Potential of compression ratio and exhaust gas dilution on improving combustion and nitrogen oxides emission performance on a PFI engine fueled with methanol

Abstract

Methanol is a promising alternative fuel, and the effects of compression ratio (CR) and exhaust gas recirculation (EGR) of a port fuel injection spark ignition engine modified from a diesel engine fueled with methanol were studied in this paper. The results showed that with the increase of the EGR rate, flame propagation slowed down, heat release rate and the pressure rise rate decreased, and the combustion lag phenomenon was more obvious in the high EGR range. Under light load and high engine speed conditions, the combustion parameters were more sensitive to the variation of EGR rate. Increasing the CR was beneficial to improve combustion stability. Higher CR and introducing exhaust gas into cylinder both can improve break thermal efficiency and fuel economy performance. EGR strategy showed great capacity on reducing NOx emissions. By Taguchi and ANOVA analysis, it was found that higher EGR rate and small CR were benefit for NOx emission, but higher CR was suitable for decreasing BSFC, 17:1 was an optimal CR for this engine. The influence ratio of EGR on NOx emissions reached 88%, and the engine operating condition had the greatest influence on BSFC, followed by EGR rate.