Analysis Of The In-Cylinder Flow And Performance In SI Engine With Variable Valve Actuation

It is one of the main reasons for low thermal efficiency of SI engine that the throttle-driven load control with its pumping losses. If removing the throttle, and controlling the in-cylinder air quantity through the variable valve actuation to regulate the effective displacement, it needn't reduce intake port and in-cylinder pressure, thereby it can avoid the pumping loss basically. But the SI engine operation is changed greatly by using this load control method.The variable valve lift (VVL) has a significant impact on the in-cylinder flow, fuel atomization, fuel-air mixing and combustion particularly.The in-cylinder flow of SI engine using a variable-valve lift load control mode at different conditions (different valve profile and different speed) is studied with three-dimensional numerical simulation method in this paper. And the simulation results are validated by the PIV experiment data. The in-cylinder flow changing and the effects on the fuel atomization, fuel-air mixing by using the VVL load control mode is discussed. And the effects on the SI engine performance with the application of different method of engine load control are studied using the one-dimensional simulation method. On this basis, the principle of SI engine pumping loss and the engine performance enhancing by using the throttle free load control mode is described.The most results are obtained as follows:During the intake stroke,there are two parts of flow entering into the cylinder.The lift flow enters directly into the cylinder along the cylinder wall on the side of intake valves;the right flow is introduced into the cylinder along the exhaust valves and the cylinder wall on this side.These two parts of flow generate two Vortices in cylinder Obviously.But the lift flow decreases rapidly, as the piston moves upward. Finally, the in-cylinder flow field evolves into a single clockwise tumble vortex.During the later compression stroke, the tumble vortex distorts and breaks up into small vortices, and the small vortices breakups because of the energy convection between the tumble vertex and small vortices. As a result, the turbulence intensity reaches the maximum value.The tumble ratio is weakened as the maximum valve lift decreases, and strengthened slightly as engine speed increases. So as low-speed and low-valve lift condition, it needs to take other measures to strengthen the tumble. As the engine speed increases, the in-cylinder turbulent kinetic energy improves. Non-dimensional turbulence intensity (to remove the piston average speed influence) is strengthened as the maximum valve lift decreases ,for example, in the case of speed 960r/min-maximum valve lift 1.7mm, the Non-dimensional turbulence intensity reaches 2.9. In addition, the peak time of turbulence intensity is retarded as the valve lift lower, for example, in the case of speed 6000r/min-maximum valve lift 1.7 mm, it reaches 48°CA, which will help to maintain the turbulence.With the low-valve lift, the strong turbulence formed by intake flow jet contributes to the fuel droplets breakup and atomization. However, it has a certain role in impeding the fuel into the cylinder.By using VVLT (Variable Valve Lift and Timing) load control mode, SI engine can achieve the Miller cycle at all the work conditions, which can significantly improve the thermal efficiency of SI engine. During the idling condition, the achieved fuel improvement economy of 25% has been obtained.