Decoupling the injection strategy-dependent regulation mechanisms of methanol on Ammonia/diesel combustion under medium load

Ammonia is a promising zero‑carbon fuel, yet its application is hindered by low flame speed and high ignition energy. While methanol serves as a high-reactivity additive, its effectiveness strongly depends on operating strategies, particularly under medium-load conditions where chemical enhancement competes with physical cooling. This study employs a validated 3D-CFD model to reveal the dual effects of diesel start of injection (SOI) and methanol energy fraction (MEFP) on an ammonia/diesel engine with 90% ammonia energy fraction. Results demonstrate that SOI dictates whether methanol acts as a promoter or inhibitor. At early SOI (−18 °CA ATDC), methanol exhibits a non-monotonic effect: it initially promotes combustion but triggers severe deterioration when MEFP exceeds 40%, primarily due to thermal quenching of the diesel ignition kernel by the high latent heat of methanol. Conversely, late SOI (−14.2 °CA ATDC) creates a thermodynamic state that counteracts this cooling, allowing methanol to consistently enhance reactivity. An optimal configuration was identified at late SOI with 10% MEFP, achieving 43.3% thermal efficiency, reducing unburned NH₃ by 97%, and maintaining near-zero NO. Chemical kinetic analysis confirms that performance collapse is driven by a severe spatial disconnect between fuel-rich regions and the OH radical pool. • SOI strictly governs methanol's role as a promoter or inhibitor. • Late SOI (−14.2° ATDC) achieves 47.3% ITE and near-zero NO. • Methanol at 10% energy fraction reduces unburned NH 3 by 97%. • Early injection causes thermal quenching of the ignition kernel. • Sustaining the OH radical pool is key to complete NH 3 oxidation.