Visualization of ammonia-methanol solution combustion under spark and passive jet ignition mode in an optically-accessible engine

Ammonia-methanol (A-M) solution is a promising carbon-neutral fuel for internal combustion engines. In this paper, the in-cylinder flame imaging and measurements of A-M solution under spark ignition (SI) and passive jet ignition (JI) mode were conducted, based on an optically-accessible engine. For 7 mol/L A-M solution combustion, the flame shows brown to purple color under stoichiometric condition and orange color under lean conditions. An increased burning rate is obtained under JI mode, with higher flame speed, shorter ignition delay and combustion duration at different excess air ratios (λ = 1∼1.6) than that in SI mode. It also allows a retarded spark timing for higher power output and thermal efficiency. As ammonia energy ratio increases from 13.7 % to 50 %, the flame development becomes slower under SI mode, while JI mode shows limited improvement of combustion. For pre-chamber nozzle designs, an area-to-volume ratio larger than 0.025 cm−1 is conducive to ignition and combustion for 7 mol/L A-M solution. Besides, the effects of slot, unequal and jet-crossing nozzle were also evaluated. Overall, this work provides the basic insights of flame characteristics and demonstrates the combustion accelerating effect of JI method for ammonia-methanol fueled engines. •Ammonia-methanol solution combustion in optical engine was studied for the first time.•In-cylinder flame shows brown, purple, or orange color under different excess air ratios.•Pre-chamber jet ignition significantly accelerates ammonia-methanol combustion.•The effects of slot nozzle, unequal nozzle and jet-crossing nozzle were evaluated.