System and method for controlling an internal combustion engine during the deceleration phase

Abstract

This method for controlling a spark-ignition internal combustion engine (10) of a motor vehicle equipped with a variable valve timing system (50, 52) and an emissions control system (40) comprising at least one three-way catalyst (42a), comprises the following successive steps: cutting off fuel injection when a deceleration phase is detected and maintaining this cut-off until a subsequent injection resumption step; overlap of the intake (51) and exhaust (53) valves; uncrossing of the intake (51) and exhaust (53) valves when either a non-zero torque demand, a predetermined reinjection speed, or engine disengagement is detected; and resumption of injection. Figure for the abbreviation: [Fig 1]

Description

Title of the invention: System and method for controlling an internal combustion engine during the lifting phase Technical field

[0001] The technical field of the invention is the control of spark-ignition internal combustion engines, and more particularly, the control of such engines with a view to reducing the emission of polluting species. Previous techniques

[0002] Motor vehicles equipped with a spark-ignition internal combustion engine (of the type operating in particular on gasoline) are equipped with a system for post-treatment of polluting species included in the exhaust gases in order to comply with anti-pollution standards. The post-treatment system generally comprises a three-way catalyst which is capable of oxidizing unburned hydrocarbons (HC) and carbon monoxide (CO), and of reducing nitrogen oxides (NOx).

[0003] Spark-ignition engines operate substantially at richness 1 (i.e. with an air-fuel mixture in stoichiometric proportions), the quantities of fuel injected into the engine generally being adjusted so that the richness is set in a closed loop around the value 1.

[0004] However, in order for the treatment efficiency of the various polluting species to be optimal, it is also known to adjust the quantities of fuel injected into the engine so as to adjust the quantity of oxygen stored in the catalyst, also called OS (English acronym for: Oxygen Storage), to a set value. This set value is a predetermined value which is strictly between zero and the maximum oxygen storage capacity of the catalyst, also called OSC (English acronym for: Oxygen Storage Capacity).

[0005] This predetermined value is both sufficiently far from zero to allow sufficient oxidation of HC and CO, and sufficiently far from the OSC value to allow sufficient reduction of NOx. It can be chosen experimentally and depends on a plurality of parameters including at least the flow rate of the combustion gases passing through the catalyst and the temperature of the catalyst.

[0006] Reference will be made in particular to publication FR-A1-310110 which discloses such a method for adjusting the quantity of oxygen OS stored to a set value which is determined as a function of the flow rate of the combustion gases passing through the catalyst and the temperature of the catalyst.

[0007] Generally speaking, the closer the quantity of oxygen OS actually present in the catalyst is to zero, the more the oxidation efficiency of HC and CO decreases. Conversely, the closer the amount of oxygen OS is to the OSC, the lower the NOx reduction efficiency.

[0008] During certain driving situations, for example when changing gear in a gearbox or during a so-called foot-lift phase during which the driver completely lifts his foot off the accelerator pedal, the fuel injection is automatically cut off to reduce fuel consumption and air is sent into the post-treatment system. The quantity of oxygen stored in the catalyst then increases, for example up to the OSC, and polluting species, more particularly NOx, are no longer treated effectively when fuel is reinjected again.

[0009] When resuming fuel injection, when the amount of oxygen stored in the catalyst has reached the OSC meaning that the catalyst is saturated with oxygen, a strategy for purging or lowering the oxygen load of the catalyst is generally carried out. The catalyst purging strategy comprises increasing the richness of the injected air-fuel mixture to a richness greater than 1, i.e. increasing the proportion of fuel in the injected air-fuel mixture so that the proportion of fuel is greater than that present in the stoichiometric air-fuel mixture, so as to rapidly decrease the amount of oxygen stored in the catalyst. However, the catalyst purging strategy considerably increases the vehicle's fuel consumption and CO2 emissions.

[0010] There are several solutions for limiting the flow of air supplied to the exhaust during the fuel injection cut-off phases, so as to limit the oxygen saturation of the catalyst.

