Naturally aspirated and spark-ignited internal combustion engine

Abstract

The invention relates to a naturally aspirated and spark-ignition internal combustion engine (1), comprising: - an engine block (10) delimiting at least one cylinder (11), - a fresh air intake line (20) in each cylinder, and - an exhaust line (80) for burnt gases from each cylinder, which comprises an upstream catalyst and a downstream catalyst. According to the invention, the internal combustion engine further comprises a fresh air supply line (50) which opens into the exhaust line, between the catalysts, and which comprises means for regulating the fresh air flow, and the exhaust line comprises a richness sensor (42). Figure for the abstract: Fig.1

Description

Title of the invention: Atmospheric and spark-ignition internal combustion engine Technical field of the invention

[0001] The present invention relates generally to motor vehicles equipped with an internal combustion engine.

[0002] It applies more precisely to an atmospheric internal combustion engine with spark ignition, comprising: - an engine block delimiting at least one cylinder, - a fresh air intake line in each cylinder, and - an exhaust line for burnt gases from each cylinder, which includes an upstream catalyst and a downstream catalyst.

[0003] It relates to a method for controlling an internal combustion engine as mentioned above. State of the art

[0004] Within an increasingly restrictive legislative framework and with a view to preserving the environment, we are currently seeking technical solutions to improve the operation of internal combustion engines, in particular to reduce the quantity of pollutants released into the atmosphere.

[0005] To reduce these polluting emissions, a spark-ignition engine generally includes, in its exhaust line, a three-way catalyst making it possible to oxidize at least part of the unburned hydrocarbons (HC) and carbon monoxide (CO), and to reduce at least part of the nitrogen oxides (NOx) which are emitted in the burnt gases.

[0006] Several methods and devices for adjusting the richness are known which aim to improve the efficiency of the catalyst.

[0007] For example, it is known to use a control loop aimed at maintaining the richness of the burnt gases leaving the cylinders at a value equal to 1. For this purpose, a richness sensor (commonly called a "lambda sensor") is used, mounted in the exhaust line, upstream of the catalyst. The regulation then consists of subtracting from the output voltage of this sensor a setpoint voltage corresponding to a richness value equal to 1. The error signal is then compared to zero in a binary comparator. Thus, when the setpoint voltage is higher than the output voltage of the sensor, the air-fuel mixture is enriched using a regulator, by increasing the flow rate of fuel injected into the engine. Conversely, when the setpoint voltage is lower than the output voltage of the sensor, leans the mixture by reducing the flow of fuel injected into the engine. The resulting richness of the mixture then oscillates around the stoichiometric value.

[0008] In this way, the catalyst operates within its "catalytic window", so that it is able to carry out both the aforementioned oxidation and reduction reactions.

[0009] It can be noted here that when the catalyst leaves its catalytic window and is close to oxygen saturation, it favors the oxidation reactions of carbon monoxide, to the detriment of the reduction reactions of nitrogen oxides. Conversely, when the catalyst is devoid of oxygen, it favors the reduction reactions of nitrogen oxides but this situation is unfavorable to the oxidation reactions of carbon monoxide.

[0010] The quantity of oxygen stored in the catalyst is known to be an important parameter for ensuring good simultaneous treatment of the aforementioned pollutants. It is therefore appropriate to maintain a stable quantity of oxygen in the catalyst in order to ensure optimal decontamination.

[0011] To prevent this quantity of oxygen from drifting over time, document FR-A1-3033364 discloses a method in which the quantity of oxygen stored in the catalyst and the oxygen storage capacity of the catalyst are calculated, in particular as a function of the richness measured downstream of the catalyst. A set value for the oxygen stock in the catalyst is then deduced. Then, using a second regulator, it is possible to modify the richness setpoint (previously equal to 1) to regulate the quantity of oxygen to said setpoint value. The richness setpoint then used then oscillates around 1, deviating from this value by a few hundredths at most.

