A Detailed Comparison of Emissions and Combustion Performance Between Optical and Metal Single-Cylinder Diesel Engines at Low Temperature Combustion Conditions

A detailed comparison of cylinder pressure derived combustion performance and engine-out emissions is made between an all-metal single-cylinder light-duty diesel engine and a geometrically equivalent engine designed for optical accessibility. The metal and optically accessible single-cylinder engines have the same nominal geometry, including cylinder head, piston bowl shape and valve cutouts, bore, stroke, valve lift profiles, and fuel injection system. The bulk gas thermodynamic state near TDC and load of the two engines are closely matched by adjusting the optical engine intake mass flow and composition, intake temperature, and fueling rate for a highly dilute, low temperature combustion (LTC) operating condition with an intake O-2 concentration of 9%. Subsequent start of injection (SOI) sweeps compare the emissions trends of UHC, CO, NOx, and soot, as well as ignition delay and fuel consumption. The effect of EGR composition is also investigated to determine the level of chemical equivalency required for adequate EGR simulation in an optical engine. Five simulated EGR conditions are compared to evaluate the influence of water vapor, CO, and UHC. Results show that the optical engine pressure and heat release characteristics are influenced by the larger crevice volumes and the compliance of the extended optical piston. Greater fueling rates are also needed in the optical engine to achieve an equivalent load. The same general trends in emissions behavior are found in both engines, capturing the location of minimum UHC and CO emissions with changes in SOI. In addition, quantitative emissions levels agree well. Simulated EGR composition is found to have a minor influence on combustion behavior and emissions levels, but does not affect the general emissions trends.