FR2784626A1 - Hybrid power unit for road vehicle, comprises close-coupled IC engine and electric motor, so controlled as to maintain weak-mixture engine operation with minimal pollution - Google Patents

Abstract

The IC engine (12) has direct fuel injection designed for good fuel economy under moderate loads using a stratified weak mixture. Its crankshaft (18) is directly connected to a fly-wheel integrated with the rotor (16) of the electric motor (14), which interposed between the engine and the transmission (20). An electronic management unit (22) controls engine and motor operation, primarily to meet the net torque output demanded by the driver through accelerator pedal depression. Demand is met by the engine, the motor, sometimes in generator mode, intervening to sustain weak-mixture engine operation and to set exhaust gas temperatures at values consistent with pollution control requirements. Exhaust gas treatment includes a catalytic converter with a NOx trap retaining oxides of nitrogen. For correct trap operation gas temperatures should lie between e.g. 250 deg C and 450 deg C. Above a threshold engine torque value, in weak-mixture running, the higher figure will be exceeded. To prevent this, while maintaining fuel economy, the management unit will, if the battery is adequately charged, meet the excess torque demand using the motor. To raise low gas temperatures into the trap range the motor is run as a generator, artificially increasing the engine load, the excess energy being stored in the battery. High temperatures (600 deg C) for sulfur oxide purging are obtained in the same way.

Description

Hybrid powertrain
The invention relates to a hybrid powertrain in which the electric machine is used to regulate the temperature of the exhaust gases at an exhaust gas treatment device.

 The invention relates more particularly to a powertrain for a motor vehicle, of the type comprising an internal combustion engine capable of driving at least one drive wheel of the vehicle and an electric machine which is capable of being used in a generator mode or according to an engine mode in which it participates in driving the vehicle, of the type in which, for certain operating states of the internal combustion engine, the latter is supplied with a so-called lean air / fuel mixture in which the air is in excess with respect to the fuel, and of the type in which the internal combustion engine comprises an exhaust circuit provided with an exhaust gas treatment device.

 The invention will more particularly be described in the context of a particular hybrid powertrain. This powertrain consists of an internal combustion engine fitted with an electric machine integrated into the flywheel. In such a group, the electric machine can deliver only a very small part of the maximum power of the heat engine which therefore remains the main source of driving power for the vehicle. The machine is linked to the flywheel of the heat engine, that is to say that the flywheel is actually essentially constituted by the rotor of the electric machine. The latter is therefore linked directly to the engine shaft of the heat engine, and it is generally interposed between the heat engine and a transmission member such as a gearbox.

 In this context, the electric machine is generally a machine which can be used either in motor mode or in generator mode. In generator mode, the electric machine then replaces the alternator to supply an electric current intended to be used in the electrical circuit of the vehicle or to be stored in a storage battery. On the contrary, in engine mode, the electric machine is supplied with current previously stored in the storage battery and it supplies an engine torque on the engine shaft, this engine torque therefore adding to that of the heat engine to be transmitted. to the drive wheels of the vehicle.

 Up to now, it has been known to use the electric machine in engine mode to start the heat engine, to reduce the cycles of the heat engine when it is operating at idle speed, or to provide, for a short period of time, a supplement torque allowing for example to facilitate a hill start or to facilitate overtaking.

 However, the invention is in no way limited to such a powertrain, and can therefore be applied in all cases of hybrid powertrain in which the heat engine is capable of driving the vehicle by itself.

 The invention is intended to apply to powertrains in which the internal combustion engine is capable of operating in lean mixture. When the engine is in lean mixture mode, when the driver's torque demand is not too great, fuel consumption can be limited. However, in the context of a combustion engine whose exhaust gases must be cleaned up, operation in a lean mixture causes the formation of nitrogen oxides (NOx), molecules which it is then impossible to reduce in a conventional catalysis device because the exhaust gases then form an oxidizing medium.

 Also, to prevent these nitrogen oxides from being released into the atmosphere, it is known to store them in a storage device also called a NOx or NOxtrap trap.

 Such a NOx storage device therefore makes it possible to store the NOx molecules produced when the engine operates with lean mixture, provided that the temperature of the device is within a certain range of temperatures, for example between 250 and 450 C. When, on the contrary, the engine is used by fueling you with a rich fuel mixture, that is to say having an excess of fuel compared to the air, your exhaust gases then become reducing, which allows the reduction of nitrogen oxides NOx, and which allows the NOx trap to be emptied in order to be able to operate the engine again in lean mixture.

