Method and device for controlling a spark-ignition engine for increasing the efficiency of a particulate filter

Abstract

A method is disclosed for controlling a spark-ignition engine (1) equipped with an exhaust gas treatment system comprising a particulate filter (7) and a three-way catalyst (6), by controlling the injection of fuel into a combustion chamber (8) of the engine (1), and by controlling the ignition of an air-fuel mixture present in the combustion chamber (8) of the engine (1). The fuel injection comprises one or more fuel injection sequences comprising an injection centred at the point of top dead point and triggered so as to create a delay in ignition in a mixture rich enough to generate a deposit of fine particles in the exhaust gas treatment system.

Description

[0001] DESCRIPTION

[0002] Method and device for controlling a spark-ignition engine for increasing the efficiency of a particulate filter

[0003] Technical field

[0004] The present invention relates to the control of the operating mode of internal combustion engines of the spark-ignition type (operating in particular on petrol).

[0005] More particularly, the present invention relates to the emission control of internal combustion engines related to the filtration efficiency of particulate filters mounted at the exhaust of such engines.

[0006] Particulate filters, acronym "DPF", of internal combustion engines, collect and filter the combustion gases of the engines, thus making it possible to significantly reduce the mass and the number of particles emitted at the vehicle exhaust.

[0007] It is possible to differentiate between the so-called "heavy" particles of relatively large size, for example of the order of 100 nanometres in diameter, and the microparticles which are of smaller size, for example less than 2.5 microns, and in particular less than 1 micron. The petrol engine does not emit many “heavy” particles but emits a lot of micro-particles, which are very harmful to the environment and health.

[0008] A DPF of a petrol engine operates in a two-sequence cycle. The first sequence consists of the collection of particles, which is done during the usual operating mode of the rich engine 1 , and the second sequence consists of their removal. Indeed, the first step allows the DPF to capture the particles present in the combustion gases, which leads to the formation of a layer of heavy particles, and in particular soot, on the walls of the filter. When there is too much soot deposited, the second step, called regeneration, makes it possible to clean the DPF by burning this bed of particles.

[0009] For a spark-ignition engine, there are several types of DPF regeneration. A distinction is made between passive regeneration, which is not triggered by acting on the mode of adjustment of the engine and wherein combustion occurs naturally under the action of a characteristic temperature and flow rate, and active regeneration, which is caused when an operating mode of the engine is modified by switching the operating mode to a slightly lean mixture (richness generally between 0.95 and 0.98), the effect of which is to cause combustion of the particles in the filter. In a manner known per se, this second type of regeneration of the DPF of a petrol engine can be implemented, for example, just before the mass of the fine particles reaches a critical threshold that could clog the filter, more particularly in the case where the temperature and flow rate conditions have not been met before to passively regenerate the filter.

[0010] Prior arts

[0011] On leaving the factory, the DPFs of petrol engines have a low level of overall efficiency, i.e. a low level of retention of particles emitted in the engine's combustion gases. More specifically, the filter does not retain the finest particles, which are emitted in greater numbers, because they pass easily through the walls of the filter. The level of efficiency improves over time, as a result of the formation of a soot bed, caused by the accumulation of particles emitted by the engine. The accumulation of heavy particles on the walls of the filter tends to clog it and make it relatively less permeable to the lightest particles. Subsequently, the DPF regeneration of a petrol engine does not make it possible to completely remove the soot deposits, the efficiency of its filtration with regard to the finest particles is preserved to a certain extent.

[0012] As illustrated in [Fig. 1], which graphically shows the evolution of particulate emissions at the exhaust, in number of particles (PN), as a function of the evolution of the mileage (km), and on which curves c1 and c2 respectively represent driving with a heavy foot, i.e. with little stress on the accelerator pedal of the vehicle, and driving with a light foot, the level of particles at the exhaust stabilises at about 6,000 km. As a result, the DPF operating mode of a petrol engine also becomes efficient around 6,000 km.

[0013] Document FR-A1 -3096403 describes a system for post-treatment of exhaust gases from an exhaust line of a spark-ignition internal combustion engine comprising a three-way oxidation-reduction catalytic converter, a first particulate filter mounted directly downstream of the catalytic converter; and a second particulate filter located downstream of the first particulate filter.

