The Effects of Compression Ratio and Expansion Ratio on Engine Performance Including the Second Law of Thermodynamics: Results From a Cycle Simulation

Abstract

This investigation quantified the effects of compression ratio and expansion ratio on performance, efficiency, and second law parameters for an automotive, spark-ignition engine. The well known increase in engine performance for increasing compression ratio and expansion ratio is demonstrated. These increases for brake engine performance are modest for compression ratios greater than about 10 for the conditions studied. The results demonstrated that the increasing friction and heat losses for the higher compression ratios are of the same order as the thermodynamic gains. Also, the results included the destruction of availability during combustion. For a part load condition, the availability destroyed decreased from about 23% for a compression ratio of 4 to about 21% for a compression ratio of 10. In addition, this study examined cases with greater expansion ratios than compression ratios. The overall cycle for these cases is often called an “Atkinson” cycle. For most cases, the thermal efficiency first increased as expansion ratio increased, attained a maximum efficiency, and then decreased. The decrease in efficiency after the maximum value was due to increased heat losses, increased friction, and ineffective exhaust processes (due to the reduced cylinder pressure at the time of exhaust valve opening). For part load cases, the higher expansion ratio provided only modest gains due to increased pumping losses associated with the constant load requirement. For the wide open throttle cases, however, the higher expansion ratios provided significant gains. For example, for a compression ratio of 10, expansion ratios of 10 and 30 provided brake thermal efficiencies of about 34% and 43%, respectively. Although the net thermodynamic gains are significant, large expansion ratios such as 30 may not be practical in most applications.