Furthermore, it appeared that the NOx traps could not only store nitrogen oxides
NOx, but that they could also store sulfur oxides (SOx). However, if the storage of sulfur oxides can be done at the same temperatures as the storage and storage of nitrogen oxides, the storage of sulfur oxides can only be done when the exhaust gases form a reducing medium and that when the temperature of the storage device is greater than a certain temperature, for example greater than 650.

 Also, in order for the NOx trap to fill its rote effectively, it is therefore necessary, from time to time, to purge it to free it from sulfur oxides.

As we can see, to ensure complete depollution of exhaust gases, it is therefore necessary to be able to control the temperature of the nitrogen oxide storage device
NOx. However, the temperature of this device is essentially dependent on the temperature and the flow rate of the exhaust gases passing through it.

 Thus, to control the temperature of the NOx storage device, it is necessary to control the temperature of the exhaust gases.

 However, the temperature of the exhaust gases depends essentially on the load of the engine, that is to say on the quantity of fuel which is injected at each cycle into the cylinder. However, we cannot be satisfied with acting on the motor load to vary the temperature because the load also determines the torque delivered, and it is this last parameter which is the most important because it must be constantly adjusted so to correspond as closely as possible to the torque demand which is formulated by the driver, the latter acting for example on an accelerator pedal.

 Also, in the prior art, it is proposed to be able to modify the temperature of the exhaust gases without modifying the torque supplied by the engine, in particular by varying the ignition advance.

 However, by modifying the ignition advance, the energy efficiency of the engine is significantly modified and, in order to obtain an increase in the temperature of the exhaust gases, it is necessary to modify the ignition advance in a sense such that the yield is diminished. Also, for the engine to supply the same torque requested by the driver, it will be necessary to use a larger quantity of fuel, to the detriment of the consumption of the vehicle.

 Therefore, the object of the invention is to propose a new design of a powertrain of the type described above, in which the heat engine and the electric machine are controlled so as to respond at all times to the demands of the driver with low fuel consumption, and by allowing, at any time, an irreproachable cleaning of exhaust gases.

 For this purpose, the invention provides a powertrain of the type described above, characterized in that for certain operating states of the powertrain, the mode of use of the electric machine is determined by a powertrain management unit in order to maintain an exhaust gas temperature in the treatment device within a determined temperature range.

According to other characteristics of the invention:
the treatment device is a device for storing the molecules of nitrogen oxides present in the exhaust gases;
-for certain operating states of the powertrain, the heat engine being supplied with a lean mixture, and the torque demand becoming greater than a threshold value for which the temperature of the nitrogen oxide storage device becomes greater than a temperature maximum storage, the powertrain management unit controls the electric machine in its engine mode to provide engine torque to meet the torque demand, to maintain the exhaust gas temperature within a temperature range allowing the storage of nitrogen oxides while supplying the combustion engine with a lean mixture;
the combustion engine is provided with a system for direct injection of fuel into the cylinder by means of which, for certain operating states of the engine, the latter is supplied with a stratified air / fuel mixture in which the distribution of the fuel in the cylinder is not homogeneous, and in that, for certain operating states of the powertrain, the heat engine being supplied with a lean stratified mixture, and the torque demand becoming greater than a threshold value for which the temperature of the nitrogen oxides storage device becomes higher than a maximum storage temperature, the powertrain management unit controls the electric machine in its engine mode to supply engine torque so as to meet the torque demand, in order to maintain the exhaust gas temperature within a temperature range allowing the storage of nitrogen oxides while feeding the word eur combustion with a stratified mixture;
for certain operating states of the engine, in order to maintain the temperature of the nitrogen oxide storage device above a minimum temperature, the management unit controls the electric machine in its generating mode to provide a resisting torque opposing the engine torque supplied by the combustion engine, the latter being controlled to supply a torque equal to the sum of the torque requested by the driver with the resistive torque of the electric machine, so as to cause an increase in the temperature of the gases d 'exhaust;
the electric machine is controlled in its generator mode to maintain the temperature of the nitrogen oxide storage device within a temperature range allowing a purging of the sulfur oxides contained in the nitrogen oxide storage device;
the electric machine is controlled in its generator mode to maintain the temperature of the nitrogen oxide storage device within a temperature range allowing the storage and storage of nitrogen oxides;
the combustion engine is a direct injection engine;
-the electric machine is integrated into the flywheel of the combustion engine;
the combustion engine is a spark-ignition engine.

