Abstract: Natural gas engines have become increasingly important in transportation applications, especially in the commercial vehicle sector. With increasing demand for high efficiency and low emissions, new technologies must be explored to overcome the performance limitations of natural gas engines such as limits on lean or dilute combustion, unstable combustion, low burning velocity, and high emissions of CH4 and NOx. This paper reviews the progress of research on natural gas engines over recent decades, concentrating on ignition and combustion systems, mixture preparation, the development of different combustion modes, and after-treatment strategies. First, the features, advantages, and disadvantages of natural gas engines are introduced, following which the development of advanced ignition systems, organization of highly turbulent flows, and the preparation of high-reactivity mixtures in spark ignition engines are discussed with a focus on pre-chamber jet ignition, combustion chamber design, and H2-enriched natural gas combustion. Third, the progress in natural gas dual-fuel engines is highlighted, including the exploration of new combustion modes, the development of novel pilot fuels, and the optimization of combustion control strategies. The fourth section discusses after-treatment systems for natural gas engines operating in different combustion modes. Finally, conclusions and future trends in the development of high-efficiency and clean combustion in natural gas engines are summarized.

Key words: Natural gas engines, Pre-chamber jet ignition, Combustion chamber, Fuel reforming, Dual-fuel combustion, After-treatment ')">">Natural gas engines, Pre-chamber jet ignition, Combustion chamber, Fuel reforming, Dual-fuel combustion, After-treatment ,