GB2400138A

GB2400138A - Cooling an i.c. engine turbocharger using an electrically driven supercharger

Info

Publication number: GB2400138A
Application number: GB0307480A
Authority: GB
Inventors: William Marc Edward Brotherton
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2003-04-01
Filing date: 2003-04-01
Publication date: 2004-10-06
Anticipated expiration: 2023-04-01
Also published as: GB0307480D0; GB2400138B

Abstract

The invention relates to cooling the turbocharger after the engine has been shut off. The engine (202) has a two-stage air charge boosting system including an exhaust gas driven turbocharger (220) for compressing inlet air, an electrical supply system (230), an air passageway between (265) the air inlet system (205) and the exhaust gas system (211), and a controller (240) for controlling the operation of the air charge boosting system (218,220). The turbocharger (220) has an exhaust gas driven impeller (226). The air charge boosting system includes an electrically driven supercharger (18). The controller (240) is arranged when the engine (202) is stopped to operate the supercharger (218) to force air (260) through the air passageway (265) from the air inlet system (205) into the exhaust gas system (211) in order to cool the turbocharger impeller (226). The air passageway (265) is preferably provided by an exhaust gas recirculation system (80).

Description

24001 38 Internal Combustion Engine with Turbocharger The present

invention relates to an internal combustion engine having an exhaust gas driven turbocharger, and to a method and apparatus for cooling the turbocharger after the engine has been shut off.

Motor vehicle exhaust gas driven turbochargers are used to boost an inlet air charge and hence increase engine power and torque, particularly at medium and high engine speeds Such turbochargers have an impeller situated in the exhaust gas stream and a compressor in an air inlet stream. The impeller will normally have a number of vanes or blades, which may be of fixed geometry. In order to limit the engine power to meet a desired driver demand, the turbocharger may have an exhaust gas bypass with a control valve that allows exhaust gas to bypass at least partially the turbocharger when the driver demand is met. Additionally, or alternatively, variable geometry vanes or blades may be used to control the amount of boost delivered by the turbocharger to the air inlet stream.

In all such devices, the compressor may rotate at maximum speeds of up to about 70,000 to 250,000 rpm. This places considerable demands on bearings on which the impeller and compressor rotate, both in terms of the bearing materials and lubricants. Bearings can become damaged or degraded over time owing the heat of the exhaust gases. Bearings can also be damaged over time owing to an accumulation of unburned hydrocarbons or other contaminants. Once a bearing starts to become damaged, frictional losses can cause the bearing to generate significant amounts heat, and this in addition to the heat absorbed from the exhaust gas stream can accelerate 2 - bearing damage.

One situation in which the heating of a turbocharger and its bearings can be severe is just after an engine has stopped, particularly if the turbocharger has recently been in use.

The traditional approach to dealing with this problem is to allow the engine to idle for a short while before switching the engine off. This causes cooler exhaust gasses to flow past the impeller while a continuing flow of cool inlet air flows past the compressor, either or both of which can help draw some of the residual heat from the turbocharger bearings. This, however, is inconvenient, as the driver may have to sit in the motor vehicle for some time while the engine is idling, prior to stopping the engine. This also results in a loss in average fuel economy owing the time spent idling the engine.

It has therefore been proposed to link the turbocharger into the engine cooling system. Then, engine coolant can be pumped under the action of an electric pump through the turbocharger after the engine is stopped. This however, adds mechanical cost and complexity to the engine cooling system.

It is an object of the present invention to address these issues.

Accordingly, the invention provides an internal combustion engine, comprising at least one combustion chamber, an air inlet system arranged to supply air to the combustion chamber(s), an exhaust gas system arranged to vent exhaust gas from the combustion chamber(s), an air charge boosting system including an exhaust gas driven turbocharger for compressing air to the combustion chamber(s), an electrical supply system, an air passageway between the air inlet system and the exhaust gas system, and a controller for controlling the operation of the air charge boosting system, wherein: - the turbocharger has in the exhaust gas system an exhaust gas driven impeller and has in the air inlet system a first air compressor that is driven by the impeller; - the air charge boosting system includes an electrically driven supercharger having an electric motor powered by the electrical supply system and in the air inlet system a second air compressor that is driven by the electric motor; and - the controller is arranged when the engine is stopped to operate the electric motor of the supercharger to force air through the air passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger.

