Control of an exhaust gas aftertreatment device in a hybrid vehicle

The present invention involves a method for maintaining temperature in an exhaust gas treatment device for an internal combustion engine in a hybrid vehicle. The engine has a crankshaft and at least one cylinder. The hybrid vehicle has a fuel storage device and a fuel supply device. An exhaust gas treatment device is located downstream of the cylinder. The method comprises, in an engine off mode, pumping air to the exhaust gas aftertreatment device controlling the fuel supply device so that fuel is allowed to the exhaust gas aftertreatment device. Air flow to the exhaust gas aftertreatment device can be controlled by a throttle in an inlet duct to the engine and/or the fuel supply device to control the combustion and air-fuel ratio in the exhaust gas treatment device.

TECHNICAL FIELD

The present invention relates to a hybrid vehicle, a hybrid vehicle propulsion system, and a method for an exhaust gas aftertreatment device for an internal combustion engine in a hybrid vehicle further having an additional power source. The internal combustion engine has a crankshaft and at least one cylinder, the hybrid vehicle further comprising fuel storage device and fuel supply device, the exhaust gas treatment device located downstream of the cylinder.

BACKGROUND

The drivetrain of a hybrid vehicle includes two combined power sources for the propulsion of the vehicle. Usually an internal combustion engine is combined with an additional power source connected to or including an energy storage unit which can be charged by the internal combustion engine, and, in many solutions, also by recovering the braking energy of the vehicle.

In so called hybrid electric vehicles, the additional power source is typically an electric motor connected to an energy storage unit including rechargeable electric accumulators for energy storage. In alternative hybrids, the additional power source could include, instead of electric components, a variable displacement pump communicating with a hydraulic accumulator for energy storage, essentially in the form of a pressure tank. There are also hybrid concepts in which the additional power source includes a flywheel which stored energy and is connected to a hydrostatic transmission. In further hybrid ideas, the additional power source includes an air engine connected to an air tank for energy storage. The additional power source can also be a human, such as in the case with a moped.

The drivetrain of a hybrid vehicle can be provided as a parallel hybrid arrangement, in which the engine and the additional power source (e.g. electric motor) are both connected to a mechanical transmission for delivering torque to the wheels. Where the additional power source is an electric motor, this is often provided as a combined generator and motor. Differing from parallel hybrid arrangements, in a series hybrid arrangement there is no power path from the engine to the wheels. The main task of the engine is to provide power to the additional power source and/or the energy storage unit connected thereto. Combined hybrids have features from both parallel and series hybrid arrangements, in that they have power split devices allowing the power path from the engine to be mechanically directed to the wheels, or to be directed to the additional power source or the energy storage unit connected thereto.

The operation of most hybrid vehicles with internal combustion engines usually includes a number of different modes for the power distribution in the drivetrain. For example, the vehicle can be run in a cruise mode, in which the power from the engine is split into a path to the wheels and a path to the energy storage unit, e.g. to batteries via a generator. As another example, the vehicle can be run in an energy storage unit charge mode, (for a hybrid electric vehicle usually referred to as a battery charge mode), during engine idling, in which the energy storage unit is charged by the engine, e.g. via a generator. In parallel or combined hybrid arrangements, there could also be a power boost mode in which power is provided to the wheels from both the engine and the power source.

In addition, the operation of most hybrid vehicles with internal combustion engines usually includes engine off modes, in which the internal combustion is shut down. Such modes can include a mode in which propulsion is provided only by the additional power source. An example of such a mode is a so called electric vehicle mode in a combined or parallel hybrid electric vehicle. For the vehicle to run in an engine off mode, necessary accessories are powered by the additional power source.

Usually, one or more exhaust gas aftertreatment devices, known as catalytic converters, are provided in the exhaust system of the engine. Catalytic converters convert regulated gases, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), into substances such as carbon dioxide (CO2), nitrogen (N2) and water (H2O). A catalytic converter can not fully exhibit its converting effects when its temperature is lower than its activation temperature. A problem with hybrid vehicles is that during engine off modes, the catalyst converter temperature can fall below the activation temperature, in which case HC and NOx emission are not efficiently processed in the exhaust gas aftertreatment devices when the internal combustion engine is started again.

