Influence of injection timing on wall film formation and low-temperature combustion in an air-cooled diesel engine under cold-start conditions

During cold-start conditions, the thermodynamic environment in air-cooled diesel engines is unfavorable for fuel evaporation and combustion, often resulting in spray-wall impingement and wall film formation. This study numerically investigates the effects of injection timing on mixture formation and combustion characteristics using a validated full-cycle CFD model of a heavy-duty air-cooled diesel engine. Results show that advancing the injection timing leads to lower in-cylinder temperature and pressure at the time of injection, which reduces the peak and standard deviation of the equivalence ratio, but increases wall-impinged fuel and wall film mass. The near-wall jet region exhibits a thicker film than the far-wall region, with peak film thickness rising under earlier injections. Combustion and heat release occur earlier with advancing timing, raising peak pressure and heat release rate. However, IMEP shows a non-monotonic trend due to the combined effects of combustion phasing and heat release dynamics, peaking at −12°CA. Earlier injection also delays CH 2 O and OH formation. CH 2 O appears at 6°CA ASOI for −6°CA injection, and at 14°CA ASOI for −21°CA; corresponding OH formation delays from 11°CA to 19°CA ASOI. The baseline injection timing (−6°CA) leads to a late main heat release phase and noticeable post-combustion due to increased fuel quantity. In contrast, moderate injection advancement extends ignition delay but aligns the main heat release phase closer to top dead center, improving combustion efficiency and cold-start stability. An optimal injection timing of −12°CA is identified for enhancing IMEP and overall cold-start performance.