The supercharger therefore serves two functions. When the engine speed is low, the turbocharger boost will be correspondingly slow. Because the supercharger is electrically driven, the supercharger can therefore be used to boost the air charge and hence engine power and torque at such low engine speeds. However, because such superchargers can use a significant amount of electrical power, for example 2 kW - 4 kW, and can therefore impose a significant drain on the vehicle electrical system, particularly a vehicle battery, the turbocharger will normally be used to provide power and torque boost in preference to the supercharger. The turbocharger could therefore potentially overheat when the engine is switched off. When this is the case, the - 4 supercharger can then be run to force air through the air passageway and into the exhaust gas system in order to help cool the turbocharger impeller and hence the turbocharger bearings.

The air passageway when being used to provide cooling air to the turbocharger impeller preferably has an inlet downstream of the second air compressor and an outlet upstream of the turbocharger impeller, relative to the direction of flow of inlet air and combustion gasses when the engine is in operation.

Therefore, air drawn by the supercharger can also be arranged to flow over the turbocharger compressor, which will provide an additional cooling effect to the turbocharger. The amount of air flow needed to create a significant cooling effect is significantly less than that during supercharged operation of the engine, and so the amount of electrical power drawn by the supercharger electric motor is correspondingly less. The supercharger can therefore be run for an extended time, for example between 1 and 5 minutes, after the engine is shut off, to provide a significant cooling effect for the turbocharger.

The engine will normally be a reciprocating piston internal combustion engine.

In one embodiment of the invention, the or each combustion chamber has at least one pair of valves including an inlet valve for admitting air into the combustion chamber and at least one outlet valve for venting the combustion chamber of exhaust gasses. Each of the pair of valves is then movable - 5 between closed and open orientations. The valves have an orientation when the engine is stopped in which both inlet and outlet valves are at least partially open, thereby providing said air passageway between the air inlet system and the exhaust gas system. This provides the benefit of not having to provide a separate air passageway, but using an air passageway which may be naturally present owing to the valve operation.

Optionally, the engine comprises means for controlling the rate at which the engine comes to a stop in order to ensure that there is at least one said pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop.

Preferably, there are a plurality of combustion chambers, each with at least one said pair of valves, the arrangement of combustion chambers and associated valves being such that when an engine comes to a stop there is always at least one such pair of pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop.

If this is not the case, however, then it may be necessary to use other means to ensure that the engine comes to a stop at an engine angle which will ensure that there is an air passageway through the engine combustion chamber(s). For example, the engine may be a reciprocating piston engine having a starter motor, preferably an integrated starter generator. Means may then be provided by which the starter motor can turn the engine to ensure that there is at least one said pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop. The 6 - turning may be done after the engine has come fully to a stop, depending on the engine angle, which may be monitored by an engine control unit (ECU) that receives an engine speed/angle signal from a suitable engine speed/angle sensor.

In an alternative embodiment of the invention, the air passageway between the air inlet system and the exhaust gas system is separate from the combustion chamber(s). The air passageway then includes a valve operable when the engine is stopped to permit said flow of air through the passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger impeller. The valve may be an actively operable valve, for example, moving under the control of a controller such as an ECU. Alternatively, the valve may be a passively operable valve, for example a one-way flap valve.

It is particularly advantageous if, when the engine comprises an exhaust gas recirculation (EGR) system for recirculating exhaust gas into the combustion chamber(s), the EGR system provides at least partially the air passageway between the air inlet system and the exhaust gas system. For example, the EGR system may link the air inlet system downstream of the second compressor to the exhaust gas system upstream of the impeller. During normal engine operation, the EGR system serves to recirculate exhaust gasses for improved emissions performance. Then, after the engine is stopped, the EGR system can be used to help cool the turbocharger. This arrangement therefore requires minimal additional hardware or control software, and makes further use of a supercharger which may be provided mainly to boost engine performance.

