separation chamber 102 with the air flow 106 may be filtered out via the secondary flow path 114, such that the airflow exiting the separation chamber 102 via the primary air outlet 108 and travelling along the primary flow path 110 (indicated by arrows 126) may contain a reduced proportion of debris, e.g. compared to the air flow 106. As the outlet of the secondary flow path 114 is connected to the primary flow path 110, the air flow generated along the primary flow path 110 by the air flow source 118 also generates an air flow along the secondary flow path 114 (indicated by arrow 128). As noted above, the air flow along the secondary flow path 114 carries debris that was deflected by the deflector 116 in the separation chamber 102. The air flow along the secondary flow path 114 passes through the debris trap 120, which is configured to capture debris from the airflow. In this manner, air flowing out of the debris trap 120 may have a reduced proportion of debris (e.g. compared to the air flow entering the debris trap 120). The air flowing out of the debris trap 120 (indicated by arrow 130) then exits the secondary flow path 114 and enters the primary flow path 110, where it is combined with the portion of the air flow that exited the separation chamber 102 via the primary air outlet 108 (shown by arrows 126). The combined air flow then passes from the primary flow path 110 through the air flow source 118 and is expelled via an outlet 132 of the air flow source 118. As debris carried by the incident airflow 106 entering the separation chamber 102 is deflected into the secondary flow path 114 where it is captured by the debris trap 120, the air flow exiting the outlet 132 of the air flow source 118 may be ‘clean’, i.e. it may carry a reduced proportion of debris compared to the incident air flow 106. The air filter system 100 may be designed such that a majority of the air flow in the separation chamber 102 exits the separation chamber 102 via the primary air outlet 108. In this manner, only a relatively small portion of the air flow through the air filter system 100 will pass through the debris trap 120 and experience an impedance due to the debris trap 120. As a result, a majority of the air flow through the air filter system 100 may only see a minimal impedance on its way through the air filter system 100, resulting in a low overall pressure drop across the air filter system 100. The pressure drop across the air filter system 100 may be defined as a pressure difference between a pressure at the air inlet 104 of the separation chamber 102 and a pressure at an inlet to the air flow source 118. The pressure drop thus corresponds to a weighted average of pressure drops experienced by the air flow passing through the primary air outlet 108 (which may see a relatively low impedance) and the air flow passing through the secondary air outlet 112 (which may see a relatively high impedance, due to the debris trap 120). Thus, in some embodiments, the air filter system 100 may be designed such that 80%-98% of the air flow through the separation chamber 102 exits the separation chamber 102 via the primary air outlet 108, and 2%-20% of the air flow through the separation chamber 102 exits the separation chamber 102 via the secondary air outlet 112. For example 95% of the air flow may pass through the primary air outlet 108 and 5% of the air flow may pass through the secondary air outlet 112. This may result in a very low pressure drop across the air filter system 100, whilst still providing a high filtering efficiency. The relative proportions of air flow through the primary air outlet 108 and the secondary air outlet 112 may be determined based on various parameters, such as relative dimensions (e.g. cross-sectional areas) of the primary flow path 110 and the secondary flow path 114 and the impedance caused by the debris trap 120. For example, the primary flow path 110 may have a larger cross-sectional area than the secondary flow path 114, so that a larger portion of the air flow travels via the primary air outlet 108 than the secondary air outlet 112. Additionally, the secondary flow path 114 may include a restrictor 134 for restricting air flow along the secondary flow path 114, as shown in Fig. 1. The restrictor 134 is located downstream of the debris trap 120, and defines an orifice through which the air flow in the secondary flow path 114 must pass before reaching the primary flow path 110. Thus, the restrictor 134 acts as a constriction in the secondary flow path 114, which may slow the air flow along the secondary flow path 114. The restrictor 134 may thus act to limit air flow along the secondary flow path 114, so that air will preferentially flow out of the separation chamber 102 via the primary air outlet 108. Additionally, slowing the air flow in the secondary flow path 114 may facilitate capturing of debris in the air flow by the debris trap 120, thus increasing an effectiveness of the debris trap 120. The restrictor 134 may be adjustable, e.g. to adjust a size of the orifice, so that flow speed along the secondary flow path 114 can be adjusted. Adjusting the restrictor 134 may also enable relative proportions of the air flow passing through the primary air outlet 108 and through the secondary air outlet 112 to the adjusted. For example, by adjusting the restrictor 134, the proportions of the air flow passing via the primary air outlet 108 and the secondary air outlet 112 may be adjusted between 80%-98% and 2%-20%, respectively. The air filter system 100 may further comprise a debris sensor (not shown) located in the primary flow path 110. In this manner, debris carried by the air flow along the primary flow path can be detected. The debris sensor may be located upstream of