The conflicting demands of thermodynamic benefits and costs for a variable compression ratio

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The conflicting demands of thermodynamic benefits and costs for a variable compression ratio

Wittek, Karsten / Podworny, Markus / Seibel, Jörg / Pischinger, Stefan

A variable compression ratio can help to resolve the conflict between part load and full load CR layout especially at boosted downsized gasoline engines. Depending on the VCR (variable compression ratio) concept and engine downsizing grade, the cycle simulations for NEDC (new european driving cycle) and WLTP (worldwide harmonized light vehicles test procedures) predict a fuel benefit between 5.2 and 7.2 % for the chosen vehicle and powertrain configurations. The three different types of VCR concepts have different impact on the engine architecture. Therefore, some systems can only be implemented in a new engine family while others can be applied as a derivate of an existing engine. The direct comparison of these systems shows, that there are conflicting demands of benefits and costs. The maximum thermodynamic potential of VCR can only be accomplished with continuous VCR systems but goes along with relatively high additional costs. The 2-stage VCR shows a fuel benefit decreased by 0.7 to 1.4 %, but still offers a better cost/benefit ratio compared to the continuous system. Therefore, the 2-stage VCR system seems to be a good compromise, especially for retrofitting into existing engine architectures.