A further advantage of this system is that this can help to keep an EGR system clear of soot build-ups. A reverse flow of air will tend to blow any soot back into the exhaust gas system. Because the exhaust gas system, and any catalytic converter downstream in the exhaust gas system, will still be hot during the period when the reverse flow is helping to cool the turbocharger, such dislodged soot can be cleanly oxidised in the oxygen-rich environment provided by the cooling air flow.

In a preferred embodiment of the invention, the EGR may comprise a control valve operable to control the recirculation of exhaust gas from the exhaust gas system to the air inlet system. The control valve is then also operable to control the flow of air from the second air compressor to cool the turbocharger impeller. Such use of a common component to perform two distinct functions helps to reduce the materials and manufacturing cost of the system.

Similarly, the controller may control both the operation of the second compressor and the control valve to control the flow of air from the second air compressor to cool the turbocharger impeller.

The controller may be integrated within an engine control unit (ECU) and may, in principle be either a unitary controller, or distributed amongst a plurality of distinct controllers.

The engine may comprise a turbocharger temperature sensor connected to the controller for monitoring the temperature of the turbocharger. The controller is then arranged to operate - 8 - the electric motor of the supercharger to force air through the air passageway from the air inlet system into the exhaust gas system when the monitored temperature is above a first predetermined level, and to cease said operation of the electric motor when said monitored temperature is below a second predetermined level. The second predetermined level may be below the first predetermined level.

Additionally or alternatively, the controller may be arranged to monitor the operation of the engine and the turbocharger and to estimate therefrom the temperature of the turbocharger and to operate the electric motor of the supercharger in response to the level of the estimated temperature to cool the turbocharger. The estimation is preferably based on a rolling record of engine and turbocharger operation.

Also according to the invention, there is provided a method of operating an internal combustion engine, said engine comprising: at least one combustion chamber; an air inlet system; a fuel supply system; an exhaust gas system; an air charge boosting system including a turbocharger having a linked impeller and a first compressor, and a supercharger having a linked electric motor and a second compressor; an electrical supply system; and an air passageway between the air inlet system and the exhaust gas system; the method comprising the steps of: a) supplying to the combustion chamber(s) inlet air through the air inlet system and fuel from the fuel supply system, and combusting said supplied inlet air and fuel inside the combustion chamber(s) in order to run the engine; - 9 - b) venting from the combustion chamber(s) into the exhaust gas system exhaust gasses from said combustion of the air and fuel; c) using the exhaust gasses from the engine to power the impeller, and hence power the linked first compressor to compress said inlet air; d) ceasing running of the engine, and then using the electrical supply system to operate the electric motor and hence power the linked second compressor to compress downstream air in said air inlet system; e) allowing said compressed downstream air to flow through the air passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger.

The turbocharger may comprise an exhaust gas bypass conduit which bypasses the impeller, and a bypass valve which can be used to control the turbocharger speed by controlling the flow of exhaust gases through the exhaust gas bypass. Such a bypass valve can therefore also be used advantageously to control the cooling of the turbocharger impeller.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically a part of a motor vehicle with an internal combustion engine according a first embodiment of the invention, having a two-stage air compression system in which an electrically driven - 10 supercharger forces air through partially open engine cylinder valves to cool a turbocharger when the engine is stopped; Figure 2 shows schematically a part of a motor vehicle with an internal combustion engine according a second embodiment of the invention, similar to the first embodiment, but having a dedicated air passageway for providing the cooling air flow to the turbocharger) Figure 3 is shows schematically a part of a motor vehicle with an internal combustion engine according a third embodiment of the invention, similar to the second embodiment, but having an exhaust gas recirculation (EGR) system which also provides an air passageway for providing the cooling air flow to the turbocharger; and Figure 4 shows a flow chart illustrating a method according to the invention of using a two-stage air compression system in which an electrically driven supercharger forces air through partially an air passageway to cool a turbocharger when the engine is stopped.

Figure 1 shows a motor vehicle 1, having an internal combustion engine 2, which may be either a compression- ignition or a spark-ignition type engine, according to a first embodiment of the invention. The engine 2 has four in-line combustion chambers, marked I, II, III, and IV, each of which is formed from a cylinder with a reciprocating piston (not shown). An air inlet system 5 is supplies inlet air 8 to the combustion chambers 4 via an inlet manifold 10. - 11

An exhaust gas system 11 is arranged to vent exhaust gases from the combustion chambers via an exhaust manifold 14.