Other characteristics and advantages of the invention will appear on reading the following detailed description for the understanding of which reference will be made to the appended drawings in which:
FIG. 1 is a schematic view illustrating a powertrain according to the invention;
FIGS. 2A to 2D are graphs illustrating the operation of an engine according to the state of the art;
FIGS. 3A to 3D are graphs illustrating, under the same conditions, the operation of a powertrain according to the invention, optimized to limit fuel consumption and pollution;
FIG. 4 is a flowchart illustrating the main steps of a first method of controlling a powertrain integrating the teachings of the invention;
FIGS. 5A to 5D are graphs illustrating the management of a powertrain according to the state of the art when it is a question of raising the temperature of the exhaust gases to allow purging of the sulfur oxides
SOx;
FIGS. 6A to 6D are graphs illustrating the management of an engine according to the invention to allow the storage of the sulfur oxides retained in the storage device; and
FIG. 7 is a flow diagram illustrating the main steps of a second method of controlling a powertrain according to the invention making it possible to perform the purge function.

 FIG. 1 shows schematically a powertrain 10 of the hybrid type, more particularly a powertrain 10 consisting of a heat engine 12, for example an internal combustion engine with alternating pistons, which is provided with a machine electric 14 integrated into its flywheel. The rotor 16 of the electric machine 14 is therefore integral in rotation with the motor shaft 18 of the heat engine 12 so that the electric machine 14 is interposed between the heat engine 12 and a transmission member 20 which can for example be a gearbox fitted with a clutch.

 A central management unit 22 controls the operation of the heat engine 12 and of the electric machine 14 as a function of various parameters and in particular as a function of a torque Cd requested by the driver.

 The driver of the vehicle manifests his request for torque by acting on an interface member such as an accelerator pedal.

The heat engine 12 supplies a motor torque Cmot while the electric machine 14 imposes on the motor shaft
This is positive when the electric machine is used as a motor and negative when the electric machine is used as a generator. Thus, the powertrain 10 supplies the transmission device with a torque Cgmp which is equal to the algebraic sum of the torques
Cmot and Cme.

 When used as a generator, the electric machine 14, whose rotor 16 is a) ors driven in rotation either by the motor shaft 18, or by the transmission 20, produces current which is likely to be used by an electrical circuit of the vehicle or by being stored in a storage battery.

 In accordance with the teachings of the invention, the heat engine 12 is an engine capable of operating with a lean mixture, that is to say a fuel mixture in which the air is in excess relative to the quantity of fuel. Of course, the engine is used in lean mixture only for relatively low loads, that is to say relatively low torque demands of the driver, the maximum power of the engine can only be obtained with higher richnesses. or equal to the unit richness, that is to say when the fuel mixture has an excess of fuel relative to the stoichiometric equilibrium of the combustion reaction.

 In the preferred embodiment of the invention, the internal combustion engine 12 is a spark-ignition engine with direct fuel injection.

 Direct fuel injection makes it possible to use particularly poor carbide mixtures, the ignition of the mixture being favored by the fact that direct injection makes it possible to supply the engine with a stratified lean load, that is to say a charge in which the fuel which is injected into the cylinder is not homogeneously distributed therein at the time of ignition, the fuel then being grouped together as much as possible in the vicinity of the spark plug, so to present a sufficient local concentration for the initiation of combustion. Compared to a homogeneous lean mixture, a layered lean mixture can therefore allow correct operation of the engine with even less fuel, this to the benefit of the consumption of the powertrain.

 According to the invention, the engine 12 is also provided with an exhaust gas pollution control system. Conventionally, in the case of a positive-ignition engine, it comprises a three-way catalyst which makes it possible to significantly reduce the content of the exhaust gases in hydrocarbon (HC), in nitrogen oxides (NOx) and carbon monoxide (CO).

 However, such a three-way catalyst generally works only for richness values very close to the unit value, that is to say only when the fuel mixture has the stoichiometric ratio between the quantities of air fuel introduced into the mixture for combustion. The stoichiometric ratio is approximately 1 g of fuel for 14.7 g of air.

