Internal combustion engine and method for controlling such an engine

Abstract

The invention relates to an internal combustion engine (1) comprising a combustion chamber, a fuel injection circuit (60), a fresh gas intake line (20) equipped with at least one intake valve (17A), an exhaust line (80) for exhausting the exhaust gases and equipped with at least one exhaust valve (17B) and a catalytic converter (83), a variable valve lift system, and a computer (100), and the invention more particularly relates to a method for controlling such an engine, which comprises steps of acquiring an operating parameter of the engine and of determining a nominal control setpoint of the variable valve lift system as a function of said parameter. According to the invention, if the amount of oxygen in the catalytic converter is below a predetermined oxygen threshold and if the pressures allow it, provision is made for said nominal setpoint to be corrected so as to advance the opening of the intake valve and / or retard the closing of the exhaust valve.

Description

Description Title of the invention: Internal combustion engine and method for controlling such an engine Technical field of the invention

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

[0002] It relates more particularly to an internal combustion engine comprising: - a combustion chamber, - a fuel injection circuit in the combustion chamber, - a fresh gas intake line into the combustion chamber which is equipped with at least one intake valve, - an exhaust line for the burnt gases outside the combustion chamber which is equipped with at least one exhaust valve and a catalyst, - a variable lift system for the intake valve and / or the exhaust valve, and - a computer suitable for controlling the variable lift system.

[0003] The invention then relates to a method for controlling such a motor. 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, particularly to reduce the quantity of pollutants released into the atmosphere.

[0005] To reduce its polluting emissions, a spark-ignition engine generally has a three-way catalyst in its exhaust line to oxidize at least some of the unburned hydrocarbons (HC) and carbon monoxide (CO), and to reduce at least some of the nitrogen oxides (NOx) emitted in the engine's combustion 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 that includes an oxygen sensor (or lambda sensor) mounted in the exhaust line, upstream of the catalyst. The output voltage of this sensor is subtracted from a set voltage generally corresponding to a richness value equal to 1. The error signal is then compared to zero in a binary comparator. Thus, when the set voltage is higher than the output voltage of the sensor, the air-fuel mixture is enriched using a regulator, generally proportional-integral (PI) type. Conversely, when the setpoint voltage is lower than the probe output voltage, the mixture is leaner. 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 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 carbon monoxide oxidation reactions, to the detriment of nitrogen oxide reduction reactions. Conversely, when the catalyst is devoid of oxygen, it favors nitrogen oxide reduction reactions but this situation is unfavorable to carbon monoxide oxidation reactions.

[0010] The amount of oxygen stored in the catalyst is therefore a very important parameter to ensure good simultaneous treatment of the three pollutants mentioned above. It is therefore necessary to maintain a stable amount of oxygen in the catalyst to ensure optimal decontamination. This is one of the reasons why the richness is regulated around a value of 1.

[0011] Under certain conditions, however, it is known to regulate this richness around a different value, temporarily. Typically, at high loads, such as on motorways, the richness can be raised to a value slightly higher than 1, for example 1.002, in order to promote the reduction of nitrogen oxide emissions (these emissions being highly regulated), even if it means disadvantaging carbon monoxide emissions.

[0012] But we understand that this situation cannot continue as long as we would like since the catalyst would otherwise gradually lose its oxygen and would end up no longer being able to oxidize the carbon monoxide and unburned hydrocarbons. Presentation of the invention

[0013] In order to overcome the aforementioned drawback of the state of the art, the present invention proposes a solution for maintaining the oxygen level in the catalyst at an optimal value even when the richness setpoint is greater than 1.

[0014] More particularly, the invention proposes a method for controlling a motor as defined in the introduction, in which steps are provided: - acquisition of at least one engine operating parameter (typically its speed, its load and the required torque), of a quantity of oxygen stored in the catalyst, and of a pressure difference between the pressure of the fresh gases in the intake line and the pressure of the burnt gases in the exhaust line, - determination of a nominal control setpoint for the variable lift system depending on said operating parameter, and - if the quantity of oxygen is lower than a determined oxygen threshold and if the pressure difference is higher than a determined pressure threshold, correction of said nominal setpoint so as to advance the opening of the intake valve and / or delay the closing of the exhaust valve.

[0015] Thus, thanks to the invention, when the quantity of oxygen is reduced in the catalyst and there is therefore a risk that the latter can no longer oxidize the carbon monoxide and unburned hydrocarbons, the computer controls the variable valve lift system so as to generate a scavenging phenomenon through the combustion chamber.