[0011] For example, it is possible to close the throttle body at the intake to mechanically restrict the flow of air drawn into the intake and therefore the flow of air supplied to the exhaust. In addition to a generally limited gain, this results in a reduction in the pressure of the intake manifold which can lead to an increase in oil consumption.

[0012] For example, in the event of zero torque demand, which is the case when lifting the foot, it is possible to disengage the engine from the transmission and run it at idle, or even turn it off. However, this type of solution is incompatible with mechanical gearboxes, because it requires an automatic transmission system or at least a controlled clutch.

[0013] Document FR-A1-3120253 describes a method for controlling a spark-ignition engine with the objective of not saturating a catalyst mounted on the exhaust with oxygen, when there is no torque demand, typically in a foot-lift phase. According to this document, the mass of air admitted by the engine is reduced either by recirculating exhaust gases to the intake through an EGR (or "Exhaust Gas Recycling" in English) type recirculation circuit, or by using a variable valve timing system, and fuel is injected after combustion so as to increase the richness without burning the fuel in the cylinder, i.e. without producing torque. However, such fuel injection increases the vehicle's consumption and CO2 emissions. In addition, late fuel injection leads to the generation of an exothermic reaction in the catalyst, which can damage it. Statement of the invention

[0014] The invention aims to eliminate NOx emission peaks during an acceleration phase following a foot lifting phase with an engine control method optimized during the accelerator pedal lifting phase making it possible to avoid oxygen saturation of the pollution control system while eliminating the problems of fuel consumption and catalyst deterioration described above.

[0015] The subject of the invention is a method for controlling a spark-ignition internal combustion engine of a motor vehicle equipped with a variable valve timing system and a system for decontaminating polluting species comprising at least one three-way catalyst.

[0016] The method comprises successive steps of: - cutting off the fuel injection when a foot lifting phase is detected and maintaining this cut-off until a later stage of resuming injection, - crossing of intake and exhaust valves, - uncrossing of the intake and exhaust valves carried out when either a torque demand which is not zero, or a predetermined reinjection speed, or engine disengagement is detected, and - resumption of the injection.

[0017] Advantageously, the injection resumption step is carried out when a flow of fresh air admitted into the engine exceeds a predetermined value so as to avoid combustion misfires.

[0018] Preferably, a torque setpoint is assigned a zero value in the decoupling step, which is maintained until injection has resumed.

[0019] According to an advantageous characteristic, from the resumption of injection the value of the torque setpoint is made to converge towards the value of a torque request from the driver.

[0020] For example, convergence is achieved using a filter or a time ramp.

[0021] According to another characteristic, the crossing of the intake and exhaust valves per engine cycle has a duration ranging from 40°Vil to 120°Vil.

[0022] According to another aspect, the invention relates to a system for controlling a spark-ignition internal combustion engine of a motor vehicle provided with a valve timing system and a system for decontaminating polluting species comprising at least one three-way catalyst, comprising an electronic control unit and configured for implementing a control method as described above.

[0023] According to another aspect, the invention relates to a spark-ignition internal combustion engine comprising a valve timing system and a system for removing polluting species comprising at least one three-way catalyst, the engine being provided with a control system configured for implementing a control method as described above.

[0024] According to another aspect, the invention relates to a motor vehicle equipped with an engine as described above. Brief description of the drawings

[0025] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0026] [Fig-1] illustrates, schematically, the structure of a combustion engine internal of a motor vehicle equipped with an engine control system according to the invention;

[0027] [Fig.2] illustrates a flowchart of a method for controlling the motor of [Fig.l] according to the invention; and

[0028] [Fig.3] illustrates the evolution over time of different parameters of an engine controlled by a control method according to the invention. Detailed description of at least one embodiment

[0029] In the example illustrated in [Fig.l], the internal combustion engine 10 is of the spark-ignition type and comprises, in a non-limiting manner, three cylinders 12 in line, a fresh air intake manifold 14, an exhaust manifold 16 and a turbo-compression system or turbocharger 18.

[0030] The cylinders 12 are supplied with air via the intake manifold 14, or distributor, itself supplied by a pipe 20 provided with an air filter 22 and a compressor 18a of the turbocharger 18 of the engine 10.