the connection of the secondary flow path 114 to the primary flow path 110, to ensure that any detected debris exited the separation chamber via the primary air outlet 108. The air filter system 100 may include a controller (not shown) which is configured to control operating parameters of the air filter system 100 based on an output signal from the debris sensor. Operating parameters that can be adjusted by the controller may include a speed of the air flow source 118 (to control air flow speed through the system), a size of the orifice defined by the restrictor 134, and / or a speed of any other air flow sources used with the air filter system 100. For example, the controller may be configured to adjust the operating parameters to minimise an amount of debris detected in the primary flow path 110. The debris sensor may be configured to detect a property of the debris, such as a size or mass of the debris. The controller may then adjust the operating parameters to improve filtering efficiency for the detected size or mass of the debris and / or decrease the pressure drop across the air filter system. Although the air filter system 100 was described above as having a debris sensor in the primary flow path 110, the air filter system 100 can additionally or alternatively include a debris sensor in other parts of the air filter system, such as in the separation chamber 102 or the secondary flow path 114. The air filter system 100 may also include air pressure sensors at various locations in the air filter system 100, in order to monitor a performance of the system. For example, the air filter system 100 may include a pressure sensor at the air inlet 104, a pressure sensor in the primary flow path 110, and / or a pressure sensor in the secondary flow path 114. In the example shown, the secondary flow path 114 is connected to the primary flow path 110, so that the airflow source 118 causes air flows along both the primary and the secondary flow paths 110, 114. However, in other embodiments, the outlet of the secondary flow path 114 need not necessarily be connected to the primary flow path 110. For example, the outlet of the secondary flow path 114 may be connected to a separate air flow source which is arranged to cause an air flow along the secondary flow path 114, so that air flowing out of the debris trap 120 may not be recombined with the air flow along the primary flow path 110. It should be noted that the schematic diagram of Fig. 1 is intended to represent the connections and functional relationships between various components of the air filter system 100, and is not intended as a representation of the physical shapes and arrangements of those components. Figs. 2-7 illustrate an air filter system 200 that is an embodiment of the invention. The air filter system 200 represents a specific implementation of the air filter system 100 discussed above. Accordingly, features in Figs. 2-7 corresponding to features of the air filter system 100 will be indicated using the same reference numerals as in Fig. 1. All features discussed above in relation to the air filter system 100 may be shared with the air filter system 200. Fig. 2 shows a schematic diagram of the air filter system 200. The arrows shown in Fig. 2 depict a direction of air flow through the air filter system 200. The air filter system 200 comprises a separation chamber 102 with an air inlet 104, a primary air outlet 108 and a secondary air outlet 112. An air flow source (not shown in Figs. 2-7) is connected to the primary air outlet 108 via a primary air flow path 110, in order to generate an air flow through the separation chamber 102 from the air inlet 104 towards the primary air outlet 108. Fig. 3a, which shows a schematic cutaway view of a portion of the air filter system 200, depicts the separation chamber 102 in more detail. The separation chamber 102 contains a deflector 116 which is arranged to deflect debris carried by the air flow in the separation chamber 102 towards the secondary air outlet 112. The deflector 116 is formed of a hollow body, which is suspended in the separation chamber 102 by means of vanes 202 which extend between the deflector 116 and a sidewall of the separation chamber 102. The vanes 202 are aerodynamically shaped, to reduce a drag on the air flow through the separation chamber 102. An annular air flow passage is defined between the deflector 116 and the sidewall of the separation chamber 102, such that the air flow through the separation chamber 102 passes through the annular air flow passage. The primary air outlet 108 is arranged at an opposite end of the separation chamber 102 relative to the air inlet 104, with the deflector 116 being positioned in the separation chamber 102 between the air inlet 104 and the primary air outlet 108. In this manner, the air flow through the separation chamber 102 must pass around the deflector 116. The secondary air outlet 112 is arranged around the primary air outlet 108. In particular, the secondary air outlet 112 surrounds and is concentric with the primary air outlet 108. Fig. 5 shows a view of the concentric primary and secondary outlets 108, 112, looking towards the outlets from within the separation chamber 102. As can be seen, the primary air outlet 108 has a circular shape, with the secondary air outlet 112 forming a ring around the primary air outlet 108. Ribs 204 extend across the secondary air outlet 112 from the sidewall of the separation chamber 102 to support a lip 206 that surrounds and defines the primary air outlet 108. The ribs 204 act to divide the ring-shaped secondary air outlet 112 into multiple (in the example shown, five) secondary air outlets arranged around the primary air outlet 108. The deflector 116 is shaped to deflect