Natural gas is a promising fuel for internal combustion (IC) engines with minimal modification, whereas its low power output and slow flame propagation speed remain a challenge for automobile manufacturers. To find a method of improving the natural gas engines, methane combustion with different additions was comparatively studied. High-speed direct photography and simultaneous pressure were performed to capture detailed combustion evolutions. First, the results of pure methane combustion confirm its good antiknock property, and no pressure oscillation occurs even there is an end-gas auto-ignition, indicating that high compression ratio and high boosting are effective ways to improve the performance of natural gas engines. Second, adding heavy hydrocarbons can greatly improve engines' power output, but engine knock should be considered if low antiknock fuel was used. Third, as a carbon-free and gaseous fuel, hydrogen addition can not only increase methane flame propagation speed but reduce cyclic variations. However, a proper fraction is needed under different load conditions. Last, oxygen-enriched combustion is an effective way to promote methane combustion. The heat release becomes faster and more concentrated, specifically, the flame propagation speed can be increased by more than 2 times under 27% oxygen concentration condition. The current study shall give insights into improving natural gas engines' performance.