Catalyst heating operation in compression-ignition engines is critical to ensure rapid light-off of exhaust catalysts during cold-start. This is typically achieved by using late post injections for increased exhaust enthalpy, which retardability is constrained by acceptable CO and unburned hydrocarbons emissions, since they are directly emitted through the tailpipe due to the inactivity of the oxidation catalyst at these conditions. Post-injection retardability has shown to be affected by the cetane number of the fuel, but it is unclear how other fuel properties affect the ability to retard the combustion. This study aims to understand the impact of the distillation characteristics of the fuel on the performance of catalyst heating operation and on post-injection retardability.
In this study, experiments are performed in a single-cylinder medium-duty diesel engine fueled with three full boiling-range diesel fuels with different distillation curves using a five-injection strategy (two pilot, one main, two post) optimized for catalyst heating operation. The two post-injections are block-shifted to more retarded timings for three different first-post to second-post fuel split ratios and at a constant engine load. Decreasing the volatility of the fuel leads to higher exhaust enthalpy values and lower CO and unburned hydrocarbon emissions. The increase in exhaust enthalpy is caused by an additional fuel requirement to compensate for higher heat losses caused by higher flame-wall interactions with the less volatile fuels. The decrease in CO and unburned hydrocarbon emissions is caused by lower formation of overly-lean regions that do not burn properly using with the less volatile fuels. Thus, low volatility fuels improve the retardability of post injections. Chemical kinetic simulations are performed to better understand autoignition reactivity differences between fuels, concluding that fuel effects are more important for the pilot injections due to the low in-cylinder temperature and high in-cylinder pressure at the time of injection.