Hydrogen�s high flammability, low ignition energy, clean burning, and high flame speed are attractive advantages of using internal combustion engines to attain high performance and efficiency as well as achieving �real� clean emission with only water and carbon dioxide as exhaust. To overcome volumetric efficiency loss, high NOx emissions and abnormal combustion, the influence of air�fuel ratio is studied. Optimization of the air�fuel ratio effect and understanding of the combustion processes are actively being studied in the automotive industry and research institutions. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2019.