The engine 1 has a two-stage air charge boosting system including an electrically driven supercharger 18 and an exhaust gas driven turbocharger 20. The turbocharger 20 and supercharger 18 have respectively a first rotary compressor 21 and a second rotary compressor 22 for compressing the inlet air 8 to the combustion chambers 4. The first and second compressors 21,22 are in series in a main air inlet conduit 3, with the supercharger compressor 22 being upstream of the turbocharger compressor 21. A mass air flow sensor (MAF) 55 lies upstream of the supercharger compressor 22, and monitors the amount of uncompressed air 7 drawn in by the air inlet system 5.

The engine 1 is supplied with fuel 19, which may be diesel or gasoline fuel, in a conventional manner, for example by means of direct fuel injectors 12, one for each of the combustion chambers 4. Each combustion chamber 4 has one or more inlet valves 27 at the downstream end of the air inlet manifold 10, and one or more outlet valves 29 at the upstream end of the exhaust manifold 14. Not shown, for clarity, is a spark ignition system, which may be used in the case of a gasoline engine. The operation of the fuel injectors 12 and valves 27,29 to create combustion events is conventional, and will therefore not be further described.

The turbocharger 20 has a rotary turbine impeller 26 that lies in a main exhaust conduit 13. The impeller 26 is rotationally linked by a shaft 17 to the turbocharger compressor 21. The shaft 17, compressor 21 and impeller 26 - 12 are rotationally supported by bearings 23. The bearings 23 are preferably sealed ball type bearings.

When exhaust gasses 16' flow past the impeller 26, the impeller is forced to rotate at speeds of up to 250k rpm, and this in turn rotates the turbocharger compressor 21 at the same speed, thereby compressing air 24, and permitting the engine power and torque to be boosted. In the compression process, the temperature of the compressed air 24 is elevated.

Downstream of the turbocharger 20 is an intercooler 25 which reduces the temperature of the hot compressed air 24 immediately downstream of the turbocharger compressor 21 in order to increase the density of inlet air 8 in the inlet manifold 10.

The turbocharger 20 operates in the conventional manner, by boosting the amount of air supplied to the cylinders 4. Such turbocharging systems become effective above a certain minimum engine rpm, for example 1,500 to 2,000 rpm, once the flow of exhaust gas 16 has become sufficient to drive the impeller 26. This results in the effect known as "turbo-lag" in which engine torque is relatively unresponsive to driver demand until the engine has reached the minimum rpm for significant turbo boost.

The engine 2 is therefore provided with the electrically- driven supercharger 18, powered by an electric motor (M) 29 which is active only intermittently at lower engine speeds in order to increase engine torque according to driver demand.

At full power, the supercharger rotary compressor 22 may be - 13 operating at up to 250k rpm. The supercharger 18 does, however, draw a significant amount of current 28 from a vehicle electrical system comprising a battery 30 and an integrated starter generator (ISG) 31, for example up to 300 Amps. This exceeds the amount that a typical vehicle electrical system 30,31 can be expected to supply continuously. Therefore, once the engine speed has reached a point where the turbocharger 20 can compress the inlet air 8 to meet driver demand, the supercharger 18 is deactivated.

In this example, both the supercharger 18 and turbocharger 20 are provided with air bypasses 32,34. The supercharger bypass 32 includes a bypass valve 36 for controlling the air flow through and past the supercharger 18. Similarly, the turbocharger 20 includes a wastegate valve 38 so that air can bypass the impeller 26 when the turbocharger 20 is not in use, and to limit the pressures and turbocharger speed when the turbocharger 20 is in full use.

It should be noted however, that if the engine is a diesel engine, then the turbocharger may be fitted with Variable Geometry Turbines (VGT) that have variable flow/pressure characteristics to suit all required operating conditions. A wastegate bypass is not required with such VGT devices.

Engine operation is controlled by an engine control unit (ECU) 40. The ECU 40 receives a number of input signals indicative of engine operation, such as an engine speed/angle signal (RPM) 41 from an engine speed/angle sensor 51, an engine temperature signal (ET) 42 from an engine temperature sensor 52, a turbocharger temperature signal (TT) 43 from a turbocharger temperature sensor 53, a turbocharger speed - 14 signal (TC) 44 from a turbocharger speed sensor 54, and a mass airflow signal (AF) 45 from the mass airflow sensor (MAF) 55.