debris outwards towards the secondary air outlet 112, whilst guiding the air flow towards the primary air outlet 108. For illustration purposes, the deflector 116 is depicted on its own in Fig. 3b, which shows a cross-sectional view of the deflector 116. As indicated in Fig. 3b, the deflector 116 includes a deflection surface 208 arranged towards its upstream end (i.e. the end of the deflector 116 facing towards the air inlet 104), and a guide surface 214 arranged towards its downstream end (i.e. the end of the deflector facing towards the primary air outlet 108). The deflection surface 208 is configured to deflect debris towards the secondary air outlet 112, whilst the guide surface 214 is configured to guide the air flow towards the primary air outlet 108. The deflection surface 208 is arranged so that it is angled relative to the direction of incident air flow (indicated by arrow 210 in Fig. 3a), so that debris carried by the air flow may be deflected laterally outwards (relative to the direction of incident air flow 210) and towards the secondary air outlet 112. Debris carried by the air flow may be deflected due to bouncing off the deflection surface 208, and / or due to the sudden change in air flow direction as the air flow is forced to pass around the deflection surface 208. As illustrated, the deflection surface 208 may have a conical shape, with a tip of the cone pointing towards the incident air flow 210. In the example shown, the tip of the cone-shaped deflection surface 208 is rounded, such that the deflection surface 208 terminates at a convex rounded tip 209 at the upstream end of the deflector 116. The deflection surface 208 and the guide surface 214 join together at a curved side surface 211 of the deflector 116. The guide surface 214 extends towards the primary air outlet 108, to guide the air flow towards the primary air outlet 108. In particular, the guide surface may have a conical shape that tapers from the side surface 211 to a tip 212 located at the upstream end of the deflector 116. Thus, the deflector 116 has a shape that bulges outwards from its upstream end towards the side surface 211, and that then tapers from the side surface 211 towards the tip 212 at its upstream end. The tip 212 may be pointed, which may enhance an aerodynamic performance of the deflector 116. The tip 212 of the deflector 116 is centred relative to the primary air outlet 108 and protrudes into the primary air outlet 108, such that air flowing along the guide surface 214 is led directly to the primary air outlet 108. The curved side surface 211 acts to steer the air flow back towards the primary air outlet 108, following deflection of the air flow caused by the deflection surface 208 and the outwards bulging of the deflector 116. The deflector 116 is shaped so that the air flow attaches itself to the outer surface of the deflector 116 (including the deflection surface 208, side surface 211 and guide surface 214), so that the air flow follows the guide surface 214 towards the primary air outlet 108. In contrast, an inertia of deflected debris may prevent it from being steered with the air flow towards the primary air outlet 108, so that it continues on its deflected trajectory and exits the separation chamber 102 via the secondary air outlet 112. In other words, whilst the air flow may readily follow the curved side surface 211 and the guide surface 214 to reach the primary air outlet 108, the inertia of the deflected debris may prevent it from following such a tight curve so that it cannot reach the primary air outlet 108 and instead passes though the secondary air outlet 112 arranged around the primary air outlet 108. The separation chamber 102 is arranged such that the annular air flow passage defined between the deflector 116 and the sidewall of the separation chamber 102 progressively narrows towards a widest point of the deflector 116 near the curved side surface 211. For example, as shown in Fig. 3a, a width 216 of the air flow passage near an upstream end of the deflector 116 is greater than a width 218 of the air flow passage near the curved the side surface 211. This progressive narrowing is followed by a subsequent widening of the air flow passage, as a width of the deflector 116 tapers towards the tip 212. This progressive narrowing and subsequent widening of the air flow passage around the deflector 116 acts as a Venturi passage that accelerates the air flow in the separation chamber 102 as it passes around the curved side surface 211. This may enhance the curved side surface 211’s ability to accelerate the air flow towards the primary air outlet 108, as well as increase an inertia of debris in the air flow and so make it less likely to reach the primary air outlet 108. As mentioned above, the primary air outlet 108 comprises a lip 206. As shown in Fig. 3a, the lip 206 protrudes into the separation chamber 102 and flares outwards from the primary air outlet 108. As a result, the lip 206 acts as a funnel to channel the air flow into the primary air outlet 108. The lip 206 also acts as a barrier between the primary air outlet 108 and the secondary air outlet 112, to prevent debris from bouncing out of the secondary air outlet 112 into the primary air outlet 108. Additionally, an upstream portion of the primary flow path 110 located adjacent to the primary air outlet 108 forms a Venturi passage 220, to accelerate the air flow through the primary air outlet 108 and along the primary flow path 110. The Venturi passage 220 corresponds to a narrowing and subsequent widening of the primary flow path 110 in a region adjacent to the primary air outlet 108. This may serve to increase a flow of air out of the separation chamber 102 via the primary air outlet 108. The deflector 