[0012] On the other hand, it often happens that the catalyst is split into two separate blocks. A first block, called the upstream catalyst, is then positioned as close as possible to the cylinders, so as to heat up quickly after a cold start of the internal combustion engine. Thus, it is quickly able to treat the polluting emissions after this start. The second block, called the downstream catalyst, is not positioned in the same place, due to the lack of space in the engine compartment of the vehicle. It is generally installed under the floor of the vehicle.

[0013] A disadvantage of this type of architecture is that when the engine operates at high speed and high load, the temperature of the upstream catalyst increases sharply, with the risk of exceeding a temperature threshold beyond which the mechanical strength of this catalyst is no longer ensured.

[0014] The solution previously used to avoid this consisted of regulating the richness at the cylinder outlet to a value greater than 1 (we speak of a rich mixture). Indeed, in this configuration, we note that the temperature of the burnt gases drops sharply (for example, of the order of 120°C for a richness of 1.1 instead of 1) for two reasons. The first reason is that the fuel injected at low temperature evaporates into the gases, which causes their temperature to decrease. The second reason is that, when the richness is high, the amount of oxygen in the upstream catalyst drops, so that the highly exothermic reactions of carbon monoxide oxidation no longer occur.

[0015] This technical solution is however not compatible with the new standards for reducing polluting emissions, precisely because it causes carbon monoxide emissions. Presentation of the invention

[0016] In order to overcome the aforementioned drawback of the state of the art, the present invention proposes to blow, at high speed and high load, fresh air between the two catalysts, so that the carbon monoxide can be treated in the downstream block.

[0017] More particularly, the invention provides an internal combustion engine as defined in the introduction, in which: - a fresh air supply line is provided which opens into the exhaust line, between the upstream catalyst and the downstream catalyst, and which includes means for regulating the flow of fresh air circulating in the supply line, and in which - the exhaust line has a richness sensor located downstream of the outlet of the supply line into the exhaust line.

[0018] Also proposed according to the invention is a method for controlling an internal combustion engine as mentioned above, in which it is provided to acquire a load from the internal combustion engine, and in which: - as long as the load is below a load threshold, it is planned to regulate the richness of the burnt gases to a first set value and to block the circulation of fresh air in the supply line, - otherwise, it is planned to regulate the richness of the burnt gases to a second set value which is strictly higher than the first set value and to allow the circulation of fresh air in the supply line.

[0019] Thus, it remains possible to control the engine so that the mixture of fuel and fresh air is stoichiometric as long as the engine is not over-stressed, i.e. outside a high speed and high load range, and so that the mixture is rich when the engine is under heavy stress, so that it can deliver the desired torque while ensuring effective treatment of pollutants.

[0020] Therefore, this solution allows the engine to offer high performance in terms of available power and torque, without affecting the processing of pollutants.

[0021] Other advantageous and non-limiting characteristics of the motor according to the invention, taken individually or in all technically possible combinations, are the following: - the intake line including an air filter, the supply line originates in the intake line, downstream of the air filter; - the supply line opens into an area of the exhaust line where the pressure is lower than atmospheric pressure when the internal combustion engine is operating at full speed and full load; - said area of the exhaust line comprises a venturi neck inside which the supply line opens; - the engine block delimits at least two cylinders and the exhaust line comprises an exhaust manifold which is delimited by the engine block; - said richness sensor is located downstream of the downstream catalyst.

[0022] Other advantageous and non-limiting characteristics of the method according to the invention, taken individually or in all technically possible combinations, are the following: - the internal combustion engine comprising a crankshaft which rotates at a current speed, the load threshold varies according to the current speed and is equal to the maximum load that the internal combustion engine can provide at the current speed when the richness of the burnt gases at the outlet of each cylinder is maintained equal to 1; - the first setpoint value is equal to 1, to within 5%; - the second setpoint value is at most equal to 1.15; - when the circulation of fresh air in the supply line is permitted, the flow of fresh air circulating in the supply line is regulated so that the richness measured by said richness sensor remains equal to 1.

