Combustion and emission characteristics of a direct-injection engine fueled with natural gas/hydrogen blends under various ignition timings

In this paper, an experimental study on combustion and emission characteristics in a direct-injection spark-ignited engine fueled with CNG/hydrogen blends under various ignition timings was investigated. The results show that the ignition timing has significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter of the direct-injection natural gas engine. The turbulence flow in combustion chamber generated by the fuel jet maintains high and relative strong mixture stratification is presented when decreasing the interval between the end of injection and the ignition timing, giving fast burning rate and high thermal efficiency. The maximum rate of heat release increase with the advance of ignition timing and it gives little variation with hydrogen addition under the same ignition timing. This indicated that hydrogen addition has little influence on combustion parameters at the constant ignition timing. The exhaust HC emission decreases and the exhaust CO₂ concentration increases with the advance of ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ show little variation with hydrogen addition. The exhaust NO_x increases with the advance of ignition timing and this behavior tends to be more obviously at large ignition advance operating condition. The effective thermal efficiency of CNG/hydrogen mixture combustion increases compared to these of natural gas combustion when hydrogen fraction is over 10%.