Performance evaluation of a PODE/diesel direct injection compression ignition engine with ammonia port injection for partial energy substitution

Ammonia (NH3) has emerged as a potential low-carbon fuel for heavy-duty applications, particularly in diesel-dominated sectors. The use of NH3 can partially replace diesel fuel and reduce carbon dioxide (CO2) emissions by lowering the carbon content in the fuel mixture. However, its carbon reduction potential is limited by the ammonia torque replacement ratio (ATRR), defined as the ratio of the torque produced by ammonia combustion to the total engine torque. To further enhance decarbonization, polyoxymethylene dimethyl ether (PODE), a low C/H ratio fuel, can be mixed with diesel. This study experimentally investigated the combustion and emission characteristics of a PODE/diesel direct injection (DI)compression ignition (CI) engine integrated with ammonia port fuel injection (PFI) for partial energy replacement, focusing on how engine control parameters influence these characteristics, an area that has received limited attention in existing literature. The results indicated that the ammonia PFI-PODE/diesel DI CI mode increased the contribution of the premixed combustion stage and reduced that of the diffusion combustion stage, which is consistent with the characteristics of low-reactivity-fuel PFI/high-reactivity-fuel DI CI engines when the equivalence ratio of the port-injected fuel-air mixture is too lean to sustain stable flame propagation. Moreover, partially replacing PODE/diesel with ammonia eliminated the separation of the premixed combustion stages of PODE and diesel, changing the triple peaks in the heat release rate to two. In terms of emissions, NH3-PODE/diesel operation reduced soot and nitrogen oxides (NOx) emissions, in addition to CO2 reduction, but led to increased unburned ammonia and nitrous oxide (N2O) emissions, necessitating aftertreatment to control engine-out N2O, a more potent greenhouse gas than CO2. Increasing the ATRR further reduced CO2 emissions, but ammonia slip became a major issue, reducing combustion efficiency to an unacceptable level, which limited further increases in ATRR. Advancing the injection timing improved thermal efficiency by reducing phasing loss, which in turn enhanced the greenhouse gas (GHG) reduction potential, but could not be set to maximum brake torque timing due to the maximum pressure rise rate limit. Increasing injection pressure improved thermal efficiency and further reduced GHG emissions, but at the cost of higher unburned ammonia emissions. Overall, NH3-PODE/diesel operation demonstrated stable performance and significant GHG reduction potential, making it promising for heavy-duty applications, particularly in off-road sectors.