Auto-Ignition and Flame Characteristics of Ammonia Reactivity-Controlled Compression Ignition Combustion in Comparison with Methane

Ammonia(NH3), as a hydrogen carrier, is the key tobeing carbon-neutral for internal combustion engines. However, thepoor combustion properties of NH3 hindered the developmentof NH3 engines. In the current study, an optical engineis adopted and the auto-ignition and flame characteristics of NH3 reactivity-controlled compression ignition (RCCI) combustion(in comparison to CH4) are optically studied. The experimentalresults show that there exists a critical injection timing for theeffect of pilot fuel on NH3 combustion. Before the criticaltiming, the combustion phase (auto-ignition) will delay because ofthe less over-rich pilot fuel. In addition, low-temperature heat releaseis observed for NH3/n-heptane under earlyinjection timing conditions. When comparing the two fuels, the auto-ignitiontiming is delayed while the combustion duration (CA05-90) is shorterfor NH3 combustion. The main reason is that the low chemicalreactivity of NH3 inhibits the auto-ignition of the pilotfuel (n-heptane), while the next oxidation pathwaysof NH3 are extremely fast. Flame images show that, forNH(3) RCCI combustion, the highest probability of auto-ignitionis in the bottom and NH3 flame nearly propagates from theedge to the center, which is mainly due to the temperature inhomogeneityof the cylinder. Meanwhile, for CH4 combustion, the highestprobability of auto-ignition is in the center of the cylinder, andCH(4) flame nearly propagates from the center to the edge.The current study revealed the similarities/differences between NH3 and CH4 under RCCI conditions and can give someinsights into the application of NH3 engines.