[0031] The turbocharger 18 essentially comprises a turbine 18b driven by the exhaust gases and the compressor 18a, mounted on the same shaft as the turbine 18b, and ensuring compression of the air distributed by the air filter 22 or air box, in the aim of increasing the quantity (mass flow) of air admitted into the cylinders 12 of the engine 10 for an identical volume flow.

[0032] The internal combustion engine 10 comprises an intake circuit Ca and an exhaust circuit Ce.

[0033] The intake circuit Ca comprises, from upstream to downstream in the direction of air circulation:

[0034] - the air filter 22;

[0035] - a flow meter 26 arranged in the intake pipe 20 downstream of the air filter 22 to measure the actual value of the mass flow rate of air entering the engine 10;

[0036] - an air intake valve 9;

[0037] - the compressor 18a of the turbocharger 18;

[0038] - a throttle body 30 or a gas intake valve in the engine;

[0039] - a heat exchanger 32 configured to cool the intake gases cor corresponding to a mixture of fresh air and recirculated gases after their compression in the compressor 18a;

[0040] -pressure and temperature sensors 33 for measuring the pressure and temperature in the intake manifold 14; and

[0041] - the intake manifold 14.

[0042] The compressor is associated with a bypass circuit equipped with an inlet discharge valve 39 which opens in the event of sudden closure of the throttle body 30, to prevent the compressed air, located between the compressor 18a and the throttle body 30, from passing through the compressor 18a in reverse and damaging it, when, for example, the driver of the vehicle suddenly lifts his foot off the accelerator pedal.

[0043] The exhaust circuit Ce comprises, from upstream to downstream in the direction of circulation of the burnt gases:

[0044] - the exhaust manifold 16;

[0045] - the turbine 18b of the turbocharger 18; and

[0046] - a system 40 for depolluting the combustion gases of the engine, comprising including a three-way catalyst.

[0047] As regards the exhaust manifold 16, the latter recovers the exhaust gases resulting from the combustion and evacuates them to the outside, via a gas exhaust duct 34 opening at the inlet of the turbine 18b of the turbocharger 18 and via an exhaust line 36 mounted downstream of the turbine 18b.

[0048] The engine 10 further comprises a partial recirculation circuit 38 of the exhaust gases to the intake, called the “EGR” or “exhaust gas recirculation” circuit in English.

[0049] The engine 10 may not be equipped with an EGR circuit, without departing from the scope of the invention.

[0050] This circuit 38 is here, in a non-limiting manner, a low-pressure exhaust gas recirculation circuit. It is connected to the exhaust line 36, at a point located downstream of said turbine 18b, and in particular downstream of the gas depollution system, and returns the exhaust gases to the fresh air supply pipe 20, upstream of the compressor 18a of the turbocharger 18, in particular downstream of the flow meter 26. The flow meter 26 only measures the flow of fresh air alone.

[0051] As illustrated, this recirculation circuit 38 comprises, in the direction of circulation of the recycled gases, a cooler 38a, a filter 38b, and a valve 38c configured to regulate the flow rate of the low-pressure exhaust gases. The valve 38c is arranged downstream of the cooler 38a and the filter 38b and upstream of the compressor 18a.

[0052] The system 40 for depolluting the combustion gases of the engine comprises a first post-treatment device 42 comprising at least one three-way catalyst 42a which can be electrically heated, with at least one first oxygen sensor 43a mounted upstream of the first post-treatment device 42.

[0053] The first upstream oxygen sensor 43a is generally used to regulate in a closed loop the value of the richness of the air-fuel mixture in the engine around a set value, for example the value 1 corresponding to an air-fuel mixture in stoichiometric proportions.

[0054] Furthermore, a second optional oxygen sensor 43b, for example of the binary or proportional type, can be mounted downstream of the first post-treatment device 42 so as to be able to correct the set value of the richness regulation loop, in particular with the aim of adjusting the quantity of oxygen stored inside the catalyst 42a.