[0016] This phenomenon involves circulating fresh gases from the intake line to the exhaust line without burning them in the cylinders, taking advantage of the moment when all the valves are open at the same time. Thus, fresh gases loaded with oxygen reach the catalyst, which increases the quantity of oxygen stored in this catalyst.

[0017] Other advantageous and non-limiting characteristics of the control method according to the invention, taken individually or in all technically possible combinations, are the following:

[0018] - during the acquisition step, the computer acquires a richness value of the burnt gases, during the determination step, the computer determines a nominal control setpoint for the fuel injection circuit as a function of the richness value, and during the correction step, the computer controls the fuel injection circuit according to a corrected setpoint distinct from the nominal setpoint, so as to maintain a richness of a mixture of fresh gas and fuel admitted into the combustion chamber equal to a determined target value;

[0019] - the corrected setpoint is determined in open loop, based on a total flow of fresh gas admitted into the combustion chamber and a flow of swept fresh gas passing from the intake line to the exhaust line without being burned in the combustion chamber;

[0020] - said pressure threshold is equal to zero;

[0021] - the nominal control setpoint of the variable lift system is determined as a function of an engine speed and an engine load parameter;

[0022] - the oxygen threshold is a predetermined constant;

[0023] - during the acquisition step, the computer acquires a torque request that the engine must develop, during the determination step, the computer determines a nominal control setpoint for a general intake valve as a function of the torque request, and during the correction step, it is planned to correct the nominal control setpoint for the general intake valve as a function of the advance to the opening of the intake valve and / or the delay in closing the exhaust valve.

[0024] The invention also provides an internal combustion engine comprising:

[0025] - a combustion chamber,

[0026] - a fuel injection circuit in the combustion chamber,

[0027] - a fresh gas intake line into the combustion chamber which is equipped with at least one intake valve,

[0028] - an exhaust line for the burnt gases outside the combustion chamber which is equipped with at least one exhaust valve and a catalyst,

[0029] - a variable lift system for the intake valve and / or the exhaust valve, and

[0030] - a computer adapted to control the variable lift system and programmed to implement a control method as mentioned above.

[0031] Preferably, the intake line comprises a fresh gas compressor.

[0032] 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

[0033] 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.

[0034] On the attached drawings:

[0035] [Fig.l] is a schematic view of an internal combustion engine capable of implementing the control method according to the invention;

[0036] [Fig.2] is a schematic sectional view of the engine block of the internal combustion engine of [Fig.l];

[0037] [Fig.3] is a diagram illustrating the main steps of the control method according to the invention.

[0038] In the description, the terms "upstream" and "downstream" will be used according to the direction of gas flow, from the point where fresh air is taken from the atmosphere to the outlet of the burnt gases into the atmosphere.

[0039] In [Fig.l], there is shown schematically an internal combustion engine 1 of a motor vehicle, which comprises an engine block 10 delimiting a combustion chamber formed by cylinders. Here these cylinders are four in number but they could be fewer (for example three) or more.

[0040] Upstream of the cylinders 11, the internal combustion engine 1 has an intake line 20 which takes fresh air from the atmosphere and which opens into a re- air distributor 25 arranged to distribute fresh air to each of the four cylinders 11 of the engine block 10.

[0041] 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, a compressor 22 which compresses the fresh air filtered by the air filter 21, a main air cooler 23 which cools this compressed fresh air, and a general intake valve 24 (also called a “throttle body”) which makes it possible to regulate the flow of fresh air opening into the air distributor 25.

[0042] 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 which have been previously burned in the cylinders 11 discharge, to an exhaust silencer 87 allowing the burnt gases to be expanded before they are discharged into the atmosphere. It also comprises, in the direction of flow of the burnt gases, a turbine 82 allowing the compressor 22 to be driven, and a catalyst 83 for the treatment of the burnt gases. Alternatively, the compressor 22 could be driven differently, for example by an electric motor. As a further alternative, but not preferentially, provision could be made for the intake line not to include a compressor.

[0043] Here, the engine does not have a gas circulation line. Alternatively, it could, for example, also have a burnt gas recirculation line from the exhaust line to the intake line, at high or low pressure, better known as the EGR line. It could also have additional pollution control devices.

[0044] The internal combustion engine 1 further comprises a fuel injection circuit 60, which comprises an injection pump 62 arranged to take fuel from a tank 61 in order to supply it under pressure via a distribution rail 63 into injectors 64.

