Combustion deterioration mechanisms during the transient starting process of a highly-boosted diesel engine

Abstract

Severe combustion deterioration and cycle-to-cycle variations during the transient starting of highly boosted diesel engines primarily stem from turbocharger lag and air-fuel mismatch. To overcome the limitations of conventional single-cycle simulations in capturing consecutive in-cylinder dynamics, a 1D-3D coupled multi-cycle CFD methodology is proposed. Macroscopic boundaries, notably exhaust backflow, were dynamically mapped from a 1D system model into a 3D model. Results reveal a three-stage transient combustion evolution: establishment, deterioration, and stabilization. Induced by intake lag, the exhaust backflow acts as the primary driver for deterioration by elevating the internal exhaust gas recirculation (EGR) rate and distorting spatial oxygen distributions. Furthermore, sensitivity analysis on the high-pressure indicated mean effective pressure (IMEP) demonstrates that intake air mass and internal EGR dominate transient performance. Additionally, maintaining a slightly enriched combustion state (λ≈1) rather than conventional lean combustion effectively enhances starting capability.