The ECU 40 generates under software control a number of output signals for controlling engine operation, including a fuel injection signal (EFI) 46 for scheduling the fuel injectors 12, a supercharger speed signal (SC) 47 for controlling the speed of the supercharger motor 29, and an ISG signal 48 for controlling the engine start operation of the integrated starter generator 31.

When the turbocharger 20 has been used for an extended time immediately followed by engine shut off, then the turbocharger bearings 23 can become excessively heated. Such a pattern of usage is monitored by the ECU 40, together with the turbocharger temperature (TC) 41. If the turbocharger temperature exceeds a first predefined temperature limit of C following engine shut down, then the ECU sends a signal 47 to the supercharger motor 29 to operate the rotary compressor 22 at up to 20k rpm. This moderately compresses the inlet air 8.

The engine will normally come to a stop with one pair of inlet and outlet valves 27,29 partially open, as drawn schematically for the combustion chamber 4 marked I. This permits an air flow 60 through an air passageway 65 between the air inlet system 5 and the exhaust gas system 11 formed by the combustion chamber I. The ECU 40 monitors the air flow 60 from the mass air flow signal 45, and if this is insufficient to provide adequate 15 cooling, it may be that one or the other of the valves 27,29 is nearly closed. The ECU 40 therefore uses the ISG signal line 48 to command the integrated starter generator 31 to rotate the engine angle slightly, for example up to 20 in either direction, until an increased inlet airflow 7 is detected from the mass airflow sensor 55. When the air flow has thus been increased, the ECU 40 continues to maintain the engine angle by appropriate control of the integrated starter generator 31.

The ECU 40 also acts to close the wastegate valve 38 so that all of the cooling air flow 60 is directed towards the impeller 26 in order to provide a maximum cooling effect for the impeller side of the turbocharger 20. At the same time, there will be some cooling of the compressor side of the turbocharger 20 owing to the airflow 60 past the turbocharger compressor 21. The supercharger 18 may need to be operated in this manner for between 1 and 5 minutes, until the monitored turbocharger temperature (TC) 44 has dropped to a second predefined temperature limit of 150 C. When this happens, the ECU 40 shuts down the operation of the supercharger 18.

Reference now is made to Figure 2, which shows a second embodiment of an internal combustion engine 102 according to the invention, in which components similar to those of the first embodiment are illustrated with corresponding reference numerals incremented by 100.

The second embodiment 102 differs from the first embodiment in having a dedicated air passageway 165 for providing cooling airflow to the turbocharger 120. The air passageway has an upstream end 165' joined to the inlet manifold À 16 110, and a downstream end 165" joined to the exhaust manifold 114. A control valve 59 receives a control valve signal (V) 49 from the ECU 140, which causes the control valve 59 either to open or close. Normally, the control valve 59 is closed, and there is no air flow through the air passageway 165.

When, however, the ECU 140 acts to cool the turbocharger 120, the supercharger 118 is first spun up under the control 147 of the ECU 140 to between lOk rpm and 20k rpm. The ECU 140 then sends a signal 49 to open the control valve 59, allowing a cooling air flow 160 to pass through the air passageway 165, and so cool the turbocharger 120 in a similar manner to that described above.

The third embodiment 202 differs from the second embodiment 102 in having an air passageway 265 for providing cooling airflow to the turbocharger 120 which is shared with an exhaust gas recirculation (EGR) system 80. In normal operation of the engine 202, the EGR system 80 allows a proportion of the exhaust gasses 216 to be recirculated 85 from the exhaust manifold 214 back into the inlet manifold 210, in order to improve the emissions performance of the engine 202. The flow of the recirculated exhaust gasses 85 is controlled by means of a control valve 88, which opens and closes in response to a control signal (EGR/V) 50 from the ECU 240 When the engine 202 is shut off, the ECU 240 can then activate the supercharger 218 and open the control valve 88 in order to generate a cooling flow of air 260 through the air passageway 265 from the inlet manifold 210 into the exhaust manifold 214, in order to cool the turbocharger 220 in a similar manner to that described above. In particular, - 17 the turbocharger bypass valve 238 is closed to maximise the rate of the cooling of the turbocharger impeller 226.

