Study on the effect of ammonia energy ratio on the combustion characteristics of ammonia-diesel dual-fuel under CO2/O2

In order to investigate the effect of ammonia energy ratio (AER) on the ignition characteristics of ammonia-diesel dual-fuel in a CO 2 /O 2 atmosphere, a simplified diesel-ammonia (SDA) mechanism is developed, in which five NH 3 -related kinetic parameters are updated on the basis of the B2PLYP/def2-TZVP method with dispersion correction, while a diesel surrogate consisting of 70 % n-heptane and 30 % toluene is adopted, and the final mechanism contains 307 species and 1763 reactions. Meanwhile, a visual rapid compression machine (RCM) system is established, and experiments are carried out at ammonia energy ratios of 0 %, 10 %, 20 %, 30 %, and 40 % under three background gas conditions, namely 47 %CO 2 /53 %O 2 , 51 %CO 2 /49 %O 2 , and 55 %CO 2 /45 %O 2. Furthermore, ignition delay time, heat release rate, and reaction pathways are analyzed by combining numerical calculation with experimental observation. The results show that the SDA mechanism can accurately predict the ignition delay time of the ammonia–diesel dual-fuel system in a CO 2 /O 2 atmosphere, with maximum and average relative errors of 8.49 % and 3.02 %, respectively. Under the condition with 30 %AER, the peak heat release rate (HRR) in the 55 %CO 2 atmosphere decreases by about 66 % compared with that in the 47 %CO 2 atmosphere, whereas the spectral width increases by about 43 %. Moreover, under high AER and CO 2 /O 2 atmosphere, recycling and termination reactions related to NH 2 and H 2 NO are enhanced, low-temperature hydrocarbon chain-branching reactions are suppressed, and the coupled effect finally leads to delayed ignition, weakened heat release, and lower flame luminosity.