[0023] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Detailed description of the invention

[0024] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0025] In the attached drawings:

[0026] [Fig-1] is a schematic view of an internal combustion engine according to the invention;

[0027] [Fig.2] represents two embodiments of zone II of [Fig.l];

[0028] [Fig.3] is a graph illustrating the variations in the maximum torque of the engine of [Fig.l] as a function of its speed, at richness equal to 1 and at maximum richness;

[0029] [Fig.4] is a graph illustrating the variations in the values of three parameters along the exhaust line of the internal combustion engine of [Fig.l], when the engine is under load, at richness equal to 1 and at maximum richness.

[0030] In the description, the terms "upstream" and "downstream" will be used according to the normal direction of gas flow, from the point of sampling of fresh air in the atmosphere to the outlet of the burnt gases into the atmosphere.

[0031] In [Fig.l], a motor vehicle internal combustion engine 1 is schematically represented.

[0032] This engine can be used in any type of vehicle (truck, bus, plane, boat, etc.). It could even be used in other ways, for example in a generator. Here, we will consider it as being used in a car.

[0033] Such a car comprises a chassis and bodywork elements which together delimit a passenger compartment for the vehicle's passengers and an engine compartment.

[0034] The internal combustion engine 1 is here a naturally aspirated engine, therefore without a turbocharger or mechanical compressor type supercharging system. It is also a spark ignition engine. It then uses as fuel gasoline (unleaded type 95 or 98), or alcohol, or LPG or even hydrogen.

[0035] It comprises an engine block 10 which is housed in the engine compartment and which internally delimits cylinders in which pistons slide. Here these cylinders 11 are three in number but they could be fewer (for example two or even one) or more (for example four, six or eight). The pistons are conventionally coupled to a crankshaft by connecting rods, which makes it possible to rotate this crankshaft at a rotational speed called “rpm”.

[0036] The cylinders 11 are generally closed on top by a cylinder head. This cylinder head has gas inlet and outlet passages which can be closed by valves.

[0037] Upstream of the cylinders 11, the internal combustion engine 1 comprises an intake line 20 which takes fresh air from the atmosphere and which opens into an air distributor 25 arranged to distribute the fresh air to each of the three cylinders 11 of the engine block 10.

[0038] This intake line 20 comprises, in the direction of flow of the fresh air, an air filter 21 which filters the fresh air taken from the atmosphere, and a general intake valve 24 (also called “throttle box”) which makes it possible to regulate the flow of fresh air opening into the air distributor 25.

[0039] At the outlet of the cylinders 11, the internal combustion engine 1 comprises a line exhaust 80 which extends from an exhaust manifold 81 into which the gases which have been previously burned in the cylinders 11 discharge, to an exhaust silencer (not shown) allowing the burnt gases to be expanded before they are discharged into the atmosphere. It also includes means for depolluting the burnt gases.

[0040] Preferably, the exhaust manifold 81 is delimited by the cylinder head. In other words, the exhaust manifold and the cylinder head form a single-piece device. They are formed from a single piece. The exhaust manifold is thus located as close as possible to the cylinders, so that the burnt gases which leave it and flow into the pollution control means have as high a temperature as possible. An advantage of this positioning is that the temperature of the pollution control means rises rapidly after a cold start of the engine. These pollution control means are in fact designed to operate within a predefined temperature window, in particular from a given lower temperature threshold, called the priming temperature threshold, which is often high (of the order of 350°C). The position of the pollution control means, as close as possible to the cylinders, therefore makes it possible to reach this temperature as quickly as possible.

[0041] The internal combustion engine 1 further comprises a fuel injection circuit 60, which comprises an injection pump 62 arranged to take the fuel from a tank 61 in order to bring it under pressure via a distribution rail 63 into injectors 64. The latter open directly into the cylinders 11 in the case of a direct injection engine, or otherwise into the air intake line 20, in particular into the air distributor 25. It will be noted here that the duration of opening of the injectors 64 will be used to regulate the flow rate of fuel injected into the cylinders 11.

[0042] In the context of the invention, this engine 1 is a controlled ignition four-stroke engine, which means that it comprises spark plugs adapted to generate sparks in the cylinders 11 in order to initiate the combustion of the mixture of fresh air and fuel in the cylinders 11 at the desired moment (between the compression and expansion strokes).

