AbstractTo improve the thermal efficiency of a shale-gas engine and reduce the cycle variations in the engine, this study adopts the experimental method and simulation to examine the engine’s combustion process and cycle variations under different ignition modes. Nonlinear dynamics are employed to explore the dynamic characteristics of the combustion process. The influence of the number of spark plugs on flame propagation in the cylinder is evaluated. The results indicate that when dual spark plugs are used for ignition, the turbulence intensity increases, and the flame propagation speed increases. This approach is conducive to improving the power of the shale-gas engine. Synchronous dual ignition positively affects engine power and combustion stability. The cycle variation coefficient, correlation dimension, and maximum Lyapunov exponent of synchronous dual ignition reach their lowest values, which are 1.56%, 1.73%, and 0.073%, respectively. The phase space trajectory distribution area of the synchronous dual ignition is small. The dense distribution of Poincaré map scatter points and return map points indicate that the combustion process tends to be stable. This study pays particular attention to the significance of reducing the cycle variations of a shale-gas engine and improving its stability.