Given increasingly stringent emission targets, engine efficiency has become of foremost importance. While increasing engine compression ratio can lead to efficiency gains, it also leads to higher in-cylinder pressure and temperatures, thus increasing the risk of knock. One potential solution is the use of a Variable Compression Ratio system, which is capable of exploiting the advantages coming from high compression ratio while limiting its drawbacks by operating at low engine loads with a high compression ratio, and at high loads with a low compression ratio, where knock could pose a significant threat. This paper describes the design of a model for the evaluation of fuel consumption for an engine equipped with a VCR system over representative drive cycles. The model takes as inputs; a switching time for the VCR system, the vehicle characteristics, engine performance maps corresponding to two different compression ratios, and a drive cycle. Two different types of transmission, a Continuously Variable Transmission and a Step Ratio Transmission, were also considered in the model.
As an initial step, the compression ratio switching time was assumed to be constant, independent of the engine working conditions. Successively, an engine speed-load dependent switch time was introduced based on reported values for a real VCR system. Finally, the effect of transients during compression ratio changes was accounted for through the introduction of a worsening percentage applied to the mass of fuel injected. Results showed improvements between 0.3% and 0.7% in fuel economy over a static 10:1 compression ratio, depending on the selected vehicle and drive cycle. Also, with a constant switching time less than 20 engine cycles, the compression ratio switching procedure is always completed in the time increment covered in the FTP-75 cycle. Simulations utilizing a speed-load dependent switch time based on a prototype VCR system proved capable of staying below this threshold. However, the analysis of the effect of transients demonstrated that an increase of about the 2% in the mass of fuel injected during changes in compression ratio could possibly counter balance all the fuel economy benefits coming from the adoption of a VCR system.