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Abstract

The purpose of the work was to determine the conditions under which pilot liquid diesel flame (as a source of heat) can ignite natural gas directly injected into the cylinder for diesel engines. The work included experimental and numerical simulation of a conventional diesel and natural gas fueling. The engine was operated at medium speed and load conditions of 1250 RPM and 3 bar (brake mean effective pressure). Measurements and computations were performed by varying the fuel injection timing for baseline diesel (100% diesel) and for diesel-gas (30% pilot diesel and 70% natural gas). High speed flame photography based on the endoscope technique was used to obtain combustion flame pictures of the firing engine. Measured cylinder pressure data were analyzed by a multi-zone combustion model for mass burning rate. Three-dimensional numerical simulation based on the KIVA code was used to predict thermal and flow field in the engine chamber. The KIVA code was modified for diesel-gas combustion. A single-step chemical reaction coupled with a mixing-controlled combustion model were implemented in the modified KIVA code to predict ignition and combustion. Successful ignition of natural gas by a pilot diesel flame depends strongly on the relative injection timing of the pilot diesel and natural gas. With the beginning of natural gas injection 3 degrees after the injection of the pilot diesel, a successful ignition is possible over a wide range of injection timings. Successful ignition with late injection timing is associated with low emissions of oxides of nitrogen (NOx). With pilot injection, the natural gas is ignited by contact with the pilot diesel combustion products rather than the combustion enhanced compression of the unburned fuel and air. Ignition of the natural gas depends strongly on the combustion duration of the pilot diesel. Under the conditions studied, the diesel-gas combustion has about the same ignition delay and combustion duration as baseline diesel. Combustion with the diesel-gas case is apparently smoother (i.e. lower cycle-to-cycle variations) than with baseline diesel.