Investigation of Fuel Effects on Dilute, Mixing-Controlled Combustion in an Optical Direct-Injection Diesel Engine

Abstract

Effects of fuel type on dilute diesel combustion and emissions were investigated using an optically accessible diesel engine. A number 2 diesel fuel, a primary reference fuel blend, and a soy-derived biodiesel were evaluated at an engine speed of 1200 rpm, and for intake-oxygen mole fractions ranging from 21% down to 8%. Diagnostics included conventional heat-release analysis; the measurement of in-cylinder natural luminosity; in-cylinder imaging of the start of fuel injection; and the measurement of engine-out emissions of nitrogen oxides (NOx), smoke, HC, and CO. Data previously obtained using the fuel diethylene glycol diethyl ether are presented to provide further insight. Results reveal that reduced oxygen mole fractions produce a slower heat release and lower levels of soot formation. However, engine-out particulate matter levels are also dramatically influenced by subsequent soot oxidation, which is greatest at higher oxygen mole fractions. NOx emissions are observed to drop dramatically as oxygen mole fractions are reduced, due to the associated reductions in combustion temperatures. Overall, changes in oxygen mole fractions produce similar trends in emissions results for each of the fuels investigated; however, differences do exist that may provide opportunities for implementing effective fuel-specific emissions-control strategies. A parameter called the overlimit function is introduced to help evaluate and optimize operating strategies for engine systems that must comply with multiple simultaneous constraints (e.g., emissions levels, efficiency, peak cylinder pressure, and ringing intensity) as operating conditions are varied.