[0045] To control the various components of the internal combustion engine 1 and in particular the intake valve 24 and the injectors 64, a computer 100 is provided comprising a processor (CPU), a random access memory (RAM), a read only memory (ROM), analog-to-digital converters (A / D), and various input and output interfaces.

[0046] Thanks to its input interfaces, the computer 100 is adapted to receive input signals relating to engine operating parameters from different sensors.

[0047] It is thus particularly suitable for receiving a signal which is relative to the angle of depression a of the accelerator pedal, or to a pressure exerted on this pedal. It is also suitable for acquiring the engine speed, as well as its load.

[0048] It is also intended to receive a signal relating to the richness / . of the mixture of fuel and fresh air blown into the combustion chamber. This signal is sent here by a dioxygen sensor 40 placed in the exhaust line 80, upstream of the catalyst 83.

[0049] Thanks to a predetermined mapping 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.

[0050] Finally, thanks to its output interfaces, the computer 100 is adapted to transmit these output signals to the various components of the engine, in particular to the intake valve 24 and the injectors 64.

[0051] In [Fig.2], the engine block 10 of the internal combustion engine 1 of [Fig.l] is shown in section.

[0052] This engine block 10 is made up of four main parts, including a cylinder block 10C, an oil pan 10D which is fixed under the cylinder block 10C and which contains oil intended to lubricate the various parts of the engine, a cylinder head 10B which is fixed on the cylinder block 10C and a cylinder head cover 10A which covers the cylinder head 10B.

[0053] In this [Fig.2] we can see the interior of one of the cylinders 11 of the engine block 10.

[0054] This cylinder has a symmetry of revolution around an axis Al here vertical. It houses a cylindrical piston 14 of revolution around the axis Al, in such a way that this piston 14 is adapted to slide in this cylinder 11 along the axis Al, according to an alternating rectilinear movement (or back and forth movement).

[0055] This piston 14 has a peripheral skirt which is transversely pierced with two openings for receiving an axis on which is engaged an upper end of a connecting rod 13. The lower end of this connecting rod 13 is linked, by means of an eccentric connection, to a crankshaft 12 (also called “engine shaft”).

[0056] Thus, the alternating rectilinear movement of the piston 14 makes it possible to rotate the crankshaft 12 of the internal combustion engine 1 around its longitudinal axis, called the engine axis A2.

[0057] We can also see in this [Fig.2] the end of one of the fuel injectors 64, which opens directly into the cylinder 11 (we speak of “direct injection”).

[0058] The engine here being with spark ignition, it also includes a spark plug (not visible) located near this injector 64.

[0059] For the admission of fresh air to the cylinder 11, the cylinder head 10B is pierced with one (or two) intake ducts 16A which extend from the air distributor 25 to an intake opening 15A provided in the lower face of the cylinder head, opposite the cylinder 11.

[0060] For the exhaust of burnt gases outside the cylinder 11, the cylinder head 10B is pierced with one (or two) exhaust ducts 16B which originate in a exhaust opening 15B provided in the lower face of the cylinder head, opposite the cylinder 11, close to the intake opening 15 A, and which opens into the exhaust manifold 81.

[0061] To regulate the flow rates of fresh air inlet and burnt gas outlet in each cylinder 11, the cylinder head 10B houses intake valves 17A and exhaust valves 17B whose flared ends close the intake 15A and exhaust 15B openings of the fresh air intake 16A and burnt gas exhaust 16B ducts.

[0062] It also houses distribution means adapted to control the position of these intake 17A and exhaust 17B valves, so that the internal combustion engine operates according to four thermodynamic cycles called intake, compression, explosion and exhaust cycles.

[0063] These distribution means are here of the variable type (in English we speak of VVT system for 'Variable Valve Timing'). They thus make it possible to shift the moment of opening and / or closing of the intake and / or exhaust valves in relation to the top dead center (i.e. in relation to the moment when the piston reaches the highest point in the cylinder 11). In other words, the computer can vary the moment of opening and / or closing of at least one of the valves.

[0064] These means of distribution include for example: - two camshafts 18A, 18B which are mounted so as to be able to rotate in the cylinder head 10B and which carry cams 19A, 19B arranged to periodically press on the intake 17A and exhaust 17B valves so that each valve “lifts” regularly in order to periodically release a passage for fresh air or burnt gases via the intake 15A and exhaust 15B openings, - a timing chain (not shown) which connects the crankshaft 12 in rotation to two pinions respectively coupled to the two camshafts 18 A, 18B, and - control systems (not shown) which allow each of these two pinions to be angularly offset relative to the camshaft which carries it, by an angle of several degrees in one direction or the other.

