Comparison of E10 and E85 spark ignited stratified combustion and soot formation

The combustion process, soot formation and oxidation in spark ignited, spray guided, globally lean, stratified combustion of E10 and E85 was investigated and compared in a single cylinder optical engine with an outward opening piezo actuated injector. Two engine loads and several injection sequences were studied. Cylinder pressure measurements, high speed video recordings, OH∗ chemiluminescence and soot incandescence imaging were used. Stable ignition and combustion of both fuels was achieved with a spark at the end of the last the fuel spray for all injection strategy cases except for the single injection of large fuel quantities. The average rate of heat release and work output increased and the work variability decreased for both fuels with multiple injections of fuel for both engine loads. The changes for each case and variable were approximately equal for the two fuels. Single injections of E85 led to pool fires on the piston surface which were the only source of soot formation. Pool fires persisted after the end of conditions favourable to soot oxidation. For E10 the source of soot formation was a combination of pool fires and gas phase soot formation at 3.6 bar while at 5 bar pool fires were the only source. Two injections led to a significant reduction of the soot formation for both fuels and both engine loads. Pool fires remained the sole source of soot formation for E85 while the gas phase was the primary source for E10 and largely ceased during the combustion process while soot oxidizing conditions were present. The spatial distribution of combustion was remarkably similar for both fuels when multiple injections were used.