 Also, in order to ensure efficient exhaust gas discharge when the engine is used with a lean mixture, a device for storing nitrogen oxides (NOx) is also provided, also called a NOx trap or NOxtrap. Indeed, when the engine is supplied with a lean mixture, the emissions of HC hydrocarbons and carbon monoxides are very low, while on the contrary the excess of oxygen tends to favor the formation of molecules of oxides of nitrogen (NOx).

 Thus, when the engine is used in a lean mixture, the nitrogen oxides are stored progressively in the storage device. On the contrary, when the engine is supplied with a stoichiometric mixture or with a rich mixture, it is then possible to destock the molecules of nitrogen oxides obtained in the storage device.

According to a first aspect of the invention, the central management unit 22 of the powertrain 10 will therefore control the heat engine 12 and the electric machine 14 so that, when the engine operates with lean mixture, the temperature of the gases exhaust such that these maintain the storage device inside a temperature window between a low temperature Tmin, for example 250, and a high temperature
Tmax, for example 450. Within this temperature range, the storage and storage reactions of the
NOx can occur.

 FIGS. 2A to 2D have illustrated a conventional method for managing the temperature of the exhaust gases in the case of an engine operating in a lean mixture.

 The speed V of the vehicle as a function of time has been illustrated in FIG. 2A. We are here in the case where the driver wishes to cause the vehicle to accelerate, this acceleration allowing the vehicle to pass from speed V1, equal for example to 70 km / h, to speed V2, equal for example to 100 km / h , this between instants t1 and t3. To cause this acceleration, it is therefore necessary to control the heat engine 12 so that it provides additional torque allowing this acceleration.

 FIG. 2B shows the temperature of the exhaust gases which would result from the control of the heat engine if the latter were forced to remain in operation in a lean mixture. We see that, until time t1, the engine is controlled to maintain the vehicle at the speed V1 of 70km / h and the temperature of these exhaust gases, at the NOx storage device, is for example equal to the temperature T1 of 400 C. We are then in the storage window so that the storage operation can actually be carried out.

 Then, from the instant t1, the additional torque requested from the heat engine 12, always controlled in lean mixture, would cause an increase in the temperature of the exhaust gases, these being able to reach during acceleration a temperature equal to temperature t2, for example 500 C, that is to say greater than the temperature Tmax for storage of nitrogen oxides. Indeed, we see on the curve that beyond the instant t2, the temperature of the exhaust gases would come to exceed 450.

 There is therefore a threshold torque Cs beyond which the heat engine cannot go, when it is supplied with a lean mixture, without the temperature of the exhaust gases exceeding a temperature for which the NOx storage device reaches a temperature above the maximum storage temperature which is approximately 450. This torque Cs is less than the maximum torque that the engine is capable of providing in lean mode.

 Also, to prevent nitrogen oxides from being released into the atmosphere, we can see in Figure 2C which illustrates the richness R of the fuel mixture supplied to the engine, that, from time t2, we are forced to order the heat engine 12 according to the prior art so that it is supplied with a stoichiometric fuel mixture, this in order to allow pollution control of the exhaust gases by the three-way catalyst. However, such a switchover of the operation of the heat engine is harmful with regard to the consumption of the engine, this being due on the one hand to the fact that the engine efficiency is less good for the unit richness than when using a lean mixture, and this being further reinforced by the fact that the mode changeover causes a transient phase during which the efficiency of the engine is particularly poor.

 Illustrated in FIG. 2D is the temperature variation curve of the NOx storage device when the engine is controlled according to the state of the art, in accordance with what is illustrated in FIG. 2C. We therefore see that from time t2, when switching to stoichiometric mode, the temperature of the exhaust gases increases significantly at the level of the storage device, always to obtain the same acceleration which allows the vehicle to pass from 70 to 100 km / h. The maximum temperature can this time reach 600 C and this is further reinforced by the fact that, at the unit richness, there is a combustion of CO and HC in the NOxtrap, a particularly exothermic reaction which contributes to the rise in temperature . As can be seen in Figure 2D, this rise in temperature extends well beyond the end of the acceleration period and tends to maintain the temperature in the storage device at a temperature above the maximum storage temperature of 450, this although, as can be seen more particularly in FIG. 2B, it would be theoretically possible to maintain the vehicle at its new speed V2 by supplying the engine with lean mixture and then obtaining a temperature T3 of the exhaust gases in the storage device of approximately 430, temperature compatible with the storage reaction.

