Performance and Fuel Economy Enhancement of Pressure Charged SI Engines through Turboexpansion - An Initial Study

One of the most expedient routes to improving in-vehicle fuel economy is to reduce the swept volume of an engine and run it at a higher BMEP for any given output. This can be achieved through pressure charging. However, for maximum fuel economy, particularly at part-load, the compression ratio (CR) should be kept as high as possible. This is at odds with the requirement in pressure charged engines to reduce the CR at higher loads due to the knock limit. Lotus has studied a pressure charging system which will allow a high compression ratio to be maintained at all times. This is achieved by deliberately over compressing the charge air, intercooling it at the resulting elevated pressure, and then expanding it, via a turbine, to the desired plenum boost pressure, ensuring a plenum temperature which can potentially become sub-ambient at full-load. Due to its high CR, it is possible that an engine fitted with such a charging system could therefore achieve fuel consumption equal to or better than a Variable Compression Ratio unit, while permitting conventional engine architecture and assembly lines. This paper analyses the likely performance of an engine fitted with such a charging system, and presents details of rig tests and results of performance modelling conducted to investigate one potential layout, and discusses the 'Nomad' engine project, which is intended, amongst other things, to prove the ability of a turboexpansion system to facilitate aggressive engine downsizing.