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The current boom in natural gas from shale formations in the United States has reduced the price of natural gas to less than the price of petroleum fuels. Thus it is attractive to convert high horsepower diesel engines that use large quantities of fuel to dual fuel operation where a portion of the diesel fuel is replaced by natural gas. The substitution is limited by emissions of unburned natural gas and severe combustion phenomena such as auto-ignition or knock of the mixture and high rates of pressure rise during the ignition and early phase combustion of the diesel and natural gas-air mixture. In this work, the combustion process for dual fuel combustion was investigated using 3D CFD. The combustion process was modeled using detailed chemistry and a simulation domain sensitivity study was conducted to investigate the combustion to CFD geometry assumptions. A baseline model capturing the onset of knock was validated against experimental data from a heavy-duty dual-fuel engine.

The CO2 advantage coupled with the low NOX and PM potential of natural gas (NG) makes it well-suited for meeting future greenhouse gas (GHG) and NOX regulations for on-road medium and heavy-duty engines. However, because NG is mostly methane, reduced combustion efficiency associated with traditional NG fueling strategies can result in significant levels of methane emissions which offset the CO2 advantage due to reduced efficiency and the high global warming potential of methane. To address this issue, the unique co-direct injection capability of the Westport HPDI fuel system was leveraged to obtain a partially-premixed fuel charge by injecting NG during the compression stroke followed by diesel injection for ignition timing control. This combustion strategy, referred to as DI2, was found to improve thermal and combustion efficiencies over fumigated dual-fuel combustion modes.

Enhancement of fuel/air mixing is one path towards enabling future diesel engines to increase efficiency and control emissions. Air-assist fuel injections have shown potential for low pressure applications and the current work aims to extend air-assist feasibility understanding to high pressure environments. Analyses were completed and carried out for traditional high pressure fuel-only, internal air-assist, and external air-assist fuel/air mixing processes. A combination of analytical 0-D theory and 3D CFD were used to help understand the processes and guide the design of the air-assisted setup. The internal air-assisted setup was determined to have excellent liquid fuel vaporization, but poorer fuel dispersion than the traditional high-pressure fuel injections.