[0043] Taking into account the type of engine, the means for depolluting the burnt gases of the engine comprise two “three-way” catalysts 82, 84 making it possible to oxidize at least a portion of the unburned hydrocarbons (HC) and carbon monoxide (CO) contained in the burnt gases, and to reduce at least a portion of the nitrogen oxides (NOx) emitted. On recent engines, they generally also comprise a particulate filter 83.

[0044] Here, the upstream catalyst 82 is housed in the same enclosure as the particle filter 83, which enclosure is for example fixed to the engine block 10. The upstream catalyst 82 is in any case located in the engine compartment.

[0045] The downstream catalyst 84 is for its part located at a greater distance from the engine block 10 than the upstream catalyst 82. It could be housed in the engine compartment but, for reasons of space, it is preferably located under the chassis.

[0046] These two catalysts 82, 84 are distinct. In other words, their metal envelopes are separate and their temperatures are generally different. Thus, if the upstream catalyst 82 quickly reaches its ignition temperature after a cold start of the engine, this is not necessarily the case for the downstream catalyst 84.

[0047] In the context of the invention, the internal combustion engine 1 further comprises a fresh air supply line 50 which opens into the exhaust line 80, between the upstream 82 and downstream 84 catalysts. This supply line 50 is designed to bring oxygen into the downstream catalyst 84 as needed. It then comprises means for regulating the fresh air flow, here formed by a regulating valve 51.

[0048] This supply line 50 here originates in the intake line 20, downstream of the air filter 21. Alternatively, it could take air directly from the atmosphere, and then be provided with its own air filtering system.

[0049] A pump could be provided to force, when the control valve 51 is open, the circulation of fresh air towards the exhaust line 80. However, for reasons of economy, it will be preferable to use a solution without an electrical system. This solution will consist of connecting the supply line 50 in an area of the exhaust line 80 where the pressure is lower than the pressure in the intake line 20 (therefore lower than atmospheric pressure).

[0050] This area could be formed by the intrados of an elbow formed by the exhaust line 80. However, here, it will be formed by a Venturi type neck.

[0051] As shown in [Fig.2], such a neck forms a narrowing of the passenger section, which forces the acceleration of the burnt gases circulating in the exhaust line 80, and therefore a reduction in their pressure. In practice, it has a so-called convergent-divergent shape.

[0052] In the mode illustrated on the left part of [Fig.2], the venturi neck 85 is located in a pipe of the exhaust line 80, and it is formed by this pipe whose section narrows before returning to its initial dimension. In this mode, the pipe has an opening at the level of its zone of smallest section, into which the supply line 50 opens.

[0053] On the contrary, in the mode illustrated on the right part of [Fig.2], the venturi neck is an element 86 which is fixed inside the pipe and which has a maximum diameter strictly less than the internal diameter of the pipe. This element 86 then has a section which narrows before returning to its initial dimension. In this mode, the element 86 has an opening at the level of its zone of smallest section, into which the supply line 50 opens.

[0054] To control the various components of the internal combustion engine 1 and in particular the intake valve 24, the regulating valve 51 and the injectors 64, a computer 100 comprising a processor (CPU), random access memory (RAM), read only memory (ROM), analog-to-digital converters (A / D), and various input and output interfaces.

[0055] Thanks to its input interfaces, the computer 100 is adapted to receive from different sensors input signals relating to engine operating parameters.

[0056] It is thus particularly adapted to receive a signal which is relative to the depression angle a of an accelerator pedal of the motor vehicle, or to a pressure exerted on this pedal by the foot 30 of the driver of the vehicle. It is also adapted to acquire the engine speed.

[0057] It is also provided to receive signals relating to the richness of the mixture of fuel and fresh air blown into the cylinders 11. It is recalled that the richness designates the ratio of the mass flow rate of fuel to the mass flow rate of air, divided by the ratio of the mass flow rate of fuel to the mass flow rate of air in stoichiometric proportions.

