Infiniti’s New VC-T Engine

Sep 21, 2016

Infiniti QX50

The upcoming Infiniti QX50 doesn’t normally get pulses racing for many people, no matter how shapely its preview, the QX Sport concept, for example. No midsize SUV does, to be fair. But it has something special under the hood, the world’s first production variable compression ratio engine. That means the QX50’s 2.0 liter turbo four, which makes 268 hp and 288 lb-ft of torque, will have up to 27 percent better fuel economy. Here’s how it works.

The trend of moving to smaller, turbocharged engines carries with it one big falsehood. Under low load when the turbo isn’t needed, these engines are less efficient than an equivalent engine without a turbo. This means if you never need the extra power, you’re wasting fuel.

Turbocharged (and supercharged) engines use a lower compression ratio to prevent detonation. When you force extra air in a cylinder and mix it with fuel, it’s more likely to prematurely go boom. Lowering the compression ratio prevents this problem, but it’s less efficient. Infiniti’s VC-T promises the best of both worlds, with a compression ratio that ranges from 8.0:1 for high-power turbo needs to a 14.0:1 ratio for fuel-sipping efficiency.

Infiniti QX50 gauge panel

Instead of having the pistons connected to the crankshaft, Infiniti’s engine has a pivot arm with a connection on each end. One end connects to the piston, the other end connects to a second lower shaft, which is controlled by an actuator arm. At any given time the engine’s pistons move up and down according to the lobes on the crankshaft. But the actuator arm can change the angle of the pivot arm up and down. That is, the pistons still move in the same motion with the same stroke, but phase the entire stroke up or down. Move the pivot up and there’s less room at the top, which means a higher compression ratio. Move the pivot down and compression ratio goes down too. As an added bonus, the lower shaft eliminates the need for counter-rotating balance shafts.

Infiniti says this system works constantly and can vary the compression ratio to any number between 8:1 and 14:1. It also uses electronic variable valve timing on the intake valves to switch into Atkinson-cycle combustion for greater efficiency. The exhaust valve uses a more common oil pressure-based cam phaser. Other nifty features include a cylinder head with integrated exhaust manifold, electronic wastegate control for the turbo, and a variable-displacement oil pump. Both port and direct fuel injection are used as well.

Infiniti says that the variable compression ratio technology can be scaled to other sizes of four-cylinder engines, but is impractical for V6 and V8 engines. Speaking of V6s, the power and torque of the VC-T is right in line with the 3.5-liter VQ used in the QX60 and various front-wheel-drive Nissans. Like those vehicles, the VC-T in the QX50 will be mounted transverse but offer some form of all-wheel drive. It’s easy to see this engine taking over from the VQ as Infiniti’s parent company spreads the VC-T into other applications.

For the near future the VC-T will be exclusive to the Nissan QX50, which starts production in 2017. Dropping this engine in the rear-drive Q60 might have been more buzz-worthy, but Nissan (and Infiniti) chose a more pragmatic plan. The transverse setup guarantees more widespread adoption, spreading the cost and corporate fuel economy benefits over a wider net. Putting all that aside, the VC-T is an amazing piece of groundbreaking technology. Amazing enough that yes, Infiniti has many excited for a midsize SUV.