Effect of peak firing pressure constraint on thermal efficiency of a Sabathe-Miller engine

Abstract

How to Improve the thermal efficiency of internal combustion engine is an eternal topic since it is directly related to energy consumption and pollution emissions. Among lots of technologies applied on heavy duty diesel engine, Miller cycle which increases the effective expansion ratio in relation to the compression is proved to be a good choice to balance engine performance with fuel economy and CO2/NOx/smoke emissions. However, limitation of peak firing pressure (PFP) will restrict the intake pressure or compression ratio of miller engines which in turn influence the thermal efficiency, and their in-depth trade-off relationship is not clear yet. Using the engine cycle simulation, this paper explores the most reasonable cylinder pressure waveform limited by PFP and found there exists an optimized pressure ratio values which slightly affected by other operating conditions for Sabathe-Miller cycle engine. Thereafter, effects of PFP limit on the performance of a large single cylinder engine were investigated considering the effect of engine load, miller degree, equivalence ratio as well as engine speed. It is found that under full load condition, the maximum brake thermal efficiency occurs when pressure ratio (PR) is about 1.2, and PFP constraint plays a dominant role on brake efficiency and could contribute more than 4.0% efficiency by increasing PFP constraint from 250bar to 350bar. Compared with full load condition, the brake thermal efficiency increases 4.6% and 6.3% respectively under fuel-rich (extremely high CR) and lean-burn (low heat loss) half load conditions. Meanwhile, miller degree was optimized for Sabathe-Miller cycle, and it plays a key role on reducing combustion temperature instead of increasing thermal efficiency compared with effect of PFP constraint.