Method for controlling a gaseous fuel engine during cold start-up

Abstract

This method of control after starting a spark-ignition internal combustion engine (10) of a motor vehicle equipped with a pollution control system (40) comprising at least one three-way catalyst (42a), comprises: - a dual-fuel step carried out when the temperature of the catalyst is less than or equal to a predetermined minimum catalyst efficiency initiation temperature, in which an indirect and early injection of a larger quantity of gaseous fuel is carried out in association with a single direct and late injection of a smaller quantity of liquid fuel and a withdrawal of the engine ignition advance from an optimal advance maximizing engine efficiency, and - a single-fuel step carried out when the catalyst (42a) exceeds the initiation temperature, in which the energy required for the operation of the engine is supplied by an indirect injection of gaseous fuel.Figure for the abbreviation: [Fig 1].

Description

Title of the invention: Method for controlling an engine using gaseous fuel during the cold start phase Technical field

[0001] The technical field of the invention is the control of spark-ignition internal combustion engines operating selectively in single-fuel mode or in dual-fuel mode of gaseous fuel and liquid fuel, and more particularly, the control of such engines in the cold start phase and with a view to reducing the emission of polluting species. Previous techniques

[0002] Motor vehicles equipped with a spark-ignition internal combustion engine are equipped with a system for after-treatment of polluting species included in the exhaust gases in order to meet anti-pollution standards. The after-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] It is known that the efficiency of a post-treatment system depends on its temperature. By efficiency, we mean the proportion of pollutant molecules of a given type that enter the system, that the system manages to treat. The efficiency begins to reach acceptable values, for example between 50% and 90%, when the temperature of the catalyst is for example between approximately 250°C and 300°C. The efficiency of a catalyst is zero until the temperature has reached a value of the order of 150°C.

[0004] Immediately after starting a thermal engine, it is therefore necessary to take measures to ensure that the vehicle does not release too many polluting emissions into the outside atmosphere when it is a cold start, that is to say, a start carried out when the catalyst has not reached a sufficient temperature threshold.

[0005] Conventionally, while the catalyst is cold, delayed combustions are carried out inside the engine cylinders to heat the catalyst, so as to degrade the combustion efficiency and increase the thermal losses of the engine. For example, it is common to delay the ignition advance.

[0006] However, the ignition under-advance remains limited by combustion instabilities. Indeed, increasingly erratic combustions generate significant torque differences from one cycle to another and result in significant speed instabilities which generate unacceptable increases in noise and vibrations, and increases in pollutant emissions, particularly unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) when said instabilities lead to poor combustion.

[0007] As is known from the prior art, there are internal combustion engines operating selectively in monofuel mode or in dual fuel mode of gaseous fuel and liquid fuel.

[0008] In most cases, such an engine is equipped with a first indirect gaseous fuel injection system associated with a second direct liquid fuel injection system. The gaseous fuel is generally liquefied petroleum gas, acronym LPG, or natural gas for vehicles, acronym CNG. The liquid fuel is generally gasoline.

[0009] A known solution for reducing combustion instabilities in the case of spark-ignition and direct injection engines consists of carrying out stratified charge injection. Such injection consists of obtaining in the combustion chamber a heterogeneous fuel mixture whose rich fractions are located in the vicinity of the ignition point, typically a spark plug, in order to promote the ignition of the mixture.

[0010] Thus, the actual starting of the engine and then its cold operating phase are generally carried out in monofuel mode, with direct and stratified injection of liquid fuel, typically gasoline.

[0011] However, it is desirable to increase the use of gaseous fuels. Indeed, their characteristics allow them to degrade air quality less and reduce greenhouse gas emissions compared to a liquid fuel such as gasoline. On the other hand, these gaseous fuels are generally less expensive than liquid fuels and therefore make it possible to reduce the cost of using motor vehicles.

[0012] It is thus desirable to use gaseous fuels in the cold operating phase of the engine following its start-up, that is to say during the temperature rise phase, also called "activation", of the catalyst.

[0013] However, it is not possible to carry out stratified charge injections with indirect gaseous fuel injection systems. Indeed, in indirect gaseous fuel injection systems, the injection is carried out in the intake manifold, therefore too far from the ignition point to be able to reach sufficiently rich fractions which promote ignition.

