Efficient downsizing engine technologies for real world driving

Abstract

Increasing demands for ever more fuel efficient and environmentally friendly vehicles, deriving from both political and social motivation, have made the fuel optimisation of the whole powertrain the main focus of development programmes.

One of the main outcomes of this drive has been the trend for the downsizing of IC engines, where the specific power output has increased to enable smaller displacement and number of cylinder power units to be used. This offers significant de-throttling benefits at part load, together with the potential for reductions in specific friction, which derive in the most part from reductions in the number of cylinders used, and consequently the number of bearing surfaces required.

However, in order to achieve increased specific power output levels, IC engines generally require boosting, which leads to a potential reduction in operating efficiency at high load due to increased tendency to knocking, and the need to overfuel at high power levels to keep exhaust gas temperatures within material specifications for exhaust gas components. The increased tendency to knock not only forces the combustion phasing to be retarded, but also requires lowered compression ratios to be employed, in order to maintain high load combustion quality. This however also has a negative impact on part-load operating efficiency.

The result of these effects is that the benefits of downsizing diminish as specific power outputs increase, and the optimum level of downsizing depends to a large extent on driving style and the types of roads (and thereby the average speeds and acceleration / deceleration rates) on which the vehicle is being used.

In order to take downsizing further and gain the full potential of the fuel consumption benefits, as well as to maintain these benefits over a wide range of applications, a selection of advanced complementary technologies have been investigated.