Infiniti's variable-compression engine: witchcraft explained

Infiniti introduced the world's first variable compression-ratio engine, a technology it claims to have been working on for two decades, at the 2017 Los Angeles Auto Show.

A variable compression-ratio engine is a feat of automotive engineering, flummoxing automakers since long before Infiniti showed its first car ever back at the 1989 Detroit Auto Show. The technology represents a fundamental rethink of how an engine operates.

Infiniti’s senior powertrain performance manager Chris Day knows well how the engine works and explained it in simpler terms in Los Angeles last year.

An engine's compression ratio—what Day called the “DNA” of the engine—is a simple way of expressing the difference in a combustion chamber's volume between the bottom of the stroke (the bigger number) and the top of the stroke (the one).

A high compression ratio is more thermally efficient, and thermal efficiency is king for fuel efficiency. On the flipside, a lower compression ratio—combined with a turbocharger to force feed the larger combustion chambers—packs plenty of power.

Infiniti VC-Turbo

Infiniti VC-Turbo

Infiniti VC-Turbo

Infiniti VC-Turbo

Infiniti VC-Turbo

Infiniti VC-Turbo

Engines with these combustion ratios are not uncommon. The Toyota Prius operates a 13:1 ratio while the twin-turbocharged Mercedes-AMG E63S operates at 8.6:1. Infiniti's VC-Turbo straddles both—and the middle too—fluctuating between 8:1 and 14:1. Bouncing between those two ratios helps the VC-Turbo to an estimated 27 percent improvement in fuel economy over Infiniti's old 3.7-liter V-6-powered QX50 while boasting more torque.

“We've introduced a system within the engine that is unique to the industry, providing best-in-class fuel economy and acceleration,” Day told Motor Authority.

In the 2019 Infiniti QX50, the first production model to get the VC-Turbo, it produces 268 horsepower and 280 pound-feet of torque to the 325 hp and 267 lb-ft in the V-6-powered 2017 QX50. But the VC-Turbo generates its horsepower and torque peaks at lower engine speeds—5,600 and 4,400 rpm, respectively, to the VQ's 7,000 and 5,200 rpm. It's also about 60 pounds lighter than the V-6.

But the numbers are a small part the story, and understanding the rest requires understanding the engine itself.

In a normal engine, a crankshaft spins around in the belly of the engine, raising and lowering the pistons across a fixed range—or stroke—and a never-changing compression ratio. But in the VC-Turbo, a 0.5-kilowatt electric motor powers the variable compression actuator (VCA)—the brain of the engine—on the side of the block and draws its information directly from the ECU. A hook-like actuator link ties the VCA to a non-rotating control shaft, while a control link attaches to the heart of the engine, the multilink.

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