Browse Publications Technical Papers 2019-01-0069
2019-01-15

Development of a Highly Flexible Variable Valve Train System for Combustion System Investigations 2019-01-0069

For the optimization and extension of operating range of future combustion systems aiming at ultra-low emissions and high efficiency, the control of gas exchange plays a major role. Important parameters for optimization are e.g. volumetric efficiency, residual gas control, in-cylinder charge motion and precise control of the level of homogeneity or inhomogeneity of the charge as required by the particular combustion mode. In addition, advanced operating modes such as Miller or Atkinson cycle or gasoline compression ignition demand a high degree of variability in cam timing. A highly flexible variable valve train has been designed for the investigation and development of such new combustion processes. This novel valve train is based on a mechanically fully variable actuation concept using two independently rotating cam disks per valve. By this arrangement, new degrees of freedom in the design of the valve opening curve arise. As an example, it is possible to vary both the valve lift and the opening duration of a secondary opening event of the exhaust valves during the intake process (“second event”) without affecting the main exhaust valve lift. This functionality can be used to precisely adjust the amount of residual gas on an individual cycle basis. Another option is to separately control valve lift and opening duration, which can be used on the inlet to optimize volumetric efficiency, including Miller and Atkinson cycle operation. It is also possible to keep valve closing deceleration constant while varying the opening duration. For a single cylinder research engine using a four-valve cylinder head, this valve train has been designed in a way so that all four valves can be controlled independently, opening the possibility of inlet valve phasing combined with a second event on the exhaust. This allows precise control of charge motion and – in conjunction with the timing of direct injection – of the degree of homogenization of the in-cylinder mixture. The layout was verified and optimized using multi-body simulation coupled with structural analysis by FEM. For validation of the design methodology and of the multi-body simulation, the new valve train has been manufactured and tested on a mechanical component test bench. This allowed the verification of valve lift and acceleration as well as of the friction caused by the valve train. In a “backward” design approach, the valve lift curves and the timing variation range required for the generation of particular variations of in-cylinder charge motion were defined by 3D CFD simulation. On this basis, the layout of the valve train components for realization in the research engine was defined. The new valve train will be employed for the development of highly efficient and low-emission combustion systems on the test bench.

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