A Study of a Variable Compression Ratio System with a Multi-Link Mechanism

This paper presents a variable compression ratio (VCR) system that has a new piston-crankshaft mechanism with multiple links. This multi-link mechanism varies the piston position at top dead center (TDC), making it possible to change the compression ratio of the engine continuously. Previous attempts have been made to achieve variable compression ratio with this type of method, but it was difficult to avoid various undesirable effects such as an increase in the engine size, substantial weight increases, increased engine block vibration due to a worsening of piston acceleration characteristics and increased friction resulting from a larger number of sliding parts. At the stage of developing the basic design of the multi-link geometry, emphasis was placed on selection of a suitable link geometry and optimization of the detailed dimensions with the aim of essentially resolving these previous issues. As a result of the studies conducted at that stage, a VCR link geometry was found that optimally resolves these various issues. Besides achieving the desired VCR capability, this VCR system also provides several new and additional benefits including:

1. (1)
   a reduction of second-order vibration by making the piston stroke resemble simple harmonic motion (similar to second-order balancer function);
2. (2)
   lower piston friction loss as a result of reducing the piston-side thrust load in the expansion stroke; and
3. (3)
   a compact package that can be housed in the crankcase of existing engines.

The basic characteristics of a prototype VCR engine that incorporates these new benefits are described on the basis of a theoretical analysis and experimental results obtained in tests conducted with the prototype engine.