[0014] The first filter located downstream of the catalytic converter generally remains empty enough to be able to store the fairly large particles, while the second filter located downstream of the first DPF remains full enough to store the relatively fine particles. Each of the filters consists of a regeneration system composed of differential pressure sensors and sensors capable of determining the mass of the particles. This regeneration system operates in such a mode that the first filter downstream of the catalytic converter empties while the second filter downstream of the first DPF remains protected from regenerations of the first filter downstream.

[0015] However, the efficiency of treating fine particles remains insufficient because when the engine and particulate filters are new, the first DPF is not yet sufficiently full. This document does not mention how to obtain a satisfactory initial filling of the first filter.

[0016] Explanation of the invention

[0017] Therefore, the present invention has the purpose of accelerating the deposition of fine particles in a particulate filter of a petrol engine placed downstream of a three-way catalyst in order to increase the filtration efficiency of the particulate filter.

[0018] The invention relates to a method for controlling a spark-ignition engine equipped with an exhaust gas treatment system comprising a particulate filter and a three-way catalyst, by controlling the injection of fuel into a combustion chamber of the engine and by controlling the ignition of an air-fuel mixture present in the combustion chamber of the engine.

[0019] The fuel injection comprises one or more fuel injection sequences, comprising an injection centred at the top dead centre of the engine and triggered so as to create an ignition delay in a mixture rich enough to generate a deposit of fine particles in the exhaust gas treatment system.

[0020] According to one embodiment of the invention, the injection centred at the top dead centre of the engine is the last injection during an injection sequence.

[0021] Advantageously, the fuel injection(s) are caused at engine cycle angles between 180° and -20° of the crankshaft, i.e. between bottom dead centre and 20° after top dead centre over an engine compression cycle.

[0022] Preferably, this interval is reduced to angles comprised between 80° and - Ignition can be triggered after the end of the last injection centred at top dead centre, with a gap, separating the end of the last injection and ignition, between 3° and 8° of crankshaft, so that ignition occurs between -4° and -28° of crankshaft.

[0023] According to one embodiment, this method is implemented during the catalyst heating phase. This method is thus implemented at the start of the vehicle, when the engine is cold, making it possible to accelerate the deposition of fine particles in the DPF of a petrol engine.

[0024] The invention also relates to a system for controlling a spark-ignition engine equipped with an exhaust gas treatment system comprising a particulate filter and a three-way catalyst.

[0025] This system comprises injection control means for causing one or more fuel injection sequences comprising an injection centred at top dead centre and triggered so as to create a delay in ignition in a mixture rich enough to generate a deposit of particles in the exhaust gas treatment system.

[0026] Advantageously, the control means are configured so that said injection centred on top dead centre is the last injection of the sequence.

[0027] According to one embodiment, the control means are configured so that the injections are caused at angles of the engine cycle of between 180° and -20°, preferably between 80° and -12° of crankshaft.

[0028] Advantageously, the system comprises ignition control means configured to cause ignition after the end of the last injection with a gap of between 3° and 8° of crankshaft, so that ignition occurs between -4° and -28° of crankshaft.

[0029] The invention also relates to a motor vehicle comprising a control system as defined above.

[0030] Brief description of the drawings

[0031] 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 wherein:

[0032] - [Fig. 1 ], which has already been mentioned, shows graphically the evolution of particulate emissions (PN) at the exhaust as a function of mileage (km);

[0033] - [Fig. 2] shows schematically the structure of an internal combustion engine of a motor vehicle engine equipped with a control system according to the invention; - [Fig. 3] shows graphically an embodiment of the invention according to which a sequence of three injections is implemented in the combustion chamber of the engine between an intake bottom dead centre and an exhaust bottom dead centre;

[0034] - [Fig. 4] shows graphically the number of particles (PN) calculated according to each phase of the method according to the invention;

[0035] - [Fig. 5] shows graphically the particle emissivity trend according to different combustion parameters; and

[0036] - [Fig. 6] shows schematically the walls of the DPF of a spark-ignition internal combustion engine, in new condition and after formation of the particle bed.

[0037] Detailed description

[0038] [Fig. 2] shows schematically the general structure of an internal combustion engine 1 of a motor vehicle, of the spark-ignition type (operating in particular on petrol), and equipped with a control system according to the invention.

