Study on stable combustion mechanism and emission characteristics of a diesel/methanol dual-fuel injection system under constant pressure differential control

Abstract

Pressure differential fluctuations can significantly disturb pilot diesel storage and ignition stability in pressure-differential-driven dual-fuel injection systems, thereby limiting the achievable methanol substitution rate (MSR). To address this issue, a constant-pressure-differential-controlled diesel/methanol dual-fuel injection system was developed and experimentally investigated on a 0.498 L single-cylinder compression-ignition engine. A physics-based causal framework, namely Δ P - m s - η - MSR, was established to elucidate the mechanism by which pressure differential regulation influences combustion stability and methanol substitution capability. The results demonstrated that the front-end diesel storage quantity ( m s ) was primarily governed by the pressure differential (Δ P ) and exhibited an approximately linear relationship with Δ P . By stabilizing Δ P with a mechanical feedback control unit, fluctuations in m s and pilot ignition energy were effectively suppressed, resulting in a significant improvement in cycle-to-cycle combustion consistency. Compared with the conventional dual-rail control strategy, the coefficient of variation of indicated mean effective pressure ( COV IMEP ) decreased by approximately 80%. The enhanced ignition stability reduced unnecessary diesel storage while maintaining reliable pilot ignition. Consequently, the maximum MSR increased from 80% to 88%, and the pilot diesel utilization efficiency ( η ) increased from 43.48% to 64.52%, corresponding to an increase of 21.04 percentage points. As the MSR increased, soot and NOx emissions decreased by 99.57% and 67.83%. At full load, BTE reached a maximum of 38.71% at MSR 57%, then decreased-indicating a trade-off between efficiency and maximum methanol substitution. The results confirm that pressure differential stabilization is the fundamental prerequisite for achieving high-MSR combustion in pressure-differential-driven dual-fuel injection systems.