[0014] The current use of gaseous fuels is thus accompanied by a more limited withdrawal of the ignition advance due to combustion instabilities and the impossibility of achieving stratified charges. This results in a slower temperature rise rate of the catalyst than if liquid fuels were used and therefore lower catalyst efficiency in the transient phase and less controlled emissions of polluting species. Statement of the invention

[0015] In view of the above, the invention aims to use gaseous fuel in the cold operating phase of a motor vehicle engine after its start-up, while controlling the emission of polluting species.

[0016] The subject of the invention is a method for controlling a spark-ignition internal combustion engine of a motor vehicle equipped with a system for decontaminating polluting species comprising at least one three-way catalyst.

[0017] The method comprises, after each start of the engine: - a step of operating the engine in dual-fuel mode carried out when the catalyst temperature is less than or equal to a predetermined minimum catalyst efficiency initiation temperature (Ta), in which for each engine cycle, an indirect and early injection of a larger quantity of gaseous fuel is carried out, combined with a single direct and late injection of a smaller quantity of liquid fuel and a reduction in the engine ignition advance compared to an optimal advance maximizing engine efficiency, and - a step of operating the engine in single-fuel mode carried out when the temperature of the catalyst exceeds the priming temperature or when the duration of operation of the engine in dual-fuel mode exceeds a predetermined maximum duration, in which the energy necessary for the operation of each engine cycle is supplied by indirect injection of gaseous fuel.

[0018] Such a method makes it possible to use a dual-fuel mode operation which maximizes the use of gaseous fuel in the cold operating phase of the engine after its start, while controlling the emission of polluting species. Indeed, a reduced quantity of liquid fuel is used to produce a stratified charge making it possible to achieve a strong ignition advance reduction which maximizes the heat available for heating the catalyst. Thus, the method improves the efficiency of the catalyst in the transient phase, because it accelerates its temperature rise.

[0019] Preferably, for each cycle of the engine operating in dual-fuel mode, the quantity of liquid fuel injected is regulated in an open loop relative to a predetermined setpoint depending on the instantaneous operating point of the engine, while the quantity of gaseous fuel injected is regulated via a richness control loop.

[0020] For example, for each cycle of the engine operating in dual-fuel mode, the The value of the ratio between the quantity of injected liquid fuel and the total quantity of injected fuel depends on the value of the ignition advance withdrawal, so as to preserve the stability of combustion.

[0021] Advantageously, for each cycle of the engine operating in dual-fuel mode, the indirect injection of gaseous fuel is carried out during an expansion or exhaust time of the cycle, during a closing of the intake valves of the engine, which promotes the homogeneity of the mixture of gaseous fuel and fresh air.

[0022] Alternatively, for each cycle of the engine operating in dual-fuel mode, the indirect injection of gaseous fuel is carried out during an intake stroke of the cycle, during an opening of the intake valves of the engine. However, this does not allow for heterogeneity of the fuel charge and to obtain a stratified mixture.

[0023] According to a characteristic of the invention, for each cycle of the engine operating in dual-fuel mode, the indirect injection of gaseous fuel is associated with a direct injection of liquid fuel which is carried out during a compression stroke of the cycle, after the closing of the intake valves of the engine, and preferably just before ignition, so as to allow the stratified charge to be produced.

[0024] According to another characteristic, for each cycle of the engine operating in single-fuel mode, a unit richness setpoint is used to regulate the quantity of gaseous fuel injected and an optimal value of the ignition advance maximizing the efficiency of the engine.

[0025] According to another aspect, the invention relates to a motor vehicle spark-ignition internal combustion engine equipped with a direct liquid fuel injection system, an indirect gaseous fuel injection system, a control system and a system for decontaminating polluting species comprising at least one three-way catalyst, implementing a method as described above.

[0026] According to another aspect, the invention relates to a motor vehicle equipped with an engine as described above. Brief description of the drawings

[0027] 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:

[0028] [Fig.l] illustrates, schematically, the structure of an internal combustion engine of a motor vehicle equipped with an engine control system according to the invention; and

[0029] [Fig.2] illustrates a flowchart of a method for controlling the motor of [Fig.l] according to the invention. Detailed description of at least one embodiment

[0030] 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.

[0031] 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.

[0032] 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, with the aim of increasing the quantity (mass flow rate) of air admitted into the cylinders 12 of the engine 10 for an identical volume flow rate. Alternatively, the engine 10 may be of the atmospheric type.

[0033] The internal combustion engine 10 comprises an intake circuit Ca and an exhaust circuit Ce.

