Comparative Study on the Effect of Premixed Equivalence Ratio on Engine Characteristics of Ammonia-Fueled Engine Under Diesel Pilot Ignition Versus Spark Ignition Combustion Mode

The adoption of zero-carbon fuel like ammonia will play a key role in the achievement of carbon neutrality targets. This work reports a comparative study on the effect of premixed equivalence ratio (Φ_premix aried from 0.8 to 1.25) on combustion and emission characteristics of ammonia-fueled engine operating initially under (a) compression ignition (CI) mode (ignited by dodecane pilot injection), and then converted to (b) spark ignition (SI) mode (by replacing fuel injector with spark plug). The experiments were performed in a single-cylinder engine (compression ratio = 16.4:1). The ammonia energy fraction was maintained at 95% during the CI mode and 100% ammonia in the SI mode. The power output and indicated thermal efficiency are lower in SI mode than in CI mode, certainly due to the occurrence of multiple auto-ignition sites. Indeed, the unburned ammonia emissions were observed to be higher in SI mode as compared to CI mode, especially in rich conditions. As expected, the carbon-based emissions reduced significantly in SI operating mode are very low, only due to lubricant oil leakages. N₂O emissions were higher at near stoichiometric-rich conditions (Φ_premix = 1.05–1.25) emissions but remain negligible in SI mode, except at the highest equivalence ratio (1.25). As N₂O has very high global warming potential (265 times CO₂ after 100 years) CO₂—equivalent impact was evaluated by considering both N₂O and CO₂ emissions.