Thermal efficiency and emissions improvement of the lean burn high compression ratio HPDI NG engine at different combustion modes

Abstract

High pressure direct injection (HPDI) technology has great potential in achieving high thermal efficiency and is currently one of the most advanced combustion technologies for natural gas (NG) engines. However, with the tightening of emission regulations, it is essential to further explore how HPDI NG engines can limit pollutant emissions while still achieving high thermal efficiency. In this study, based on NG mixture-limited combustion (NMLC) and NG slightly premixed combustion (NSPC) modes, the potential of the change in excess air ratio (EAR) in achieving high indicated thermal efficiency (ITE) and low pollutant emissions at different compression ratios (CR) is systematically investigated by constructing a three-dimensional (3D) model. The conclusion of the investigation shows that: (1) in the NMLC mode, the ITE reaches its maximum of 45.9 % after increasing the EAR to 3.0 at medium CR (CR = 17), and the CO and soot emissions can be kept at lower values. (2) In the NSPC mode, at medium–high CR, an increase in EAR to 2.5 increases the high ITE limit further (up to 47.8 %), while the very low CO and soot emissions are maintained but NOx emissions increased. (3) Overall, in both modes, CO, CO2 and soot emissions can be reduced to a greater extent by adjusting EAR and CR. In comparison, the NMLC mode has greater potential to reduce NOx emissions, while the NSPC mode has more advantages in terms of improving ITE limits and achieving near-zero CO and soot emissions.