Experimental Study on Combustion Characteristics of CNG Direct Injection in a Spark Ignition Engine

Natural gas (CNG) for spark ignition (SI) engines has the potential to reduce CO2 emissions by more than 25% compared with gasoline operation. Currently, all spark ignited CNG engines that are in mass production have port fuel injection. They suffer loss of peak torque and power relative to gasoline engines due to reduction in volumetric efficiency. Direct injection (DI) can overcome this drawback by injecting the fuel after intake valve closure leading to significant improvements in engine performance. This study compares the effects of early and late injection (before and after intake valve closure respectively) on engine performance metrics like combustion durations, thermal efficiency, emissions and lean operation at the world wide mapping point (WWMP) i.e. at 3.3 bar IMEPn and 1500 RPM. The results show that late injection compared to early injection causes faster combustion as a result of higher turbulence at the time of ignition and that the resultant increase in thermal efficiency is offset by increased pumping losses to maintain the IMEPn. The faster combustion due to injection induced turbulence reduces the cyclic variations which also aids in extending the lean limit at the WWMP.