In-cylinder thermochemical fuel reforming for high efficiency in ammonia spark-ignited engines through hydrogen generation from fuel-rich operations

Abstract

The increasing trend towards global carbon neutrality is driving interest in ammonia fuel as a potential zero-carbon solution for transportation. However, due to the non-ideal combustion characteristics of ammonia, it is necessary to mix it with hydrogen to achieve better engine performance. While generating hydrogen from ammonia on-board is a key technology, there has been limited research in this area, which is hindering the introduction of ammonia engines to the market. This paper proposes an initial study on the feasibility of thermochemical fuel reforming (TFR) technology for generating hydrogen from ammonia fuel-rich operations. A zero-dimensional engine model, validated against experimental results, is used to assess the potential of the TFR approach and identify any barriers to implementation. The results indicate that the optimum air-fuel ratio for hydrogen generation is an equivalence ratio of two, as it maximizes hydrogen production rates while minimizing operational complexities. Moreover, increasing intake temperature, pressure, and compression ratio enhances hydrogen generation from the ammonia-rich mixture. However, owing to the limited reactivity of the ammonia fuel, the rate of hydrogen production remains relatively small (less than 1%/cycle), significantly falling short of the demand. Additionally, the combustion of the fuel-rich mixture in the TFR cylinder results in approximately 20%/cycle of hydrogen in the emitted gases, potentially enhancing the efficiency of working cylinders, but it also poses the risk of engine component failures due to the high pressure rise rate. In conclusion, the use of solely ammonia to produce hydrogen in the TFR cylinder proves to be inefficient, warranting further investigations into the potential benefits of incorporating additives to promote hydrogen production. Alternatively, if a catalyst is utilized to promote hydrogen production from ammonia, a separate electrically heated hydrogen generation system independently of the engine system proves to be more effective, allowing continuous hydrogen production, unlike the intermittent high-temperature environment provided by TFR technology.