The Turboexpansion Concept - Initial Dynamometer Results

Abstract

<div class="htmlview paragraph">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.</div> <div class="htmlview paragraph">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”.</div> <div class="htmlview paragraph">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.</div>