116 and the primary air outlet 108 are arranged such that the direction of air flow incident on the deflector 116 (indicated by arrow 210) is substantially aligned with a direction of air flow through the primary air outlet (indicated by arrow 222). In this manner, air flow within the separation chamber 102 and through the primary air outlet 108 is along substantially the same direction, which may promote rapid air flow through the separation chamber 102. Moreover, a longitudinal axis of the deflector 116 (i.e. a axis linking the rounded tip 209 at the upstream end and the tip 212 at the downstream end) may be substantially aligned with the incident air flow 210 and centred about the primary air outlet 108. This may result in a substantially uniform air flow around the deflector 116, thus improving an efficiency of the air flow through the separation chamber 102. As indicated above, the secondary air outlet 112 is divided into multiple secondary air outlets 112 by the ribs 204. A respective secondary outlet passage 224 is connected to each of the secondary air outlets, with the secondary outlet passages 224 being combined into the secondary flow path 114 by a manifold 226. This can be seen, for example, in Fig. 4a which shows a perspective view of a portion of the air filter system 200. In the example shown, there are five secondary outlet passages 224 (one for each secondary air outlet 112) which are joined together at an upstream end of the manifold 226. Note however that, for illustration purposes, Fig. 2 only depicts two secondary outlet passages 224. A single secondary flow path 114 is connected to a downstream end of the manifold 226. In this manner, air flows from the secondary air outlets 112 through the secondary outlet passages 224 are combined in the manifold 226 and then pass along the secondary flow path 114. As the secondary air outlets 112 are arranged in a ring around the primary air outlet 108, several of the secondary outlet passages 224 partially wind around the outside of the primary flow path 110 to connect to the manifold 226. The winding secondary outlet passages 224 follow a smoothly curving path without any sharp corners or bends. This is to avoid excessive slowing of air flow through the secondary outlet passages 224, which could cause debris to drop out of the secondary outlet passages 224 back into the separation chamber 102. In particular, each secondary outlet passage 224 is arranged such that any bends in the secondary outlet passage 224 have a radius of curvature equal to or greater than four times a width (e.g. diameter) of the secondary outlet passage 224. This may avoid sharp turns in the secondary outlet passages 224, to reduce a risk of debris falling from the secondary outlet passages 224 back into the separation chamber 102. As shown in Fig. 4a and Fig. 4b (which shows another perspective view of the portion of the air filter system 100), the manifold 226 may include a removable panel 228 or door for accessing an inside of the manifold 226, e.g. to enable cleaning of the inside of the manifold 226. A debris trap 120 is connected to the secondary flow path 114 (see e.g. Fig. 2), to capture debris carried by air flowing along the secondary flow path 114. The debris trap 120 may be arranged below the secondary air outlet 112, which may encourage debris to fall into the debris trap 120 under the action of gravity. A first portion 114a of the secondary flow path 114 which extends from the manifold 226 to the debris trap 120 may thus extend in a downwards direction, which may further encourage debris to fall into the debris trap 120. A more detailed view of the debris trap 120 is shown in Fig. 6, which shows a schematic cross-sectional diagram of the debris trap 120. The debris trap 120 includes a plurality of baffles (or barriers) 230 which are configured to slow the flow of air through the debris trap 120. In particular, the baffles 230 are arranged to cause the air to flow along a convoluted path including multiple sharp changes in direction, to slow the flow of air through the debris trap 120. As a result of the slowed air flow through the debris trap 120, debris carried by the air flow drops out of the air flow and falls into a debris receptacle 232 located at a bottom of the debris trap 120. The debris receptacle 232 may be removable or emptiable, to enable a user to empty debris collected in the debris receptacle 232. The debris receptacle 232 may be in the form of a collector tray mounted at the bottom of the debris trap 120. A baffle located near an outlet of the debris trap 120 may include a hole 234 through which air can flow to exit the debris trap 120, but which inhibits exit of debris from the debris trap 120. A second portion 114b of the secondary flow path 114 connects the outlet of the debris trap 120 to the primary flow path 110. In particular, as shown in Fig. 2, the secondary flow path 114 is connected to the primary flow path 110 at a location of the primary flow path 110 downstream of the Venturi passage 220. The second portion 114b of the secondary flow path 114 may be arranged to extend in an upwards direction, to further reduce a risk of debris being entrained along the second portion 114b of the secondary flow path 114. Thus, the airflow from the secondary flow path 114 re-joins the air flow from the primary flow path 110 after having passed through the debris trap 120. In line with the discussion above relating to the air flow system 100, the air flow caused by the air flow source along the primary flow path 110 causes air to flow along the secondary flow path 114. The second portion 114b of the secondary flow path 114 includes a restrictor 134, which is configured to restrict airflow along the secondary flow path 