For vehicles in general, many catalyst heating strategies have been suggested. U.S. Pat. No. 6,161,377 and U.S. Pat. No. 6,901,749 are concerned with rapidly heating the catalytic converter during cold starts. EP0935056A2 describes shutting off ignition in one or more cylinders when cranking during engine start, to heat the cold catalyst. EP1174612A1 describes, in a diesel engine application for regeneration of a particulate filter or a NOx-trap, injecting fuel into the cylinders during an engine braking situation. However, none of these references describe hybrid vehicles.

US2004/0194452A1 discloses a hybrid vehicle where fuel supply is carried out during an initial period of cranking for resuming the operation of the internal combustion engine. U.S. Pat. No. 6,318,077B1 describes a special heater for a catalyst for rapid heating of the catalyst at a cold start. However, none of these references describe maintaining catalyst temperature during an engine off mode.

SUMMARY OF THE INVENTION

The problems described above are overcome by providing air and fuel to the exhaust gas aftertreatment device while the engine is not operating which leads to oxidation of the fuel in the exhaust gas aftertreatment device to maintain the temperature within its activation range. The air can be provided by rotating the engine by the additional power source and control of the air quantity can be accomplished by a throttle valve in an inlet duct of the engine. Alternatively, the air is provided into the exhaust system directly. Fuel is supplied by the fuel injectors, either located in the port or in the cylinder. In a gasoline engine with spark plugs, combustion in the cylinder is avoided by preventing the spark plugs from firing. In a diesel engine, combustion in the cylinder is avoided by injecting fuel into the cylinder late in the expansion stroke such that the fuel does not compression ignite. Alternatively, fuel can be supplied directly into the exhaust system.

In yet another alternative, fuel is supplied from the canister aboard the vehicle which traps fuel vapors during refueling and from fuel tank vapor generation due to changes in temperature. A purge gas valve is opened to allow air to remove the trapped vapors and induct them through the engine and into the exhaust system.

The fuel and air supplied to the exhaust gas aftertreatment device is controlled to provide a desired air-fuel ratio, which is greater than 1.0, preferably between 1.25 and 2.0.

Further, the fuel and air supplies are controlled so that the maximum temperature in the exhaust gas aftertreatment device occurs in a desired location along the length of the exhaust gas aftertreatment device. The desired location is downstream of a front edge, as much as 50% of the distance into the exhaust gas aftertreatment device.

It should be noted that the air flow can be controlled any suitable device, such as a throttle valve, and/or control of inlet valves and/or the exhaust valves in the cylinders, e.g. in the form of a variable valve timing (VVT) system and/or a cam profile shifting (CPS) system.

It is an advantage of the present invention that the hybrid vehicle emission are reduced by maintaining an activation temperature in the exhaust gas aftertreatment device even when the engine is not operating. By doing so, high emissions upon engine restart are avoided.

DETAILED DESCRIPTION

FIG. 1shows parts of a hybrid vehicle propulsion system1with a combined hybrid structure, the propulsion system1having an internal combustion engine2and an additional power source3in the form of an electric motor3. A first transmission4provides in a manner known in the art a mechanical connection between the engine2, a generator5and a second transmission6, which in turn provides a mechanical connection between the engine2via the first transmission4, the motor3and two wheels7of the vehicle. The generator5is electrically connected to a charger8, which in turn is connected to an energy storage unit9comprising a plurality of batteries, and to a converter10, the latter being connected to the motor3. Fuel storage tank11comprising a fuel tank11, and an exhaust system12with an exhaust treatment device13in the form of a catalytic converter13are provided for the engine2.

Transmissions4,6are controllable as described below and work as power split devices to allow the power path from engine2to be mechanically directed to wheels7, or to be directed via generator5and charger8to motor3or to batteries9. Transmissions4,6also allow mechanical power from motor3to be directed to engine2, as discussed closer below.

FIG. 2shows schematically certain of the hybrid vehicle propulsion system parts inFIG. 1in some detail. Internal combustion engine2comprises at least one cylinder102with a reciprocating piston103connected to a crankshaft131. Crankshaft131is mechanically connected to motor3via transmissions4,6as also depicted inFIG. 1, (transmissions4,6are depicted inFIG. 2with broken lines).

Communication between cylinder102and an inlet duct104is controlled by at least one inlet valve105, and communication between cylinder102and an exhaust duct106is controlled by at least one exhaust valve107. Downstream of the cylinder(s)102, catalytic converter13is provided. The term catalytic converter13is not intended to be limiting. Alternatively, element13is any exhaust gas aftertreatment device such as a lean NOx trap, a SCR catalyst, a diesel particulate filter, a diesel oxidation catalyst, and any other known exhaust gas aftertreatment device.

