Combustion and emission characteristics of CNG/gasoline DFSI engine with CNG direct injection

CNG was a low-carbon alternative fuel for engines, which also had the advantages of higher engine knock resistance and better fuel/air mixing. The CNG injection and ignition timing was of significant importance for the performance of Dual Fuel Spark Ignition (DFSI) engines. In this paper, the coordinated effects of CNG injection strategy and ignition timing on the combustion and emission characteristics of CNG/gasoline DFSI engines were investigated. The results showed that the CNG/gasoline engine can achieve a comparable level of BMEP to that of a pure gasoline engine by using the CNGDI plus GPI injection approach with an optimal combination of injection and igniting timing. The cyclic variations of CNG/gasoline engines were increased with the advancing of ignition timing, and these variations can be ceased by using proper CNG injection strategies (early or mild). The gaseous emissions (HC, CO, and NOx) of CNG/gasoline engines were equivalent to those of pure gasoline engines; but the PN emissions were lower than that of gasoline engines. The particle size distribution of CNG/gasoline DFSI engines followed a single peak nucleation pattern and most of the particles were in nucleation mode (d <= 50 nm) peaking at around 15 nm. This study would be beneficial for the application of CNG in gasoline engines, which was promising to alleviate the strong dependence of engines on petroleum energy and to reduce their greenhouse gas emission.