Effects of ammonia and carbon dioxide concentration variation on ammonia-diesel blended fuel combustion in CO2/O2 atmosphere

In a closed-cycle diesel engine (CCDE) with liquid oxygen, CO2 and O2 constitute the main components of the gases within the cylinders. To investigate the effects of ammonia (NH3) and carbon dioxide (CO2) concentration variation on the combustion characteristics of ammonia-diesel blended fuel (ADBF) in CO2/O2 atmosphere, a new ammonia-diesel oxidation combustion (ADOC) model is proposed, which utilized quantum chemical calculation. Firstly, the structures of the reactants and transition states are optimized using the B3PLYP and CCSD(T) methods, and the reaction mechanism is simplified using the DRGEP method. A simplified mechanism comprising 348 species and 2168 reactions is obtained, which is integrated into a physical model in Converge to simulate spray ignition and combustion processes. Secondly, rapid compression machine (RCM) platform is built and different ammonia-diesel ratios (1:9, 2:8, 3:7 and 4:6) and CO2/O2 (35 %/65 %, 39 %/61 %, 43 %/57 %, 47 %/53 %, 51 %/49 % and 55 %/45 %) are measured. Furthermore, operational stability is verified according to three top dead center pressures. Thirdly, reaction pathway for CO + HNO, orbit-based analysis of the Fukui function, maximum in-cylinder pressures (MICPs), flame areas and propagation speeds are discussed between simulation and experiment for different ammonia-diesel ratios and CO2/O2. The results showed that ADOC model can effectively simulate MICPs of ADBF combustion under CO2/O2 atmosphere and maximum error is 6.53 % at 10 %NH3 + 90 %diesel (35 % CO2 / 65 % O2). END-2 is more likely to occurs than the formation of END-1 in CO + HNO → NH + CO2. As the CO2 concentration increases (from 35 % to 43 %), both the flame propagation speed and flame area significantly decrease. Conversely, with the increase in NH3 concentration, the initial acceleration of flame propagation becomes more pronounced. Especially at a CO2 concentration of 43 %, the flame propagation speed at an NH3 concentration of 30 % shows a substantial improvement compared to that at 10 % NH3. © 2025