A Study of a Multiple-link Variable Compression Ratio System for Improving Engine Performance 2006-01-0616

The authors have previously proposed an engine system that uses a new piston-crank system incorporating a multiple-link mechanism to vary the piston's motion at top dead center and thereby obtain the optimum compression ratio matching the operating conditions. This multiple-link variable compression ratio (VCR) mechanism can be installed without increasing the engine size or weight substantially by selecting a suitable type of link mechanism and optimizing the detailed specifications.

Previous papers by the authors have made clear the features of the VCR mechanism that facilitates continuously variable control of the compression ratio [1][2]. It was shown that engine friction attributable to piston-side thrust can be reduced through an upright orientation of the upper link in the expansion strokes. Furthermore, it was also shown that piston stroke characteristics resembling simple harmonic motion can be achieved through an optimal design of the link dimensions, thereby reducing engine noise and vibration induced by the piston's second-order inertial forces as well as reducing crankshaft torque fluctuations.

In the present study, the above-mentioned multiple-link VCR mechanism was applied to a turbocharged engine to investigate its effect on engine performance. It was found that fuel economy and power output can both be improved by increasing the compression ratio and applying exhaust gas recirculation (EGR) under low loads and by lowering the compression ratio and applying higher boost pressure under high loads. The piston stroke achieved with this multiple-link mechanism resembles simple harmonic motion, unlike that of conventional engines. This motion is characterized by slower piston speed near top dead center and faster piston speed near bottom dead center. The effect of these characteristics on fuel economy and maximum power under part load operation was made clear in terms of combustion stability, time loss, cooling loss and friction.