Variable Compression Engine

One of the key variables for an internal combustion engine is its compression ratio. This is the ratio of the maximum volume within the cylinder (when the piston is at bottom dead center) and the minimum volume within the cylinder (when the piston is at top dead center). Obviously, this ratio is fixed at the point of design – the amount of travel of a piston within a cylinder is determined by the profile of the crankshaft.

At least that has always been the case. But Infiniti has just changed that with their debut of its new VC-T engine at the Paris Motor Show. Their clever technology changes compression ratios (and displacement) on the fly, and could easily be one of the biggest technological leaps for internal combustion engines in years.

The higher an engine’s compression ratio is, the more mechanical energy it converts from the combustion of a given amount of fuel mixed with a given amount of air. But too high a ratio causes knocking – premature detonation of fuel-air mixture during the engine’s compression stroke caused by high cylinder temperatures.

Over the years, engineers and chemists have come up with a number of solutions for this problem. Adding tetraethyl lead to gasoline, for example. Or using exotic blends of alcohols and other hydrocarbon fuels that have lower temperatures of combustion – this mainly happened in auto racing, but E85 ethanol/gasoline has the same effect.

More recently, the practice of direct injection (injecting fuel under high pressure into each cylinder instead of farther upstream) has been employed to reduce cylinder temperatures in turbocharged engines. That’s because forced induction engines are even more sensitive to the problem – knock is a function of heat and pressure, and adding a turbo to your engine will increase both of those plenty.

Infiniti says that the VC-T engine can alter its compression ratio from as low as 8:1 (for high performance) to as high as 14:1 (for efficiency). The company (and its parent, Nissan) have been working on the challenge for some time now – with a patent application filed in 2001 by the company for such a system.

As you can see from the illustration, the connecting rod to the piston meets the crankshaft via a multi-link, which is also connected to an actuator arm. Moving the actuator changes the angle of the multi-link, which in turn changes the travel of the piston within the cylinder.

“It is a revolutionary next-step in optimizing the efficiency of the internal combustion engine. This technological breakthrough delivers the power of a high-performance 2.0-liter turbo gasoline engine with a high level of efficiency at the same time,” said Roland Krueger, president of Infiniti Motor Company. At first glance, we’re inclined to agree with him.