Why Variable-Compression Engines Are Better

28th November 2018

Internal combustion engines have a parameter called “compression ratio.” An engine’s compression ratio refers to the amount of “squeezing” that the pistons apply to air-fuel mixtures during operation. The ratio is calculated by taking the volume in each engine cylinder when the piston is at the bottom, and dividing it by the volume when the piston is at the top. Typical compression ratios are 9:1, 10:1, and so on.

Note that today’s engines have one compression ratio. The problem is that having one fixed compression ratio at work in an engine is that compromises need to be made over the range of use. Engine designers would far prefer engines that can change compression ratios during operation.

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.  This means higher gas mileage and lower amounts of pollutants. The problem is that if an engine is coupled with a turbo- or super- charger, there will be times that the compression ratio is too high and you’d like the car’s computer to dial it back. Thats what variable compression is all about: dialing it back.

Infiniti – One of the First

Infiniti was one of the first companies to decide they wanted to perfect variable compression technology. In the last 20 years, they developed more than 100 engine prototypes and spent more than 30,000 hours testing variable compression systems. The fruit of their labor is the VC-Turbo engine.

Infiniti’s VC-Turbo is a turbocharged 2.0-liter inline-four that can operate at very high compression ratios without knocking. During times of high turbo boost, the VC-Turbo’s 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.

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 inline-fours have to balance out vibrations. They accomplished this with another new technology: active motor mounts. Sensors integrated in the upper engine mounts detect vibrations from the VC-Turbo, then creates opposing vibrations to cancel them out. All together, the design knocks nine decibels off the previous engine noise, making their new four-banger almost as quiet as a V6 design.

And Atkinson cycle

The VC-Turbo can also run in Atkinson-cycle to boost gas mileage. Our consultant at Southpoint Dodge (Austin, TX) explained that Atkinson cycle engines work by creating a small window of time in which the engine’s intake valves open slightly, drawing extra air into the combustion chambers right as the pistons begin to compress the fuel-air mixture. By reducing the engine’s volume, it lets the engine act like a smaller, more efficient engine. Atkinson-cycle engines are common in hybrid gasoline-electric cars that emphasize fuel efficiency and in which electric motors compensate for the reduced power, but it’s rare on straight-fuel engines.

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 technology that makes smaller engines perform like the big power mills of recent years. Variable ratio compression is one technique that works.


Corey is an all round tech guru who has worked at some major blue chip companies. He started Poweronemedia to share his views and knowledge with the rest of the blogging world.

Comments are closed.