Propulsion system1also comprises a control unit109, which has computational and data memory capacities, and which can be provided as one unit, or as more than one logically interconnected physical units. Control unit109is adapted to control air flow by controlling a throttle valve110. Control unit109is also adapted to control fuel supply by controlling at least one fuel injector111in the inlet duct104. In this embodiment, where the engine presents more than one cylinder, the fuel injection can be controlled individually for each cylinder, by a fuel injector being provided at a separate portion of the inlet duct104communicating with the respective cylinder, (so called port fuel injection). Alternatively, as is known in the art, a fuel injector can be provided in each cylinder102, (so called direct fuel injection). As a further alternative, one single fuel injector can be provided for more than one cylinder, or all cylinders, for example at an upstream portion of the inlet duct communicating with more than one cylinder, or all cylinders. Fuel injector(s)111communicate with the fuel tank11, via a fuel pump121.

In addition, control unit109is also adapted to determine the engine air flow based on signals received from an air flow sensor114located in the inlet duct104. As an alternative, as is known in the art, the air flow can be computed based on parameters such as the inlet manifold pressure, throttle position, engine speed, inlet temperature, and atmospheric pressure. Methods for determining these parameters are known in the art, and not explained further here.

Further, at each cylinder, a spark plug116is provided and controlled by control unit109.

Control unit109is adapted to determine the temperature of the catalytic converter13based at least partly on the air flow, the lambda value, the ambient temperature, engine load, and engine rotational speed. As an alternative, control unit109can be adapted to receive signals from a temperature sensor located in exhaust duct106between cylinder102and the catalytic converter13, or in catalytic converter13, based on which signals, the catalytic converter temperature can be determined.

Control unit109is further adapted to receive data corresponding to a state of charge (SOC) of the batteries9(FIG. 1). In addition, control unit109is adapted to adjust, as known in the art, the value of a requested torque parameter based on signals from an accelerator pedal117in the vehicle. Based at least partly on the SOC and the requested torque, control unit109is adapted to determine whether propulsion system1is to operate in a cruise mode, a battery charge mode, a power boost mode, or an electric vehicle mode, the latter being an engine off mode. Such operational modes have been briefly described above in the section “Background”.

Control unit109is further adapted to control the output torque of motor3. Control unit109is also adapted to control, in manners known in the art, activators (not shown) in transmissions4,6for control of power paths therein. By suitable control of motor3, transmissions4,6and the engine, power paths in propulsion system1can be controlled and operational modes of propulsion system1can be chosen.

In the engine off mode, in dependence as described below on catalytic converter13temperature, control unit109is adapted to control fuel injector111and spark plug116so that fuel injection and ignition are inhibited.

FIG. 3depicts a method according to a preferred embodiment of the invention. Control unit109determines201based on the SOC and the requested torque that propulsion system1is to operate in an electric vehicle mode, and fuel injection and ignition are inhibited. While fuel injection and ignition are inhibited, catalytic converter13temperature is monitored. Control unit109determines202whether catalytic converter13temperature is below a predetermined first temperature threshold value. If it is determined that catalytic converter13temperature is not below the first temperature threshold value, the fuel injection and ignition remain inhibited.

If it is determined that catalytic converter13temperature is below the first temperature threshold value, converter activation maintaining measures are taken203, in which the ignition remains inhibited, but motor3and transmissions4,6are controlled so that crankshaft131is rotated by motor3. Also, fuel injection is allowed. In addition, throttle valve110is controlled so as to be open. Thereby, by piston103movement(s), air is pumped through cylinder(s)103, and air and fuel is transported from cylinder(s)102and through exhaust duct106, in which a substantially homogenous air/fuel mixture is provided. The mixture reaches catalytic converter13where it is combusted to increase the catalytic converter13temperature. Thereby, a further decrease of catalytic converter13temperature is prevented, and it can remain active.

While measures to maintain converter activation are undertaken, catalytic converter8temperature is monitored. Thereby, control unit109determines204whether the catalytic converter13temperature is above a predetermined second temperature threshold value, which is higher than the first temperature threshold value. If it is determined that the catalytic converter13temperature is not above the second temperature threshold value, the converter activation maintaining measures are continued203. If it is determined that catalytic converter13temperature is above the second temperature threshold value, motor3and transmissions4,6are controlled205so that rotation of crankshaft131stops and injectors111are controlled201so as to inhibit fuel injection. Control unit109continues to monitor catalytic converter13temperature, and to determine202whether catalytic converter13temperature is below the first temperature threshold value.

