It’s the world’s first production variable-compression ratio engine.

By altering the distance that the piston head travels within its cylinder, the VC-Turbo can operate with a compression ratio as low as 8:1 (for maximum performance) or as high as 14:1 (for maximum efficiency). The engine is as powerful as the older naturally aspirated 3.5L VQ V6 in the previous QX50 and makes more torque, but it’s also smaller, lighter, and a lot more fuel-efficient. It can even switch between the Atkinson and regular combustion cycles.

The variable-compression ratio technology works as follows. Instead of just having a connecting rod between the crankshaft and piston head, there’s now a multilink arrangement around the crankshaft, with the con-rod attached to one end. At the other end is an actuator arm that’s connected to a harmonic drive. It’s this harmonic drive that controls the distance that the piston travels within the cylinder by changing the angle of the multilink and, therefore, the top-dead-center position of the piston.

This means that the engine actually varies in displacement; when the compression ratio is at its lowest, the engine measures 1.997L compared to just 1.970L when compression is highest. Peak power is 268hp (200kW) at 5,600rpm, and peak torque is 280ft-lbs (380Nm) at 4,400rpm. According to Infiniti, the front-wheel-drive QX50 should get 27mpg, a 35-percent improvement on the outgoing V6-engined vehicle.

Additionally, the VC-Turbo makes use of both port- and direct-fuel injection, switching between the two under low load depending upon the circumstances, then using both in concert during times of high load. According to Nissan’s chief powertrain engineer, Shinichi Kiga, the variable-compression ratio engine has been at least two decades in the making. “The breakthrough required two things. One is computer aided design; along with great simulations, it helped us optimize each part,” he explained. “Also, on the manufacturing side, we needed to be more precise with tolerances.” (Nissan is Infiniti’s parent company.)

The turbocharger is a normal single-scroll design, but the engine’s exhaust manifold is integrated into the cylinder head. Infiniti says this improves emissions, as there is a shorter path to the catalytic converter, which is now mounted next to the turbo so it gets up to temperature more quickly. Other interesting touches are a low-friction mirror bore coating, vapor-deposited on the cylinder walls (which is said to reduce cylinder friction by 44 percent), plus active engine mounts to ensure the turbo four is as smooth as the outgoing V6, an engine noted for its silky nature.

The VC-Turbo isn’t the first time someone has tried to make a variable-compression ratio (or variable displacement) engine, but it is the first time one has gone into production. Together with another cool technology revealed earlier this year—Mazda’s homogenous charge compression ignition Skyactive-X was announced over the summer—there might still be some life in the internal combustion engine.

Listing image by Jonathan Gitlin

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