Assessing the feasibility of increasing engine efficiency through extreme compression

Abstract

Operating simple-cycle chemical engines at extremely high compression ratios can, theoretically, increase thermal efficiency by nearly a factor of two. To operate at these significantly higher compression ratios, a new engine architecture is required which is inherently compatible with the higher temperatures and pressures present at these conditions. In addition, the design must manage heat transfer, piston–cylinder sealing and friction, the combustion event, and emissions in order to be successful. To test feasibility of this strategy, a single-shot, free-piston device was constructed which operates at compression ratios of up to 100:1. Air-compression experiments are used to characterize the device and its losses – a combination of heat transfer out of the cylinder and mass transfer past the piston sealing rings. Preliminary experiments using a lean, diesel-like combustion strategy are performed with indicated efficiencies ranging from 52 to 60 per cent for compression ratios of 30–100. These high efficiencies indicate initial feasibility and support further research and engineering.