Investigation of two low emissions strategies for diesel engines: Premixed charge compression ignition (PCCI) and stoichiometric combustion

Premixed Charge Compression Ignition (PCCI) and stoichiometric combustion were investigated in a single-cylinder, high-speed, direct-injection (HSDI) diesel engine to explore solutions for meeting future emission regulations.; The effects of spray targeting on emissions in PCCI combustion were examined with six included spray angle nozzles (50 to 154 degrees) by introducing high levels of EGR (55%) with a relatively low compression ratio (16.0) and an open-crater type piston bowl. The soot and CO production were minimized when the spray was targeted at the top edge of the piston bowl, regardless of the spray angle, while NOx formation was not affected by the targeting. The results from narrow angle nozzles (50 and 85 degrees) indicated that soot could be optimized by targeting sprays at the bottom of the piston bowl. CO emission increased but was significantly reduced by targeting sprays at the inner surface of the bowl, with a corresponding increase in soot emission. In the standard diesel combustion regime, the soot and CO increased with retarded start of injection (SOI) timing. -20 degree ATDC SOI timing provides a rough boundary between standard diesel and PCCI combustion, as seen from the heat release rate data.; Stoichiometric diesel combustion was characterized experimentally to explore the possible use of a three-way catalyst with a diesel engine to control NOx emissions, as well as carbon monoxide and unburned hydrocarbon emissions. The effects of fuel injection pressure, boost pressure, swirl, intake air temperature, combustion regime, and engine load were assessed under stoichiometric conditions. The results indicated that stoichiometric operation could be achieved with relatively minor fuel economy and soot impacts. The fuel consumption at stoichiometric operation was increased by about 7% compared to the best fuel economy case of standard diesel combustion (at around 0.8 of equivalence ratio). Soot emissions under stoichiometric operation were significantly affected by engine load and the combustion regime. Boost pressure and engine load were found to be the most dominant factors for NOx generation, and the intake air temperature and combustion regime had a relatively minor effect while injection pressure and swirl did not have significant effects.