Simulation of ammonia combustion in dual-fuel compression-ignition engine

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Ammonia is regarded as an attractive alternative fuel for combustion engines with a great potential to replace currently used petroleum-based energy carriers. Its direct combustion does not result in emission of carbon dioxide and it can be produced from renewable sources. The energy density of ammonia is reasonable, it is relatively easy to store and has well established infrastructure for production, storage, handling and distribution. However, ammonia is corrosive to some materials, difficult to ignite due to high octane number and high autoignition temperature, and has low heating value. Therefore theoretical and experimental studies are needed to enhance the performance and expand the operating range of ammonia-fuelled engines. This study investigates the feasibility of introducing ammonia to dual-fuel compression-ignition engine, where a pilot dose of diesel oil initiates the ignition of ammonia-air mixture. Numerical simulations of the combustion process were carried out using a one-dimensional model, parametrized according to the specifications of a four-cylinder, turbocharged compression-ignition engine with conventional direct injection supply system, modified slightly for ammonia induction. The model was validated by comparison with empirically determined data. The simulations were performed for different combinations of ammonia-diesel oil ratios at various engine load. Ammonia was treated as an energy replacement for diesel oil. Engine performance and emissions of selected pollutants in dual-fuel operation mode were compared to that with diesel oil only. The results showed that different ammonia–diesel oil ratios can be used to achieve the same engine torque. A higher than rated power, depending on engine load conditions, can be obtained with the addition of an extra dose of ammonia. The influence of ammonia on the emissions of particular pollutants varied and was discussed in detail. Acquired combustion characteristics and engine performance parameters allow for a positive verification of the proposed model. The drawn conclusions may be useful for the development of dual-fuel engine control algorithms.