Variable Compression Ratio Engines are Coming

Compression ratio is a very important term used for internal combustion engines. It is the ratio between the volume of the cylinder when the piston is at its bottom dead center and the volume of cylinder when the piston is at its top dead center. In essence, compression ratio defines the ratio to which a charge of incoming air-fuel mixture is compressed between the piston and cylinder head before combustion takes place. Once an engine is designed, its compression ratio is fixed for its lifetime, until now.

After 20 years of development, Nissan is finally ready to display a new four-cylinder turbocharged engine that can vary its compression ratio.

Now I know that you all think, what has Nissan to do with motorcycles… But not forget that Nissan has some interest in making a motorcycle. In 2014 Nissan revealed to friend and foe a 1500cc three-cylinder turbocharged motorcycle engine that was capable of producing 400 horsepower and weighing only 40 kilograms. For comparison the average MotoGP engine delivers around 260 horsepower.

There are even some rumors that Nissan is interested in the Italian Bimota, which makes great motorcycles but has no engine production themselves…

But back to the new Nissan variable compression ratio engine. In terms, the system works by varying the stroke length which allows the engine to have infinite variations between a compression ratio of 8:1 and 14:1. This groundbreaking technology allegedly produces a never seen before leapfrog improvement in terms of both power and efficiency.

Exact performance figures have not been released, but Nissan claims the torque and efficiency are unbelievable good. That in combined with the lowest possible exhaust emission ever.

Advantages of high compression ratio are that more power can be extracted out of each drop of fuel, hence higher combustion efficiency. Tight squeezing of air and fuel mixture in the small combustion chamber results in greater bang, which translates into more power. A typical high-performance engine runs with a compression ratio as close as possible to 14:1.

High-compression ratios have some disadvantages as well – engine knocking due to poor fuel quality is one of them. Thanks to a small clearance volume, inlets and exhaust valves may hit the piston during the cycle. Also, there is a limit to which air can be compressed between the small clearance volume. High pressure generated during combustion tends to overstress the engine and its components. Hence we cannot increase power beyond a certain limit for a certain capacity of engine. Technologies like variable valve timing and variable geometry turbochargers have successfully stretched the band, but only to reach a new limit.