Injection timing optimization for low-carbon isopropanol–diesel blends in compression-ignition engines

This study investigates the combustion behavior, emission characteristics, and fuel economy of isopropanol (IPA)–diesel blends in a compression ignition engine operating under idle conditions (750 rpm, 20 Nm), with a particular focus on the effects of injection timing in ultra-low-load regimes. Owing to its low cetane number, high latent heat of vaporization, and inherent oxygen content, IPA significantly altered the ignition and combustion processes. The prolonged ignition delay extended the premixing phase, thereby enhancing particulate oxidation and markedly reducing smoke emissions, while slightly retarding combustion phasing and decreasing brake thermal efficiency (BTE). Stable operation was achieved up to 25% IPA, with the coefficient of variation of IMEP (COVimep) maintained below 3%. Compared to neat diesel, brake specific fuel consumption (BSFC) increased by 9–12%; however, the corresponding fuel cost penalty remained within 2%. Emission results showed simultaneous reductions in NOx (up to 6.6%), smoke opacity (up to 29.1%), and CO₂ (up to 4.76%), whereas CO and HC increased under certain injection strategies due to localized incomplete combustion. Overall, IPA–diesel blends effectively alleviate the NOx–smoke trade-off under idle operation, offering mechanistic insight into how fuel physicochemical properties govern combustion and emissions in ultra-low-load conditions.