Is The Petrol Engine Revolution Here?

The claimed breakthrough? Variable compression ratios, or what Infiniti will be badging VC-T (variable compression turbocharged) when it unveils its new engine at the Paris Motor Show at the end of September.

The variable compression ratio engine has been a holy grail for automotive engineers since internal-combustion pioneer Ricardo built the first almost 100 years ago. Many others have tried since –Lotus did a two-stroke version, project Omnivore, and at the Geneva show in 2000 Saab unveiled SVC (Saab variable compression) as “an entirely new concept for combining high performance with low exhaust emissions”. Like the company itself, it never made it.

VCR engines have existed for decades in labs too, but their fearsome complexity has always ruled them out for production.

Until now, says Infiniti. Its “world first” VC-T engine for the Paris salon is “production ready” it says, though not as yet specifying when or which models will get it. “VC-T technology is a revolutionary next-step in optimizing the efficiency of the internal combustion engine,” said Infiniti chef Roland Krueger.

Not much is known about the engine, other than it is a four-cylinder with the power of a 2.0-litre turbo (200hp-plus) but much greater efficiency. This is the result of a compression ratio that automatically varies between 8:1 (for high performance) and 14:1 (for high efficiency).

As with previous prototypes, the variable ratio is achieved by altering the height the pistons reach, so in effect constantly changing the displacement of the engine. In the past this has been achieved by physically moving the cylinder head on the block, sloping it at different angles in order to increase or decrease the displacement within the cylinders.

Infiniti’s solution appears simpler and far more elegant. Instead of the cylinder head moving, the height the pistons reach is adjusted via an actuator arm working with a Harmonic Drive unit and control shaft connected to the crankshaft. The Harmonic Drive unit detects when a change in compression ratio is needed – when you floor it, for example – triggering the linkage to allow the pistons to reach whatever height best delivers the appropriate compression ratio for the driving situation.

It sounds complex but certainly not as complex as previous solutions. Infiniti in fact claims its new engine is actually lighter and more compact than a conventional unit, as well as quieter and smoother.

So, a real step forward? Infiniti thinks so: “More than 20 years in development, the VC-T engine represents a major breakthrough in internal-combustion powertrain technology…and delivers multiple customer benefits.”