Partially premixed combustion (PPC) is intended to improve fuel efficiency and minimize the engine-out emissions. PPC is known to have the potential to reduce emissions of nitrogen oxides (NOx) and soot, but often at the expense of increased emissions of unburned hydrocarbons (HC) and carbon monoxide (CO). PPC has demonstrated remarkable fuel flexibility and can be operated with a large variety of liquid fuels, ranging from low-octane, high-cetane diesel fuels to high-octane gasolines and alcohols. Several research groups have demonstrated that naphtha fuels provide a beneficial compromise between functional load range and low emissions. To increase the understanding of the influence of individual fuel components typically found in commercial fuels, such as alkenes, aromatics and alcohols, a systematic experimental study of 15 surrogate fuel mixtures of n-heptane, isooctane, toluene and ethanol was performed in a light-duty PPC engine using a design of experiment methodology. The impacts of oxygen concentration, injection pressure, combustion phasing and premixed fraction were investigated for all fuel blends. The investigation also produced data for future kinetics modeling that can expand the understanding of fuel effects on PPC.
The experiments were performed at 8 bar IMEPg and at an engine speed of 1500 rpm using a single injection strategy.
HC and CO emissions increased greatly with increasing ethanol and decreased greatly with increasing n-heptane concentration. NOx emissions increased with an increasing oxygen fraction and decreased with increasing ethanol concentration. Although soot levels were near zero in many of the experiments, we observed that high levels of n-heptane increased soot, while high ethanol reduced it. The premixed fraction was impossible to predict due to data variation. No correlation was found between the premixed fraction and soot levels.