Variable-Compression Engine

Infiniti Conquers the Variable-Compression Engine

FindItMore | Internal combustion engines have a specification called “compression ratio.” This refers to the volume in each engine cylinder when the piston is at the bottom divided by the volume when the piston is at the top. Typical compression ratios are 9:1, 10:1, and higher. The problem is that having one fixed compression ratio, say 10:1, at work in an engine at all times means that compromises need to be made in the rest of the engine’s design. The holy grail for engine designers would be an engine that can change compression ratios on the fly.

Why multiple compression ratios are better

A high compression ratio means you’re squeezing the air-fuel mixture in the engine’s combustion chambers tightly, which in turn means more power and fuel efficiency. But engineers are caught in a tug of war. They would like a compression ratio that’s high for the sake of power and efficiency, but not so high that engine knock damages the engine. Wouldn’t it be nice to be able to change the engine’s compression ratio on the fly?

Infiniti engineers figure it out

Infiniti was one of the first companies to decide they wanted to perfect variable compression technology. As a result, their engineers have spent the last 20 years working on the technology. In the process, they developed more than 100 engine prototypes and spent more than 30,000 hours on testbeds. The fruit of their labor is the VC-Turbo engine.

It happens seamlessly

Moving seamlessly between compression ratios, the VC-Turbo engine responds to the car’s driving conditions and continuously changes its compression ratio. Transforming on demand from 8:1 for high performance to 14:1 for high efficiency, it delivers performance when you need it and ultimate efficiency when you don’t.

The folks at Warren Henry Infiniti in Miami explains in simple terms what we have: an engine that combines the power of a high-performance 2.0-liter gas engine with the torque and efficiency of an advanced diesel. In particular, when used in the VC-Turbo, during times of high turbo boost, the engine management computer signals an electric motor to lower the compression ratio. When the motor isn’t using the turbocharger as much, the motor lengthens the pistons’ reach, which ups the compression ratio. And it’s all seamless to the driver.

Fancy motor mounts too

The engineers wanted variable compression and a smooth running engine too. They accomplished this by ditching the two balance shafts that conventional in-line-fours have to balance out vibrations. They accomplished this with another new technology: active motor mounts. Sensors integrated into the upper engine mounts detect vibrations from the VC-Turbo, then create opposing vibrations to cancel them out. Altogether, the design knocks nine decibels off the previous engine noise, making their new four-banger almost as quiet as a V6 design.

Times are a-changing

Toughening fuel economy and emissions standards are squeezing internal combustion engine engineers. However, car companies expect gas-only motors to stick around for a while. The key to the future is a technology that makes smaller engines perform like the big power mills of recent years. Variable ratio compression is one technique that works.


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