Analysis Of Unthrottled Engine Operation With Variable Valve Actuation

The part-load fuel consumption potential of gasoline engine is always limited by the way of restrictor load controlling mode which bring pumping loss. Using variable valve actuation can remove the Throttle from gasoline engine and control engine load by the intake valves. However, there are lots of ways to adjust intake valve motion for unthrottled load control and the different unthrottled load control modes will effect on the process of intake stroke and other Performances of the engine.Firstly, a one-dimensional simulation model is built in the research for a gasoline engine which is based on GT-Power. In this study, the intake valve opening position and the intake valve duration are independent while the peak intake valve lift is adjust to achieve the desired engine load. The research studies pumping loss, the initial flow velocity of the intake stroke, the combustion duration, the ignition delay period and the fuel economy performance under different unthrottled load control modes. And the research reaches the following conclusion.1. Not all the unthrottled load control modes can reduce the pumping loss for gasoline engine. The pumping loss increases as the intake valve duration gets longer with the same intake valve opening position. And the pumping loss increases as the intake valve opening position gets delayed with the same intake valve duration. The pumping loss declines most obviously when it comes to the unthrottled load control mode with the earliest intake valve opening position and the shortest intake valve duration. It can decline 40%-50% of the pumping loss for part load at all speeds.2. For different unthrottled load control modes the initial flow velocity of the intake stroke increases as the intake valve duration gets longer with the same intake valve opening position. And the initial flow velocity of the intake stroke increases as the intake valve opening position gets delayed with the same intake valve duration.3. For different unthrottled load control modes the combustion duration and the ignition delay period decrease as the intake valve duration gets longer with the same intake valve opening position. And the combustion duration and the ignition delay period decrease as the intake valve opening position gets delayed with the same intake valve duration.4. For different unthrottled load control modes the fuel economy performance increases as the intake valve duration gets shorter with the same intake valve opening position. And the fuel economy performance decreases the intake valve opening position gets delayed with the same intake valve duration. The fuel economy performance increases mostobviously when it comes to the unthrottled load control mode with the earliest intake valve opening position and the shortest intake valve duration. It can increase 5%-6% of the economy performance at low load and 3%-4% of the economy performance at medium load.Secondly a three- dimensional simulation model is built in the research which is based on AVL-FIRE for further study on tumble motion, swirl motion and turbulence during intake stroke and compression stroke with different load control modes. The study chooses two typical load control modes in 2200r/min, 25% load. “a” unthrottled load control mode is the one which has better economy performance and less pumping loss with the intake valve duration is 144°CA and the peak intake valve lift is 3.6mm. “b” unthrottled load control mode is the one which has better combustion with the intake valve duration is 238°CA and the peak intake valve lift is 0.9mm. And the research reaches the following conclusion.1. While “a” unthrottled load control mode can improve the initial flow velocity of the intake stroke with its lower intake valve lift, the intake valve open duration is short and the intake valve shuts early,which cause that fresh airflow cannot continue to provide enough energy for the air movement in cylinder, therefore it limits the strength of the overall air flow movement in cylinder, tumble motion, swirl motion and turbulence are weaker than the original machine.2. “b” unthrottled load control mode has low peak intake valve lift, which will product high-speed air motion around the intake valve seat. It drives high speed of the air motion in cylinder. And in this mode, the high-speed air motion of the intake valve on the left side forms the anticlockwise tumble motion near the left side of the cylinder. It makes the tumble motion near the right side of the cylinder weaker which limits the tumble ratio. “b” unthrottled load control mode can also make obvious single swirl motion at the mid of compression stroke which makes swirl ratio higher. “b” unthrottled load control mode can effectively improve turbulence intensity in the cylinder, and the peak of turbulence intensity appeared late, its peak is approximately double of the original machine load control mode. The turbulent kinetic energy in the cylinder near the ignition time can be significantly greater than other load control modes.