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Multidimensional simulations are being used to assist the development of a directly injected natural gas system for heavy-duty diesel engines. In this method of converting diesel engines to natural gas fueling, the gas injection takes place at high pressure at the end of the compression stroke. A small amount of pilot diesel fuel is injected prior to the natural gas to promote ignition. Both fuels are injected through a centrally located injector. The mathematical simulations are sought to provide a better understanding of the injection and combustion process of pilot-ignited directly-injected natural gas. The mathematical simulations are also expected to help optimize the injection process, looking in particular at the tip geometry and at the injection delay between the two fuels. The paper presents the mathematical model, which is based on the KIVA-II code. The model includes modifications for underexpanded natural gas jets, and includes a turbulent combustion model.

The ignition and combustion of natural gas directly injected into a multi-cylinder two-stroke diesel engine and ignited by a pilot liquid diesel injection has been investigated experimentally and with the aid of numerical simulation. Measurements of cylinder pressure and thermal efficiency were supplemented by endoscopic observation of flame development and three-dimensional numerical simulation of the ignition and combustion process. With gas/diesel fueling and appropriate injection timing, ignition delay and combustion duration can be about the same as with 100% diesel liquid fueling. Flame photography indicates that, for the same liquid diesel flow rate, subsequent injection of natural gas has a negligible effect on the ignition delay of the liquid fuel. Relative ignition timing is of major importance in obtaining successful combustion.