A Study Concerning the Noise and Vibration Characteristics of an Engine with Multiple-Link Variable Compression Ratio Mechanism

The authors have previously proposed an engine system that uses a new piston-crank system incorporating a mulitiple-link mechanism to vary the piston's position 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 dimentions. We have previously reported that this system provides additional benefits by reducing second-order inertial forces and the friction losses caused by piston side thrust in the combustion and expansion processes.

This paper first describes the further improvements made to the link dimensions since our previous report. An improved link geometry is presented in which the centerline of the cylinder bore is offset in the opposite direction from that of existing internal combustion engines. That improved link geometry reduces the acceleration of moving parts, which is a prerequisite for high-speed engine operation and also further reduces second-order inertial forces.

The noise and vibration characteristics of a prototype turbocharged gasoline engine incorporating the improved multiple-link VCR mechanism were compared with those of an internal combustion engine having a conventional piston-crank system and nearly the same configuration except for the major moving parts. It was found that the radiation noise level of the multiple-link VCR engine showed less sensitivity to torque, with the result that its noise level in the high-load region was lower than that of the conventional engine. It was also observed that the multiple-link mechanism worked to reduce fluctuations in crankshaft axial torque.