114. As discussed in relation to the air filter system 100, the restrictor 134 can be adjusted to adjust the proportion of air flow through the secondary flow path 114, as well as air flow speed along the secondary flow path 114. Arrows in Fig. 2 indicate the directions of air flow through the air filter system 200. When the air flow source (not shown) is activated, air is caused to flow into the separation chamber 102 via the air inlet 104. The airflow passes around the deflector 116, with a majority of the air flow being directed towards the primary air outlet 108, whilst debris carried by the air flow is deflected outwards towards the secondary air outlets 112. In this manner, a majority of the air flow may exit the separation chamber 102 via the primary air outlet 108 to travel along the primary flow path 110 towards the air flow source. The deflected debris and a small portion of the air flow exit the separation chamber 102 via the secondary air outlets 112 to pass through the secondary outlet passages 224, the manifold 226, the first portion 114a of the secondary flow path 114 and the debris trap 120. Debris may fall out of the air flow into the debris receptacle 232, with the air flow exiting the debris trap 120 to travel along the second portion 114b of the secondary flow path 114 to re-join the rest of the air flow travelling along the primary flow path 110. The combined air flow may then pass through the air flow source, and be expelled via an outlet of the air flow source. In line with the discussion for the air filter system 100, the air filter system 200 may be arranged such that 80%-98% of the air flow through the separation chamber 102 exits the separation chamber 102 via the primary air outlet 108, and 2%-20% of the air flow through the separation chamber 102 exits the separation chamber 102 via the secondary air outlet 112, to achieve a low pressure drop across the air filter system 200. The air filter system 200 may be manufactured as multiple parts which are assembled together. In particular, a section 236 of the separation chamber 102 (indicated by the dashed lines in Fig. 1), including the deflector 116 and the vanes 202 may be integrally formed as one part. This may be achieved using additive manufacturing, including selective laser sintering or multi-jet fusion as these techniques may provide a high-quality surface finish resulting in good aerodynamic qualities of the part. The part illustrated in Figs. 4a and 4b may also be formed as a single integral part. This part includes an upper portion of the separation chamber 102, the primary and secondary outlets 108, 112, the secondary outlet passages 224, the manifold 226, part of the first portion 114a of the secondary flow path 114, part of the primary flow path 110, and part of the second portion 114b of the secondary flow path 114 that connects to the primary flow path 110. As a result, the part shown in Figs. 4a and 4b has a complex shape, such that additive manufacturing techniques including selective laser sintering or multi-jet fusion may be well-suited to manufacture of this part. Fig. 5 shows a view of the part of Figs. 4a and 4b, looking towards the primary and secondary outlets 108, 112. As can be seen, a flange 238 is provided on the part, to enable connection to a mating flange on the section 236 of the separation chamber 102. Fig. 7 shows a perspective view of the air filter system 200, where the part of Figs 4a and 4b is connected via its flange 238 to the section 236 of the separation chamber 102. Other parts of the air filter system 200 may be formed separately using suitable manufacturing techniques, and assembled as required. Fig. 8 shows a schematic side view of a debris trap 700 that may be used in an air filter system according to an embodiment of the invention. For example, the debris trap 700 may be used as the debris trap 120 in the system 100 described above. As another example, the debris trap 700 may be used with the air filter system 200, e.g. instead of (or in addition to) the debris trap illustrated in Fig. 6. The debris trap 700 includes a cyclone separator 702, which is configured to separate debris from the air flowing along the secondary flow path 114, and a debris receptacle 704 which is configured to receive and hold the debris separated from the air flow. A sectional side view of the cyclone separator 702 is shown in Fig. 9. The cyclone separator 702 includes an air inlet 706 which is connected to receive the air flow from the secondary flow path 114. The cyclone separator 702 further includes an air outlet 708, which is in fluid communication with the air inlet 706 via a cyclone chamber 710. The air outlet 708 may be connected to the primary flow path 110, so that the air flowing along the secondary flow path 114 rejoins the primary flow path 110 after passing through the debris trap 700. The cyclone chamber 710 is configured to cause form a cyclone with the air flow received via the air inlet 706, as shown by the dashed lines. Cyclonic motion of the air flow in the cyclone chamber 710 causes debris carried by the air flow to separate from the air flow. The air flow exits the cyclone chamber 710 via the air outlet 708, whilst the debris drop towards a bottom of the cyclone chamber 710. The debris exit the cyclone chamber 710 via a debris outlet 712 of the cyclone chamber 710 located at a bottom of the cyclone chamber 710. The debris outlet 712 of the cyclone chamber 710 is connected to an inlet 714 of the debris receptacle 704, so that the debris receptacle 704 can receive and collect the debris that were separated from the air flow by the cyclone separator 702. A sectional side view of the debris receptacle 704 is shown in Fig. 10. The inlet 714 is arranged towards a