Investigation of Methyl Decanoate Combustion in an Optical Direct-Injection Diesel Engine

An optically accessible heavy-duty diesel engine was used to investigate the impact of methyl decanoate (MD) on combustion and emissions. Specific goals of the study were to produce experimental data for validating engine combustion models using MD (a biodiesel surrogate), as well as to determine if MD could enable soot-free leaner-lifted flame combustion (LLFC), a mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. An ultralow sulfur diesel certification fuel (CF) was used as the baseline fuel, and experiments were conducted at two fuel-injection pressures with three levels of charge-gas dilution; start of combustion and duration of fuel injection were held constant. In addition to conventional pressure-based and engine-out emissions measurements, exhaust laser-induced incandescence, in-cylinder natural luminosity, and in-cylinder chemiluminescence diagnostics were used to provide detailed insight into combustion processes. Results indicate that MD effectively eliminated soot emissions but that soot formation still occurred in-cylinder, with equivalence ratios at the flame lift-off length in excess of approximately 3. Nevertheless, the oxygen content of MD sufficiently limited soot formation and promoted soot oxidation such that very little soot remained at exhaust-valve open. Nitrogen oxides (NOx) emissions for MD relative to CF showed different trends depending on fuel-injection pressure, with distinct fuel effects influencing NOx formation depending on engine operating condition. Hydrocarbon (HC) and CO emissions were higher for MD compared to CF and corresponded to lower fuel-conversion and combustion efficiencies. These differences were attributed to the lower-load conditions of MD, resulting from its lower energy density and the need to limit fuel-injection duration to obtain valid lift-off length measurements.