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Natural gas is a viable alternative to gasoline and diesel fuel because it is a clean burning fuel that is available from a large domestic reserve through a mature infrastructure. The heavy dependence of the small engine sector on oil, much of which is imported from foreign countries and the small engine sector's negative impact on the air quality in urban areas are two pervasive problems that can be helped by using Compressed Natural Gas (CNG) as a small engine fuel. In addition, CNG is typically over 80% methane, which is produced by the decay of organic material, so while natural gas is not renewable its use enables much of the infrastructure required for a methane-based renewable energy system. In order to determine the emissions benefit of using CNG as compared to gasoline in a small engine, a 750 cc 90 degree V Twin port-fuel-injected production engine rated at 29 horsepower (HP), designed and built by Kohler Inc.

The purpose of the present paper is to develop an engine simulation tool in a commercial CFD code to study the spray and mixing process that can be used to access the performance of a Gasoline Direct Injection (GDI) engine. The ignition, combustion and pollutant formation are strongly dependent on the quality of the fuel-air mixture. The fuel is injected directly into the combustion chamber by high-pressure fuel injector. The fuel atomization and evaporation process takes place due to the interaction of the small fuel particles generated by the injector and the in-cylinder air motion. Experimental study on the spray and mixing process is difficult and expensive, which has been recognized as a major obstacle towards the optimization of the combustion chamber geometry, engine components and the injection strategies.