Atmospheric spark-ignition internal combustion engine

Abstract

According to the invention, the internal combustion engine also comprises a fresh air supply line (50) which opens into the exhaust line, between the catalysts, and which comprises means for regulating the flow of fresh air, and the exhaust line comprises a richness probe (42).

Description

[0001] ATMOSPHERIC SPARK-IGNITION INTERNAL COMBUSTION ENGINE

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates generally to motor vehicles equipped with an internal combustion engine.

[0004] More specifically, it applies to an atmospheric, spark-ignition internal combustion engine comprising :

[0005] - an engine block delimiting at least one cylinder,

[0006] - a fresh air intake line into each cylinder, and

[0007] - an exhaust line for burnt gases leaving each cylinder, which includes an upstream catalyst and a downstream catalyst.

[0008] It relates to a method for controlling an internal combustion engine as described above.

[0009] STATE OF THE ART

[0010] Within an increasingly restrictive legislative framework and with a view to preserving the environment, technical solutions are currently being sought to improve the operation of internal combustion engines, in particular to reduce the quantity of pollutants released into the atmosphere.

[0011] To reduce these pollutant emissions, a spark-ignition engine generally includes a three-way catalyst in its exhaust line to oxidise at least some of the unburnt hydrocarbons (HC) and carbon monoxide (CO), and to reduce at least some of the nitrogen oxides (NOx) emitted in the burnt gases.

[0012] Several methods and devices for adjusting richness are known, with the aim of improving catalyst efficiency.

[0013] For example, it is known to use a control loop designed to maintain the richness of the burnt gases leaving the cylinders at a value equal to 1. To do this, a richness probe (commonly known as a "lambda probe") mounted in the exhaust line, upstream of the catalyst, is used. Regulation then consists of subtracting from the output voltage of this probe a setpoint voltage corresponding to a richness value equal to 1. The error signal is then compared to zero in a binary comparator. When the setpoint voltage is higher than the probe output voltage, the air-fuel mixture is enriched by a regulator, increasing the flow of fuel injected into the engine. Conversely, when the setpoint voltage is lower than the probe output voltage, the mixture is made leaner by reducing the flow of fuel injected into the engine. The resulting mixture richness then fluctuates around the stoichiometric value.

[0014] In this way, the catalyst operates within its "catalytic window", so that it is able to carry out both the oxidation and reduction reactions mentioned above

[0015] It can be noted here that when the catalyst leaves its catalytic window and is close to oxygen saturation, it favours carbon monoxide oxidation reactions, to the detriment of nitrogen oxide reduction reactions. Conversely, when the catalyst is devoid of oxygen, it favours nitrogen oxide reduction reactions, but this situation is unfavourable to carbon monoxide oxidation reactions.

[0016] The quantity of oxygen stored in the catalyst is known to be an important parameter in ensuring good simultaneous treatment of the above-mentioned pollutants. It is therefore important to maintain a stable quantity of oxygen in the catalyst to ensure optimum depollution.

[0017] To prevent this quantity of oxygen from drifting overtime, a process is known from document FR- A1-3033364 in which the quantity of oxygen stored in the catalyst and the oxygen storage capacity of the catalyst are calculated as a function, in particular, of the richness measured downstream of the catalyst. A setpoint value for oxygen storage in the catalyst is then deduced. Then, thanks to a second regulator, it is possible to modify the richness setpoint (previously equal to 1) to regulate the quantity of oxygen to the said setpoint value. The richness setpoint then used oscillates around 1 , deviating from this value by a few hundredths at most.

[0018] On the other hand, the catalyst is often split into two separate blocks. The first block, called the upstream catalyst, is positioned as close as possible to the cylinders, so that it heats up quickly after the internal combustion engine is cold-started. In this way, it is quickly able to treat the polluting emissions after this start-up. The second unit, called the downstream catalyst, is not positioned in the same place, because of the lack of space in the vehicle's engine compartment. It is generally installed under the vehicle floor.

[0019] One disadvantage of this type of architecture is that when the engine is running at high speed and under heavy load, the temperature of the upstream catalyst rises sharply, with the risk of exceeding a temperature threshold beyond which the mechanical strength of the catalyst can no longer be guaranteed.

[0020] The solution previously used to avoid this was to regulate the richness at the cylinder outlet to a value greater than 1 (known as a rich mixture). In fact, in this configuration, the temperature of the burnt gases falls sharply (for example, by around 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 in the gases, causing their temperature to fall. The second reason is that, when the richness is raised, the quantity of oxygen in the upstream catalyst drops, so that the highly exothermic carbon monoxide oxidation reactions no longer take place.

