Background of the invention The advantages of operating an engine in a premixed charge controlled auto-ignition mode when it is operating under low and medium power conditions are well known and documented. They include even running, high combustion efficiency and very low NOx emissions. For these reasons, it is desirable to operate an engine over as large a range as possible in this mode but there are conditions imposed by the combustion process itself that preclude its use outside certain limits.

For example, CAI cannot be used at idling because the temperature of the residual gases is too low to trigger auto-ignition. At the other end of the scale, if the proportion of residual gases is too low, the temperature of the trapped charge will be reduced too much by the excessive mass of the fresh intake charge and once again will be too low to trigger auto-ignition. There are therefore upper and lower limits of the ratio of the masses of the residual gases and the fresh intake charge beyond which controlled auto-ignition cannot occur.

Object of the invention The present invention seeks to enable the operating range within which an engine can operate reliably with auto-

ignition to be extended, in particular to raise the upper power limit of the engine while operating in this mode.

Summary of the invention In accordance with the present invention, there is provided an internal combustion engine that is operable under low and medium power conditions with controlled auto- ignition triggered by retained residual gases and has an exhaust turbocharger, wherein, within the mass flow range encountered during controlled auto-ignition, the turbocharger is operative to increase the exhaust back- pressure at the same time as increasing the intake charge pressure sufficiently to maintain a mass ratio of residual gases to fresh intake charge that will support controlled auto-ignition, the overall mass of the trapped charge being thereby increased to enable the auto-ignition mode to be sustained automatically across a wider engine power range.

While it would be possible to use an exhaust throttle to raise the exhaust back-pressure and any form of supercharger to boost the intake pressure such a solution would not be ideal. An exhaust throttle is wasteful of power, while a supercharger (no matter how it is driven) will act as a power drain on the engine. Furthermore, the two separate devices would need to be controlled in synchronism in order to maintain the desired mass ratio of residual gases to fresh intake charge.

Instead, the invention uses a dedicated exhaust-gas powered turbocharger that provides the desired changes in the exhaust and intake pressures automatically while operating in the controlled auto-ignition mass flow range.

Such a turbocharger would not, without adjustment or alteration, be suitable for increasing maximum engine output power when not operating in controlled auto-ignition mode as it would throttle the intake system unduly and the turbine

of the charger would be driven at speeds beyond its safe limits.

It is important therefore to provide means for reducing the turbine speed when it is not acting to extend the upper power range of the controlled auto-ignition mode.

One way of achieving this is to bypass the turbocharger when the engine is not operating in the controlled auto- ignition mode.

It is however preferred instead to use a turbocharger having a variable geometry. In this case, the turbocharger can be operated with a high exhaust back pressure and a high intake charge pressure when the engine is operating at low and medium power in the controlled auto-ignition mode, and with a lower back pressure and a higher intake charge flow when the engine is operating at high power with conventional charge ignition. Thus by using a variable geometry turbocharger, the high power advantages of having a conventional turbocharger when operating with conventional ignition are combined with improved economy, smooth running and reduced NOx emissions when operating within the extended power range achievable with the aid of the present invention in the controlled auto-ignition mode.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an engine having an exhaust turbocharger with a wastegate, and Figure 2 is a schematic representation of a variable geometry turbocharger.

Detailed description of the preferred embodiments Figure 1 shows an engine 10 having an intake manifold 12 and a main butterfly throttle 14. The engine has an exhaust manifold 15 which leads by way of an exhaust turbocharger 16 to the exhaust after-treatment system represented simply by an arrow 17. The exhaust gas flow drives a turbine blade 30 which is connected by way of a shaft 34 to an impeller 32 that acts to pressurise the air upstream of the main throttle 14. Ambient air is drawn in through the usual filter represented by an arrow 13 and flows through a pipe 18 to the compressor side of the turbocharger 16. The air pressurised by the impeller 32 flows by way of a conduit 20 to an intercooler 44 which reduces the temperature of the intake air before it is fed to intake manifold 12.