top of the debris receptacle 704, with the debris receptacle 704 having container or enclosure arranged to hold (store) the received debris. In this manner, debris 716 may build up within the debris receptacle 704, as illustrated in Fig. 10. The debris receptacle 704 further includes an outlet valve 718 through which the debris receptacle 704 can be emptied, i.e. the debris 716 can be evacuated via the outlet valve 718. The outlet valve 718 is arranged towards a lower end of the debris receptacle 704, and may be laterally offset from the inlet 714 (e.g. relative to a vertical direction). The outlet valve 718 may be connected to an outlet of the debris receptacle 704 via a suitable connector, such as a screw 720 which seals the outlet valve 718 to the outlet of the debris receptacle 704. The outlet valve 718 can be opened in order to empty the debris 716 from the debris receptacle 704. In some implementations, the outlet valve 718 may be in the form of a flexible suction valve, which is arranged such that the valve is closed when the air filter system is in operation. In particular, during operation of the filter system, a pressure inside the filter system (including inside the debris receptacle 704) will tend to be lower than a surrounding atmospheric pressure (i.e. there will be a negative pressure inside the filter system), due to the air flows generated within the system. Accordingly, the flexible suction valve may be arranged to seal the outlet of the debris receptacle 704 when the pressure in the air filter system is lower than the atmospheric pressure, to prevent debris from being released from the debris trap during operation of the filter system. In other implementations the outlet valve 718 may be an actuatable valve, which can be controlled to move the outlet valve 718 between an open state and a closed state, in order to control emptying of the debris receptacle 704, e.g. during operation of the air filter system. In such a case, an inlet valve may be provided at the inlet 714 of the debris receptacle 704, to control opening and closing of the inlet 714. In this manner, the inlet 714 can be sealed whenever the outlet valve 718 is opened, so as to prevent pressure losses of the air filter system via the debris receptacle 704. By way of example, the outlet valve 718 and the inlet valve may each be implemented using a respective pinch valve. Such a pinch valve includes a deformable membrane which can be pinched together when actuated to form a seal. A pinch valve can be pneumatically, hydraulically, and / or mechanically actuated, depending on the specific implementation of the pinch valve. Other types of valve may also be used as the inlet valve and the outlet valve 718, such as a solenoid valve or a ball valve. The debris receptacle 704 may further include an actuator for conveying debris towards the outlet 718, to facilitate emptying debris from the debris receptacle 704. An example of such an actuator is illustrated in Fig. 11, which shows a sectional top view of the debris receptacle 704, where a section is taken along the plane D-D indicated in Fig. 10. In the example of Fig. 11, the actuator is in the form of a screw (or Archimedes screw) 722. The screw 722 extends within the debris receptacle 704, from a first end of the debris receptacle 704 located under the inlet 714 towards the outlet 718. The screw 722 includes a shaft with a helical thread such that, when the shaft is rotated about its longitudinal axis, the helical thread conveys (transports) the debris 716 towards the outlet 718. The debris receptacle 704 may comprise a motor (not shown) for rotating the screw 722. Accordingly, when the outlet valve 718 is open, the screw 722 to be rotated to expel debris from the debris receptacle 704 via the outlet 718. Other types of actuators may be used instead of the screw 722 for conveying debris towards the outlet valve 718. For example, the actuator may comprise an air flow source (e.g. blower, fan, compressed air source) arranged to blow the debris towards the outlet valve 718. As a further example, the actuator may comprise a movable member (e.g. a spatula or brush) arranged to push the debris towards the outlet valve 718. The valve(s) and / or actuator of the debris receptacle 704 may be controllable by the air flow system to provide automated emptying of the debris receptacle 704. For example, the air flow system may include a controller for controlling emptying of the debris receptacle 704. The controller may be the same or a separate controller to the controller mentioned above. The controller may be implemented, for instance, via a suitable computing system (e.g. an onboard computer of the vehicle), and / or via a suitable microcontroller or the like. To perform emptying of the debris receptacle 704, the controller may be configured to close the inlet valve to seal the inlet 714, and to open the outlet valve 718. The controller may then be configured to operate the actuator (e.g. the screw 722), to convey debris towards the outlet valve 718. Once evacuation of debris via the outlet 718 is completed, the outlet valve 718 may be closed, following which the inlet valve may be re-opened to allow debris to enter the debris receptacle 704. An amount of debris emptied from the debris receptacle 704 may be controlled by controlling an amount of time the outlet valve 718 is opened. This allows, for example, the debris receptacle 704 to be emptied in small doses, e.g. by opening the outlet valve 718 for short time periods. Emptying of the debris receptacle 704 may be initiate manually by a user. For example, the controller may be configured to perform emptying of the debris receptacle 704 in response to an instruction received from a user, e.g. received via a