System and method for controlling an internal combustion engine in the phase of foot lift

Abstract

This 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 pollutant species comprising at least one three-way catalyst (42a), comprises the following successive steps: - cutting off the fuel injection when a foot lift phase is detected and maintaining this cut-off until a subsequent step of resuming the injection, - crossing of intake (51) and exhaust (53) valves, - uncrossing of the intake (51) and exhaust (53) valves carried out when either a torque request that is not zero, or a predetermined reinjection speed, or a disengagement of the engine is detected, - resuming injection.

Description

[0001] DESCRIPTION

[0002] System and method for controlling an internal combustion engine in the phase of foot lift

[0003] Technical field

[0004] 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.

[0005] Prior art

[0006] Motor vehicles equipped with a spark- ignition internal combustion engine (of the type running in particular on petrol) are equipped with a system for post-treatment of the polluting species comprised in the exhaust gases in order to meet the antipollution standards. The after-treatment system typically includes a three-way catalyst that is capable of oxidising unbumt hydrocarbons (HC) and carbon monoxide (CO), and reducing nitrogen oxides (NOx).

[0007] Spark- ignition engines operate substantially at richness 1 (i.e. with an air- fuel mixture in stoichiometric proportions), the amounts of fuel injected into the engine being generally adjusted so that the richness is adjusted in a closed loop around the value 1.

[0008] However, in order for the treatment efficiency of the different polluting species to be optimal, it is also known to adjust the amounts of fuel injected into the engine so as to adjust the amount of oxygen stored in the catalyst, also known as OS (acronym for: Oxygen Storage), on a set value. This set value is a predetermined value that is strictly comprised between zero and the maximum oxygen storage capacity of the catalyst, also known as OSC (acronym for: Oxygen Storage Capacity).

[0009] This predetermined value is both far enough from zero to allow sufficient oxidation of HC and CO, and far enough from the OSC value to allow sufficient reduction of NOx. It can be chosen experimentally and depends on a plurality of parameters comprising at least the flow rate of the combustion gases passing through the catalyst and the temperature of the catalyst. Reference will be made in particular to publication FR-A1-310110 which discloses such a method for adjusting the amount 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.

[0010] Generally speaking, the closer to zero the amount of OS oxygen actually present in the catalyst, the more the oxidation efficiency of HC and CO decreases. Conversely, the closer the amount of OS oxygen comes to the OSC, the lower the NOx reduction efficiency.

[0011] During certain driving situations, for example during a gear shift of a gearbox or a phase called lifting of the foot (foot lift) during which the driver completely lifts the foot off the accelerator pedal, the fuel injection is automatically cut off to decrease the fuel consumption and air is sent into the after-treatment system. The amount of oxygen stored in the catalyst then increases, for example up to the OSC, and polluting species, more particularly NOx, are no longer effectively treated as soon as fuel is reinjected again.

[0012] When resuming fuel injection, while the amount of oxygen stored in the catalyst has reached the OSC meaning that the catalyst is saturated with oxygen, a strategy of purging or lowering the oxygen load of the catalyst is usually carried out. The catalyst purge strategy includes 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 such 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 purge strategy significantly increases the vehicle’s fuel consumption and CO2 emissions.

[0013] There are several solutions to limit the flow of air brought to the exhaust in the cut-off phases of the fuel injection, so as to limit the oxygen saturation of the catalyst.

[0014] For example, it is possible to close the throttle housing at the intake to mechanically restrict the flow of air drawn in at the intake and therefore the flow of air brought to the exhaust. In addition to a generally limited gain, this results in a drop in the pressure of the intake manifold which can lead to an increase in oil consumption. For example, in the case of a zero torque demand, which is the case of a foot lift, it is possible to disengage the engine from the transmission and make it run at idle speed, or even cut it off. However, this type of solution is incompatible with mechanical gearboxes, as it requires an automatic transmission system or at least a controlled clutch.

[0015] Document FR-A1-3120253 describes a method for controlling a sparkignition engine with the objective of not saturating an exhaust-mounted catalyst 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 to the exhaust gas intake via an EGR (Exhaust Gas Recycling) 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, that is to say without producing torque. However, such fuel injection increases the consumption and CO2 emissions of the vehicle. In addition, the late injection of fuel leads to an exothermic reaction in the catalyst, which can damage it.

