The Research On Flow Characteristic And Performance Optimization Of Variable Intake System Of Motorcycle Engine

Variable inlet technology can improve engine'performance at part load by organizing variable intake flow and changing singular intake charge motion to variety inlet port charge'size and movement suitable for running load of engine. This paper developed a variable intake system on a single cylinder 125 ml engine with four valves. The system made intake charge variety along with the load of engine, and so, it improved part load performance.The developed variable intake system realized five intake mode. A series of experiments were carried out on steady state flow port test-bed for the variable inlet port performance measurement. The flow coefficient of five intake mode'port increase with valve lift by obvious different ratio. For small variable intake mode, variable intake flow coefficient higher than customary intake mode at small valve lift, and variable intake flow coefficient lower than customary intake mode at large valve lift. For large variable intake mode, tumble and swirl ratio higher than customary intake mode. The in-cylinder air motion is the resultant swirl—an inclined-axis swirl instead of single swirl or tumble.The variable intake device can adjust the inclined axis swirl in the cylinder.A three-dimensional steady model of CFD (Computational Fluid Dynamics) was developed to simulate the flow of inlet port and cylinder. The result showed that the swirl intensity weaken and its space become smaller at variable intake side; meanwhile swirl became bigger and swirl center moved to the center of the cylinder at customary intake side with flow area decrease. The model simulated three kind of variable intake position. The result showed that swirl ratio increase but flow coefficient became smaller at rest variable intake position, so three were impossible to optimize variable intake position.A three-dimensional transient model of CFD was developed to simulate the intake compress and combustion in the cylinder. The result showed that tubulence energy form at intake stroke, and variable swirl ratio after compress stroke effected tubulence energy. Tubulence energy increased with swirl ratio in the cylinder, and result that combustion ratio and cylinder pressure increase.The engine performance experiment was carried out. The result showed that the power increased 6.37%,5.32%,4.93% at 25%,35% 50% throttle opening; and improvement became smaller with larger load; so suitable variable intake can improve engine performance at small and middle load.Optimization was carried out for the entire engine, the result showed that the variety of long and diameter of intake manifold had effect to engine performance ; the variety of diameter and long of exhaust manifold had effect to engine performance. There were a most optimized intake-exhaust valves timing along with variable engine load.