The third embodiment provides significant advantages, particularly by making use of an EGR system 80 that may already be present in a motor vehicle engine. This arrangement also helps to keep the EGR system 80 free from potentially clogging soot, by blowing any such soot back into the exhaust system 211, where it will be oxidised in an oxygen-rich environment by the residual heat in the exhaust manifold 214, exhaust conduit 213 or downstream catalytic converter (not shown).

The EGR system 80 links the exhaust gas system 211 at a point upstream of the turbocharger impeller 226 to the air inlet system 210 downstream of the turbocharger compressor 221.

Therefore, in all of the illustrated embodiments 2,102,202, the air passageway 65,165,265 joins the air supply system downstream of the turbocharger compressor 21,121,221. Cooling is therefore provided on both sides of the turbocharger 20,120,220.

Figure 4 shows a flow chart 90 illustrating a method according to the invention of using a two-stage air compression system in which an electrically driven supercharger forces air through an air passageway to cool a turbocharger when the engine is stopped. First, air and fuel are burnt 91 in a reciprocating piston engine, and exhaust gasses are vented 92 into an exhaust system. When it is necessary to boost engine power or torque, the exhaust gasses are used 93 to drive a turbocharger in order to compress inlet air. During normal operation of the engine, an EGR - 18 system is used 94 to recirculate a proportion of exhaust gasses back into the air inlet system in order to improve engine emissions. After the engine is turned off 95, an engine control unit (ECU) continues to monitor 96 turbo charger temperature, and if the temperature is not inside limits 97, then the ECU runs the supercharger and the EGR system to provide 98 a cooling air flow to both sides of the turbocharger in the air inlet system and in the exhaust gas system. The ECU then continues to monitor 96 the turbocharger temperature, and when this has dropped within limits 97, the ECU ceases operation 99 of the supercharger and ECU.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.

It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention, as defined by the appended claims. For example, it is not necessary that the supercharger and turbocharger be in series, as a parallel arrangement would still result in a cooling air flow past the turbocharger impeller. - 19

