Lean Ammonia Combustion in an Optically Accessible Combustion Chamber Using Hydrogen Jet Ignition

Abstract

Ammonia is a promising carbon-free fuel for deep-sea shipping, but its low reactivity makes it difficult to reliably combust. Hydrogen prechamber jet ignition can be used to address this challenge, yet the minimum hydrogen energy share (HES) needed to maintain stable, lean ammonia combustion under a stratified direct-injection charge needs to be investigated. This paper presents the optical and emission characteristics of a direct-injected ammonia lean-burn combustion using hydrogen-fueled prechamber jet ignition. The effect of HES on the combustion process was investigated and visualized under these conditions. All of the tests were conducted in a modified single-cylinder, optically accessible combustion chamber at a fixed lean equivalence ratio of 0.6, with HES varied from 6% to 10%. Combustion performance, flame development, and NOx/N2O/unburned NH3 emissions were characterized. The results show that stable lean ammonia combustion can be sustained with HES as low as 6%, with combustion performance and flame propagation improving markedly above this threshold; 7–8% HES was identified as the optimum, balancing peak pressure, combustion phasing, and emissions. Combustion visualization imaging further revealed a temporal delay between OH* emission and heat release rate across all conditions, attributed to the ammonia decomposition pathway, and identified the jet penetration zone as the dominant region of OH*/NH* radical formation. These findings establish a lower HES bound for stratified ammonia/hydrogen ignition and provide the spatially resolved radical formation profile for this combustion mode.