Thermal efficiency and emission characteristics of a lean burn HCNG engine

An experimental research was conducted on a 6-cylinder CNG S. I. engine using variable compositions of natural gas/hydrogen mixtures to analyze the effect of hydrogen blending ratio on engine's thermal efficiency and emission characteristics. Tests were conducted under several engine operating conditions. The results show that hydrogen addition can extend the lean operation limit, increase burn rate and decrease the spark advance for best torque. At fixed spark timing, hydrogen addition was not found to have obvious advantage in thermal efficiency. However, NO_x emission increases with the increase of hydrogen fraction. While at maximum break torque spark timing (MBT), hydrogen enrichment can increase engine's indicated thermal efficiency, decrease emissions of unburned methane and CO, also mitigate the trade-off relations between NO_x and unburned hydrocarbon. The advantages of hydrogen addition are also evident in its ability to allow the engine operate efficiently on much leaner fuel air mixtures, thus helps to decrease NO_x emissions and heat transfer loss.