Characterization of the system behaviour of a variable compression ratio (VCR) connecting rod with eccentrically piston pin suspension and hydraulic moment support

摘要

Making the geometrical compression ratio an adjustable parameter is considered the last remaining significant technological step in combustion engine design. Numerous prototype engines with variable compression ratio (VCR) were designed and tested. The implementation of VCR technology increases the averaged energy conversion efficiency in stoichiometric spark ignition (SI) engines ranging from 3 to 8% depending on the driving cycle, the fuel and the degree of turbo charging. The approach of a connecting rod with an adjustable length, a so called VCR connecting rod, is considered a promising solution because it can be implemented in almost every reciprocating engine regardless the type (inline, V or boxer engine) with a minimum of changes on existing engine layout. One design approach for a VCR connecting rod consists on an eccentrically piston pin suspension in combination with two hydraulic cylinders keeping the eccentric in a desired angular position. A change in compression ratio (CR) is initiated by means of actuating a hydraulic switch valve on the VCR connecting rod. The gas and mass forces acting on the piston pin result into a moment acting on the eccentric. In order to optimize the design of a VCR connecting rod with respect to robustness, weight and manufacturing costs, the dynamic behaviour and any arising phenomena must be known and understood. The aim of this work was to investigate this dynamic behaviour by means of fired engine testing. For this purpose, a VCR connecting rod was instrumented with sensors and the measured data was acquired synchronously to the crank angle. During these tests, operational parameters such as engine speed and load and the oil feed stream conditions were assessed for the two compression ratios stages and during the transitions from one to another compression ratio.