[0021] However, this technical solution is not compatible with the new standards for reducing polluting emissions, precisely because it causes carbon monoxide emissions.

[0022] PRESENTATION OF THE INVENTION

[0023] In order to remedy the aforementioned drawback of the state of the art, the present invention proposes blowing fresh air between the two catalysts at high speed and high load, so that the carbon monoxide can be treated in the downstream block.

[0024] More particularly, an internal combustion engine as defined in the introduction is proposed according to the invention, in which :

[0025] - a fresh air supply line is provided which opens into the exhaust line, between the upstream catalyst and the downstream catalyst, and which comprises means for regulating the flow of fresh air circulating in the supply line, and in which

[0026] - the exhaust line comprises a richness probe located downstream of the opening of the supply line into the exhaust line.

[0027] The invention also proposes a method for controlling an internal combustion engine such as that mentioned above, in which it is provided to acquire a load of the internal combustion engine, and in which :

[0028] - as long as the load is below a load threshold, it is provided to regulate the richness of the burnt gases to a first setpoint value and to block the circulation of fresh air in the supply line,

[0029] - otherwise, the richness of the burnt gases is regulated at a second set-point value which is strictly greater than the first set-point value and fresh air is allowed to circulate in the supply line. In this way, it is still possible to control the engine so that the mixture of fuel and fresh air is stoichiometric as long as the engine is not overloaded, i.e. outside a high-speed, high-load range, and so that the mixture is rich when the engine is overloaded, so that it can deliver the desired torque while ensuring effective treatment of pollutants.

[0030] This solution allows the engine to offer high performance in terms of available power and torque, without compromising pollutant treatment.

[0031] Other advantageous and non-limiting features of the engine according to the invention, taken individually or in any technically possible combination, are as follows:

[0032] - the intake line comprising an air filter, the supply line originates in the intake line, downstream of the air filter;

[0033] - the supply line opens out into a zone 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;

[0034] - the said zone of the exhaust line comprises a venturi throat into which the supply line opens;

[0035] - the engine block delimits at least two cylinders and the exhaust line comprises an exhaust manifold which is delimited by the engine block;

[0036] - said richness probe is located downstream of the downstream catalyst.

[0037] Other advantageous and non-limiting features of the method according to the invention, taken individually or in any technically possible combination, are as follows:

[0038] - the internal combustion engine having a crankshaft which rotates at a current speed, the load threshold varies as a function of the current speed and is equal to the maximum load that the internal combustion engine can deliver at the current speed when the richness of the gases burnt at the outlet of each cylinder is kept equal to 1 ; - the first setpoint is equal to 1 , with 5% accuracy;

[0039] - the second setpoint is a maximum of 1.15 ;

[0040] - 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 the said richness probe remains is kept equal to 1.

[0041] Of course, the various features, variants and embodiments of the invention may be associated with one another in various combinations insofar as they are not incompatible or mutually exclusive.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] The following description is based on the annexed drawings, which are given by way of nonlimiting examples, and will make it clear what the invention consists of and how it can be implemented.

[0044] On the attached drawings :

[0045] [Fig. 1] is a schematic view of an internal combustion engine in accordance with the invention;

[0046] [Fig. 2] shows two embodiments of zone II in figure 1;

[0047] [Fig. 3] is a graph illustrating the variations in the maximum torque of the engine in Figure 1 as a function of its speed, with fuel richness equal to 1 and with maximum fuel richness ;

[0048] [Fig. 4] is a graph illustrating the variations in the values of three parameters along the exhaust line of the internal combustion engine shown in Fig. 1 , when the engine is under load, at a richness equal to 1 and at maximum richness.

[0049] In the description, the terms "upstream" and "downstream" will be used according to the normal direction of flow of the gases, from the point where the fresh air is taken from the atmosphere to the point where the burnt gases exit into the atmosphere.

[0050] Figure 1 shows a schematic diagram of a motor vehicle internal combustion engine 1.

[0051] This engine could be used in any type of vehicle (lorry, bus, plane, boat, etc.). It could even be used in other ways, for example in a generator set. We'll assume here that it is fitted to a car.

[0052] Such a car comprises a chassis and bodywork elements which together define a passenger compartment and an engine compartment.

