Ignition and combustion characteristics of diesel piloted ammonia injections

Abstract

Ammonia is considered a potential carbon-free alternative to fossil fuels. However, its unfavorable combustion characteristics and propensity to form fuel NO${}_x$ pose a challenge for its use as fuel for internal combustion engines. The high-pressure dual fuel (HPDF) direct-injection of ammonia could offer the potential to reduce ammonia slip and decrease NO${}_x$ formation. The feasibility of this combustion process has not yet been shown experimentally in literature. This work examines the ignition and combustion characteristics of diesel piloted liquid ammonia sprays under engine-relevant conditions in a rapid-compression-expansion-machine (RCEM). By examining heat release rates (HRRs) under a variety of spatial and temporal spray interaction configurations, charge conditions as well as different diesel pilot amounts and injection durations, the fundamental prerequisites for successful combustion of liquid ammonia sprays are revealed. Strong interaction of the two fuels is found necessary to properly ignite ammonia. Misfiring due to deterioration of the pilot mixture formation can be avoided by injecting diesel first. A strong correlation between main ignition delay and burnout rate suggests a significant influence of wall quenching effects. An investigation of less reactive charge conditions suggests poor suitability of the combustion process for low-load engine operation. While reliable ammonia ignition was achieved for diesel pilot amounts as small as 3.2% of the total injected LHV, ignition is increasingly delayed for smaller pilot amounts. For an operating point, which showed favorable ignition behavior and high conversion rates, pilot fuel amount and injection duration are found to have a major influence on the combustion process.