[0058] In the context of the invention, the exhaust line 80 is equipped with three richness probes. These are in particular oxygen probes, that is to say probes which make it possible to determine a residual concentration of oxygen in the gases.

[0059] A first richness probe, called upstream probe 40, is placed upstream of the upstream catalyst 82. This upstream probe 40 is of the proportional type, which means that the signal that it emits has a characteristic (typically its voltage) which is proportional to the measured richness.

[0060] A second probe, called an intermediate probe 41, is placed at a location between the two upstream 82 and downstream 84 catalysts (typically at the outlet of this filter). This probe can be of the binary type, which means that the signal it emits has a characteristic (typically its voltage) which indicates only whether the mixture is rich or lean or stoichiometric. Such a probe is less expensive than a proportional probe. It is based on a measurement of the oxygen content of the burnt gases. In practice, it delivers a substantially constant and high voltage when the richness is greater than a threshold very slightly greater than 1 (for example beyond a richness of 1.02). It delivers a substantially constant and low voltage when the richness is lower than another threshold very slightly lower than 1 (for example below a richness of 0.98).Between these two extreme voltage values, it delivers a voltage which varies almost proportionally with the richness.

[0061] A third probe, called downstream probe 42, is placed downstream of the downstream catalyst 84. This can once again be an oxygen probe. However, new standards could require the placement downstream of the last catalyst, i.e. the downstream catalyst 84, of a nitrogen oxide concentration sensor. Such a sensor makes it possible to determine the richness of the gases, so it will be preferable to use this sensor rather than a dedicated probe, for cost reasons. For simplification, this sensor will hereinafter be called downstream probe 42 and it will be considered that it delivers a voltage which behaves like that of the binary intermediate probe 41.

[0062] It may be noted that the three richness probes are “physical” probes. Alternatively, one and / or the other of these probes could be of the “software” type, that is to say be formed by an algorithm (based for example on an observer) making it possible to calculate the richness as a function of various other parameters of the engine.

[0063] Other probes (or sensors) could also be used. For example, it would be possible to use a sensor to measure the temperature of the upstream catalyst 82, a sensor to determine the engine speed, and a sensor to determine the position of the accelerator pedal of the vehicle on which the engine is mounted.

[0064] Thanks to predetermined maps on the test bench and stored in its read-only memory, the computer 100 is adapted to generate, for each operating condition of the engine, output signals.

[0065] Among these maps, the computer 100 stores data characterizing the graph illustrated in [Fig.3].

[0066] This graph represents the variations of the load (or torque) C of the engine as a function of its speed co.

[0067] Here, for the sake of simplicity, we can consider that the load corresponds to the torque exerted by the crankshaft of the internal combustion engine 1. But in practice, it is rather the ratio of the work provided by the engine at a given speed to the maximum work that it could provide at this same speed.

[0068] In [Fig. 3], the curve Cs represents the variations in the maximum torque that the engine can provide when the richness measured by the upstream probe 40 is kept equal to 1, as a function of the engine speed. The curve Cmax also represents the variations in the maximum torque that the engine can provide when the richness measured by the upstream probe 40 is allowed to deviate from the value 1, as a function of the engine speed. For example, said richness can be increased to a maximum value strictly greater than 1 so as to maintain the temperature of the components of the engine exhaust line, in particular the exhaust manifold and the upstream catalyst, below a maximum temperature value corresponding to a thermomechanical resistance limit.

[0069] We note that these two curves Cs, Cmax are merged under a cos regime threshold.

[0070] In the following, the engine will be said to operate at high speed and at full load when its speed is higher than the cos speed threshold and its load is included between the two curves Cs, Cmax.

[0071] Thanks to its output interfaces, the computer 100 is adapted to transmit output signals to the various components of the engine, in particular to the intake valve 24, to the regulation valve 51 and to the injectors 64.

[0072] Thanks to its memory, the computer stores a computer application, consisting of computer programs comprising instructions whose execution by the processor allows the computer to implement the method described below.

[0073] When the engine is started, the fresh air taken from the atmosphere by the intake line 20 is filtered by the air filter 21, mixed with the fuel, then burned in the cylinders 11.

