Influence of natural gas and hydrogen properties on internal combustion engine performance, combustion, and emissions: A review

•NG is considered a cleaner burning fossil fuel than oil or coal.•Hydrogen is a sustainable and environmentally friendly energy source that should be more widely used.•Experimental findings have concluded that hydrogen is better suited for use in gasoline engines due to its high self-ignition temperature.•Hydrogen addition to diesel fuel leads to a reduction in some exhaust emissions and fuel consumption, however NOx emissions were increased. This paper provides a comprehensive overview of the physical properties and applications of natural gas (NG) and hydrogen as fuels in internal combustion (IC) engines. The paper also meticulously examines the use of both NG and hydrogen as a fuel in vehicles, their production, physical characteristics, and combustion properties. It reviews the current experimental studies in the literature and investigates the results of using both fuels. It further covers the challenges associated with injectors, needle valves, lubrication, spark plugs, and safety requirements for both fuels. Finally, the challenges related to the storage, production, and safety of both fuels are also discussed. The literature review reveals that NG in spark ignition (SI) engines has a clear and direct positive impact on fuel economy and certain emissions, notably reducing CO2 and non-methane hydrocarbons. However, its effect on other emissions, such as unburnt hydrocarbons (UHC), nitrogen oxides (NOx), and carbon monoxide (CO), is less clear. NG, which is primarily methane, has a lower carbon-to-hydrogen ratio than diesel fuel, resulting in lower CO2 emissions per unit of energy released. In contrast, hydrogen is particularly well-suited for use in gasoline engines due to its high self-ignition temperature. While increasing the hydrogen content of NG engines reduces torque and power output, higher hydrogen input results in reduced fuel consumption and the mitigation of toxic exhaust emissions. Due to its high ignition temperature, hydrogen is not inherently suitable for direct use in diesel engines, necessitating the exploration of alternative methods for hydrogen introduction into the cylinder. The literature review suggests that hydrogen in diesel engines has shown a reduction in specific exhaust emissions and fuel consumption and an increase in NOx emissions. Overall, the paper provides a valuable and informative overview of the challenges and opportunities associated with using hydrogen and NG as fuels in IC engines. It highlights the need for f