Influence of different volume percent hydrogen/natural gas mixtures on idle performance of a CNG engine

In order to study the influence of the 0 - 50 vol % hydrogen addition on idle performance, an experimental research was conducted on a six-cylinder throttle body injection natural gas engine. Experiments have been made under various excess air ratios and ignition timings. The results show that hydrogen addition remarkably decreased CH₄ emission whereas it had no significant effect on the reduction of CO. NO_x emission was relatively low at idle compared to other emissions. Hydrogen addition combined with ignition timing retardation was an effective way to reduce idle emission. The COV in IMEP and partial-burn ratio could be simultaneously reduced by hydrogen addition, which indicated the improvement of idle stability. In turn, the improvement of idle stability decreased the fuel consumption. Also, the curve for COV in IMEP was smoother versus ignition timing after hydrogen addition, which is desirable at idle because small idle speed error was adjusted by ignition timing in many electric idle control units. It can be concluded hydrogen addition is an effective and applicable approach to improve idle stability and decrease emission.