Investigation into partially premixed combustion fueled with N-butanol-diesel blends

Partial Premixed Compression Ignition (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 NOX and smoke emissions and also improve thermal efficiency. As a clean and renewable biofuel, n-butanol has many superior properties, such as good miscibility in diesel, low cetane number and high volatility, which make it an attractive alternative or blending component to diesel fuel to achieve PPCI. In this paper, PPCI combustion in a four-cylinder light-duty diesel engine fueled with n-butanol-diesel blends was achieved through early or late injection condition in which the whole amount of fuel was delivered before ignition. The aim of the project was to evaluate the potential in realizing Partial Premixed low temperature combustion with different fuel reactivity and oxygen content. The exploration strategies were focused on the early or late injection timing, injection pressure and load rate to evaluate the effects of n-butanol fuel characteristics on PPCI combustion and emissions. The effects of injection timing and load rate on PM mass-size distribution have also been investigated. Results show that both early and late injections have long premixed duration, which is helpful to form more homogeneous mixtures, making a great improvement on smoke emission. With the increase of n-butanol blending ratio, smoke emission can be reduced by up to 70%, while NOX shows a slight increase under moderate EGR rate. When the load rate is increased, the premixed combustion fraction decreases apparently, leading to a dramatic increase of soot mass due to the short ignition delay. It is meaningful to find that PPCI combustion can be achieved with reasonable injection timing, lower injection pressure, moderate EGR without penalties in fuel consumption when using high n-butanol blending ratios. Moreover, n-butanol-diesel blends can effectively reduce the soot mass when compared with pure diesel, but has little influence on the PM size distribution. Results indicate that n-butanol-diesel blends are more conducive to expand PPCI operating condition and improve engine performance and emissions.