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The high-pressure direct injection (HPDI) of natural gas permits diesel engines to retain their performance and high fuel economy while reducing regulated emissions. In the work presented in this paper, a pilot diesel fuel ignites directly injected natural gas, and both fuels are injected through a single injector. Recently the HPDI engine achieved a combined NOx+nmHC emissions of 2.38 g/bhp-hr during official certification tests performed under the US EPA specified FTP cycle for heavy-duty diesel engines. NOx, nmHC and PM emissions were reduced by 45%, 85% and 71%, respectively, compared to the 1998 EPA emissions requirement. These results are consistent with previously reported results on a two-stroke engine. The present study clearly demonstrates that a combination of gas injection timing and pressure can significantly reduce NOx emissions while retaining the overall thermal efficiency.

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.

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 high-pressure direct injection (HPDI) of natural gas (NG) permits diesel engines to retain their high fuel economy while reducing regulated emissions. In the work presented in this paper, directly injected natural gas is ignited by pilot diesel fuel, and both fuels are injected through a single injector. The injector concept is discussed, along with the description of the instrumented Detroit Diesel two-stroke 6V-92TA DDEC II engine used for the experiments. Measurements of the performance and emissions with the HPDI of NG confirm the retention of the high efficiency of the diesel engine and demonstrate reductions in nitrogen oxide (NOx) emissions near 50% at high load using the same injection timing as for diesel fueling. Methane (CH4) and non-methane hydrocarbon (nmHC) emissions were found to be as low as those measured for diesel fueling at high loads, but were higher at low load operation. The gas injection pressure was found to affect the low-load emissions.

A new injector has been designed for sequential injection of high-pressure natural gas and a quantity of liquid diesel fuel directly into diesel engine cylinders late in the compression stroke. Injected a few degrees before the natural gas, the pilot liquid fuel auto-ignites and serves, as it burns, to ignite the gaseous fuel which enters the chamber as an underexpanded sonic jet generating high local turbulence. Tests on a single-cylinder two-stroke engine with full electronic control have demonstrated the capability of this fueling method to nearly match conventional diesel engine efficiency over a wide range of load and substantially reduce the emissions of oxides of nitrogen (NOx), particulate mater (PM) and carbon dioxide (CO2).