 As can therefore be seen in FIG. 2C, we are then forced to keep the engine in stoichiometric operating mode well beyond the instant t4 from which we could envisage, apart from pollution control problems, to control the engine of again in lean mixture to maintain the vehicle at speed V2 of 100 km / h.

 FIGS. 3A to 3D illustrate a mode of control of a powertrain according to the invention which allows, during an acceleration of the type which has just been described, to keep the heat engine 12 in operation in a lean mixture, this of course without causing emission of nitrogen oxides into the atmosphere. The graphs in Figures 3A and 3B are identical to those in Figures 2A and 2B.

 On the graph of FIG. 3C, which illustrates the mode of use of the electric machine 14, it can be seen that from the instant t2 beyond which the torque requested by the driver becomes greater than the threshold torque Cs that the heat engine is capable of supplying without the temperature of the storage device exceeding the maximum temperature Tmax of storage of NOx, the electric machine 14 is controlled to operate according to its engine mode in which it supplies engine torque to the drive wheels of the vehicle , of course by taking electrical energy previously stored in the storage battery.

 FIG. 3C illustrates an operating state of the powertrain 10 in which, outside the acceleration period, the electric machine is used as a generator, for example to recharge the accumulator battery. However, for other engine operating states, the electric machine 14 could be at rest, or even be used as an engine, depending on other operating parameters of the vehicle.

The invention resides in the fact that one of the parameters according to which the management unit 22 controls the electric machine 14 is, directly or indirectly, the temperature of the NOx storage device.

Consequently, it can be seen in FIG. 3D that the temperature of the storage device remains within the temperature range for which the storage reactions of the
NOx are possible. At the same time, as can be seen in FIG. 3A, the entire powertrain 10 delivers sufficient torque for the vehicle to accelerate as desired by the driver.

 At time t3, when the acceleration stops and the demand for torque drops, we then find ourselves at a level of torque requested by the driver which is less than the threshold torque defined above. It is then possible to progressively reduce the intervention of the electric machine 14 so as to no longer drive the vehicle except with the heat engine, the latter never having ceased to be supplied with a lean mixture.

 Thus, thanks to the invention, the tilting of the operating mode of the heat engine is avoided from an operating mode in lean mixture to an operating mode in stoichiometric mixing, this to the benefit of the group's yield and fuel consumption. powertrain.

 To implement a method of controlling a powertrain according to the invention, the main steps of the flow diagram illustrated in FIG. 4 can therefore be followed.

On this flowchart, we can see in step 100 that we first check whether the requested torque Cd is lower or not than the threshold torque Cs defined above. If so, then it suffices to control the heat engine 12 so that it delivers this torque Cd requested by the driver, without there being any need to modify the operating mode and the engine being able to operate in lean mixture.

Otherwise, the motor torque that the electric machine 14 must supply is calculated in step 110.
Cme is equal to the difference of the torque Cd requested by the driver from which the threshold torque Cs is subtracted.

 In step 120, it is checked, in particular as a function of the state of charge of the storage battery, whether or not it is possible for the electric machine to supply this torque.

If possible, the powertrain 10 is controlled by the management unit 22 so that the heat engine 12 provides a torque equal to its threshold torque Cs while the electric machine 14 supplies the engine torque Cme calculated at l step 110. Thus, the powertrain 10 supplies the vehicle with the total torque Cgmp =
Cs + Cme which is equal to the torque Cd requested by the driver.

 In the case where, for example, the state of charge of the battery would not allow the electric machine 14 to supply sufficient torque, we would then be obliged to satisfy the demand for torque Cd without causing emission of oxides nitrogen, to cause a change in the engine operating mode, the heat engine 12 then being supplied with a stoichiometric mixture and being controlled to supply an engine torque Cmot equal to the torque requested by the driver.

 In the example which has just been described, a method of controlling the powertrain 10 has been described which makes it possible, in certain cases, to avoid the tilting of the heat engine from an operating mode in lean mixture towards a mode operating in homogeneous mixture.

 However, in the context of a direct injection heat engine, provision may be made to implement the same kind of control strategy to avoid the tilting of the heat engine from an operating mode of lean mixture stratified to an operating mode in a homogeneous lean mixture, this in order to further improve the efficiency of the powertrain.

 Another application case of the invention is envisaged for purging the sulfur oxides contained in the device for storing nitrogen oxides.

