Hydrogen production in dedicated reforming cylinder of in-cylinder reforming gas recirculation ammonia engine

Ammonia has been regarded as a promising alternative fuel for the maritime sector, but current ammonia engines still face challenges such as high unburned ammonia and limited thermal efficiency owing to the poor combustion characteristics of ammonia fuel, especially for the medium- and high-speed engines. The addition of hydrogen has recently attracted significant attention as an effective strategy to enhance ammonia combustion performance. However, the direct use of hydrogen in maritime applications remains challenged by storage and transportation constraints. Since ammonia serves as an excellent hydrogen carrier, it provides the feasibility to generate hydrogen via ammonia decomposition or reforming, thereby enabling ammonia‑hydrogen co-combustion through single ammonia fuel supply. Compared with the route that couples an external ammonia reformer, the in-cylinder reforming gas recirculation route achieved through ammonia-rich combustion in one or more dedicated reforming cylinder(s) can avoid the use of catalysts and additional energy consumption. Unfortunately, existing studies have mainly focused on evaluating the feasibility of the in-cylinder reforming route, leaving significant potential for further exploration. Therefore, this study numerically investigates the effects of intake temperature, wall temperature, piston profile, and compression ratio on the excess ammonia conversion, hydrogen production, and thermal efficiency of the reforming cylinder. Results indicate that wall temperature management can enhance the performance of the reforming cylinder owing to the improved excess ammonia conversion in the near-wall region, with the potential to increase the conversion from around 84% to 93%. Although increasing the intake temperature promotes excess ammonia conversion, it also increases in-cylinder heat transfer losses and reduces the thermal efficiency of the reforming cylinder. Compared with wall and intake temperature management, which usually require thermal barrier coatings or additional energy input, optimizing the combustion chamber parameters, such as the compression ratio, is more feasible. Increasing the compression ratio enhances thermal efficiency but comes at the cost of reduced ammonia conversion and hydrogen production and a compression ratio of 19 exhibits relatively superior overall performance. At a compression ratio of 19, the excess ammonia conversion reaches approximately 84%, enabling the reforming cylinder to supply hydrogen accounting for 21.7% of the total energy input to the remaining cylinders. Under most conditions, the hydrogen produced by one reforming cylinder, when recirculated into the other three cylinders, corresponds to around a 20% hydrogen enrichment level.