Claims

Claims 1. An internal combustion engine, comprising at least one

combustion chamber, an air inlet system arranged to supply air to the combustion chamber(s), an exhaust gas system arranged to vent exhaust gas from the combustion chamber(s), an air charge boosting system including an exhaust gas driven turbocharger for compressing air to the combustion chamber(s), an electrical supply system, an air passageway between the air inlet system and the exhaust gas system, and a controller for controlling the operation of the air charge boosting system, wherein: - the turbocharger has in the exhaust gas system an exhaust gas driven impeller and has in the air inlet system a first air compressor that is driven by the impeller; - the air charge boosting system includes an electrically driven supercharger having an electric motor powered by the electrical supply system and in the air inlet system a second air compressor that is driven by the electric motor; and; - the controller is arranged when the engine is stopped to operate the electric motor of the supercharger to force air through the air passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger.
2. An internal combustion engine as claimed in Claim 1, in which the or each combustion chamber has at least one pair of valves including an inlet valve for admitting air into the combustion chamber and at least one outlet valve for venting the combustion chamber of exhaust gasses, each of said valves being movable between closed and open orientations, wherein the valves have an orientation when the engine is stopped in which both inlet and outlet valves are at least partially open, thereby providing said air passageway between the air inlet system and the exhaust gas system.
3. An internal combustion engine as claimed in Claim 2, in which the engine comprises means for controlling the rate at which the engine comes to a stop in order to ensure that there is at least one of said pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop.
4. An internal combustion engine as claimed in Claim 2, in which there are a plurality of combustion chambers, each with at least one of said pair of valves, the arrangement of combustion chambers and associated valves being such that when an engine comes to a stop there is always at least one such pair of pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop.
5. An internal combustion engine as claimed in Claim 2, in which the engine is a reciprocating piston engine and comprises a starter motor and means by which the starter motor can turn the engine to ensure that there is at least one of said pair of inlet and outlet valves which are both at least partially open when the engine comes to a stop.
6. An internal combustion engine as claimed in Claim 1, in which the air passageway between the air inlet system and the exhaust gas system is separate from the combustion chamber(s), said air passageway including a valve operable - 21 when the engine is stopped to permit said flow of air through the passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger.
7. An internal combustion chamber as claimed in Claim 6, in which said valve is an actively operable valve.
8. An internal combustion chamber as claimed in Claim 6, in which said valve is a passively operable valve.
9. An internal combustion engine as claimed in Claim 1, comprising an exhaust gas recirculation (EGR) system for recirculating exhaust gas into the combustion chamber(s), wherein the EGR system links the air inlet system downstream of the second compressor to the exhaust gas system upstream of the impeller, the EGR system providing at least partially the air passageway between the air inlet system and the exhaust gas system.
10. An internal combustion engine as claimed in Claim 9, in which the EGR comprises a control valve operable to control the recirculation of exhaust gas from the exhaust gas system to the air inlet system, said control valve also being operable to control the flow of air from the second air compressor to cool the turbocharger impeller.
11. An internal combustion engine as claimed in Claim 10, in which the controller controls both the operation of the second compressor and the control valve to control the flow of air from the second air compressor to cool the turbocharger impeller. - 22
12. An internal combustion engine as claimed in any preceding claim, in which the first compressor is in series with the second compressor.
13. An internal combustion engine as claimed in Claim 12, in which the second compressor is upstream of the first compressor.
14. An internal combustion engine as claimed in Claim 12 or Claim 13, in which the air passageway joins the air supply system downstream of the first compressor.
15. An internal combustion engine as claimed in any preceding claim, comprising a turbocharger temperature sensor, said temperature sensor being connected to the controller for monitoring the temperature of the turbocharger, the controller being arranged to operate the electric motor of the supercharger to force air through the air passageway from the air inlet system into the exhaust gas system when said monitored temperature is above a first predetermined level, and to cease said operation of the electric motor when said monitored temperature is below a second predetermined level.
16. An internal combustion engine as claimed in Claim 15, in which said second predetermined level is below said first predetermined level.
17. An internal combustion engine as claimed in any preceding claim, in which the controller is arranged to monitor the operation of the engine and the turbocharger and to estimate therefrom the temperature of the turbocharger and - 23 to operate the electric motor of the supercharger in response to the level of the estimated temperature to cool the turbocharger.
18. A method of operating an internal combustion engine, said engine comprising: at least one combustion chamber; an air inlet system; a fuel supply system; an exhaust gas system; an air charge boosting system including a turbocharger having a linked impeller and a first compressor, and a supercharger having a linked electric motor and a second compressor; an electrical supply system; and an air passageway between the air inlet system and the exhaust gas system; the method comprising the steps of: a) supplying to the combustion chamber(s) inlet air through the air inlet system and fuel from the fuel supply system, and combusting said supplied inlet air and fuel inside the combustion chamber(s) in order to run the engine; b) venting from the combustion chamber(s) into the exhaust gas system exhaust gasses from said combustion of the air and fuel; c) using the exhaust gasses from the engine to power the impeller, and hence power the linked first compressor to compress said inlet air; d) ceasing running of the engine, and then using the electrical supply system to operate the electric motor and hence power the linked second compressor to compress downstream air in said air inlet system; - 24 e) allowing said compressed downstream air to flow through the air passageway from the air inlet system into the exhaust gas system in order to cool the turbocharger.
19. A method as claimed in Claim 18, in which the turbocharger comprises an exhaust gas bypass conduit which bypasses the impeller, and a bypass valve, comprising the steps of: f) in step c) controlling turbocharger speed by using the bypass valve to control the flow of exhaust gases through the exhaust gas bypass; and g) in step e) using the bypass valve to control the cooling of the turbocharger impeller.
20. A method as claimed in Claim 19, in which in step g) the bypass valve is closed to maximise the rate of the cooling of the turbocharger impeller.
21. An internal combustion engine, substantially as herein described, with reference to Figures 1-4 or as shown in Figures 1, 2 or 3 of the accompanying drawings.
22. A method of operating an internal combustion engine, substantially as herein described, with reference to Figures 1-4 or as shown in Figure 4 of the accompanying drawings.