An experimental study on the combustion and emission characteristics of a diesel engine under low temperature combustion of diesel/gasoline/n-butanol blends

Based on a small-scale high-speed diesel engine, the low temperature combustion and emission characteristics of four different fuels were experimentally investigated at different EGR ratios. The following fuels were studied: pure diesel (D100), a diesel and gasoline blend with a volume ratio of 70:30 (D70G30), a diesel and n-butanol blend with a volume ratio of 70:30 (D70B30) and a blend of diesel, gasoline and n-butanol with a volume ratio of 70:15:15 (D70G15B15). During the combustion of all four kinds of fuels, when the EGR ratio was smaller than 25%, the increase of EGR ratio sorted little effects on the maximum pressure rising ratio, as well as on the emission of soot, CO and total hydrocarbons (THC). Conversely, the emission of NOx decreased significantly as the EGR ratio increased. At the same EGR ratio, the variation of fuel characteristics by adding gasoline or n-butanol into diesel did not sort significant effects on the NOx emission, whereas it was found to greatly affect the maximum pressure rising ratio. When the EGR ratio was greater than 25%, the emissions of soot, CO and THC increased rapidly with the rising EGR ratio due to the decrease in excess air coefficient and to the excessively long ignition delay period. The D70B30 blended fuel exhibited more remarkable results in soot emission reduction than the D70G30 blended fuel. When the EGR ratio was close to 40%, NOx emissions almost approached zero. As the EGR ratio increased, the number concentration of nucleation-mode particles (Dp < 50 nm) during the combustion of the examined fuels first decreased and then increased; the number concentration of accumulation-mode particles (50 nm < Dp < 1000 nm) increased continuously. When the EGR ratio was smaller than 25%, the total particulate matters (PM) number concentration during the combustion of the four fuels did not present a clear trend; when the EGR ratio was higher than 25%, the total PM number concentration during the combustion of the four tested fuels increased rapidly, with a descending order of rising amplitude, D100 > D70G30 > D70G15B15 > D70B30. At a fixed EGR ratio, the count median diameters (CMD) corresponding to the peak values of the number concentration of both nucleation-mode and accumulation-mode particles had an ascending order of D70B30 < D70G15B15 < D70G30 < D100. The emissions of soot, NOx, CO and THC and the number concentration of particles reached their optimum values during the combustion of D70B30 at an EGR ratio of approximately 25%.