Effect of the novel continuous variable compression ratio (CVCR) configuration coupled with spark assisted induced ignition (SAII) combustion mode on the performance behavior of the spark ignition engine

Abstract

The paradox of low efficiency and high fuel consumption at low-load and high efficiency and knocking combustion at high-load are pushing the engine community to optimize compression ratio (CR) in the spark ignition (SI) engine. However, it is difficult to balance the performance under a wide load range with using a fixed CR. In this paper, a novel continuous variable compression ratio (CVCR) system was proposed and successfully used in a SI engine, which could continuously change the auxiliary combustion chamber volume, thereby achieving different compression ratios from 10.0:1 to 17.8:1. The results indicated that the total mass of residual exhaust gas in the cylinder of the engine equipped with CVCR system was increased, and the residual gas fraction (RGF) in the auxiliary cylinder was increased, while the RGF in the main cylinder wasn’t changed dramatically with increasing CR. In addition, combustion rate was increased and combustion duration was decreased with increasing CR. The heat transfer rate and its percentage in the total fuel energy was decreased with the increase of the CR. The BSFC and effective thermal efficiency of the engine equipped with CVCR system could be improved by 15.85% and 15.57% by adopting CR 17.8:1 and 10% external EGR under the operating condition of 2000 r/min and BMEP 9.59 bar, respectively. However, the intensity of knocking combustion was increased with CR and limited the utilization of the high CR in the SI engine. Using external exhaust gas recirculation strategy could greatly alleviate the knocking combustion and reduced the heat transfer rate through the combustion chamber wall, which was beneficial to increase the indicated thermal efficiency and adiabatic efficiency.