This study investigates neat n-butanol, as a cleaner power source, to directly replace conventional diesel fuels for enabling low temperature combustion on a modern common-rail diesel engine. Engine tests are performed at medium engine loads (6～8 bar IMEP) with the single-shot injection strategy for both n-butanol and diesel fuels. As indicated by the experimental results, the combustion of neat n-butanol offers comparable engine efficiency to that of diesel while producing substantially lower NO_x emissions even without the use of exhaust gas recirculation. The greater resistance to auto-ignition allows n-butanol to undergo a prolonged ignition delay for air-fuel mixing; the high volatility helps to enhance the cylinder charge homogeneity; the fuel-borne oxygen contributes to smoke reduction and, as a result, the smoke emissions of n-butanol combustion are generally at a near-zero level under the tested engine operating conditions. However, the n-butanol combustion process typically features rapid heat release within a short combustion duration, which leads to high pressure rise rates. The use of exhaust gas recirculation is effective to postpone the ignition and slow down the combustion rates, but deteriorated HC and CO emissions and, in certain cases, misfire incidences are observed with high rates of exhaust gas recirculation. When low temperature combustion is enabled under the tested conditions, the application of n-butanol replacing diesel offers improved engine efficiency and requires less use of exhaust gas recirculation.
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