Auto-Ignition and Flame Characteristics of Ammonia Reactivity-Controlled Compression Ignition Combustion in Comparison with Methane
Lin Chen, Ren Zhang, Haiqiao Wei
Abstract
Ammonia (NH 3 ), as a hydrogen carrier, is the key to being carbon-neutral for internal combustion engines. However, the poor combustion properties of NH 3 hindered the development of NH 3 engines. In the current study, an optical engine is adopted and the auto-ignition and flame characteristics of NH 3 reactivity-controlled compression ignition (RCCI) combustion (in comparison to CH 4 ) are optically studied. The experimental results show that there exists a critical injection timing for the effect of pilot fuel on NH 3 combustion. Before the critical timing, the combustion phase (auto-ignition) will delay because of the less over-rich pilot fuel. In addition, low-temperature heat release is observed for NH 3 / n -heptane under early injection timing conditions. When comparing the two fuels, the auto-ignition timing is delayed while the combustion duration (CA05-90) is shorter for NH 3 combustion. The main reason is that the low chemical reactivity of NH 3 inhibits the auto-ignition of the pilot fuel ( n -heptane), while the next oxidation pathways of NH 3 are extremely fast. Flame images show that, for NH 3 RCCI combustion, the highest probability of auto-ignition is in the bottom and NH 3 flame nearly propagates from the edge to the center, which is mainly due to the temperature inhomogeneity of the cylinder. Meanwhile, for CH 4 combustion, the highest probability of auto-ignition is in the center of the cylinder, and CH 4 flame nearly propagates from the center to the edge. The current study revealed the similarities/differences between NH 3 and CH 4 under RCCI conditions and can give some insights into the application of NH 3 engines.