[0074] On leaving the cylinders 11, the burnt gases are treated by the pollution control means, then expanded in the exhaust silencer before being released into the atmosphere.

[0075] The method implemented by the computer 100 comprises, when the engine operates in this way, several main steps which can now be described.

[0076] The computer 100 is programmed to implement these steps recursively, that is to say in a loop and at regular time steps.

[0077] The first of these steps consists of acquiring engine parameters.

[0078] Thus, the computer acquires at least the engine speed co. It can also acquire other parameters, such as the temperature of the upstream catalyst 82.

[0079] The computer also acquires at this stage the angular position a of the accelerator pedal 30.

[0080] During a second step, the computer determines a torque request desired by the driver.

[0081] The desired torque request corresponds, for example, to the torque that the driver would like the engine to develop.

[0082] This request can for example be calculated taking into account the engine speed co and the angular position a of the accelerator pedal 30.

[0083] The torque request can otherwise be calculated differently, in particular when the vehicle is driven (partially) autonomously. For example, the torque request may depend on the activation of a cruise control.

[0084] Consequently, it will hereinafter be referred to more generally as “torque setpoint Ce”.

[0085] During a third step, the computer 100 reads in its map the current value Cs(co) of maximum torque at richness equal to 1 (given by the curve Cs), and possibly also the current value Cmax(co) of maximum torque at optimum richness (given by the curve Cmax), taking into account the engine speed co.

[0086] During a fourth step, the calculator 100 calculates a richness setpoint.

[0087] This richness setpoint corresponds to the richness value that one wishes to measure at the upstream probe 40. In other words, while the intake valve 24 is controlled as a function of the torque setpoint Ce, the fuel injectors 64 are controlled so as to regulate the richness of the burnt gases at the upstream probe 40 according to this richness setpoint.

[0088] Within the framework of the invention, two situations are distinguished.

[0089] The first situation corresponds to the case where the torque setpoint Ce is less than or equal to the value Cs(co) (zone A of [Fig.3]).

[0090] In this situation, the richness setpoint is intended to be equal to a first value VAL1, which is here substantially equal to 1 (to within 5%). In addition, the regulating valve 51 is kept closed, to block the passage of fresh air.

[0091] Thus, as long as the engine speed remains below the cos speed threshold, or when the speed exceeds this threshold but the torque setpoint Ce remains restricted, the engine richness will be regulated around the value 1.

[0092] We could predict that the first value VAL1 is exactly equal to 1 and constant.

[0093] Alternatively, it could be provided to calculate this first value VAL1 so that the quantity of oxygen stored in the upstream catalyst 82 remains constant and favorable to both the oxidation reactions and the reduction reactions. The first value VAL1 will then be calculated in particular as a function of the measurement carried out by the intermediate richness probe 4L. Document FR-A1-3033364 describes, for example, such a method.

[0094] Then, the first value VAL1 will oscillate around 1, deviating from this value by a maximum of 5%.

[0095] The second situation corresponds to the case where the torque setpoint Ce is strictly greater than the value Cs(co) (zone B of [Fig.3]).

[0096] In this situation, the richness setpoint is intended to be equal to a second value VAL2, which is strictly greater than the first value VAL1 (and in practice strictly greater than 1). In addition, the control valve 51 is controlled to open.

[0097] Thus, at high speed and full load, it is intended to enrich the combustion mixture entering the cylinders 11 so that the engine can develop a greater torque than at richness equal to 1 and so as to cool the burnt gases which pass through the upstream catalyst 82. In fact, the surplus fuel cools the burnt gases and causes the oxygen to disappear from this upstream catalyst 82, so that no exothermic oxidation reaction can occur there. In practice, a reduction in temperature in the upstream catalyst 82 can exceed 100°C. The pollutants not treated in the upstream catalyst 82 are, on the other hand, treated in the downstream catalyst 84, thanks to the supply of oxygen from the supply line 50.