 Indeed, such storage of sulfur oxides SOx can only be done when the temperature in the storage device is higher than a threshold temperature Ts, for example equal to 650 C. Of course, such storage of sulfur oxides , which is the consequence of a reduction reaction, can only occur when the exhaust gases form a reducing medium, that is to say only when the heat engine 12 is supplied with a fuel mixture whose richness is at least 1 or more.

 However, even when the engine is supplied with a stoichiometric mixture, it is rare that under normal operating conditions of the vehicle, the exhaust gases allow the storage device to reach such a temperature.

 Also, when the vehicle is traveling at a stabilized speed, for example at speed V1 of 100 km / h, in order to obtain a rise in temperature, without modifying the torque supplied by the heat engine 12, it is necessary, depending on the state of the technique, to modify the ignition advance of the engine 12 in order to degrade the efficiency. Thus, during the entire purge time interval between the instants t1 and t2, the value A of the ignition advance is reduced, as shown in FIG. 5B, and at the same time, for compensate for the drop in efficiency, the apap opening angle of the air intake butterfly valve is increased as shown in FIG. 5C, which necessarily corresponds, given that there is a unit richness, to a increase in the amount of fuel introduced into each cylinder during each cycle.

 In this way, it can be seen in FIG. 5D that it is possible, thanks to this device, to obtain an exhaust gas temperature which reaches the temperature Ts necessary for the storage of sulfur oxides, that is to say the temperature of 650 C. However, by using this process according to the state of the art, it can be seen that, during the entire duration of the purging of the sulfur oxides, it is necessary to supply the heat engine with more fuel, without this is also made necessary by the driver's desire to accelerate, or by the presence of a slope to climb.

 On the contrary, thanks to the method according to the invention, we will cause, as we can see in FIGS. 6A to 6D, a rise in temperature of the exhaust gases obtained, as we can see in FIG. 6C, by the fact that the heat engine is supplied during this period with a quantity of fuel which makes it possible to obtain such a rise in temperature.

 However, thanks to the invention, it is not necessary to reduce the efficiency of the motor to maintain a constant speed. In fact, according to the invention, the central management unit controls the electric machine 14 so that, during the duration of the purge, the machine 14 produces electric current, that is to say that it works in generator mode. The electric machine 14 then absorbs a torque and the management unit 22 controls the machine 14 in such a way that the absorbed torque is equal to the excess of torque supplied by the heat engine compared to the torque requested by the driver.

 Thus, unlike the prior art, the rise in temperature which is obtained by burning a larger quantity of fuel is not lost since the additional energy supplied by this fuel is transformed into electrical energy which is stored in the storage battery and which can be used later.

 As in the first embodiment of the invention, the electric machine 14 could, outside the purge time, be used in engine mode or be at rest, depending on other operating parameters of the vehicle.

 Illustrated in FIG. 7 is a flowchart illustrating the main steps of a method making it possible, according to the invention, to purge the sulfur oxides contained in the device for storing nitrogen oxides.

 First of all, in step 200, the management unit 22 of the powertrain 10 controls the thermal engine so that it is supplied with a fuel mixture in stoichiometric proportions, that is to say tell unitary wealth. Then, in step 210, it is checked whether the temperature T of the NOx storage device is higher or not than the minimum temperature Ts for purging sulfur oxides, that is to say about 650.

 If yes, which may be the case under heavy load when the driver requests a high torque, the central management unit leaves the electric machine 14 at rest and controls the heat engine so that it supplies all of the torque. CD requested by the driver.

 Otherwise, that is to say if the temperature T of the NOx storage device is lower than the temperature Ts of 650, the value of the torque Cme which the electric machine 14 must absorb is evaluated in step 220. so that the corresponding increase in the torque supplied by the heat engine is at the origin of a sufficient increase in the temperature of the exhaust gases in order to reach the necessary temperature of the NOx storage device. The torque Cme is therefore a negative torque and it is checked, still in step 220, in particular as a function of the state of charge of the battery, whether it is actually possible for the electric machine 14 to absorb such a torque.

 If so, the heat engine is therefore controlled to supply the torque Cmot which is equal to the torque Cd requested by the driver minus the torque Cme supplied by the electric machine 14, which is negative since the electric machine 14 then absorbs power.

 In step 230, it is checked whether the temperature T of the NOx storage device is much higher than the threshold level Ts of 650. If so, the management unit 22 continues to control the powertrain 10 in this manner until the end of the purge operation of the sulfur oxide molecules.

