Combustion and emissions of an ammonia-gasoline dual-fuel spark ignition engine: Effects of ammonia substitution rate and spark ignition timing

Abstract

To alleviate global warming, the combustion of zero-carbon ammonia fuel in engines is considered to diminish carbon emissions in the transportation sector. Nevertheless, ammonia presents challenges such as low laminar flame speed, high minimum ignition energy, and high auto-ignition temperature. To address these issues, the combustion performance of ammonia engines is enhanced through the use of gasoline as a promoter. This study examined the combustion and emissions characteristics of a four-cylinder spark ignition engine operating on an ammonia-gasoline dual-fuel, with a compression ratio of 11.5:1 under different operation conditions. The influences of ammonia substitution rate, spark timing, and engine load were investigated. The findings revealed that raising the ammonia substitution rate from 0 % to 80 % significantly affected how the flame kernel forms, slowing down the flame propagation speed. Increasing the ammonia substitution rate lowers in-cylinder combustion temperature, reducing heat transfer loss to coolants and improving thermal efficiency by 3.2 %. In comparison to pure gasoline, the combustion of the ammonia-gasoline mixture produced higher NOx emissions, which was primarily from fuel-NOx source. Meanwhile, emissions of unburned ammonia gradually increased, while CO and HC emissions decreased. The main source of N₂O emissions was the low-temperature oxidation pathway of ammonia, and a higher ammonia substitution rate or a delay in spark timing resulted in higher N₂O emissions. At higher engine loads, the ammonia-gasoline engine exhibited more stable operation and improved thermal efficiency. Nevertheless, these benefits came at the cost of increased emissions of unburned NH₃, HC, and CO, while the emissions of N₂O and NOx were reduced.