[0098] The second value VAL2 of the richness setpoint is between 1 and a maximum threshold VALs (which is between 1.1 and 1.15). This second value VAL2 varies according to the torque setpoint Ce. It is chosen to be all the greater as the torque setpoint Ce is greater than the value Cs(co). For example, it can be provided that the second value VAL2 varies linearly between 1 and the maximum threshold VALs, according to the following formula:

[0099] VAL2 = 1 + VALs. (Ce- Cs(co)) / (Cmax(co)-Cs(co))

[0100] During a fifth step, the computer 100 calculates a setpoint for the control valve 51.

[0101] In practice, the setpoint is calculated so that the valve remains closed as long as the torque setpoint Ce is lower than the value Cs(co), i.e. when the richness is regulated around the value 1.

[0102] On the other hand, it is calculated so that the valve opens when the torque setpoint Ce is greater than the value Cs(co).

[0103] The regulating valve 51 could be controlled in a very simple way, namely in a bistable manner. It could thus be designed to open fully when the richness is greater than 1. Indeed, it can be assumed that the reduction reactions occur entirely in the upstream catalyst 82, so that only oxidation reactions remain to be generated in the downstream catalyst 84 to treat the polluting emissions of the engine. In other words, a significant supply of oxygen will not harm the production of these reactions.

[0104] However, preferably (in order to prevent the quantity of oxygen in the downstream catalyst 84 from being too high after the closing of the regulating valve 51), it will be preferable to regulate the opening of the regulating valve 51 so that the richness measured by the downstream probe 42 remains equal to 1.

[0105] During a sixth step, the computer 100 transmits the previously calculated instructions to the intake valve 24, to the injectors 64 and to the regulation valve 51.

[0106] Thus: - the flow of fresh air in the intake line 20 depends on the position of the accelerator pedal, - the fuel flow is regulated so that the richness measured by the upstream probe 40 remains substantially equal to the calculated richness setpoint (i.e. to the first value VAL1 or to the second value VAL2 depending on the situation), and - the flow of fresh air blown into the exhaust line 80 by the supply line 50 is regulated so that the richness measured by the downstream probe 42 remains sens- possibly equal to 1.

[0107] In [Fig.4], a part of the length of the exhaust line is shown 80.

[0108] The variations in the values of three parameters relating to the burnt gases, along this exhaust line 80, have also been represented by three graphs.

[0109] On these three graphs, the continuous line curve corresponds to the case where the richness measured by the upstream probe 40 is regulated around 1 and where the torque setpoint Ce is on the curve Cs of [Fig.3]. On the other hand, the dotted curve corresponds to the case where the richness measured by the upstream probe 40 is regulated around a value greater than 1 and where the torque setpoint Ce is on the curve Cmax of [Fig.3].

[0110] The first parameter illustrated is the temperature T of the burnt gases.

[0111] We note that the temperature T is initially equal to a temperature threshold Tmax beyond which the mechanical strength of the upstream catalyst 82 is no longer ensured. This upstream catalyst 82 is located so close to the cylinders 11 that the temperature of the burnt gases passing through it is substantially equal to that of the gases leaving the cylinders 11.

[0112] It can be seen that, in the first situation (solid line), the temperature increases very slightly in the upstream catalyst 82 then drops regularly. On the contrary, in the second situation, the temperature drops more at the junction between the supply 50 and exhaust 80 lines, due to the supply of fresh air, then it rises sharply at the level of the downstream catalyst 84 where the exothermic oxidation reactions occur.

[0113] It is also observed that this temperature rise is much greater than the drop caused by the supply of fresh air. It is therefore understood that it is because the downstream catalyst 84 is sufficiently far from the upstream catalyst (so that the temperature of the burnt gases has sufficiently decreased) that it is possible to oxidize the pollutants (in particular carbon monoxide CO and unburned hydrocarbons HC) without exceeding the temperature threshold Tmax in the downstream catalyst 84.

[0114] The second parameter is the richness R of the burnt gases. It can be seen that it is strictly greater than 1 upstream of the junction between the supply 50 and exhaust 80 lines, and equal to 1 downstream.