[0055] The gas depollution system 40 further comprises a second post-treatment device 44 which is here a fine particle filter. The system 40 may also comprise a third oxygen probe 43c, for example of the binary type, mounted downstream of the second device 44, for example for diagnostic purposes.

[0056] The engine 10 also comprises a variable timing system 50 for the intake valves 51 of the engine and possibly also a variable timing system 52 for the exhaust valves 53 of the engine. The variable timing system 50 for the intake valves is equipped with a sensor 54 which makes it possible to know at any time its angular position, which corresponds to determined moments of opening and closing of the intake valves 51 in the combustion cycle of the engine (these moments generally being measured in crankshaft degrees, noted °Vil, relative to a top dead center position). The variable timing system 52 for the exhaust valves, if present, is also equipped with a sensor 55 which makes it possible to know at each instant its angular position, which also corresponds to determined instants of opening and closing of the exhaust valves 53 in the combustion cycle of the engine.

[0057] The engine is associated with a fuel circuit comprising, for example, fuel injectors (not referenced) injecting gasoline directly into each cylinder from a fuel tank (not shown).

[0058] Furthermore, the engine is equipped with a control system 60 comprising an electronic control unit 61 configured to control the various elements of the internal combustion engine from data collected by sensors at different locations in the engine.

[0059] The electronic control unit 61 comprises a calculation module 62, a measurement module 63 and a control module 64.

[0060] [Fig.2] illustrates a flowchart of a control method according to the invention.

[0061] The method begins with a prior step 65 of detecting a lifting phase. foot, corresponding to a period during which the driver does not press the accelerator pedal. During a foot lifting phase, the engine torque demand is zero.

[0062] The method continues with a step 66 of cutting off the fuel injection. The injection cutting is maintained until a subsequent step 69 of resuming the injection.

[0063] During the following crossover step 67, the electronic control unit 61 controls the variable timing system 50, 52 to carry out a crossover phase of the intake 51 and exhaust 53 valves, that is to say a phase during the intake time during which the intake valves 51 and the exhaust valves 52 are open at the same time. By thus combining the setpoints of the positions of the camshaft phasers, it is possible to limit the flow of fresh air admitted into the engine and discharged to the exhaust. As a result, the supply of oxygen to the catalyst 42a is limited in the lifting phase and the oxygen saturation and the capacity to treat NOx are better controlled during the following reinjection.

[0064] It should be noted that if the engine 10 is equipped only with a variable valve timing system at the intake 50, the crossover phase is achieved by the sole offset of the intake valves 51, without variation in the timing of the exhaust valves 53. For example, the mass of air admitted into the combustion chambers can be reduced via a Miller cycle or an Atkinson cycle. The Miller or Atkinson cycles provide for the intake valves to close respectively early or late relative to the bottom dead center. In both cases, the engine's pumping losses are limited and the engine's fuel consumption is improved.

[0065] Preferably, the crossing of the intake 51 and exhaust 53 valves by engine cycle has a duration ranging from 40°Vil to 120°Vil.

[0066] The crossover phase is maintained until a decrossing step 68 which is carried out when the control system detects either a torque demand which is not zero, or a reinjection speed, or a disengagement of the engine.

[0067] Detecting the reinjection speed or "re-coupling" speed consists of detecting a predetermined speed from which reinjection is desired. The value of the predetermined speed depends in particular on the gearbox ratio engaged, the engine temperature and the altitude. It is also possible to anticipate valve uncrossing by monitoring the approach to the reinjection speed. Thus, by considering the instantaneous value and the rate of decrease of the current engine speed, it is possible to control valve uncrossing, so that the valves reach a position allowing sufficient air to be admitted into the combustion chamber to avoid combustion misfires at the moment when the engine reaches the predetermined reinjection speed.

[0068] Engine disengagement is detected when the engine is disengaged from the transmission to engage another gear in the gearbox or to mark the stopping of the vehicle. In the event of engine disengagement, the valves are controlled to uncross so that the valves reach a position allowing sufficient air to be admitted into the combustion chamber to avoid combustion misfires when injection is resumed.

