Numerical investigation of ammonia-rich combustion produces hydrogen to accelerate ammonia combustion in a direct injection SI engine

Abstract

Ammonia is a carbon-free fuel with significant potential to minimize carbon emissions. However, ammonia has weak combustion properties, necessitating more study to improve its combustion performance in engines. A numerical simulation was conducted to evaluate the impact of fuel composition and injection-ignition synergy strategy on the performance of an ammonia-hydrogen spark ignition engine with liquid ammonia direct injection and hydrogen port injection. Specifically, two distinct injection modes were investigated: injection after intake valve close (IAIVC) and injection before top dead center (IBTDC). The outcomes reveal that the IBTDC mode generates a strong stratification of ammonia near the top dead center, resulting in ammonia-rich combustion, then leading to enriched hydrogen production and finally enhancing ammonia combustion and shorting the combustion duration. Liquid ammonia in-cylinder direct injection reduces the combustion temperature and decreases NO emissions. Optimizing the injection timing and spark timing based on a split injection strategy results in lower fuel consumption and emissions. Specifically, NO emissions decrease from 30.5 g/kWh to 21.7 g/kWh at a similar ITE (≈43.5%), and ITE increased from 43.3% to 44.3% for similar NO emission (≈30.0 g/kWh), respectively, with the reduction in both NH3 and N2O emissions.