Effects of Fuel Ignition Quality on Critical Equivalence Ratio for Autoignition

Many advanced combustion modes utilize a premixed charge, which can vary from partially premixed to fully premixed, to simultaneously reduce emissions of nitrogen oxides (NOx) and particulate matter (PM). However, the partially premixed charge results in excessive emissions of total hydrocarbons (THC) and carbon monoxide (CO) from incomplete combustion. High ignition quality fuels, or fuels with high cetane numbers, have been in shown previous studies to enable reductions of THC and CO emissions during advanced diesel combustion operation. Many factors were suggested which contribute to the lower THC and CO emissions produced by the combustion of a high cetane fuel. Among the proposed factors was that a high cetane number fuel will have a lower combustion lean limit than a low cetane number fuel, thus resisting incomplete combustion. This theory was examined in the present work where the critical equivalence ratio (φ), the minimum equivalence ratio at which a fuel can autoignite, was identified for diesel, a high temperature Fischer―Tropsch (HTFT) fuel, and a low temperature Fischer-Tropsch (LTFT) fuel. The fuels were vaporized and premixed with air heated to 260 °C and then fed into a modified cooperative fuels research (CFR) octane rating engine, at compression ratios of 4, 5, 6, and 8. Equivalence ratio (φ) was gradually increased until the critical φ was determined. At the lowest compression ratio of 4, the diesel fuel did not autoignite while the HTFT and the LTFT fuels did at critical φ of 0.76 and 0.35, respectively. At the highest compression ratio of 8, the critical φ of the fuels began to converge where diesel, HTFT, and LTFT had critical φ values of 0.23, 0.20, and 0.17, respectively. However, in the presence of simulated EGR (O2 10.7 vol %, CO2 8 vol %, and N2 81.3 vol %) at a compression ratio of 8, the critical φ diverged dramatically, where the diesel, HTFT, and LTFT had critical φ values of 1.00, 0.77, and 0.38, respectively. n-Hexane, n-heptane, and n-dodecane were used as single-component surrogates of similar ignition quality as the diesel, HTFT, and LTFT, respectively. The single-component surrogates had leaner critical φ than their multicomponent counterparts did.