[0034] The intake circuit Ca comprises, from upstream to downstream in the direction of air circulation:

[0035] - the air filter 22;

[0036] - 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;

[0037] - an air intake valve 9;

[0038] - the compressor 18a of the turbocharger 18;

[0039] - a throttle body 30 or a gas intake valve in the engine;

[0040] - 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;

[0041] -pressure and temperature sensors 33 for measuring the pressure and temperature in the intake manifold 14; and

[0042] - the intake manifold 14.

[0043] 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.

[0044] The exhaust circuit Ce comprises, from upstream to downstream in the direction of circulation burnt gases:

[0045] - the exhaust manifold 16;

[0046] - the turbine 18b of the turbocharger 18; and

[0047] - a system 40 for depolluting the combustion gases of the engine, comprising including a three-way catalyst.

[0048] 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.

[0049] The engine 10 may be equipped with a partial recirculation circuit for the exhaust gases at the intake, called an “EGR” circuit or “exhaust gas recirculation” in English, without departing from the scope of the invention.

[0050] 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.

[0051] 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.

[0052] 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 setpoint value of the richness regulation loop, in particular with the aim of adjusting the quantity of oxygen stored inside the catalyst 42a.

[0053] The gas depollution system 40 further comprises, in the direction of air circulation, a second post-treatment device 44 and a third post-treatment device 45. In the example of [Fig. 1], the second device 44 is a fine particle filter and the third device 45 is a three-way underbody catalyst.

[0054] The system 40 may further comprise a nitrogen oxide concentration sensor 43c.

[0055] 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 times of opening and closing of the intake valves 51 in the combustion cycle of the engine. The variable timing system 52 of the exhaust valves, if present, is also equipped with a sensor 55 which makes it possible to know at any time its angular position, which also corresponds to determined times of opening and closing of the exhaust valves 53 in the combustion cycle of the engine.

[0056] 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.

[0057] The electronic control unit 61 comprises a calculation module 62, a measurement module 63 and a control module 64.

[0058] The engine is associated with a circuit (not shown) for direct injection of liquid fuel comprising a fuel tank such as gasoline, a low-pressure pump which draws fuel from the tank, a high-pressure pump which makes it possible to inject the liquid fuel at high pressure into a common rail equipped with three injectors which each open into a cylinder of the engine. The liquid fuel is typically gasoline. Alternatively, the liquid fuel may be a mixture of gasoline and ethanol, or even pure ethanol.

[0059] The engine is also associated with a circuit (not shown) for indirect injection of gaseous fuel comprising a pressurized liquefied gas tank, a pressure reducer, a common injection rail provided with injectors of the expanded gas in the intake manifold. The gaseous fuel is typically LPG or CNG.

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

[0061] The method is implemented by the electronic control unit 61 at each starting the engine 10. For example, the actual starting of the engine can be carried out in single-fuel mode, by direct injection of liquid fuel according to prior art methods which will not be detailed here.

[0062] When the start-up is carried out, the method begins with a step 65 of measuring and verifying the temperature of the catalyst 42a.

[0063] If the measured catalyst temperature is less than or equal to a predetermined minimum catalyst efficiency ignition temperature Ta, the method continues with step 66 of operating the engine in dual-fuel mode.

[0064] If the measured temperature of the catalyst is higher than the ignition temperature Ta, the method continues with step 67 of operating the engine in single-fuel mode.

[0065] During step 66 of operation of the engine in dual-fuel mode, for each cycle of the engine 10, an indirect and early injection of a larger quantity of gaseous fuel is carried out, associated with a single direct and late injection of a smaller quantity of liquid fuel and with a reduction in the ignition advance of the engine 10 relative to an optimal advance maximizing the efficiency of engine 10.

[0066] Preferably, the quantity of injected liquid fuel is regulated in an open loop relative to a predetermined setpoint value depending on the instantaneous operating point of the engine, while the quantity of injected gaseous fuel is regulated via a richness control loop. For example, the setpoint values ​​of the quantity of injected liquid fuel are stored in a memory of the electronic control unit 61 in the form of maps.

[0067] The value of the ratio between the quantity of liquid fuel injected and the total quantity of fuel injected depends on the value of the withdrawal of the ignition advance so as to preserve the stability of the combustion. For example, the more the withdrawal is increased compared to an optimal advance, the more the fraction of liquid fuel injected late must be increased in order to increase the local richness of the mixture near the spark plugs and thus promote combustion stability.