[0016] Explanation of the invention

[0017] The aim of the invention is to eliminate NOx emission peaks during an acceleration phase following a foot lift phase with an optimised engine control method during the accelerator pedal lift phase making it possible to avoid oxygen saturation of the depollution system while eliminating the fuel consumption and catalyst deterioration problems described above.

[0018] The subject of the invention is a method for controlling a motor vehicle sparkignition internal combustion engine equipped with a variable valve timing system and a pollution control system comprising at least one three-way catalyst.

[0019] The process comprises the successive steps of: cutting off the fuel injection when a foot lift phase is detected and maintaining this cut-off until a subsequent step of resuming the injection, crossing of intake and exhaust valves, uncrossing of the intake and exhaust valves carried out when either a torque demand that is not zero, or a predetermined reinjection speed, or a disengagement of the engine is detected, and resuming injection.

[0020] Advantageously, the step of resuming the injection is carried out when a flow of fresh air admitted into the engine exceeds a predetermined value so as to avoid misfires.

[0021] Preferably, the uncrossing step is assigned a zero value to a torque set point that is maintained until injection has resumed.

[0022] According to an advantageous characteristic, from the resumption of injection, the value of the torque set point is converged to the value of a torque request from the driver.

[0023] For example, convergence is achieved using a filter or time ramp.

[0024] 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.

[0025] According to another aspect, the invention relates to a control system for a motor vehicle spark- ignition internal combustion engine provided with a valve timing system and a system for depolluting pollutant species comprising at least one three- way catalyst, comprising an electronic control unit and configured to implement a control method as described above.

[0026] According to another aspect, the invention relates to a spark-ignition internal combustion engine comprising a valve timing system and a system for depolluting pollutant species comprising at least one three-way catalyst, the engine being provided with a control system configured to implement a control method as described above.

[0027] According to another aspect, the invention relates to a motor vehicle provided with an engine as described above.

[0028] Brief description of the drawings

[0029] Other aims, characteristics and advantages of the invention will become apparent on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which: [Fig 1] schematically illustrates the structure of a motor vehicle internal combustion engine provided with an engine control system according to the invention;

[0030] [FIG. 2] illustrates a flowchart of a method of controlling the motor of [FIG. 1] according to the invention; and

[0031] [Fig 3] illustrates the evolution over time of different parameters of a motor controlled by a control method according to the invention.

[0032] Detailed description of at least one embodiment

[0033] In the example shown on Figure 1, the internal combustion engine 10 is of the spark-ignition type and comprises, by way of example, three cylinders 12 in line, a fresh-air intake manifold 14, an exhaust manifold 16 and a turbo-compression system or turbocharger 18.

[0034] The cylinders 12 are supplied with air via the intake manifold 14, or distributor, itself supplied by a pipe 20 fitted with an air filter 22 and a compressor 18a of the turbocharger 18 of the engine 10.

[0035] 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, which compresses the air distributed by the air filter 22 or airbox, in order to increase the quantity (mass flow) of air admitted to the cylinders 12 of the engine 10 for an identical volume flow.

[0036] The internal combustion engine 10 thus comprises an intake circuit Ca and an exhaust circuit Ce.

[0037] The intake circuit Ca comprises, from upstream to downstream in the direction of air circulation:

[0038] - the air filter 22;

[0039] - a flow meter 26 located in the intake duct 20 downstream of the air filter 22 to measure the actual value of the mass flow of air entering the engine 10;

[0040] - an air intake valve 9;

[0041] - the compressor 18a of the turbocharger 18;

[0042] - a throttle housing 30 or a gas intake valve in the engine; - a heat exchanger 32 configured to cool the intake gases corresponding to a mixture of fresh air and recirculated gases after they have been compressed in the compressor 18a;

[0043] -pressure and temperature sensors 33 for measuring the pressure and temperature in the intake manifold 14; and

[0044] - the intake manifold 14.