[0053] The internal combustion engine 1 in this case is an atmospheric engine, i.e. without a turbocharger or mechanical compressor type supercharging system. It is also a spark-ignition engine. It uses petrol (unleaded 95 or 98), alcohol, LPG or even hydrogen as fuel

[0054] It comprises an engine block 10 which is housed in the engine compartment and which internally delimits cylinders in which pistons slide. There are three cylinders 11 in this example, but there could be fewer (e.g. two or even one) or more (e.g. four, six or eight). The pistons are usually coupled to a crankshaft by connecting rods, which enable the crankshaft to rotate at a speed called "rpm". Cylinders 11 are generally closed at the top by a cylinder head. This cylinder head has gas inlet and outlet passages that can be closed by valves.

[0055] Upstream of the cylinders 11 , the internal combustion engine 1 comprises an intake line 20 which takes fresh air from the atmosphere and opens into an air distributor 25 arranged to distribute the fresh air to each of the three cylinders 11 of the engine block 10.

[0056] 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 known as a "butterfly box") which regulates the flow of fresh air discharging into the air distributor 25.

[0057] At the outlet of the cylinders 11 , the internal combustion engine 1 comprises an exhaust line 80 which extends from an exhaust manifold 81 into which the gases previously burnt in the cylinders 11 discharge, to an exhaust silencer (not shown) which allows the burnt gases to be expanded before they are discharged into the atmosphere. It also includes means for cleaning up the burnt gases.

[0058] Preferably, the exhaust manifold 81 is bounded by the cylinder head. In other words, the exhaust manifold and cylinder head form a one-piece device. They are formed from a single one-piece component. This means that the exhaust manifold is as close as possible to the cylinders, so that the burnt gases leaving it and discharging into the depollution means have as high a temperature as possible. One advantage of this positioning is that the temperature of the depollution means rises rapidly after the engine is cold-started. These depollution means are designed to operate within a predefined temperature window, in particular from a given lower temperature threshold, known as the ignition temperature threshold, which is often high (of the order of 350°C). The position of the depollution means, as close as possible to the cylinders, means that this temperature can be reached as quickly as possible.

[0059] The internal combustion engine 1 also comprises a fuel injection circuit 60, which comprises an injection pump 62 arranged to draw fuel from a tank 61 in order to feed it under pressure via a distribution rail 63 to injectors 64. These 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 should be noted here that the duration of opening of the injectors 64 will be used to regulate the flow of fuel injected into the cylinders 11.

[0060] In the context of the invention, this engine 1 is spark-ignited and four-stroke, which means that it has spark plugs adapted to generate sparks in the cylinders 11 in order to initiate combustion of the mixture of fresh air and fuel in the cylinders 11 at the desired moment (between the compression and expansion strokes).

[0061] Depending on the type of engine, the means of depolluting the engine's burnt gases comprise two "three-way" catalysts 82, 84 which oxidise at least some of the unburnt hydrocarbons (HC) and carbon monoxide (CO) contained in the burnt gases, and reduce at least some of the nitrogen oxides (NOx) emitted. Recent engines generally also include a particulate filter 83. Here, the upstream catalyst 82 is housed in the same enclosure as the particulate filter 83, which enclosure is attached to the engine block 10, for example. In any case, the upstream catalyst 82 is located in the engine compartment.

[0062] The downstream catalyst 84 is 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.

[0063] These two catalysts 82,84 are distinct. In other words, their metal shells are separate and their temperatures are generally different. So while the upstream catalyst 82 quickly reaches its ignition temperature after a cold engine start, the same is not necessarily true for the downstream catalyst 84.

[0064] In the context of the invention, the internal combustion engine 1 also 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 supply oxygen to the downstream catalyst 84 as required. It then comprises means for regulating the flow of fresh air, in this case formed by a regulating valve 51.

[0065] This supply line 50 starts 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.

[0066] A pump could be provided to force the circulation of fresh air towards the exhaust line 80 when the control valve 51 is open. However, for reasons of economy, it is preferable to use a solution without an electrical system. This solution consists of connecting the supply line 50 in a zone of the exhaust line 80 where the pressure is lower than the pressure in the intake line 20 (therefore lower than atmospheric pressure).

[0067] This zone could be formed by the lower surface of a bend formed by the exhaust line 80. Here, however, it will be formed by a Venturi-type throat.

[0068] As shown in Figure 2, such a throat forms a temporary narrowing of the cross-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.

[0069] In the mode illustrated on the left of Figure 2, the venturi throat 85 is located in a pipe of the exhaust line 80, and is formed by this pipe, the cross-section of which narrows before returning to its initial size. In this mode, the pipe has an opening in its area of smallest cross-section, into which the supply line 50 opens.

