The Research Development in Direct Injection Spark-Ignition Natural Gas Engine

Abstract

Natural gas is thought to be one of the most promising alternatives to traditional vehicle fuels. Nowadays, the natural-gas-fueled engine has been realized in both the spark-ignition engine and the compression-ignition engine. Due to the complicated fueling systems in dual fuel mode and the loss in volumetric efficiency in port injection mode, direct injection spark-ignition natural gas engine can be utilized to avoid these defects and its mixture preparation flexibility will improve the fuel economy. The ability to increase the compression ratio can improve the engine performance. In addition, natural gas direct injection combustion can avoid smoke emission from gasoline direct injection combustion. Due to the high injection pressure requirement, special gas injector should be developed to match the injection and flow characteristics. Meanwhile, the arrangement of spark plug and fuel injector is very sensitive to the engine performance and emissions. The injection timings and ignition timings are very important operating parameters and the control of these parameters will determine the mixture concentration distribution in the cylinder and thus the combustion characteristics. Natural gas direct injection combustion can realize high combustion stability with less cycle-by-cycle variation and the lean burn limit can be extended compared with that of the port injection mode, and also the HC emission can be reduced based on appropriate charge stratification and gas flow condition. However, the particle number concentration and NOx emission will increase with the improvement of combustion status. Adding hydrogen into direct injection natural gas engine is expected to improve the engine performance and decrease engine emissions. By using the swirl injection system, this engine can realize the increase of brake thermal efficiency and the reduction of the brake NOx, HC, CO and CO2 emission simultaneously, when the hydrogen fraction exceeds 10 %. Nevertheless, further research in spark-ignition system and fuel injection system should be conducted before the product stage of this kind of engine.