[0045] The compressor is associated with a bypass circuit equipped with an inlet relief valve 39 which opens in the event of sudden closure of the throttle housing 30, to prevent the compressed air, located between the compressor 18a and the throttle housing 30, from passing through the compressor 18a and degrading it, when, for example, the driver of the vehicle abruptly lifts the foot of the accelerator pedal.

[0046] The exhaust circuit Ce comprises, from upstream to downstream in the direction of flow of the burnt gases:

[0047] - exhaust manifold 16;

[0048] - turbine 18b of turbocharger 18; and

[0049] - a system 40 for depolluting the combustion gases of the engine, comprising in particular a three-way catalyst.

[0050] The exhaust manifold 16 collects the exhaust gases produced by combustion and discharges them to the outside via an exhaust gas duct 34 opening out at the inlet to the turbine 18b of the turbocharger 18 and via an exhaust line 36 mounted downstream of the turbine 18b.

[0051] Engine 10 also includes a partial exhaust gas recirculation circuit (38), called EGR circuit.

[0052] The motor 10 may not be provided with an EGR circuit, without thereby departing from the scope of the invention.

[0053] This circuit 38 is in a non-limiting way a low-pressure exhaust gas recirculation circuit. It is connected to the exhaust line 36, at a point downstream of said turbine 18b, and in particular downstream of the gas pollution control system, and returns the exhaust gases to the fresh air supply line 20, upstream of the compressor 18a of the turbocharger 18, in particular downstream of the flow meter 26. The flow meter 26 measures only the flow of fresh air alone. As illustrated, this recirculation circuit 38 comprises, in the recycled gas flow direction, a cooler 38a, a filter 38b, and a valve 38c configured to regulate the flow of low-pressure exhaust gases. Valve 38c is located downstream of cooler 38a and filter 38b, and upstream of compressor 18a.

[0054] The engine exhaust gas pollution control system 40 includes a first after- treatment device 42 comprising at least one three-way catalytic converter 42a which can be electrically heated, with at least one first oxygen probe 43a mounted upstream of the first after-treatment device 42.

[0055] The first upstream oxygen probe 43a generally serves 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.

[0056] In addition, an optional second oxygen probe 43b, for example of the binary or proportional type, may be mounted downstream of the first post-processing device 42 so as to be able to correct the set point value of the richness regulation loop, in particular for the purpose of adjusting the amount of oxygen stored inside the catalyst 42a.

[0057] The gas depollution system 40 further comprises a second after-treatment device 44 which here is a fine particle filter. The system 40 may also include a third oxygen sensor 43c, for example of the binary type, connected downstream of the second device 44, for example for diagnostic purposes.

[0058] The engine 10 also comprises a variable timing system 50 for the intake valves 51 of the engine and optionally also a variable timing system 52 for the exhaust valves 53 of the engine. The variable timing system 50 of the intake valves is equipped with a sensor 54 which makes it possible to know at each instant its angular position, which corresponds to determined instants of opening and closing of the intake valves 51 in the combustion cycle of the engine (these instants being generally measured in crankshaft degrees, denoted °Vil, with respect to a top dead centre position). The variable timing system 52 of the exhaust valves, if it is 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. The engine is associated with a fuel circuit comprising, for example, fuel injectors (not referenced) injecting petrol directly into each cylinder from a fuel tank (not shown).

[0059] In addition, 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 on the basis of data collected by sensors at different locations on the engine.

[0060] The electronic control unit 61 comprises a calculation module 62, a measurement module 63 and a control module 64.

[0061] Figure 2 illustrates a flowchart of a control method according to the invention.

[0062] The method begins with a prior step 65 of detecting a foot lift phase, corresponding to a period during which the driver does not press the accelerator pedal. During a foot lift phase, the engine torque demand is zero.

[0063] The method continues with a step 66 of cutting off the fuel injection. The cutoff of the injection is maintained until a subsequent step 69 of resuming the injection.

[0064] During the next crossing step 67, the electronic control unit 61 controls the variable timing system 50, 52 to perform a crossing phase of the intake valve 51 and exhaust valve 53, i.e. a phase during the intake time when the intake valve 51 and exhaust valve 52 are open at the same time. By thus combining the set points of the camshaft phase shifters positions, 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 in the foot lift phase is limited and the oxygen saturation and the ability to treat NOx is better controlled during the next reinjection.