[0070] In contrast, in the mode illustrated on the right-hand side of figure 2, the venturi throat is an element 86 which is fixed inside the pipe and which has a maximum diameter strictly smaller than the inside diameter of the pipe. This element 86 then has a cross-section which narrows before returning to its initial size. In this mode, the element 86 has an opening in its area of smallest cross-section, into which the supply line 50 opens.

[0071] To control the various components of the internal combustion engine 1 and in particular the intake valve 24, the control valve 51 and the injectors 64, a computer 100 is provided comprising a processor (CPU), a random access memory (RAM), a read-only memory (ROM), analogue-to- digital (A / D) converters and various input and output interfaces.

[0072] Thanks to its input interfaces, the computer 100 is adapted to receive input signals relating to engine operating parameters from various sensors.

[0073] In particular, it is adapted to receive a signal relating to the angle of depression a of an accelerator pedal of the motor vehicle, or to pressure exerted on this pedal by the foot 30 of the driver of the vehicle. It is also adapted to acquire the engine speed.

[0074] It is also designed to receive signals relating to the richness of the mixture of fuel and fresh air injected into the cylinders 11 . Richness is defined as the ratio of the fuel mass flow rate to the air mass flow rate, divided by the ratio of the fuel mass flow rate to the air mass flow rate in stoichiometric proportions.

[0075] In the context of the invention, exhaust line 80 is equipped with three richness probes. In particular, these are oxygen probes, i.e. probes that make it possible to determine a residual concentration of oxygen in the gases.

[0076] A first richness probe, called an 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 it emits has a characteristic (typically its voltage) which is proportional to the richness measured.

[0077] A second probe, called the intermediate probe 41 , is placed at a point between the two upstream catalysts 82 and downstream catalysts 84 (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 above a threshold very slightly greater than 1 (for example above a richness of 1.02). It delivers a substantially constant and low voltage when the richness is below 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 that varies almost proportionally with the richness.

[0078] A third probe, called the downstream probe 42, is placed downstream of the downstream catalyst 84. Once again, this may be an oxygen probe. However, new standards could require a nitrogen oxide concentration sensor to be placed downstream of the last catalyst, i.e. the downstream catalyst 84. Such a sensor makes it possible to determine the richness of the gases, so it is preferable to use this sensor rather than a dedicated probe, for reasons of cost. For simplicity, this sensor will hereinafter be referred to as the downstream probe 42 and it will be assumed that it delivers a voltage which behaves like that of the intermediate binary probe 41.

[0079] It should be noted that the three richness probes are "physical" probes. Alternatively, one and / or other of these probes could be of the "software" type, i.e. be formed by an algorithm (based, for example, on an observer) enabling the richness to be calculated as a function of various other engine parameters.

[0080] Other probes (or sensors) could also be used. It would be possible, for example, 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.

[0081] Using maps predetermined on the test bench and stored in its read-only memory, the computer 100 is adapted to generate output signals for each engine operating condition.

[0082] Among these maps, the computer 100 stores data characterising the graph illustrated Figure 3. This graph shows the variations in engine load (or torque) C as a function of engine speed co. For simplicity's sake, we can assume that the load corresponds to the torque exerted by the crankshaft of the internal combustion engine 1. In practice, however, it is more the ratio of the work supplied by the engine at a given speed to the maximum work it could supply at the same speed.

[0083] In Figure 3, the curve Cs shows the variations in maximum torque that the engine can deliver when the richness measured by the upstream probe 40 is kept equal to 1 , as a function of engine speed. The Cmax curve also shows the variations in the maximum torque that the engine can deliver 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, this richness can be increased to a maximum value strictly greater than 1 so as to maintain the temperature of the engine exhaust line components, in particular the exhaust manifold and the upstream catalyst, below a maximum temperature value corresponding to a thermomechanical resistance limit.

[0084] We can see that these two curves Cs, Cmax are merged under a threshold of regime w(S) .

[0085] In the following, the engine will be said to be operating at high speed and full load when its speed is above the speed threshold oosand its load is between the two curves Cs, Cmax.

[0086] Thanks to its output interfaces, th100 is adapted to transmit output signals to the various engine components, in particular the intake valve 24, the control valve 51 and the injectors 64.

[0087] By means of its memory, the computer stores a computer application, consisting of computer programs comprising instructions whose execution by the processor enables the computer to implement the process described below.

[0088] When the engine is started, the fresh air taken from the atmosphere through the intake line 20 is filtered by the air filter 21, mixed with the fuel and then burned in the cylinders 11.

