A practical means of varying an engine?s compression ratio has been a goal of automakers for ages. How hard the air?and sometimes fuel?is squeezed before ignition, plays a crucial role in overall efficiency. Varying the compression ratio in a running engine is daunting because the ideal ratio changes with speed and load. One expedient now in wide use is combining the Atkinson cycle?which separates the effective compression ratio from the expansion ratio?with variable intake-valve timing. But the current rise in global mileage standards is forcing every maker to search for additional means of curbing fuel consumption.
At this fall?s Paris auto show, Infiniti will present a significant stride in the pursuit of a practical variable-compression-ratio engine called Variable Compression-Turbocharged (VC-T). This boosted 2.0-liter four-cylinder, the product of two decades of research and 300 patents, will power unspecified Infiniti models beginning in 2018.
Venturing well beyond the simpler Atkinson cycle approach, VC-T has four multi-link mechanisms inside its crankcase to offer computer control over each cylinder?s compression ratio. The key enabler (labeled Multi-link in the illustration above) is a diamond-shaped component that replaces the connecting rod?s big end. When the electronically controlled harmonic drive mechanism rotates, the actuator arm, control shaft, lower link, and finally the multi-link all move to vary the piston?s stroke within the cylinder bore. While ma...