[0039] Engine 1 here is an in-line four-cylinder engine 2. It comprises an intake manifold 3 ensuring the intake of fresh air into the cylinders 2, an exhaust manifold 4 ensuring the collection of exhaust gases, and an exhaust line 5 equipped with a system for after-treatment of the exhaust gases, comprising a three-way oxidation catalyst 6 and a DPF particulate filter 7 placed downstream of the catalyst 6, considering the direction of the exhaust gas flow.

[0040] The position of the pistons in the cylinders 2 is measured as a function of the crankshaft angle, in crankshaft °. According to the cycles of the engine 1 , the piston rises and falls in the combustion chamber 8.

[0041] Moreover, each cylinder 2 is equipped with a fuel injector 9 and a spark plug 10, making it possible to trigger the combustion of the air-fuel mixture, both located not far from the top dead point TDP comb of the combustion chamber 8.

[0042] The fuel injection and ignition phases of the air-fuel mixture are controlled by a control system 12 comprising injection control means 13 and ignition control means 14.

[0043] With reference to Figure 3, the injection control means 13 make it possible to cause one or more fuel injection sequences 15 such that an injection 16 is centred at the top dead point TDP comb of the combustion chamber 8, and such that, in the case of a sequence of several fuel injections 15, the last injection 16 of the sequence is centred at the top dead point TDP comb of the combustion chamber 8. This late injection is performed just before the AVA ignition.

[0044] The fact of creating one or more injections 15 of which one 16 is caused just before the AVA ignition, generates a rich and poorly homogenised air-fuel mixture, i.e. an air-fuel mixture composed mainly of fuel, at the time of the AVA ignition. This process induces a portion of the fuel to be unburned and, therefore, greater formation of hydrocarbons (HC), carbon monoxide (CO) and fine or micro-particle particles, occurs in the combustion chamber 8. These particles are then released through the exhaust line 5 of the engine 1 , and are filtered first by the three-way catalyst 6, which in particular allows the HC and CO of the engine to be oxidised and the NOx to be reduced, and then by the particulate filter 7 located downstream of the three-way catalyst 6, which treats the particles according to the sequential mode already described above.

[0045] In one embodiment, the late fuel injection, centred on the top dead centre, is carried out during the heating phase of the three-way catalyst 6, which takes place at the start of the vehicle when the engine 1 is still cold. This embodiment makes it possible to create a large number of particles at a time when the three-way catalyst 6 is not yet fully efficient. If the three-way catalyst 6 is not functional, the filtering performed by this component is not optimal and the particles are thus sent in greater numbers into the DPF of a petrol engine 7 directly. Thus, the process of depositing particles 21 on the DPF walls of a petrol engine 7 is accelerated and, consequently, the efficiency of its filtration as well.

[0046] A spark-ignition engine 1 operates in four stages. The first is the intake stage, where air is sucked in via the intake duct 3. The second stage is compression, where the air-fuel mixture is compressed until the compression piston reaches the top dead point TDP comb. The third stage is that of the explosion, where the spark ignites and causes the combustion of the air-fuel mixture. Finally, the fourth stage is that of the exhaust which allows the engine to discharge its burnt gases towards the exhaust pipe.

[0047] The injection control means 13 are configured to trigger one or more fuel injections 15 during the compression phase of the engine 1 , between 180° and -20° of crankshaft. Preferably, this interval can be reduced to an interval between 80° to - 12° of crankshaft. The injection or injections 15 caused by the injection control means 13 make it possible to create an ignition delay AVA. This AVA ignition is caused by the ignition control means 14, and is triggered with a delta deviation between 3° and 8° of crankshaft with the end of the last injection 16. Since the end of the last injection can be between -1 ° and -20° of crankshaft, the AVA ignition is caused between -4° and - 28° of crankshaft.

[0048] According to an embodiment of the invention shown in [Fig. 3], a sequence of three injections 15 is caused between an intake bottom dead point BDP a and an exhaust bottom dead point BDP e. The first injection 18 is created from 80° of crankshaft, and ends before the second injection 19 starting at 40° of crankshaft. The injection sequence then includes a third and last injection 16, centred at the top dead point TDP comb of the combustion chamber of the engine 1 , triggered at about 10° of crankshaft and stopped at about -10° of crankshaft. In this embodiment of the invention, the AVA ignition is caused at approximately -15° of crankshaft, with a delta deviation of 5° of from the end of the last injection 16.