[0065] Such a control system is well known to those skilled in the art and is not itself the subject of the present invention. It will therefore not be described in more detail here.

[0066] It should only be noted that the phase shift of a camshaft is considered zero if the complete closing of the exhaust valves (of the cylinder shown) occurs at the precise moment when the intake valves begin to open, while the piston is at top dead center between the exhaust and intake cycles.

[0067] The phase shift, which is measured in degrees, corresponds to the angle between, on the one hand, the angular position of the camshaft when the piston 14 is at top dead center between the exhaust and intake cycles, and, on the other hand, the angular position that this camshaft would present if the phase shift were zero.

[0068] This phase shift makes it possible to generate in particular a “scavenging phenomenon” by opening the intake valves 17A while the exhaust valves 17B are not yet closed (between the exhaust and intake cycles). Thus, fresh gases circulating in the intake line 20 can be sent directly into the exhaust line 80, without being first burned in the cylinder 11.

[0069] When the engine is started, fresh air taken from the atmosphere through the intake line 20 is filtered by the air filter 21, compressed by the compressor 22, cooled by the main air cooler 23, and then burned in the cylinders 11.

[0070] On leaving the cylinders 11, the burnt gases are expanded in the turbine 82, treated in the catalyst 83, then expanded again in the exhaust silencer 87 before being released into the atmosphere.

[0071] The camshafts 18A, 18B are then controlled so that the phase shift generally remains zero. However, in certain operating phases, this phase shift is adjusted to a non-zero value, for example to generate a recirculation of burnt gases from the exhaust to the intake (this is called internal EGR) or to take advantage of the scavenging phenomenon.

[0072] In practice, the phase shift setpoint transmitted to the control system is deduced from the engine's operating point, i.e. its speed and load. This setpoint is then called "nominal" and is determined independently of the quantity of oxygen stored in the catalyst 83.

[0073] According to the invention, the computer 100 is programmed to implement a method for controlling the engine 1, recursively, that is to say in a loop and at regular time steps.

[0074] This control process involves several main steps illustrated in [Fig.3],

[0075] The first step E0 consists of acquiring a desired torque request. This corresponds, for example, to the torque that the driver would like motor 1 to develop.

[0076] This request can for example be calculated taking into account the engine speed and the angular position a of the accelerator pedal 30 (received via an angular speed sensor and a position sensor connected to the computer 100).

[0077] Otherwise, it can be calculated differently, particularly when the vehicle is driven (partially) autonomously.

[0078] During a second step E2, the computer deduces from this torque request and the engine operating point the control instructions for the components of engine 1. These instructions are determined so as to adjust the fresh air flow and the fuel flow to desired values.

[0079] In practice, the computer determines the phase shift setpoint as a function of the operating point of engine 1, and the control setpoint of the intake valve 24 as a function of the torque request, taking into account the boost pressure and the phase shift of the camshafts.

[0080] The fuel flow is regulated so that the richness X of the mixture admitted into the cylinders 11 (oxygen and fuel) remains substantially equal to the target value, generally equal to 1.

[0081] To do this, the computer 100 uses, for example, a feedback loop taking into account the measurements made by the oxygen probe 40.

[0082] This feedback loop receives as input the target value C, here equal to 1. It provides as output a fuel flow rate instruction to be injected, presented here in the form of an instruction T ; of injector opening duration 64 in each cycle.

[0083] The difference e between the measured richness X and the target value C,. allows, using a regulator (for example of the PID type), to determine a correction value Te for the opening time of the injectors 64.

[0084] Thus, the instruction T ; of opening duration transmitted to the injectors 64 is equal to the sum of this correction value Te and a predetermined value f (taking into account the operating point of the engine, i.e. its speed and its load).

[0085] This feedback loop therefore makes it possible to adjust the quantity of fuel injected according to the position of the intake valve 24.

[0086] It should be noted that at high load and constant speed, the target value C,. may be increased, for example here equal to 1.002 in order to promote the treatment of nitrogen oxides.

[0087] During a third step E4, the calculator calculates the current value of the quantity of oxygen contained in the catalyst 83. This quantity of oxygen is commonly noted OS (from the English 'oxygen storage').

[0088] Various calculation methods could be employed.

[0089] Typically, it will be possible to use the one described in document FR3033364, which is based on the following equation.