 If this temperature of 650 is never reached, a strategy of modification of the advance is then implemented which, in replacement of the strategy defined above or in addition to it, allows, as we have seen in the description of the state of the art, to increase the temperature of the exhaust gases.

 In the second embodiment of the invention, a method of controlling an engine has been described in which it was sought to maintain the temperature of the nitrogen oxide storage device above the threshold level Ts for purging the sulfur oxides. Of course, the same process could be adapted to maintain said temperature above the minimum temperature Tmin for storing and storing nitrogen oxides.

 Likewise, the scope of the invention can be extended to any powertrain in which the operating mode of the electric machine is determined as a function of the operating temperature of an exhaust gas treatment device, whatever his nature.

Claims (10)

 characterized in that for certain operating states of the powertrain (10), the mode of use of the electric machine (14) is determined by a management unit (22) of the powertrain in order to maintain a temperature of the exhaust in the treatment device within a determined temperature range.  1. Powertrain for a motor vehicle, of the type comprising an internal combustion engine (12) capable of driving at least one drive wheel of the vehicle and an electric machine (14) which is capable of being used in a generator mode or according to an engine mode in which it participates in driving the vehicle, of the type in which, for certain operating states of the internal combustion engine (12), the latter is supplied with a so-called lean air / fuel mixture in which I the air is in excess with respect to the fuel, and of the type in which the internal combustion engine (12) comprises an exhaust circuit provided with an exhaust gas treatment device,  2. Powertrain according to claim 1, characterized in that the treatment device is a storage device for the nitrogen oxide molecules present in the exhaust gases.  3. Powertrain according to claim 2, characterized in that, for certain operating states of the powerplant, the heat engine (12) being supplied with a lean mixture, and the torque demand (Cd) becoming greater than a value of threshold (Cs) for which the temperature of the nitrogen oxide storage device becomes higher than a maximum storage temperature (Tmax), the powertrain management unit controls the electric machine (14) in its engine mode to supply an engine torque (Cme) so as to meet the torque demand (Cd), in order to maintain the temperature of the exhaust gases within a temperature range allowing the storage of nitrogen oxides while supplying the combustion engine ( 12) with a lean mixture.  4. Powertrain according to claim 3, characterized in that the combustion engine is provided with a system for direct injection of fuel into the cylinder by which, for certain operating states of the engine, the latter is supplied with a stratified air / fuel mixture in which the distribution of fuel in the cylinder is not homogeneous, and in that, for certain operating states of the powertrain, the heat engine (12) being supplied with a lean layered mixture, and the torque demand (Cd) becoming greater than a threshold value (Cs) for which the temperature of the nitrogen oxide storage device becomes greater than a maximum storage temperature (Tmax), the management unit (22) of the powertrain (10) controls the electric machine (14) in its engine mode to supply engine torque (Cme) so as to meet the torque demand (Cd), in order to maintain the temperature d he exhaust gases in a temperature range allowing the storage of nitrogen oxides while supplying the combustion (12) with a layered mixture.  5. Powertrain according to claim 2, characterized in that, for certain operating states of the engine, to maintain the temperature of the nitrogen oxide storage device above a minimum temperature (Ts), the unit of management (22) controls the electric machine (14) in its generator mode to provide a resistant torque (Cme) opposing the engine torque provided by the combustion engine (12), the latter being controlled to provide a torque (Cmot) equal to the sum of the torque (Cd) requested by the driver with the resistive torque (Cme) of the electric machine (14), so as to cause an increase in the temperature of the exhaust gases.  6. Powertrain according to claim 5, characterized in that the electric machine (14) is controlled in its generator mode to maintain the temperature of the nitrogen oxide storage device within a temperature range allowing purging of the sulfur oxides contained in the nitrogen oxides storage device.  7. Powertrain according to claim 5, characterized in that the electric machine is controlled in its generator mode to maintain the temperature of the nitrogen oxide storage device within a temperature range allowing the storage and storage of the oxides of nitrogen.  8. Powertrain according to any one of claims 5 to 7, characterized in that the combustion engine (12) is a direct injection engine.  9. Powertrain according to any one of the preceding claims, characterized in that the electric machine (14) is integrated into the flywheel of the combustion engine (12).  10. Powertrain according to any one of the preceding claims, characterized in that the combustion engine (12) is a spark-ignition engine.