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Measurements of performance and emissions of a Detroit Diesel 1-71 engine fueled with natural gas have been made using high-pressure direct-injection (HPDI). Natural gas is injected late in the compression cycle preceded by pilot injection of conventional liquid diesel fuel. With 6 nozzle holes for both natural gas and diesel pilot there was instability in engine operation at low load and wide scatter in emission measurements. Guided by numerical simulation results it was found experimentally that data reproducibility and engine operating stability could both be much improved by using unequal jet numbers for injection of natural gas and pilot diesel. In the range of 100 to 160 bar, combustion rate and NOx emissions increased with gas injection pressure. Best thermal efficiency results were obtained for a gas pressure of 130 bar. By adjusting beginning of injection, NOx reductions of up to 60 % from the diesel baseline could be obtained, while preserving conventional diesel efficiency.

Pilot-ignited high-pressure direct injection (HPDI) of natural gas in diesel engines results in lower emissions while retaining high thermal efficiency. As a study of HPDI technique, three-dimensional numerical simulations of injection, ignition and combustion were conducted. In particular, the effects on engine combustion of the injection interlace angle between the pilot diesel sprays and natural gas jets were investigated. Numerical simulations revealed ignition and combustion mechanisms in the engine with different injection interlace angles. The results show that altering the interlace angle changes the contact areas between the pilot diesel sprays and the natural gas jets; this affects the heat release rate. Statistical analysis was done to evaluate the expected value and variance of “closeness” between diesel sprays and natural gas jets for different injector tip configurations.