suitable user interface of the air filter system. Additionally or alternatively, the controller may be configured to automatically perform emptying of the debris receptacle 704, e.g. based on one or more predetermined criteria. For instance, the controller may be configured to perform emptying of the debris receptacle 704 at regular time intervals. In some cases, a sensor may be provided in the debris receptacle 704 for detecting a filling level of the debris receptacle 704. Then, when a predetermined filling level (or threshold) is reached, the controller may be configured to perform emptying of the debris receptacle. Various types of sensor may be used for detecting the filling level of the debris receptacle 704. By way of example, a weight sensor (e.g. a strain gauge or other sensor capable of detecting weight) may be used for detecting weight of the debris in the debris receptacle 704, to determine the filling level. As another example, a weight-sensitive switch may be used, which is activated when a predetermined weight of debris is in the debris receptacle 704. As a further example, the sensor may include one or more light gates arranged at different heights in the debris receptacle 704, for detecting when the debris reach one or more respective filling levels in the debris receptacle 704. Furthermore, the controller may be configured to control emptying of the debris receptacle 704 as a function of one or more driving conditions (parameters) of the vehicle. This can include controlling emptying of the debris receptacle 704 as a function of vehicle speed. For instance, the controller may be configured to only empty the debris receptacle 704 when the vehicle is travelling below a predetermined speed. This can also include controlling emptying of the debris receptacle 704 as a function of location of the vehicle. For instance, the controller may be configured to only empty the debris receptacle 704 when the vehicle is at a predetermined location (e.g. on a race track or road). A GPS system in the vehicle may thus be used by the controller to determine when the vehicle is at a location where emptying of the debris receptacle 704 is allowed. The controller may also be configured to only empty the debris receptacle 704 when there are no other vehicles within a predetermined distance. The controller may make use of a proximity sensor, for detecting proximity of other vehicles, in order to determine when it is allowed to empty the debris receptacle 704. Fig. 12 shows a schematic side view of a vehicle 800 having a downforce system with an air filter system 802 according to an embodiment of the invention. For example, the air filter system 802 may correspond to the air filter system 100 or the air filter system 200 described above. In the example shown, the vehicle 800 is a car, however other types of vehicle may also be used. For illustration purposes, wheels of the vehicle 800 are not depicted in Fig. 8, so that parts of the downforce system can be seen clearly. The downforce system includes a barrier 804 that extends downwards towards a ground surface (e.g. a road) 806 on which the vehicle 800 is disposed. The barrier 804 is connected at an upper end to an upper surface 808 that is disposed on an underside of the vehicle 800. The upper surface 808 may be formed by a plate that is mounted on the underside of the vehicle 800. Alternatively, all or part of the upper surface 808 may be formed by a portion of the underside of the vehicle 800. A rim 810 is disposed along a lower edge of the barrier 804. The barrier 804 is arranged to surround a region under the vehicle 800, to thereby restrict flow of air into the region under the vehicle 800. For example, the barrier 804 may define an oval-shaped region under the vehicle 800. In use, the rim 810 may contact the ground surface 806 or may come into close proximity with the ground surface 806, to restrict air flow into the region under the vehicle 800. The barrier 804 may be formed of a flexible piece of material, such as Nylon, rubber, or thin composite materials. In this manner, the barrier 804 allows the rim 810 to move relative to the upper surface 808, e.g. in response to changes in height of the vehicle 800 above the ground surface 806. This may also enable the rim 810 to be lifted away from the ground surface 806 (e.g. using an actuator, not shown) when the downforce system is not in use. The air filter system 802 is connected to an air outlet 812 of the barrier 804, the air outlet 812 being formed in the upper surface 808. In particular, the air outlet 812 is connected to the air inlet (e.g. air inlet 104) of the air filter system via an air flow path 814. In this manner, when the air flow source (e.g. air flow source 118) of the air filter system 802 is activated, air is caused to flow out of the region under the vehicle defined by the barrier 804 via the air flow path 814 to the air filter system 802. Debris carried by the air flow is captured by the air filter system 802, and clean air is ejected via an outlet 816 at a rear of the vehicle 800. Due to the evacuation of air from the region under the vehicle, a pressure differential across the barrier 804 arises, resulting in a downforce acting on the vehicle 800. As the downforce system is used, the debris trap of the air filter system 802 may fill up with debris over time, and so may need to be emptied 5 by a user to prevent it from over-filling. The debris trap may be located in an area which is easily accessible by a user, to facilitate emptying of the debris trap. For example, where the air filter 200 is used, the debris trap 120 may be arranged such that the debris receptacle 232 is located at or near an underside of the vehicle 800, so that a user can easily reach it.