[0115] The third parameter is the carbon monoxide content Tco. It is observed that in the first situation, it is low at the outlet of the cylinders 11 and that it decreases when passing through the upstream catalyst 82 or even when passing through the downstream catalyst 84. On the other hand, in the second situation, it is much higher at the outlet of the cylinders 11 and it does not decrease when passing through the upstream catalyst 82, due to the absence of oxygen. On the other hand, it is zero when passing through the downstream catalyst 84, thanks to the supply of oxygen coming from the supply line 50.

[0116] The present invention is in no way limited to the embodiment described and shown, but those skilled in the art will be able to provide any variant in accordance with the invention.

[0117] Thus, within the framework of the invention, it would be possible, for example, to place the downstream probe 42 between the outlet of the supply line 50 and the downstream catalyst 84.

[0118] According to another variant, the regulation valve 51 could be controlled as a function of the temperature of the upstream catalyst 82 or of the richness measured by the intermediate richness probe 41. Indeed, after a cold start, the upstream catalyst 82 has a temperature still very far from the temperature threshold Tmax and it stores a quantity of oxygen which could make it possible to delay the opening of the regulation valve 51.

Claims

Claims

1. A naturally aspirated and spark-ignition internal combustion engine (1), comprising: - an engine block (10) delimiting at least one cylinder (11), - an intake line (20) for fresh air in each cylinder (11), and - an exhaust line (80) for burnt gases from each cylinder (11), which comprises an upstream catalyst (82) and a downstream catalyst (84), characterized in that: - it further comprises a fresh air supply line (50) which opens into the exhaust line (80), between the upstream catalyst (82) and the downstream catalyst (84), and which comprises means (51) for regulating the flow of fresh air circulating in the supply line (50), and in that - the exhaust line (80) comprises a richness sensor (42) located downstream of the outlet of the supply line (50) in the exhaust line (80).

2. Internal combustion engine (1) according to claim 1, wherein, the intake line (20) comprising an air filter (21), the supply line (50) originates in the intake line (20), downstream of the air filter (21).

3. Internal combustion engine (1) according to one of claims 1 and 2, in which the supply line (50) opens into a zone of the exhaust line (80) where the pressure is lower than atmospheric pressure when the internal combustion engine (1) is operating at full speed and at full load.

4. Internal combustion engine (1) according to claim 3, wherein said zone of the exhaust line (80) comprises a venturi neck (85, 86) inside which the supply line (50) opens.

5. Internal combustion engine (1) according to one of claims 1 to 4, wherein the engine block (10) delimits at least two cylinders (11) and the exhaust line (80) comprises an exhaust manifold (81) which is delimited by the engine block (10).

6. Internal combustion engine (1) according to one of claims 1 to 5, wherein said richness probe (42) is located downstream of the downstream catalyst (84).

7. Method for controlling an internal combustion engine (1) according to one of claims 1 to 6, in which it is provided to acquire a load (C) from the internal combustion engine (1), and in which: - as long as the load (C) is lower than a load threshold (Cs), it is intended to regulate the richness of the burnt gases to a first set value and to block the circulation of fresh air in the supply line (50), - otherwise, it is intended to regulate the richness of the burnt gases to a second set value which is strictly higher than the first set value and to allow the circulation of fresh air in the supply line (50).

8. Control method according to claim 7, in which, the internal combustion engine (1) comprising a crankshaft which rotates at a current speed (œ), the load threshold (Cs) varies as a function of the current speed (œ) and is equal to the maximum load that the internal combustion engine (1) can provide at the current speed (œ) when the richness of the burnt gases at the outlet of each cylinder (11) is maintained equal to 1.

9. Control method according to claim 7 or 8, in which the first setpoint value is equal to 1, to within 5%.

10. Control method according to one of claims 7 to 9, in which the second setpoint value is at most equal to 1.

15.

11. Control method according to one of claims 7 to 10, in which, when the circulation of fresh air in the supply line (50) is permitted, the flow of fresh air circulating in the supply line (50) is regulated so that the richness measured by said richness probe (42) remains equal to 1.

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