It should be noted that the invention is applicable to engines with any number of cylinders, i.e. one or more cylinders102. In some embodiments of the invention, where engine2has a plurality of cylinders102, and the fuel injection can be controlled individually for each cylinder, the number of cylinders102into which fuel is supplied, while undertaking the converter activation maintaining measures, can be dependent on the temperature of the exhaust gas aftertreatment device.

As an example, it can be assumed that the engine comprises four cylinders. During the measures to maintain converter activation, if the catalytic converter temperature is below a first temperature threshold value, fuel is injected into all cylinders. If the catalytic converter temperature is above a first temperature threshold value and below a second temperature threshold value, which is higher than the first temperature threshold value, fuel is injected in only two of the cylinders102. If the catalytic converter temperature is above the second temperature threshold value, the converter activation maintaining measures are aborted. By controlling the number of cylinders in which fuel is injected, a coarse temperature control of the catalytic converter13is achieved.

Regardless of the number of cylinders in which fuel is injected during the converter activation maintaining measures, the amount of fuel injected in each cylinder can be controlled, so that a fine temperature control of the catalytic converter13temperature is achieved. Additional or alternative temperature control can be provided by controlling the frequency of fuel injection pulses.

Reference is made toFIGS. 2 and 4. In some embodiments, the method comprises controlling the throttle valve110so as to control the combustion in the catalytic converter13during the converter activation maintaining measures. Based on the flow of fuel injected, the throttle is controlled so that a combustible air/fuel mixture is provided to the catalytic converter.

Referring toFIG. 4, in which a gas flow direction is indicated with an arrow F, throttle valve110is controlled to control the location of a maximum temperature in the exhaust gas aftertreatment device13. By controlling throttle valve110so that a relatively small air flow is provided, the air/fuel mixture will be combusted relatively far upstream in catalytic converter13. As a result, the temperature distribution in the exhaust gas aftertreatment device, indicated inFIG. 4by curve T1, is maximized relatively far upstream. By controlling throttle valve110so that larger air flows are provided, the air/fuel mixture is combusted further downstream in catalytic converter13. As a result, the temperature distribution in the catalytic converter, T2, T3, is highest further downstream, depending on the air flow. In other words, increasing the air flow will move the maximum temperature downstream.

Thus, the location of the maximum temperature can be changed to ensure that the temperature is kept above the activation temperature throughout the entire catalytic converter.

As an alternative to, or in addition to a throttle valve110, the air flow control can comprise control (FIG. 2), for the inlet valve(s)5and/or the exhaust valve(s)7, for example in the form of a variable valve timing (VVT) system and/or a cam profile shifting (CPS) system. Such inlet and/or exhaust valve control can be used as an alternative or in addition to the throttle valve110for controlling the combustion in the catalytic converter13during the converter activation temperature maintenance.

It should be noted that instead of the motor3, the crankshaft131can rotated by a further power source of the propulsion system1.

FIG. 5shows another schematic view of the catalytic converter13inFIG. 1, a gas flow direction being indicated with an arrow F. In an alternative to the temperature control described with reference toFIG. 4, during the converter activation temperature maintenance, throttle110is controlled so that the air flow is reduced, preferably minimized, e.g. by throttle valve110being closed, or at least kept relatively low. As will be understood, decreasing the fuel injected, while retaining a constant air flow, will move the maximum temperature downstream in the catalytic converter.

Fuel injector(s)111are used during the converter activation maintaining measures to control the location of a maximum temperature in the exhaust gas aftertreatment device. By controlling injector(s)111so that a first, relatively rich air/fuel mixture is provided to catalytic converter13, a main portion of the mixture will be combusted relatively far upstream in the catalytic converter13. As a result, a first temperature distribution in the catalytic converter, indicated inFIG. 5with the curve T1, is maximized, T1max, relatively far upstream.

The curve T2shows a second temperature distribution during the converter activation maintenance, with the same air flow as in the case of the first temperature distribution T1, but where injector(s)111are controlled so that a second air/fuel mixture, leaner than the first air/fuel mixture, is provided to catalytic converter13. As a result, the mixture will mainly be combusted further downstream in catalytic converter13, with a maximum temperature, T2max, further downstream than the maximum temperature, T1max, of the first temperature distribution T1.

