Quantitative Reasons Why Ideal Air Standard Engine Cycles are Deficient

Abstract

The ideal air standard engine (IASE) cycles provide reasonable trend predictions of items such as thermal efficiency with respect to compression ratio, but over-predict the absolute values of the efficiency, as well as cylinder pressures and temperatures, and possess other such deficiencies. This study is aimed at quantifying the reasons for these deficiencies, by comparing the results from a thermodynamic cycle simulation (i.e. a representation of actual engines) and the IASE cycles. One of the major deficiencies of IASE cycles is the assumption of no heat transfer. For one case (compression ratio of 8), the gross indicated thermal efficiency for the cycle simulation increased from 35.9% to 41.9% by eliminating all heat transfer. The 41.9% thermal efficiency was in good agreement with the IASE cycle efficiency (41.8%) for a ratio of specific heats of 1.26. Other deficiencies were the assumed instantaneous combustion and lack of true property evaluations. The maximum cylinder pressures and temperatures were always over-predicted by the IASE cycles.