Effect of the intake port flow direction on the stability and characteristics of the in-cylinder flow field of a small motorcycle engine

The in-cylinder flow is one of the important factors that affect the quality of the air–fuel mixture in the engine. One parameter that affects the in-cylinder flow is intake port design. In this study, the effect of intake port modification based on the flow direction was investigated to understand the flow characteristics of a small motorcycle engine. An experimental system with a steady flow bench for a variety of valve lifts at pressure drops of 300 mmH2O was installed and a simulation model based on the computational fluid dynamics method was setup. Looking at the results, the tangential port increases the flow coefficient fourfold compared to other models. All models show that the greater the valve lift, the greater the airflow velocity. However, the tangential port has almost double the flow velocity compared to the other models on the 6.46 mm valve lift at 35 m/s. The helical port with the same direction has double of first tumble peak compared to other models at a crank angle of 460°. However, this port decreased drastically by 60% at the second peak. Although the first tumble peak was only half of the helical port with the same direction, the tangential port decreased only 25% at the second peak. Moreover, the tangential port has the highest first and second peaks of the kinetic energy of 13.5 m2/s2 and 6 m2/s2 respectively. The cyclic scattering of kinetic energy occurs in all models, especially the tangential port where the first cycle is quite large and the second to tenth cycles are quite stable. This is a concern because the conversion of kinetic energy arises from various flow structures with different intensities, which can be a potential problem for flow stability. But in general, the tangential port is the most optimal port compared to other models where it has increased flow velocity and better vortices resulting in increased turbulence in the cylinder engine. The increased turbulence is expected to result in a more homogeneous air–fuel mixture in the cylinder engine.