[0089] As they leave the cylinders 11 , the burnt gases are treated by the depollution means, then expanded in the exhaust silencer before being released into the atmosphere.

[0090] When the engine is operating in this way, the process implemented by the computer 100 comprises several main stages, which can now be described.

[0091] The computer 100 is programmed to carry out these steps recursively, i.e. in a loop and at regular time intervals.

[0092] The first of these steps consists of acquiring engine parameters.

[0093] In this way, the computer acquires at least the engine speed co. It can also acquire other parameters, such as the temperature of the upstream catalyst 82.

[0094] At this stage, the computer also acquires the angular position a of the accelerator pedal 30.

[0095] In a second step, the computer determines a torque request desired by the driver.

[0096] The desired torque request corresponds, for example, to the torque that the driver would like the engine to develop.

[0097] This request can for example, be calculated taking into account the engine speed co and the angular position a of the accelerator pedal 30.

[0098] Alternatively, the torque request can be calculated in another way, in particular when the vehicle is (partially) autonomously driven. For example, the torque request may depend on the engagement of a speed regulator.

[0099] Consequently, it will hereafter be referred to more generally as the "Cc torque setpoint".

[0100] During a third stage, the computer 100 reads in its mapping the current value Cs(co) of maximum torque at fuel richness equal to 1 (given by the Cs curve), and possibly also the current value Cmax(cu) of maximum torque at optimum fuel richness (given by the Cmax curve), taking into account the engine speed co.

[0101] In a fourth step, the computer 100 calculates a richness setpoint.

[0102] This richness setpoint corresponds to the richness value that it is desired to measure at the upstream probe 40. In other words, while the intake valve 24 is controlled as a function of the torque setpoint Cc, 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.

[0103] In the context of the invention, two situations are distinguished.

[0104] The first situation corresponds to the case where the torque setpoint Cc is less than or equal to the value Cs(cu) (zone A in Figure 3).

[0105] In this situation, the richness set point is designed to be equal to a first value VAL1 , which is here substantially equal to 1 (with 5% accuracy). In addition, the control valve 51 is kept closed to block the passage of fresh air.

[0106] So, as long as the engine speed remains below the speed threshold ajs, or when the speed exceeds this threshold but the torque setpoint Cc remains restricted, the engine richness will be regulated around the value 1.

[0107] The first VAL1 value could be expected to be exactly equal to 1 and constant.

[0108] Alternatively, this first VAL1 value could be calculated so that the quantity of oxygen stored in the upstream catalyst 82 remains constant and favourable to both oxidation and reduction reactions. The first VAL1 value will then be calculated as a function, in particular, of the measurement taken by the intermediate richness probe 41. Document FR-A1-3033364 describes such a process, for example.

[0109] The first VAL1 value will then oscillate around 1 , deviating from this value by a maximum of 5%. The second situation corresponds to the case where the torque setpoint Cc is strictly greater than the value Cs(cu) (zone B in Figure 3).

[0110] In this situation, the richness setpoint is set 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.

[0111] Thus, at high speed and full load, the combustion mixture entering the cylinders 11 is enriched so that the engine can develop greater torque than at a fuel richness equal to 1 and so as to cool the burnt gases which pass through the upstream catalyst 82. The excess fuel cools the burnt gases and removes the oxygen from the upstream catalyst 82, so that no exothermic oxidation reaction can take place there. In practice, this results in a reduction in temperature in the upstream catalyst 82 that can exceed 100°C. Pollutants not treated in the upstream catalyst 82 are treated in the downstream catalyst 84, thanks to the supply of oxygen from the supply line 50.

[0112] 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 VAL2 value varies according to the torque setpoint Cc. It is chosen to be greater as the torque set point Cc is greater than the value Cs(oo). 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: VAL2= 1+ VALs . (Cc- Cs(w)) / (Cmax(co)-Cs(co))

[0113] In a fifth step, the computer 100 calculates a setpoint for the control valve 51.

[0114] In practice, the setpoint is calculated so that the valve remains closed as long as the torque setpoint Cc is less than the value Cs(cu), i.e. when the richness is regulated around the value 1.

[0115] On the other hand, it is calculated so that the valve opens when the torque setpoint Cc is greater than the value Cs(co).

[0116] The control 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. In fact, 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 engine's polluting emissions. In other words, a significant supply of oxygen will not hinder the production of these reactions.