[0090] [Math.l]

[0091] In this equation, the variables are defined as follows.

[0092] Q ech denotes the flow rate of burnt gases (equal for example to the sum of the flow rate of fresh gas circulating in the intake line and fuel).

[0093] Rx denotes the richness upstream of catalyst 83 (measured by oxygen sensor 40).

[0094] Toz refers to the mass rate of oxygen in the air (approximately 0.23 or 23%).

[0095] OSjnit denotes the amount of oxygen stored at time t init from the start of integration. This quantity will be initialized to a predetermined value corresponding, for example, to the saturation of the catalyst with oxygen. The start of the integration will then correspond to a moment in which we know that the catalyst is saturated with oxygen. This is typically the case when the fuel injection is cut for a fairly long time, in particular when the driver completely lifts his foot off the accelerator pedal. This value can be predetermined by preliminary tests.

[0096] OS denotes the quantity of oxygen stored at the current time t.

[0097] During a fourth step E6, the computer compares the current value OS of the quantity of oxygen stored in the catalyst with a predetermined threshold value. This threshold value will correspond to the value below which it is considered that the catalyst is no longer able to treat carbon monoxide and unburned hydrocarbons sufficiently well.

[0098] As long as this current value OS remains greater than or equal to the threshold, which means that there is still sufficient oxygen in the catalyst 83, the process resumes at the first step E0. From then on, the phase shift of the camshafts remains equal to the nominal phase shift (determined taking into account the operating point of the engine).

[0099] It is understood that if the richness of the mixture is set to a value very slightly higher than 1 to ensure good treatment of nitrogen oxides and that this setting continues for a sufficiently long time, the catalyst 83 gradually empties of its oxygen, with the risk of no longer being able to oxidize the carbon monoxide CO (nor the unburned hydrocarbon molecules).

[0100] Therefore, if the current value OS of the quantity of oxygen stored in the catalyst becomes lower than the threshold, the process continues in a fifth step E8.

[0101] This and subsequent steps aim to inject more oxygen into the catalyst so that it can store enough to process the carbon monoxide while still meeting the torque setpoint. The idea behind this is to use the scavenging phenomenon.

[0102] It is then understood that this operation can only be carried out provided that the intake pressure is higher than the exhaust pressure.

[0103] During this step E8, the computer 100 therefore determines the intake pressure and the exhaust pressure. To do this, it can base itself on measurements made by pressure sensors located in the air distributor 25 and in the exhaust manifold 81. Alternatively, it could calculate these two pressures taking into account the operating point of the engine.

[0104] Then, during step E10, the computer determines whether the exhaust pressure is lower than the intake pressure.

[0105] If this is not the case, which means that the sweeping phenomenon cannot occur. produce, the process resumes at the first stage EO. From then on, the phase shift of the camshafts remains equal to the nominal phase shift (determined taking into account the engine operating point). It will then be noted that the quantity of oxygen stored in the catalyst may then continue to decrease or on the contrary increase, for example in the presence of a foot lift phase with fuel injection cut-off.

[0106] On the other hand, if the exhaust pressure is lower than the intake pressure, the method continues in a step E12 aimed at modifying the nominal settings of the engine calculated in step E2 in order to take advantage of the scavenging phenomenon to replenish the oxygen reserve of the catalyst 83.

[0107] For this, the computer 100 can delay the closing time of the exhaust valves 17B and / or advance the opening time of the intake valves 17A so that there is a time when these valves are simultaneously open.

[0108] We then understand that in this situation, the oxygen sensor 40 will detect a richness lower than 1. This measured richness will in fact no longer be equal to the richness of the mixture admitted into the combustion chamber.

[0109] The idea then is to adjust the engine so that the richness in the combustion chamber remains equal to 1 at each cycle (without measuring it), so as to produce the required engine torque request while reducing pollutant emissions as much as possible.

[0110] For this, the calculator 100 acquires: - the total flow of fresh gas admitted into the cylinders (measured using a flow meter or by calculation), and - the flow of swept fresh gas (passing from the intake to the exhaust without being burned in the combustion chamber) from measurements of burnt gas flow and oxygen concentration measured by the oxygen probe.

[0111] It can thus deduce from the difference between these two values ​​the flow rate of fresh gases burned in the cylinders, then adjust the fuel flow in open loop, for example at the rate of one gram of fuel for 14.7 g of fresh gas if the target value is equal to 1.