[0049] Test have been carried out to test this embodiment. In [Fig. 4], which shows graphically the number of particles (PN) calculated as a function of each phase of the method according to the invention, it can be seen in particular that it is at the end of the last injection I3, which corresponds to the third injection in this embodiment, that there is the most particle formation (PN).

[0050] On a scale from 0 to 1 , the third and last injection I3, according to this embodiment, forms a number of particles (PN) equal to 1 , while the previous injections 11 and I2 vary between 0.095 and 0.334 PN.

[0051] The number of particles (PN) is 0.292 at the time of ignition advance A before decreasing at the time of the first injection phase 11 .

[0052] [Fig. 5] also shows the trend of particle emissivity, according to certain parameters, according to the different phases of the experiment, and it can be seen that there is a peak of particle creation at the end of the third injection I3.

[0053] The results illustrated in [Fig. 6], show the creation of a deposit of particles 21 , more precisely of relatively heavy particles, at the walls of the particulate filter of a petrol engine 7 on the right-hand side R of Figure 6, which was not present on the walls of the particulate filter of a petrol engine 7 on leaving the factory, i.e. in the new state (left-hand side L of Figure 6). It can thus be seen that this particle bed formation 21 allows the particle filter of a petrol engine 7 to be more efficient and to improve in particular the filtration of fine particles, more precisely the smallest particles, which are the lightest and most numerous, and which are found in the exhaust duct 5 of the engine 1 .

Claims

CLAIMS1 . Method of controlling a spark-ignition engine (1 ) equipped with an exhaust gas treatment system comprising a particulate filter (7) and a three-way catalyst (6), by controlling the injection of fuel into a combustion chamber (8) of the engine (1 ), and by controlling the ignition of an air-fuel mixture present in the combustion chamber (8) of the engine (1 ), characterised in that the injection of the fuel comprises one or more fuel injection sequences (15) comprising an injection centred (16) at the top dead point (TDP comb) and triggered so as to create an ignition delay (AVA) in a mixture rich enough to generate a deposit of fine particles (21 ) in the exhaust gas treatment system.

2. The method according to claim 1 , wherein the injection is performed in sequences (15), said injection centred (16) on top dead point (TDP comb) being the last injection of the sequence (15).

3. Method according to one of claims 1 and 2, wherein the injections (15) are caused at angles of the cycle of the engine (1 ) between 180° and -20°, preferably between 80° and -12° of crankshaft.

4. Method according to any one of claims 1 to 3, wherein the ignition (AVA) is triggered after the end of the last injection (16) with a delta (deviation) comprised between 3° and 8° of crankshaft, so that the ignition (AVA) occurs between -4° and - 28° of crankshaft.

5. Method according to any one of claims 1 to 4, carried out after starting the vehicle, during the catalyst (6) heating phase.

6. System for controlling a spark-ignition engine (1 ) equipped with an exhaust gas treatment system comprising a particulate filter (7) and a three-way catalyst (6), characterised in that the system comprises injection control means (13) for causing one or more fuel injection sequences (15) comprising a centred injection (16) at top dead point (TDP comb) and triggered so as to create an ignition delay (AVA) in a mixture rich enough to generate a deposit of particles (21 ) in the exhaust gas treatment system.

7. System according to claim 6, wherein the control means (13) is configured such that said top dead point (TDP comb) centred injection (16) is the last injection of the sequence (15).

8. System according to one of claims 6 and 7, wherein the control means (13) is configured so that the injections (15) are caused at angles of the cycle of the engine (1 ) between 180° and -20°, preferably between 80° and -12° of crankshaft.

9. System according to any one of claims 6 to 8, comprising ignition control means (14) configured to cause ignition (AV A) after the end of the last injection (16) with a delta (deviation) comprised between 3° and 8° of crankshaft, so that ignition (AV A) occurs between -4° and -28° of crankshaft.

10. Motor vehicle comprising a control system according to any one of claims 6 to 9.

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