Effects of direct injection strategies on combustion and emission characteristics of a n-butanol dual-injection engine

As a promising oxygenated renewable fuel for spark-ignition engines, n-butanol exhibits relatively high latent heat of vaporization, high viscosity, and low volatility, making mixture preparation sensitive to injection strategies. In this study, a three-dimensional CFD model was used to investigate the effects of direct-injection ratio (DIR) and direct-injection timing (DIT) on mixture formation, combustion characteristics, gross indicated efficiency (GIE), unburned n-butanol distribution, and emissions in an n-butanol dual-injection engine. Four DIRs of 40%, 60%, 80%, and 100% and four DITs of −320°CA, −300°CA, −280°CA, and −260°CA were considered under a fixed speed-load condition. In addition, the influence of direct-injection pressure (DIP) was further analyzed under the balanced DIT/DIR strategy. The results indicate that at the ignition timing of −20°CA, the mixture non-uniformity index (MNI) generally increases with increasing DIR and retarding DIT. When DIT = −320°CA, MNI increases from 0.119 to 0.294 as DIR increases from 40% to 100%; when DIR = 100%, MNI further increases from 0.294 to 0.367 as DIT is retarded from −320°CA to −260°CA, which indicates a decline in mixture uniformity. Combustion results indicate that moderate DIRs are more favorable for concentrated heat release and efficient energy conversion. The highest GIE of approximately 40.07% is obtained at DIT = −300°CA and DIR = 80%, followed by DIT = −320°CA and DIR = 60%, with a GIE of approximately 39.89%. The temperature and H 2 O 2 distributions indicate that injection strategies affect combustion development and oxidation activity. The emission results show that reducing DIR decreases CO, HC, and Soot emissions, whereas NOx emissions increase. The DIP analysis indicates that 20 MPa provides a favorable balance among mixture preparation, combustion performance, and emissions under the balanced DIT/DIR strategy. The strategy of DIT = −320°CA and DIR = 60% is identified as a more balanced operating condition when considering mixture uniformity, combustion efficiency, and emissions together within the investigated range.