EXPLAINED

VARIABLE COMPRESSION

RYAN LEWIS

TRICKY TECH IN SIMPLE TERMS

VARIABLE COMPRESSION

APRIL 2017

What is it?

Itís the most radical rethink of the four-stroke engine since its advent. Infinitiís variable compression turbo (VCT) removes the power vs efficiency compromise with its revolutionary ability to offer any compression ratio between a low of 8:1 (for higher output) and a high of 14:1 (for greater efficiency). More than 100 prototypes were built over 20 years. It may look ostensibly like a regular four-cylinder, but it isnít.

Why does it matter?

Conventional engines squash fuel and air at a fixed volume and have relatively limited flexibility. This system can favour performance or economy based on driver input. Infiniti is the first to productionise an engine with this capability. Others have tried (GM, Volvo, PSA, Renault, SaabÖ) and failed. The brand says its 200kW/390Nm 2.0-litre VCT engine uses 27 percent less fuel than the similarly powerful 3.5-litre V6 it replaces.

How does it work?

Variable compression is achieved by changing the height of the piston at the uppermost point of its stroke.

A conventional crankshaft is used, but each connecting rod and piston attaches to its own multi-link, which rotates with the crank. On the other side of each link sits a lower arm to control the linkís orientation. That arm determines the final height of the piston by pivoting the multi-link. The higher the pistonís final position, the more it compresses the mixture in the combustion chamber.

At low compression, more boost is fed from the turbo to deliver greater performance.

At high compression the engine will run on the Atkinson cycle and achieve optimal fuel efficiency. Itís the best of both worlds.

The silver bullet

This section beneath the crankshaft is where Infinitiís brilliance lies. That, and in its engine control software, which has to be far more complex than normal to deal with variable compression.