[0112] Thus, the computer is able to inject the correct mass of fuel so that the richness of the mixture burned in the cylinders 11 remains equal to the target value and the torque developed is equal to the required torque request.

[0113] In this situation where the phase shift of the camshafts is distinct from the nominal phase shift, the quantity of oxygen admitted into the catalyst 83 increases rapidly, which makes it possible to recharge the oxygen stock. This phase shift is maintained until the calculated value of the quantity of oxygen stored becomes greater than a threshold value (which is greater than or equal to the aforementioned threshold value).

[0114] Of course, the execution of the sweeping phenomenon can be interrupted earlier, especially as soon as the intake pressure becomes lower than the exhaust pressure.

[0115] The present invention is in no way limited to the embodiments described and shown, but those skilled in the art will be able to provide any variation in accordance with the invention.

[0116] Typically, the variable valve lift system might control not all of the engine's valves, but only the intake valves or only the exhaust valves, or the valves (intake or exhaust) of only some of the engine's cylinders.

[0117] According to another variant of the invention, the computer may be programmed to generate the scavenging phenomenon only if the intake pressure is higher than the exhaust pressure with a non-zero and predetermined difference.

Claims

Claims

1. Method for controlling an internal combustion engine (1) comprising: - a combustion chamber, - a circuit (60) for injecting fuel into the combustion chamber, - an intake line (20) for fresh gas into the combustion chamber which is equipped with at least one intake valve (17A), - an exhaust line (80) for the burnt gases outside the combustion chamber which is equipped with at least one exhaust valve (17B) and a catalyst (83), - a variable lift system for the intake valve (17A) and / or the exhaust valve (17B), and - a computer (100) adapted to control the variable lift system, the control method comprises steps of acquiring at least one operating parameter of the engine (1) and determining a nominal control setpoint of the variable lift system as a function of said operating parameter, characterized in that, in the acquisition step, it is provided to acquire a quantity of oxygen stored in the catalyst (13) and a pressure difference between a pressure of the fresh gases in the intake line (20) and a pressure of the burnt gases in the exhaust line (80), and in that, if the quantity of oxygen is less than a determined oxygen threshold and if the pressure difference is greater than a determined pressure threshold, a step of correcting said nominal setpoint is provided so as to advance the opening of the intake valve (17A) and / or delay the closing of the exhaust valve (17B).

2. A method of control according to claim 1, wherein: - during the acquisition stage, the calculator acquires a value of the richness of the burnt gases, - during the determination step, the computer determines a nominal control setpoint for the fuel injection circuit (60) as a function of the richness value, and - during the correction step, the computer controls the fuel injection circuit (60) according to a corrected setpoint distinct from the nominal setpoint, so as to maintain a richness of a mixture of fresh gas and fuel admitted into the combustion chamber equal to a determined target value.

3. Control method according to claim 2, in which the corrected setpoint is determined in open loop, as a function of a total flow rate of fresh gas admitted into the combustion chamber and a flow rate of swept fresh gas passing from the intake line (20) to the exhaust line (80) without being burned in the combustion chamber.

4. Control method according to one of claims 1 to 3, in which said pressure threshold is equal to zero.

5. Control method according to one of claims 1 to 4, in which the nominal control setpoint of the variable lift system is determined as a function of a speed of the engine (1) and a load parameter of the engine (1).

6. Control method according to one of claims 1 to 5, in which the oxygen threshold is a predetermined constant.

7. Control method according to one of claims 1 to 6, in which: - during the acquisition stage, the computer acquires a torque request that the engine (1) must develop, - during the determination step, the computer determines a nominal control setpoint for a general intake valve (24) as a function of the torque request, and - during the correction step, it is planned to correct the nominal control setpoint of the general intake valve (24) as a function of the advance in opening of the intake valve (17A) and / or the delay in closing of the exhaust valve (17B).

8. Internal combustion engine (1) comprising: - a combustion chamber, - a circuit (60) for injecting fuel into the combustion chamber, - an intake line (20) for fresh gas into the combustion chamber which is equipped with at least one intake valve (17A), - an exhaust line (80) for the burnt gases outside the combustion chamber which is equipped with at least one exhaust valve (17B) and a catalyst (83), - a variable lift system for the intake valve (17A) and / or the exhaust valve (17B), and - a computer (100) adapted to control the variable lift system and programmed to implement a control method in accordance with one of claims 1 to 7.

9. Engine (1) according to claim 8, wherein the intake line (20) includes a fresh gas compressor (22).

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