An Overview of VCR Technology and Its Effects on a Turbocharged DI Engine Fueled with Ethanol and Gasoline 2017-36-0357

The possibility to vary compression ratio offers a new degree of freedom that may enable so far not exploited benefits for the combustion process especially for highly boosted spark ignited engines. Numerous approaches to enable a variable compression ratio (VCR) have been tried and tested in the past. Nevertheless, none of these systems reached series production because of several reasons, ranging from too much complexity and moveable parts to deep modification required on existing engine architectures and manufacturing lines. Instead, the approach of a variable length conrod (VCR conrod) could be the solution for integration in almost any type of engine with minor modifications. It is then considered by several OEMs as a promising candidate for midterm series production. This paper shows, firstly, a discussion of the benefits of a variable compression ratio system. Secondly, an overview of the various solutions known to realize VCR is provided, as well as the reason why the approach of a VCR conrod is promising with respect to ease of integration into different engine architectures. Next, various layouts of VCR conrods are presented. Out of these, one concept is comprehensively presented in detail: an eccentrically piston pin suspension in the small eye and the principle of exploiting gas and mass forces for actuation. Examples of application are presented and possible design variants are discussed and outlook is given regarding the possibility to operate the VCR conrod in a fully variable mode by means of fast actuation, smart real-time sensing of the current compression ratio and intelligent control. A modern 3-cylinder turbocharged DI spark-ignited engine was tested with one of the concepts. Preliminary test results pointed to up to 6% higher brake efficiency with RON 95 gasoline, but only at low loads due to knock-limited combustion at high loads. The same engine was tested with ethanol (E100) showing an overall efficiency improvement of 6% over most of load range and a maximum of near 8% at low load.