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 many automakers have experimented with compound connecting-rod and moving-cylinder mechanisms, Infiniti appears to be the first to solve the durability issues associated with a crankcase stuffed with links and levers.
Infiniti claims VC-T is capable of providing any desired compression ratio between 8:1 and 14:1. In addition to both port and direct fuel-injection systems, this engine is equipped with individual cylinder ignition timing, variable intake-valve timing, and electronic boost control. It runs on the Atkinson cycle at times. To save weight and cost, the exhaust manifold is integral with the cylinder head.
Bumping the compression ratio to 14:1 during light-throttle cruising maximizes fuel efficiency. Then, when the driver dips into the throttle for passing, the compression ratio can be dropped as turbo boost rises, avoiding detonation while providing the desired performance. Infiniti hasn’t yet revealed any power, torque, or mileage ratings, however.
Thanks to timely breakthroughs—electronic controls sixty years ago, variable valve timing in the 1980s, and now Infiniti’s combination of electronics and mechanics to vary the compression ratio—there’s no apparent end in sight for the internal-combustion engine.
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