A comparative study on combustion and emission characteristics of ammonia ignited by n-heptane, n-dodecane, and PODE in an optical engine

Abstract

Combining high-reactivity fuels is acknowledged as an efficient strategy for improving ammonia combustion performance. However, there is currently a lack of direct experimental evidence regarding the suitability of various fuels and their impacts on combustion and emission characteristics. In this study, a comparative investigation was conducted to explore the influence of n-heptane, n-dodecane, and polyoxymethylene dimethyl ether (PODE) on ammonia’s combustion and emission within an optical engine. Pressure trajectories, combustion flame evolution, and nitrogen-based pollutants were addressed. Considering the poor ignition of ammonia, the high-reactivity fuel was maintained at a 30% energy ratio, and the injection timing was varied between −70 and −20 °CA aTDC. The results demonstrate that n-dodecane and PODE can enhance ammonia combustion more pronounced compared to n-heptane. Specifically, higher thermal efficiency and reduced cyclic variations are observed, alongside an extended stable operating range. Moreover, among the three highly reactive fuels, PODE exhibits optimal performance in combustion phasing and duration, leading to more complete combustion of ammonia. Combustion visualizations indicate that, when adopting PODE fuel, sequential auto-ignition becomes more prevalent, particularly at early injection timings. Regarding nitrogen-based emissions, both PODE and n-dodecane significantly reduce NH₃ and N₂O emissions, but NOx emissions increase, especially at early injection timings. Notably, PODE can achieve NOx emissions that are comparable to those of n-heptane under late injection conditions. Therefore, adopting PODE fuel with an optimized injection strategy can present substantial benefits in terms of fuel economy and reduced pollution emissions.