[0068] Preferably, the indirect injection of gaseous fuel is carried out during an expansion or exhaust time of the cycle of the engine 10, during a closing of the intake valves of the engine. Alternatively, it remains possible to carry out the indirect injection of gaseous fuel during an intake time of the cycle of the engine 10, during an opening of the intake valves of the engine.

[0069] During the following step 68, the temperature of the catalyst 42a is compared with the ignition temperature Ta, and it is verified that the operating time of the engine in dual-fuel mode does not exceed a predetermined maximum time.

[0070] When the catalyst temperature exceeds the ignition temperature Ta or when the duration of operation of the engine in dual-fuel mode exceeds a predetermined maximum duration, the method continues with step 67 of operation of the engine in single-fuel mode.

[0071] During step 67 of engine operation in single-fuel mode, the energy necessary for the operation of each cycle of the engine 10 is supplied by indirect injection of gaseous fuel. Preferably, a unit richness setpoint is used to regulate the quantity of gaseous fuel injected and an optimal value of the ignition advance maximizing the efficiency of the engine.

[0072] In the method described above, step 66 of operating the engine in dual-fuel mode is carried out if the measured catalyst temperature is less than or equal to a predetermined minimum catalyst efficiency ignition temperature Ta (step 65). Alternatively, it is possible to add to step 65 an additional condition relating to the engine temperature. For example, step 66 of operating the engine in dual-fuel mode is carried out only if the engine temperature (or the engine coolant temperature) is less than a predetermined threshold value.

Claims

Claims

1. Method for controlling a spark-ignition internal combustion engine (10) of a motor vehicle provided with a pollution control system (40) for polluting species comprising at least one three-way catalyst (42a), the method being characterized in that it comprises, after each start of the engine: - a step of operating the engine in dual-fuel mode carried out when the temperature of the catalyst is less than or equal to a predetermined minimum catalyst efficiency ignition temperature (Ta), in which for each engine cycle, an indirect and early injection of a larger quantity of gaseous fuel is carried out associated with a single direct and late injection of a smaller quantity of liquid fuel and with a reduction of the ignition advance of the engine compared to an optimal advance maximizing the efficiency of the engine,and - a step of operating the engine in single-fuel mode carried out when the temperature of the catalyst (42a) exceeds the initiation temperature (Ta) or when the duration of operation of the engine in dual-fuel mode exceeds a predetermined maximum duration, in which the energy necessary for the operation of each cycle of the engine is supplied by an indirect injection of gaseous fuel.,

2. Control method according to claim 1 wherein for each cycle of the engine operating in dual-fuel mode, the quantity of liquid fuel injected is regulated in an open loop relative to a predetermined setpoint depending on the instantaneous operating point of the engine, while the quantity of gaseous fuel injected is regulated via a richness control loop.

3. A control method according to claim 1 or 2, wherein for each cycle of the engine operating in dual-fuel mode, the value of the ratio between the quantity of liquid fuel injected and the total quantity of fuel injected depends on the value of the withdrawal of the ignition advance, so as to preserve the stability of the combustion.

4. Control method according to any one of claims 1 to 3, wherein for each cycle of the engine operating in dual-fuel mode, the indirect injection of gaseous fuel is carried out during an expansion or exhaust stroke of the cycle, during a closing of the intake valves of the engine, and the direct injection of liquid fuel is carried out during a compression stroke of the cycle, after the closing of the intake valves of the engine, preferably just before ignition.

5. Control method according to any one of claims 1 to 3, wherein for each cycle of the engine operating in dual-fuel mode, the indirect injection of gaseous fuel is carried out during an intake stroke of the cycle, during an opening of the intake valves of the engine, and the direct injection of liquid fuel is carried out during a compression stroke of the cycle, after the closing of the intake valves of the engine, preferably just before ignition.

6. Control method according to any one of claims 1 to 5, in which for each cycle of the engine operating in single-fuel mode, a unit richness setpoint is used to regulate the quantity of gaseous fuel injected and an optimal value of the ignition advance maximizing the efficiency of the engine.

7. Internal combustion engine (10) with spark ignition for a motor vehicle provided with a direct liquid fuel injection system, an indirect gaseous fuel injection system, a control system (60) and a system (40) for depollution of polluting species comprising at least one three-way catalyst (42a), implementing a method according to any one of claims 1 to 6.

8. Motor vehicle equipped with an engine (10) according to claim 7.

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