Friction Losses of a Novel Prototype Variable Expansion-Ratio, Spark Ignition, Four-Stroke Engine

The novel engine concept represents an alternative approach to achieve the benefits ideally associated with so-called Otto-Atkinson engines in which, in earlier experimental units, the induction stroke was effectively shortened, at part load, by early, or late, inlet valve closure. In the new engine the need for variable valve timing is avoided because a specially designed mechanism shortens the induction/compression strokes and lengthens the expansion/exhaust strokes for part-load operation while maintaining constant the engine compression ratio. A concern with such an engine relates to the inherent friction loss. Accordingly, a theoretical study of the mechanical loss due to the new mechanism was conducted using a commercially available code. It was found that the friction loss of the new engine configuration was approximately 10% lower than that of an otherwise comparable conventional four-stroke engine. Additionally, results are presented of motoring tests carried out on a small, low cost, single cylinder prototype of the new engine. These tests, for which allowance was made for the irrecoverable loss of work due to over-expansion in the part-load mode, confirmed by virtue of comparison with the motoring loss of an unmodified conventional engine, the predictions. These results also verified a prior expectation based on an earlier, less rigorous, analysis.