Direct comparasion of an engine working under Otto, Miller end Diesel cycles : thermodynamic analysis and real engine performance

摘要

One of the ways to improve thermodynamic efficiency ofSpark Ignition engines is by the optimisation of valvetiming and lift and compression ratio. The throttlelessengine and the Miller cycle engine are proven conceptsfor efficiency improvements of such engines.This paper reports on an engine with variable valvetiming (VVT) and variable compression ratio (VCR) inorder to fulfill such an enhancement of efficiency. Engineload is controlled by the valve opening period (enablingthrottleless operation and Miller cycle), while the variablecompression ratio keeps the efficiency high throughoutall speed and load conditions.A computer model is used to simulate such an engineand evaluate its improvement potential, while a singlecylinder engine demonstrates these results.The same base engine was run on the test bench underthe Diesel cycle, Otto cycle and Miller cycle conditions,enabling direct thermodynamic comparisons under awide variety of conditions of speed and load.The results show a significant improvement of the Millercycle over the Otto cycle engine. Comparisons of theMiller engine with the Diesel engine shown that it ispossible to have a SI engine with better efficiency than asimilar Diesel engine for most of the working conditions.