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. Due to the ability to support a high fixed CR, it is possible that an engine fitted with such a charging system could achieve fuel economy equal to or better than a Variable Compression Ratio unit, while permitting conventional engine architecture and assembly lines. The concept has been termed “Turboexpansion”.
This paper analyses some initial test bed results taken from an engine fitted with such a charging system, and discusses future development work intended to establish further the validity of the concept and hence its feasibility as a tool to facilitate aggressive engine downsizing. Additional theoretical analysis is presented which indicates constraints on the effectiveness of the system in terms of its sensitivity to the separate component efficiencies.