Ignition and combustion study of premixed ammonia using GDI pilot injection in CI engine

In this study ignition of premixed ammonia was tested using a compression-ignited (CI) pilot fuel. n-heptane (C₇H₁₆) was used as pilot fuel, injected with a GDI injector, enabling very small pilot fuel injections. With this setup, the engine was run using up to 98.5 % energy from ammonia – higher than seen in other CI-engine studies with ammonia at the time of writing. The ammonia energy contribution was varied 80–98.5 %, while keeping the total -value constant. An important result observed was an increase in combustion efficiency from 81%–90% with higher ammonia energy due to a lower local λ-value of the ammonia–air mixture, indicating that for ammonia-ignition an easily ignitable ammonia–air mixture is preferable compared to a strong pilot injection. Lean ammonia–air mixtures showed a gap in heat release between the pilot and ammonia combustion phases, which lowered the combustion efficiency. Increased ammonia energy led to reduced ammonia-slip and N₂O-emissions and increased NO-emissions, and a lower CO-formation showed better pilot fuel oxidation.