Effect of N-butanol-Diesel Blends on Partially Premixed Combustion and Emission Characteristics in a Light-duty Engine

PPCI in diesel engine is a combustion mode between conventional diesel combustion and homogeneous charge compression ignition (HCCI) combustion, which has the potential to simultaneously reduce NO_X and soot emissions and improve thermal efficiency. N-butanol as a kind of clean and renewable biofuel can effectively prolong ignition delay and enhance fuel/air mixing because of their low cetane number, high volatility fuel characteristics, which make it a better alternative fuel to achieve PPCI. In this paper, PPCI combustion in a boosted four-cylinder diesel engine fueled with n-butanol-diesel blends is realized by adjusting injection timing and EGR rate based on single injection. The results show that both early and late injection have long premixed duration, which is helpful to form more homogeneous mixture, and no diffusion combustion is found in heat release rate curve. Premixed combustion and low temperature combustion are the key factors to reduce PM and NO_X. With the increase of blending ratio, soot emission can maximumly be reduced by 70%, while NO_X shows a slight increase at low load rate. Meanwhile, the equivalent BSFC is decreased due to higher thermal efficiency. It is meaningful to find that PPCI combustion can be achieved with lower injection pressure, EGR rate without penalties in fuel consumption when using high blending ratios. However, when the load rate is increased, the premixed combustion fraction decreases apparently due to the short ignition delay. But the premixed combustion fractions of high blending ratio are still higher than pure diesel even at high load rate, which indicates that n-butanol-diesel blends is more conducive to expand PPCI operating conditions.