This paper describes a newly developed hydraulic variable valve timing control (VTC) system, targeting the internal combustion gasoline engine, with an optimum angular position locking mechanism to reduce tailpipe emissions (TPE).
In general, emission control catalysts are used as one measure to reduce TPE. However, there is the issue that catalysts cannot remove pollutants before reaching its light-off temperature at cold engine start. To address this issue, we have been using a method of increasing the valve overlap period between intake valve opening (IVO) and exhaust valve closing (EVC) by operating a VTC system at engine start. This brings engine-out emissions (EOE) back to the combustion chamber to be burned, thereby reducing EOE levels.
However, this method requires about 3 seconds for the sufficient hydraulic pressure to start VTC operations. Additionally, the air-to-fuel ratio at engine start is calibrated to a rich condition to maintain combustion stability, which also increases EOE. The amount of EOE at this time accounts for a substantial portion of the total emissions in an emission test.
Therefore, we have developed a new VTC system that adds a function for locking the initial phase at an optimum angular position within the control range before the engine starts. This paper explains the operating principle of the new VTC system with an optimum angular position locking mechanism and the EOE reduction obtained by increasing valve overlap and other resultant effects.