Experimental study and numerical simulation on in-cylinder flow of small motorcycle engine

This study aimed at experimental and numerically investigating in-cylinder flows of a small motorcycle engine under steady-state conditions. The experiment was conducted in the engine head for a variety of fixed valve lifts at two pressure drops (300 and 600 mmH2O). Besides, this study attempted to analyze the characteristics of in-cylinder flows in a small engine by applying CFD methods. Looking at the results, there was a good agreement between the results of experiment and simulation in terms of flow coefficient, air flow rate and discharge coefficient at two pressure drops. In both horizontal and vertical planes, both increased valve lift and pressure drop delivered increased velocity and vorticity magnitude. An increase in pressure drop at the beginning of valve lift opening appeared to have no effect on the swirl ratio until the valve lift reached 5 mm. However, the swirl ratio got a 25% reduction when the valve lift reached 6.25 mm. An increase in pressure drop at the intake stroke did not deliver a significant effect on the tumble ratio and accumulated air mass. However, the accumulated air mass increased 3.77% at compression stroke. An increase in pressure drop delivered a significant effect on turbulent kinetic energy (TKE), turbulent length scale and turbulent kinematic viscosity. TKE reached its peak (200%) at 470 °CA, while turbulent length scale and turbulent kinematic viscosity reached their peaks at 590 °CA where the intake valve was almost closed. The increase in turbulence in fact produced a more homogeneous in-cylinder air-fuel mixing. Besides, the increase in turbulence directly increased the rate of fire propagation. Further study would be expected to focus on modifying the design of intake port for improving the air flow characteristics of small engines. Then, this study was expected to reduce the number of experiments required for investigating optimized parameters in small engines.