Experimental investigation on ammonia combustion ignited by methanol-enriched active pre-chamber in an optical engine

To enhance the ignitability and burning rate of ammonia-fueled spark ignition engines, it is an effective way to adopt active pre-chamber (PC) ignition with high-reactivity fuel. Therefore, in this paper, the ammonia combustion characteristics using methanol-enriched active PC ignition were investigated, based on optical engine experiments with simultaneous in-cylinder flame imaging and pressure measurement. Results show that the overall combustion process is characterized by a three-stage heat release: (I) jet flame controlled combustion; (II) ammonia flame propagation; (III) post jet induced combustion. In stage I, methanol active PC jets with high-momentum and high-reactivity are discharged to ignite MC ammonia mixture, contributing to the first peak of heat release rate. In stage II, ammonia turbulent flame propagation dominates the combustion. In stage III, post jets induce the third peak of heat release rate. In this way, with a minimal methanol energy ratio (ER) of lower than 10 %, stable and fast ammonia combustion is achieved under stoichiometric (λ_MC = 1) and lean-burn (λ_MC = 1.2) condition, with cyclic variation of indicated mean effective pressure (IMEP) lower than 5 %, flame probability higher than 80 % and maximum global flame speed of about 4 m/s. A highest IEMP of 0.72 MPa and a peak indicated thermal efficiency (η_it) of 30 % is obtained at λ_MC = 1 and λ_MC = 1.2, respectively. However, at the rich limit of λ_MC = 0.9 and lean limit of λ_MC = 1.4, the quenching of jet flames leads to weak ignition and unstable combustion, which is identified by reduced flame probability distribution and increased cyclic variation of IMEP. In addition, to ensure the cyclic combustion stability, there is also an operating range of methanol ER. At λ_MC = 1.2, unstable ignition and deteriorated combustion occur below the lower limit of ER = 6 % and above the upper limit of ER = 17 %. This paper can provide data support and operation strategy for the combustion improvements of ammonia-methanol active jet ignition engines.