Finally, by controlling injector(s)111, with the same air flow as in the case of the first and second temperature distributions T1, T2, to obtain a third air/fuel mixture, being leaner than the second air/fuel mixture, the mixture will be mainly combusted even further downstream in the catalytic converter13. As a result, the temperature distribution T3in the catalytic converter will present a maximum, T3max, further downstream.

Thus, the location of the maximum temperature can be changed, during the converter activation maintenance.

Referring toFIG. 5, preferably, the fuel injection is controlled so that the location of the maximum temperature is not upstream of a threshold location, xtTmax, the distance between the threshold location and the upstream end13U of the catalytic converter13is at least twenty percent of the extension L13of the catalytic converter13in the exhaust flow direction F.

The temperature distribution control method described with reference toFIG. 5, involves adjusting the fuel injected, while retaining a constant air flow, to move the maximum temperature downstream in the catalytic converter. It should be noted that in addition to the fuel control, the air flow can be adjusted during the converter activation maintenance to control the catalytic converter temperature distribution. In such an embodiment, throttle valve110can be used during the converter activation maintenance to control the location of a maximum temperature in the exhaust gas treatment device. As mentioned above with reference toFIG. 4, by controlling the throttle valve110so that a relatively small air flow is provided, the air/fuel mixture will be combusted relatively far upstream in the catalytic converter13. By controlling the throttle valve110so that larger air flows are provided, the air/fuel mixture will be combusted further downstream in the catalytic converter13. In other words, increasing the air flow moves the maximum temperature downstream.

FIG. 6shows schematically an alternative exhaust gas aftertreatment device comprising three catalyst monoliths131,132,133, of which an upstream monolith131is located closest to the engine opposite to an exhaust flow direction F. Similarly to what was suggested above, during converter activation maintenance, fuel injection is controlled so that the location of the maximum temperature is not upstream of a threshold location, xtTmax. The threshold location is determined such that the distance between the threshold location, xtTmax, and an upstream end13U of the upstream monolith131is at least twenty percent of the extension L18of the upstream monolith in the exhaust flow direction F.

During the converter activation maintenance, to control the catalytic converter temperature distribution, the air flow control device and/or the fuel supply device can be controlled dependent at least partly on a model stored by the control unit109for a temperature distribution in the catalytic converter. The model can be based on parameters such as the air/fuel mixture, engine speed, air flow, ignition timing, and VVT-setting. As an alternative, during the converter activation maintenance, the air flow control device and/or the fuel supply device can be controlled, to control the catalytic converter temperature distribution, at least partly in dependence on signals from a plurality of temperature sensors distributed so as to detect the temperature at locations along the catalytic converter.

Embodiments of a method for controlling, during converter activation maintenance, the air flow control device and/or the fuel supply device based at least partly on a desired temperature distribution in the catalytic converter has been presented. As understood by the person skilled in the art, the mapping of air/fuel ratios to catalytic converter temperature distributions can be done in a test environment, and can involve adjusting the air/fuel ratio while monitoring the catalytic converter temperature distribution.

Referring toFIG. 7, showing parts of a hybrid vehicle propulsion system1with a parallel hybrid structure, an alternative embodiment of the invention will be described. Propulsion system1comprises an internal combustion engine2and an additional power source3in the form of a combined electric motor and generator3. The combined motor and generator3is via a converter10electrically connected to an energy storage unit9comprising a plurality of batteries. A fuel tank11, and an exhaust system12with an exhaust gas aftertreatment device13are provided for engine2.

A transmission4provides, in a manner known in the art, a mechanical connection between the engine2, the combined motor and generator3and two wheels7of the vehicle. The transmission4is controllable as described below and works as power split device to allow the power path from engine2to be mechanically directed to wheels7, or to combined motor and generator3. Transmission4also allows mechanical power from combined motor and generator3to be directed to engine2, as discussed closer below.

FIG. 8shows schematically certain of the hybrid vehicle propulsion system parts inFIG. 7in some detail, which parts correspond to the ones described above with reference toFIG. 2, except for the following details: The engine system is provided with a fuel vapor canister122, which could enclose carbon for retaining vaporized fuel as known in the art. The canister122can communicate with the fuel tank11via a vapor vent valve123. Further, the canister122is provided with a canister air inlet124. The canister122and fuel tank11can communicate with inlet duct4via a purge gas supply valve125, which is controllable by control unit109.

