Overexpanded Engine

This chapter presents a case study to quantify the effects of expansion ratio on engine performance, efficiency, and second law parameters for an automotive, spark-ignition engine. In particular, engine configurations with a wide range of expansion ratios for a fixed compression ratio are examined. The study examined expansion ratios up to four times the compression ratio. For most cases, the thermal efficiency first increased as expansion ratio increased, then attained a maximum efficiency, and then decreased. For the 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 the wide-open throttle cases, for a compression ratio of 15, the exergy destroyed during the combustion process increased slightly from about 20.2% to 21.0% as the expansion ratio increased from 15 to 55.