Heavy-duty natural gas engines offer air pollution and energy diversity benefits. However, current homogeneous-charge lean-burn engines suffer from impaired efficiency and high unburned fuel emissions. Direct injection offers the potential of diesel-like efficiencies, but requires further research. To improve understanding of the combustion process and pollutant formation mechanisms in a pilot-ignited, direct injection of natural gas engine with intake charge dilution, the effects of enhanced gaseous jet kinetic energy, gaseous fuel composition (including ethane, propane, hydrogen, and nitrogen), and filtering the recirculated gases were studied.

An experimental investigation was carried out on a single-cylinder heavy-duty engine. Fuel consumption, in-cylinder performance and gaseous and particulate emissions (total mass, size distributions, and black carbon content) were measured. The results indicated that increasing the jet kinetic energy significantly reduced particulate matter (PM) emissions due to improved fuel-air mixing, especially at high load. The addition of hydrogen to the fuel reduced emissions of carbon monoxide (CO), unburned fuel (HC) and PM. The largest effects were observed at high load conditions. The addition of ethane and propane to the fuel resulted in increases in PM and CO emissions at all operating conditions tested;​ no effect on the combustion progression was detected. The addition of nitrogen to the fuel significantly reduced emissions of CO, PM, and HC due to enhancement of the late-cycle combustion event from increased in-cylinder turbulence. Removing PM from the recirculated gases revealed that these particles had no significant effect on the combustion event or on PM emissions.

In conclusion, mixing and kinetic enhancement both reduced the gaseous fuel ignition delay. The overall combustion event was, at high load, mixing limited;​ the combustion rate was unaffected by fuel reactivity but was increased with turbulence enhancement. Emissions formation was found to be a result of multiple influences whose relative importance varied with operating condition. Increased mixing and lower fuel carbon content reduced P M emissions. Reductions in emissions through the addition of hydrogen and nitrogen to the fuel may offer a potential technique to offset increases in emissions due to variations in ethane and propane levels in natural gas.