[0069] Detecting a non-zero torque demand involves detecting the driver's torque demand issued by pressing an accelerator pedal or by re-engaging a cruise control. As soon as a non-zero torque demand is thus detected, the valves are controlled to uncross so that the valves reach a position allowing sufficient air to be admitted into the combustion chamber to avoid misfires when injection is resumed. In order to ensure that the engine is able to produce torque without misfires, the torque demand is timed. Thus, a torque setpoint is assigned a zero value which is maintained until injection has resumed.

[0070] The method continues with a step 69 of resuming the injection carried out when a flow of fresh air admitted into the engine exceeds a predetermined value so as to avoid combustion misfires. Such a step makes it possible to guarantee that the injected fuel burns correctly in the combustion chamber.

[0071] From the injection restart, the value of the torque setpoint is converged towards the value of a torque request from the driver. Preferably, the convergence is achieved using a filter or a time ramp. Thus, the torque requested by the driver is filtered according to a maximum torque gradient, i.e. a maximum time derivative of the torque. This filtering prevents the torque setpoint from resulting in a jolt felt by the driver when the torque is resumed.

[0072] [Fig. 3] illustrates the evolution over time of different parameters of a motor 10 controlled by a control method according to the invention.

[0073] Curve 70 illustrates the driver's torque demand. The zone referenced 71 corresponds to a foot lifting phase where the driver's torque demand is zero.

[0074] When the foot lifting phase 71 is initiated, the injection 72 is cut off, followed by the crossing of the valves 73, which results in a significant drop in the flow of air admitted 74 into the engine, in particular below a minimum admitted air flow value 75 which makes it possible to avoid combustion misfires.

[0075] When the foot lifting phase 71 reaches its end, the valves are controlled to uncross (end of valve crossing 73) and at the same time a torque setpoint 76 is assigned a zero value, which is maintained until injection has resumed.

[0076] The injection resumes (end of the injection cut-off 72) at the moment when the admitted air flow 74 exceeds the minimum value 75, so as to avoid combustion misfires. From this same moment, the value of the setpoint 76 is made to converge towards the value of the driver's torque request 70.

Claims

Claims

1. Method for controlling a motor vehicle spark-ignition internal combustion engine (10) provided with a variable valve timing system (50, 52) and a system (40) for depolluting polluting species comprising at least one three-way catalyst (42a), the method being characterized in that it comprises successive steps of: - cutting off the fuel injection when a foot lifting phase is detected and maintaining this cut-off until a subsequent step of resuming injection, - crossing the intake (51) and exhaust (53) valves, - uncrossing the intake (51) and exhaust (53) valves carried out when either a torque demand which is not zero, or a predetermined reinjection speed, or a declutching of the engine is detected, and - resuming injection.

2. A method according to claim 1, wherein the step of resuming injection is carried out when a flow rate of fresh air admitted into the engine (10) exceeds a predetermined value so as to avoid combustion misfires.

3. Method according to claim 1 or 2, in which, in the uncrossing step, a torque setpoint is assigned a zero value which is maintained until injection has resumed.

4. Method according to claim 3, in which from the resumption of injection the value of the torque setpoint is made to converge towards the value of a torque request from the driver.

5. A method according to claim 4, wherein the convergence is achieved using a filter or a time ramp.

6. A method according to any one of claims 1 to 5, wherein the crossover of the intake (51) and exhaust (53) valves per engine cycle has a duration ranging from 40°Vil to 120°Vil.

7. Control system (60) for a motor vehicle spark-ignition internal combustion engine (10) provided with a valve timing system (50, 52) and a pollution control system (40) polluting species comprising at least one three-way catalyst (42a), comprising an electronic control unit (61) and configured for implementing a control method according to any one of claims 1 to 6.

8. A spark-ignition internal combustion engine (10) comprising a valve timing system (50, 52) and a system (40) for depolluting polluting species comprising at least one three-way catalyst (42a), said engine being provided with a control system (60) configured for implementing a control method according to any one of claims 1 to 6.

9. Motor vehicle equipped with an engine according to claim 8.

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