稀燃天然气掺氢发动机的热效率与排放特性

Thermal Efficiency and Emission Characteristics of a Lean Burn HCNG Engine

为了分析在天然气中掺入不同体积比的氢气对发动机经济性和排放性的影响,在一台6缸火花点火天然气发动机上开展了体积掺氢比在不同工况下对热效率和排放特性影响的试验研究。结果显示掺氢可以拓宽发动机的稀燃极限,提高燃烧速度,使得最佳转矩点火提前角(MBT)相对推迟;在点火提前角不变的情况下掺氢对热效率没有明显优势,而且会使NOx排放升高。而在MBT时,掺氢可以一定程度上提高发动机的指示热效率,降低未燃CH4和CO的排放,改善NOx与未燃碳氢(主要为CH4)的trade-off关系。掺氢的优势还体现在可以让发动机高效的工作在更稀的情况下,从而有利于降低NOx的排放和传热损失。

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, NOx 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 NOx 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 NOx emissions and heat transfer loss.