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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.

An expedient route to improving in-vehicle fuel economy in 4-stroke cycle engines is to reduce the swept volume of an engine and run it at a higher BMEP for any given output. The full-load performance of a larger capacity engine can be achieved through pressure charging. However, for maximum fuel economy, particularly at part-load, the expansion ratio, and consequently the compression ratio (CR) should be kept as high as possible. This is at odds with the requirement in pressure-charged gasoline engines to reduce the CR at higher loads due to the knock limit. In earlier work, the authors studied a pressure-charging system aimed at allowing a high CR to be maintained at all times. The operation of this type of system involves deliberately over-compressing the charge air, cooling it at the elevated pressure and temperature, and then expanding it down to the desired plenum pressure, ensuring a plenum temperature which can potentially become sub-atmospheric at full-load.