[0065] It should be noted that if the engine 10 is only equipped with a variable intake valve timing system 50, the crossing phase is achieved by the sole offset of the intake valves 51, without variation of the exhaust valve timing 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 respectively early or late closing of the intake valves with respect to bottom dead centre. In both cases, the pumping losses of the engine are limited and the fuel consumption of the engine is improved. Preferably, the crossing of the intake 51 and exhaust 53 valves per engine cycle has a duration ranging from 40 °Vil to 120 °Vil.

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

[0067] The detection of the reinjection regime or “re-engagement” regime consists in detecting a predetermined regime from which it is desired to carry out the reinjection. 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 the uncrossing of the valves by monitoring the approach of the reinjection regime. Thus, by considering the instantaneous value and the rate of decrease of the current speed of the engine, it is possible to control the uncrossing of the valves, so that the valves reach a position allowing to have enough air admitted into the combustion chamber to avoid misfiring at the time when the engine reaches the predetermined reinjection speed.

[0068] Engine disengagement is detected when the engine is uncoupled from the transmission to engage another gear of the gearbox or to mark the stopping of the vehicle. In case of engine disengagement, the uncrossing of the valves is controlled, so that the valves reach a position allowing enough air to be admitted into the combustion chamber to avoid misfires when resuming injection.

[0069] Detecting a torque demand that is not zero consists of detecting the driver’ s torque demand emitted by pressing an accelerator pedal or by resetting a cruise control. As soon as a non-zero torque demand is thus detected, the uncrossing of the valves is controlled, so that the valves reach a position allowing enough air to be admitted into the combustion chamber to avoid misfires when resuming the injection. In order to ensure that the engine is able to produce torque without misfiring, the torque request is delayed. Thus, a torque set point is assigned a zero value that is maintained as long as the injection has not 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 misfires. Such a step ensures that the injected fuel bums properly in the combustion chamber. From the resumption of injection, the value of the torque set point is converged to the value of a driver’s torque request. 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 makes it possible to prevent the torque set point from resulting in a jolt felt by the driver at the moment of torque take-up.

[0071] Figure 3 illustrates the evolution over time of different parameters of a motor 10 controlled by a control method according to the invention.

[0072] Curve 70 illustrates the driver’s torque demand. The area referenced 71 corresponds to a foot lift phase where the driver’s torque demand is zero.

[0073] When the foot lift phase 71 is initiated, the injection 72 is cut off, followed by the crossing of the valves 73, which results in a significant decrease in the intake air flow 74 into the engine, in particular below a minimum intake air flow value 75 which prevents misfires.

[0074] When the foot lift phase 71 reaches its end, the uncrossing of the valves is commanded (end of the crossing of the valves 73) and at the same time a zero value is assigned to a torque set point 76, which is maintained until injection has resumed.

[0075] The injection resumes (end of the cut-off of the injection 72) at the moment when the intake air flow 74 exceeds the minimum value 75, so as to avoid misfires. From this same moment, the value of the set point 76 is converged to the value of the torque demand of the driver 70.

Claims

CLAIMS1 . 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 pollutant species comprising at least one three-way catalyst (42a), the method being characterised in that it comprises successive steps of: cutting off the fuel injection when a foot lift phase is detected and maintaining this cut-off until a subsequent step of resuming the injection, crossing of intake (51) and exhaust (53) valves, uncrossing of the intake (51) and exhaust (53) valves carried out when either a torque demand that is not zero, or a predetermined reinjection speed, or a disengagement of the engine is detected, and resuming injection.

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

3. The method according to claim 1 or 2, wherein the uncrossing step is assigned a zero value to a torque set point, which is maintained until the injection has resumed.

4. The method according to claim 3, wherein from the injection resumption, the value of the torque set point is converged to the value of a torque demand of the driver.

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

6. The method according to any one of claims 1 to 5, wherein the crossing 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) of a motor vehicle spark-ignition internal combustion engine (10) equipped with a valve timing system (50, 52) and a system (40) for depolluting pollutant species comprising at least one three-way catalyst (42a),comprising an electronic control unit (61) and configured to implement a control method according to any one of claims 1 to 6.

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

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

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