As above described the turbocharger and the engine are conventional. Such a configuration would normally be used with standard valve and ignition timings to increase the power output of the engine by increasing the mass of charge trapped in the combustion chamber of the engine operating at high load. As there is a danger of over-pressurising the intake air, a so-called wastegate is used to limit the boost pressure. The wastegate comprises a passage 38 that bypasses the turbine 30 and allows the exhaust gases to flow directly into the after-treatment system 17. The bypass passage 38 is opened and closed by means of a valve 40 which is in turn connected to an actuator 42 responsive to the boost pressure. When the boost pressure reaches its permissible maximum, the valve 40 is opened by the actuator 42, so that the mass of exhaust gases flowing over the turbine 30 is reduced.

The present invention differs from a conventional turbocharged engine in that the turbocharger is brought into operation under low and medium power operating conditions.

Furthermore, the turbocharging is used not for the purpose of increasing engine maximum output power but to extend the range in which the engine can operate with controlled auto- ignition. In conventional engines, where the turbocharger is designed to operate to boost output power, the turbocharging effect at low mass flow is negligible. Hence, it is well known that turbocharged engines derive little benefit from the presence of the turbocharger at the bottom end of the power range and there is a sudden surge of power as the turbocharger kicks in during the top end of the power range.

In the present invention, the turbocharging is used during the controlled auto-ignition mode of operation when a high proportion of internal exhaust gas recirculation is relied upon to trigger auto-ignition of the fresh charge.

Such a high proportion of retained residual gases is preferably achieved by very early closing of the exhaust valve during the exhaust stroke.

Because a large proportion of the residual gases is not discharged from the combustion chamber, the mass flow in the exhaust system during the controlled auto-ignition mode of operation of the engine is always low and would not be sufficient to drive a conventional turbocharger to its full speed. It is therefore necessary for the turbine 30 of the turbocharger 16 in the present invention to be designed such that it reaches its maximum speed during relatively low mass flows in the exhaust system. The impeller 32 must also be designed to achieve maximum boost for the reduced intake air flow when it is driven at this speed by the turbine. The effect of the turbocharger is to increase the back pressure and the boost pressure in proportion to one another and thereby allow a greater mass of combined charge to be admitted into the combustion chamber without interfering with the auto-ignition process. In other words, the power range within which controlled auto-ignition remains possible is extended.

The fact that the turbocharger must be of special design to achieve the desired results during controlled auto-ignition means that when the engine reverts to conventional valve and ignition timings, the mass flow in the exhaust system would risk an over-speed of the turbine and at the same time the impeller 32 would not be capable of building up the intake air pressure sufficiently when the mass flow demand into the engine is increased. Hence, the same turbocharger cannot be used without modification outside the controlled auto-ignition range.

The invention offers two possible solutions to this problem. The first solution is to disable the turbocharger 16 when the engine is not operating in the controlled auto- ignition mode. The second solution is to use a variable geometry turbocharger of which the performance may be modified in dependence upon the prevailing operating conditions.

The first solution may use a variant of the arrangement shown in Figure 1. The turbine is designed to reach maximum speed with low mass flows and the maximum speed is limited by the use of a modified wastegate. In controlled auto- ignition mode, the wastegate will not be open but there will be no danger of the safe speed of the turbocharger being exceeded on account of the low mass flow in the exhaust system. When the engine is operating outside the auto- ignition range, however, the wastegate (controlled by a speed sensor rather than a boost pressure sensor) will open to bypass the turbine 30 so that the turbocharger is safeguarded from damage. At this time, the impeller 32 of the turbocharger 16 will not be able to sustain a significant boost to the intake pressure to meet the increased demand in air flow from the engine. Should it be required to increase the maximum output engine power, then a second differently designed turbocharger will be required to act in series with the turbocharger 16 that is illustrated.

The second solution, which uses a variable geometry turbocharger, is shown schematically in Figure 2. Here, the turbine side of the turbocharger 16 is shown with a deflector that can be moved between the position 50a and the position 50b to divert the gas flow and thereby allow the turbine to reach its maximum speed at different values of mass flow in the exhaust system, depending on the operating conditions of the engine. The geometry of the impeller side (not shown) of the turbocharger may also be variable to adapt the delivery rate in dependence upon the engine operating conditions so as always to be able to achieve the desired boost pressure in the intake system, regardless of mass flow.