The global trend toward decarbonization is inspiring researchers to search for low or even zero carbon fuels for internal combustion engines. Ammonia (NH3) is considered a potential alternative fuel to conventional gasoline and diesel without carbon emissions. Although it is not an ideal fuel for use in engines due to its poor combustion properties, the use of hydrogen (H2), which has low ignition energy and can be decomposed from ammonia, as a combustion promoter can enhance its suitability for engine applications. A careful literature review indicates that the relatively few existing studies on NH3/H2 engines and the uncertainty of the NH3/H2 chemistry under typical high-temperature and high-pressure engine environments limit the determination of the ammonia-to-hydrogen ratio. However, this question is important when converting an existing engine to ammonia operation because it is related to compression ratio determination, aftertreatment design, ECU mapping, and on-board hydrogen production rate requirements. In other words, this outstanding question would affect the simultaneous design of other accessory systems and the development of ammonia engines. To increase the knowledge of the hydrogen concentration in the fuel feed of ammonia fueled engines in the engine community, this study discusses this question from the perspective of laminar flame speed (LFS), because it is closely related to reactivity, diffusivity, and exothermicity, which is a macroscopic parameter of premixed combustion and directly affects the efficiency and emissions of spark ignition engines. Specifically, the required proportion of hydrogen blended with ammonia is determined by comparing the NH3/H2 reaction time scale with the millisecond combustion time scale in typical engines. The results show that high temperature enhances the LFS of ammonia, hydrogen and their mixtures, while pressure has an inhibitory effect. In addition, increasing the compression ratio and avoiding fuel-lean operation are effective ways to improve the performance of ammonia combustion with less hydrogen. Considering the high required hydrogen consumption rate and the full load conditions that guarantee high combustion efficiency, off-road application may be a suitable scenario for ammonia-hydrogen engines. Before applying ammonia to automobiles, the technology for on-board hydrogen production should be effectively developed.

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