Investigation of wall temperature effects on fuel spray impingement and combustion characteristics in air-cooled diesel engines during cold start

Under extremely cold conditions, air-cooled diesel engines frequently experience fuel spray-wall impingement and the subsequent formation of liquid fuel films due to inadequate atomization and evaporation, which can lead to start-up failures. This study presents a comprehensive full-cycle CFD simulation of a heavy-duty air-cooled diesel engine to elucidate the influence of wall temperature on spray dynamics, mixture formation, and combustion characteristics during cold start. The results indicate that increasing the wall temperature from 233 K to 553 K markedly enhances fuel atomization and promotes the evaporation of wall films, thereby reducing the maximum fuel film mass from 29.2 mg to 23.3 mg and decreasing the final residual film mass by more than 60 %. Correspondingly, the in-cylinder pressure and indicated mean effective pressure increase from 56.9 bar to 69.3 bar and from 4.13 bar to 5.36 bar, respectively. The peak instantaneous heat release rate exhibits a non-monotonic trend, rising from 653.4 J/°CA at 233 K to a maximum of 1271.2 J/°CA at 393 K, and subsequently decreasing to 997.1 J/°CA at 553 K. This behavior is associated with variations in the mass of the combustible mixture within the optimal equivalence ratio range (0.8 < φ < 1.6), which reaches a maximum of 174 mg at 393 K. Furthermore, elevated wall temperatures facilitate the earlier formation of CH2O and OH radicals, accelerating the transition from low-temperature to high-temperature reactions. However, excessive wall heating induces non-uniform combustion and localized temperature gradients, increasing the risk of thermal damage and lateral wear on the piston crown. These findings underscore the need for further evaluation of thermal loads to optimize cold-start performance and combustion stability in air-cooled diesel engines operating in frigid conditions.