Shifting the Squish: Infiniti Takes Recent History’s Biggest Step in Engine Design

Shifting the Squish

From the November 2016 issue

For ages, automakers have been searching for a practical means of varying an engine’s compression ratio on the fly. How hard the air—and sometimes the fuel—is squeezed before ignition plays a crucial role in overall efficiency: Compressing the mixture as much as possible without detonation yields a longer expansion ratio and more power for every increment of fuel consumed. Since the compression ratio is determined by basic engine geometry (the cylinder volume with the piston at the bottom of its travel versus the top), that isn’t easily changed. One expedient now in wide use is combining the Atkinson cycle with variable valve timing. But Infiniti has made a ­significant stride with its Variable Compression-Turbo (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. Here’s how it works:

Shifting the Squish: Infiniti Takes Recent History's Biggest Step in Engine Design

The key enabler is a piece Infiniti calls the “multi-link,” (001) a diamond-shaped component that replaces the connecting rod’s big end and allows computer control over each cylinder’s compression ratio.

When the electronically controlled harmonic drive mechanism (002) rotates, the actuator arm (003), eccentric control shaft (004), lower link (005), and, finally, the multi-link all move to vary the piston’s stroke.

Infiniti claims VC-T is capable of providing any desired compression ratio between 8:1 and 14:1. Bumping the ratio to 14:1 during light-throttle cruising maximizes fuel efficiency. Then, when the driver dips into the throttle, the compression ratio can drop as turbo boost rises, avoiding detonation. Infiniti hasn’t yet revealed any power, torque, or mileage ratings but claims that VC-T will combine the power of a 2.0-liter turbo gasoline engine with the torque and efficiency of a similarly sized diesel. Many auto­makers have experimented with compound-connecting-­rod and moving-cylinder mechanisms, but Infiniti appears to be the first to solve the durability issues of a crankcase stuffed with links and levers.


Stroker Ace

Honda has filed a patent in Japan for an engine in which each cylinder has a different stroke, their altering displacements translating to varied power potential and fuel consumption. The goal is to take fuller advantage of cylinder deactivation. When, for the purposes of saving fuel, the engine-control computer deactivates cylinders, it will be able to pick and choose which cylinders to drop to best match power and efficiency with the demands placed on the engine. If it moves beyond the developmental phase, it will still be years out. But probably not the 20 years Infiniti has into its VC-T. –Jared Gall

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