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 lambda-value constant. An important result observed was an increase in combustion efficiency from 81%-90% with higher ammonia energy due to a lower local lambda-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.