The use of pilot-ignited, direct-injected natural gas fuelling for heavy-duty on-road applications has been shown to substantially reduce NOx and particulate matter emissions. The fuelling process involves the injection of pilot diesel near top-dead-center, followed shortly afterwards by the injection of natural gas at high pressure. The injection pressure of the gas and diesel will substantially affect the penetration of the fuel into the combustion chamber, the break-up and atomization of the diesel spray, and the mixing and nature of the turbulent gas jet. To investigate these influences, a series of experiments were performed on a single-cylinder heavy-duty engine over a range of engine operating conditions (exhaust gas recirculation fraction, engine speed, engine load). Due to the unique nature of the single-cylinder engine, it was possible to hold all other parameters constant while only varying injection pressure. The results indicated that injection pressure had a substantial impact on emissions and performance at high loads, where substantial reductions in PM and CO were observed, with only minor increases in NOx and no significant effect on tHC or fuel consumption. At low loads, no significant impact on either emissions or performance was detected. The effects of injection pressure, while still significant, were found to be reduced at increased engine speeds. Higher injection pressures were found to consistently reduce both the number density and the size of particles in the exhaust stream.Copyright © 2005 by ASME