[0117] However, preferably (in order to prevent the quantity of oxygen in the downstream catalyst 84 from being too high after closure of the control valve 51), the opening of the control valve 51 should be regulated so that the richness measured by the downstream probe 42 remains equal to 1.

[0118] In a sixth step, the computer 100 transmits the previously calculated setpoints to the intake valve 24, the injectors 64 and the control valve 51. Thus :

[0119] - the flow of fresh air into the intake line 20 depends on the position of the accelerator pedal,

[0120] - 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. the first value VAL1 or the second value VAL2 depending on the situation), and

[0121] - 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 substantially equal to 1.

[0122] Figure 4 shows part of the length of exhaust line 80.

[0123] Three graphs also show the variations in the values of three parameters relating to the burnt gases, along this exhaust line 80.

[0124] On these three graphs, the solid line curve corresponds to the case where the richness measured by the upstream probe 40 is regulated around 1 and the torque setpoint Cc is on the curve Cs in Figure 3. On the other hand, the dotted line curve corresponds to the case where the richness measured by the upstream probe 40 is regulated around a value greater than 1 and the torque setpoint Cc is on the curve Cmax in Figure 3.

[0125] The first parameter shown is the temperature T of the burnt gases.

[0126] It can be seen 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 guaranteed. 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.

[0127] It can be seen that, in the first situation (solid line), the temperature rises very slightly in the upstream catalyst 82 and then drops steadily. On the other hand, in the second situation, the temperature drops more at the junction between the supply line 50 and the exhaust line 80, due to the supply of fresh air, then rises sharply at the downstream catalyst 84 where the exothermic oxidation reactions take place.

[0128] It can also be seen that this rise in temperature 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 decreased sufficiently) that it is possible to oxidise the pollutants (in particular carbon monoxide CO and unburnt hydrocarbons HC) without exceeding the temperature threshold Tmax in the downstream catalyst 84.

[0129] The second parameter is the richness R of the burnt gases. It can be seen that this is strictly greater than 1 upstream of the junction between the supply line 50 and the exhaust line 80, and equal to 1 downstream.

[0130] The third parameter is the carbon monoxide content Teo. It can be seen that in the first situation, it is not very high at the outlet of the cylinders 11 and that it decreases as it passes through the upstream catalyst 82 and even as it passes 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 does not decrease when passing through the upstream catalyst 82, due to the absence of oxygen. On the other hand, it is cancelled out when it passes through the downstream catalyst 84, thanks to the supply of oxygen from the supply line 50. The present invention is by no means limited to the embodiment described and illustrated, but the person skilled in the art will know how to apply any variant in accordance with the invention.

[0131] For example, in the context of the invention, the downstream probe 42 could be placed between the outlet of the supply line 50 and the downstream catalyst 84.

[0132] According to another variant, the control valve 51 could be controlled as a function of the temperature of the upstream catalyst 82 or the richness measured by the intermediate richness probe 41. After a cold start, the temperature of the upstream catalyst 82 is still a long way from the Tmax temperature threshold and it stores a quantity of oxygen which could delay the opening of the control valve 51.

Claims

CLAIMS1.- Atmospheric spark-ignition internal combustion engine (1), comprising :- an engine block (10) delimiting at least one cylinder (11),- an intake line (20) for fresh air into 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), characterised 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 rate of fresh air circulating in the supply line (50), and in that- the exhaust line (80) comprises a richness probe (42) located downstream of the outlet of the supply line (50) into the exhaust line (80).2.- Internal combustion engine (1) of claim 1 , wherein the intake line (20) includes an air filter(21), and 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, wherein 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 full load.4.- Internal combustion engine (1) according to claim 3, in which the said region of the exhaust line (80) comprises a venturi throat (85, 86) into 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.- Control method for an internal combustion engine (1) according to one of claims 1 to 6, in which provision is made for acquiring a load (C) of the internal combustion engine (1), and in which:- as long as the load (C) is less than a load threshold (Cs), provision is made 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, the richness of the burnt gases is regulated at a second set-point value which is strictly greater than the first set-point value and fresh air is allowed to circulate 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 (co), the load threshold (Cs) varies as a function of the current speed (co) and is equal to the maximum load which the internal combustion engine (1) can supply at the current speed (co) when the richness of the gases burnt at the outlet of each cylinder (11) is kept equal to 1.9.- Control method according to claim 7 or 8, in which the first setpoint value is equal to 1, with5% accuracy.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, wherein, when the circulation of fresh air in the supply line (50) is permitted, the flow rate of fresh air circulating in the supply line (50) is regulated so that the richness measured by said richness probe (42) remains equal

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