Effects of Fuel Volatility on Combustion and Emissions over a Wide Range of EGR Rates in a Diesel Engine 2014-01-2659
To investigate the effects of fuel volatility on combustion and emissions in a diesel engine, a high-volatility fuel of n-heptane was blended into diesel fuel with different volumetric fractions (0%, 40%, 70%, 100%). A wide range of EGR rates from 0% to 65% were investigated, which covered both the conventional diesel combustion and low temperature combustion. Experiments under two engine load conditions, ∼5.2 bar and ∼10.5 bar gross IMEP were performed at 1500 rpm. The injection timing was fixed at 8°CA BTDC for all test cases. Results show that even if the ignition delay and combustion duration are nearly the same for all tested fuels, the premixed combustion fractions are increased for higher volatility fuels due to the improvement on mixing process during the ignition delay period. The indicated specific fuel consumption is decreased as using high-volatility fuels. The effect of fuel volatility on soot emissions depends on engine loads. At lower load, more significant reduction in soot can be achieved compared to higher load as using high-volatility fuels. The high-volatility fuel has almost no effect on NOx emissions, while it results in higher THC and CO emissions at relatively high EGR rates. As toluene is added into pure n-heptane, soot emissions are similar to that of pure n-heptane. Therefore, the reduction on soot emissions as fueling n-heptane mainly causes by the higher fuel volatility, while the effect of aromatics on soot reduction is minor. The effect of injection pressures on soot emissions also depends on engine loads. At higher load, soot emissions decrease with the increase of injection pressure for both diesel and high-volatility fuels as EGR rate is lower than 40%. The higher injection pressure can endure higher EGR rates where soot emissions do not show a notable increase for both diesel and high-volatility fuel, but the injection pressure has relatively smaller effect on peak soot emission (∼50% EGR rate). At lower load, the higher injection pressure can eliminate soot-bump region for both diesel and high-volatility fuel. High-volatility fuel can eliminate the soot-bump and achieve low soot emissions at lower injection pressure.