Assessment of an eccentric link in the connecting rod of a spark ignition engine intended for variable compression ratio operation

Abstract

Previous modelling and numerical simulation has determined that potential exists to improve the low load efficiency of spark ignition engines through the application of variable compression ratio (VCR) in conjunction with other engine control technology. A large proportion of the potential efficiency gains can be achieved using a stepped change in compression ratio. One mechanism that can achieve this is an eccentric bush (eccentric) operated in the big-end of the connecting-rod. The natural dynamics associated with the motion of the crankshaft and connecting rod can potentially move the eccentric in the required manner. The present work aims to identify whether this is a practical means to shift the position of the eccentric. Matlab simulations of the dynamic performance of the eccentric in various simulated engine operating conditions have been performed. An experimental eccentric was also operated in a motored engine to: (1) assess its performance, and (2) compare with, and validate the modelling. Further comparisons with commercial dynamics analysis software tools are also presented. The results show that when operated within limited bounds of engine speed and oil viscosity, the eccentric can be moved in the required manner by the dynamics of the engine component motion. For application as a practical mechanism, the eccentric suffers from a very high sensitivity to its bearing surfaces friction regimes. The mixed friction which results from low speed or high bearing loads prevents the required rotation of the eccentric. In a practical sense, the motion of the eccentric does not provide the consistency over the operating range of an internal combustion engine to employ it as a VCR technology.