Claims

1. An air filter system for a vehicle, the air filter system comprising:a separation chamber comprising an air inlet, a primary air outlet connected to a primary flow path and a secondary air outlet connected to a secondary flow path, the separation chamber containing a deflector;an air flow source connected to the primary air outlet via the primary flow path, wherein the air flow source is configured to generate an air flow through the separation chamber from the air inlet towards the primary air outlet;wherein the deflector is configured to deflect debris from the air flow towards the secondary air outlet; andwherein the secondary flow path comprises a debris trap for capturing debris from air flowing along the secondary flow path.

2. An air filter system according to claim 1, wherein the air flow source is further configured to generate an air flow along the secondary flow path.

3. An air filter system according to claim 2, wherein an air outlet of the secondary flow path is connected to the primary flow path.

4. An air filter system according to any preceding claim, wherein the deflector and primary air outlet are arranged such that a direction of the air flow incident on the deflector is substantially aligned with a direction of the air flow through the primary air outlet.

5. An air filter system according to any preceding claim, wherein the deflector comprises a deflection surface located at an upstream end of the deflector, the deflection surface being configured to deflect the debris from the air flow towards the secondary air outlet.

6. An air filter system according to any preceding claim, wherein the deflector comprises a guide surface configured to guide the air flow in the separation chamber towards the primary air outlet.

7. An air filter system according to any preceding claim, wherein an air flow passage in the separation chamber is defined between a sidewall of the separation chamber and the deflector, and wherein a portion of the air flow passage progressively narrows to accelerate the air flow.

8. An air filter system according to any preceding claim, wherein the secondary air outlet is arranged around the primary air outlet.

9. An air filter system according to claim 8, wherein the secondary air outlet comprises two or more secondary air outlets arranged around the primary air outlet.

10. An air filter system according to claim 9, wherein the secondary flow path comprises two or more secondary outlet passages, each of which is connected to a respective one of the secondary air outlets, and wherein the two or more secondary outlet passages are joined together by a manifold upstream of the debris trap.

11. An air filter system according to claim 10, wherein any bends in the two or more secondary outlet passages have a radius of curvature equal to or greater than four times a width of one of the two or more secondary outlet passages.

12. An air filter system according to any preceding claim, wherein the deflector is suspended inside the separation chamber, such that the air flow passes through an annular passage around the deflector.

13. An air filter according to any preceding claim, wherein the primary air outlet comprises a lip that protrudes into the separation chamber, and wherein the lip flares outwards from the primary air outlet.

14. An air filter according to any preceding claim, wherein the secondary flow path comprises a restrictor located downstream of the debris trap, the restrictor being configured to restrict air flow along the secondary flow path.

15. An air filter according to claim 14, wherein the restrictor is adjustable, to adjust the air flow along the secondary flow path.

16. An air filter system according to any preceding claim, wherein the debris trap is arranged at a height below the secondary air outlet.

17. An air filter system according to any preceding claim, wherein the debris trap comprises a plurality of baffles for slowing air flow through the debris trap.

18. An air filter system according to any preceding claim, wherein the debris trap comprises a cyclone separator configured to separate debris from the air flowing along the secondary flow path.

19. An air filter system according to any preceding claim, wherein the debris trap comprises a debris receptacle for receiving debris from the air flowing along the secondary flow path, wherein the debris receptacle is removable or emptiable.

20. An air filter system according to claim 19, wherein the debris receptacle comprises an outlet valve that is openable to empty the debris receptacle.

21. An air filter system according to claim 20, further comprising an inlet valve at an inlet of the debris receptacle, the inlet valve being closable to prevent the air flow along the secondary flow path from entering the debris receptacle.

22. An air filter system according to one of claims 19 to 21, further comprising an actuator configured to convey debris received in the debris receptacle towards an outlet of the debris receptacle.

23. An air filter system according to any preceding claim, wherein the primary flow path and the secondary flow path are arranged such that 80%-98% of the air flow through the separation chamber exits the separation chamber via the primary air outlet, and 2%-20% of the air flow through the separation chamber exits the separation chamber via the secondary air outlet.

24. An air filter system according to any preceding claim, wherein the deflector comprises a body having an adjustable shape.

25. An air filter system according to any preceding claim, further comprising: a debris sensor for detecting debris in the primary flow path; and a controller configured to control one or more operating parameters of the air filter system based on an output signal received from the debris sensor.

26. A downforce system for a vehicle, the downforce system comprising:a barrier configured to restrict a flow of air into a region under the vehicle; and an air filter system according to any preceding claim;wherein an air outlet of the barrier is connected to the air inlet of the separation chamber, such that the air flow source is configured to generate a pressure differential across the barrier.

27. A vehicle comprising an air filter system according to one of claims 1 to 25.

28. A vehicle comprising a downforce system according to claim 26.

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