Alternatively, or in addition, the purge gas supply valve125can comprise a purge gas supply pump controllable by the control unit, and adapted to pump purge gas (fuel vapors) from canister122and/or fuel tank11into inlet duct4.

FIG. 9depicts a method according to the alternative embodiment of the invention, in the hybrid vehicle propulsion system1inFIG. 7. The method corresponds to the one that has been described above with reference toFIG. 3, with some exceptions.

As inFIG. 3, control unit109determines201that propulsion system1is to operate in an electric vehicle mode, and catalytic converter13temperature is monitored. If it is determined202that catalytic converter13temperature is below a first temperature threshold value, it is determined202awhether the contents of fuel in canister122is above a predetermined canister level threshold value. The contents of canister122can be determined as is known in the art, for example based on measured values of lambda, injected fuel and air flow. If it is determined202athat the contents of fuel in canister122is below the threshold value, it is determined that no or alternative converter activation maintenance are carried out.

If it is determined202athat the contents of fuel in canister122is above the threshold value, converter activation maintenance are taken203, in which the ignition remains inhibited, but motor3and transmission4is controlled so that crankshaft131is rotated by combined motor and generator3. Also, purge gas supply valve125is controlled so that fuel from canister122is allowed into inlet duct104so as to mix with air, allowed by the open throttle valve110. Thereby, by piston103movement(s), air and fuel are pumped through cylinder(s)103, and transported through exhaust duct106, in which a air/fuel mixture is provided, which reaches catalytic converter13where it is combusted to increase catalytic converter13temperature.

As inFIG. 3, while converter activation maintenance is undertaken, control unit109determines204whether catalytic converter13temperature is above a predetermined second temperature threshold value, which is higher than the first temperature threshold value. If it is determined that catalytic converter13temperature is above the second temperature threshold value, motor3and transmissions4,6are controlled205so that rotation of the crankshaft131stops.

Referring toFIG. 10, showing parts of a hybrid vehicle propulsion system1with a series hybrid structure, a further alternative embodiment of the invention will be described. Propulsion system1comprises an internal combustion engine2and an additional power source3in the form of an electric motor3. Engine2is mechanically connected to a generator5which is electrically connected to a charger8, which in turn is connected to a plurality of batteries9, and to a converter10, the latter being connected to the motor3, which is mechanically connected to two wheels7of the vehicle. Fuel tank11and an exhaust system12with an exhaust gas aftertreatment device13in the form of a catalytic converter13are provided for the engine2.

Propulsion system1comprises an air pump132and a fuel pump133adapted to supply air and fuel into exhaust system12upstream from catalytic converter13. The fuel pump is adapted to be fed from fuel tank11. Air pump132and the fuel pump are adapted to be driven by motor3via a mechanical connection, including for example belt drives, and also a clutch134, controllable by a control unit109for engagement and disengagement of pumps132,133with motor3.

FIG. 11depicts a method according to the further alternative embodiment of the invention, in the hybrid vehicle propulsion system1inFIG. 10. The method corresponds to the one that has been described above with reference toFIG. 3, with some exceptions.

Control unit109determines201that propulsion system1is to operate in a mode in which the engine is off. As inFIG. 3, catalytic converter13temperature is monitored by the control device109. If it is determined202that catalytic converter13temperature is below a first temperature threshold value, converter activation maintenance are taken203, in which clutch134is controlled203so as to engage pumps132,133. Thereby, air and fuel are introduced into exhaust system12to form a mixture, which reaches the catalytic converter13where it is combusted to increase catalytic converter13temperature.

As inFIG. 3, while the converter activation maintenance are undertaken, control unit109determines204whether catalytic converter13temperature is above a predetermined second temperature threshold value, which is higher than the first temperature threshold value, and if it is determined that catalytic converter13temperature is above the second temperature threshold value, clutch134is controlled205so as to disengage pumps132,133.

It should be noted that instead of motor3, air pump132and fuel pump133inFIG. 10could be driven by some alternative driving device of propulsion system1.

The invention is of course also applicable to hybrid vehicles combining more than two power sources for its propulsion, one of them being an internal combustion engine.

Above, embodiments with spark ignition internal combustion engines have been described, but it should be noted that the invention is applicable to hybrid vehicle propulsion systems with alternative types of internal combustion engines, for example diesel engines. For a diesel engine, converter activation maintenance could for example involve, while the crankshaft is being rotated by the additional power source, injecting fuel during an exhaust stroke in the respective cylinder, i.e. when the respective exhaust valve is open.