Thermodynamic and exergy analysis of high compression ratio coupled with late intake valve closing to improve thermal efficiency of two-stage turbocharged diesel engines

Abstract

A Miller cycle and higher geometric compression ratio (GCR) can be adopted to improve the brake thermal efficiency (BTE) of an engine. The principle of the Miller cycle is to change the intake valve closing (IVC) time. Retard IVC (RIVC) and buffer delay IVC (BIVC) were used to realize the Miller cycle of a late IVC (LIVC), and the influence of the LIVC-coupled GCR on BTE was analyzed by one-dimensional simulation. The results showed that an increase in the IVC delay angle reduced the exhaust flow rate and pressure, thereby reducing the exhaust thermomechanical exergy (EXE) and pumping losses. But it also increased the wall heat transfer exergy. Therefore, with a delay in IVC, the BTE slightly increased at first and then decreased sharply. Simultaneously increasing the IVC delay angle and GCR can reduce the combustion irreversible exergy, and effectively improve the BTE. Under the conditions of RIVC = 35 °CA and GCR = 25.5, the BTE reached 48.3%. The BIVC had a higher inflation efficiency and a greater effective compression ratio than that of the RIVC. However, the BIVC method increased the EXE; therefore, its BTE was lower than that of the RIVC method under most conditions.