The modern automobile engine has fixed compression ratios, that currently usually hover at about 10.0:1 or 10.5:1 for normally aspirated engines and much lower for turbocharged engines. Cars with direct injection can go higher, but we’re quite limited because compression ratios are fixed. A turbocharged engine running off boost makes less power than a normally aspirated engine of similiar cubic capacity because of the reduced compression ratio. While this has been minimized with very fast spooling turbos these days, it is still not ideal.
The answer is a variable compression ratio engine. Saab showcased a concept version of a variable compression ratio engine back in the year 2000, and it was called the SVC, or Saab Variable Compression. The project never did go anywhere. The SVC was a turbocharged 1.6 litre engine that used a pivoting cylinder head to change compression ratios from 8.0:1 all the way up to a high 14.0:1 according to engine operating status. The pivoting head adjusted the slope of the upper part in relation to the lower part of the engine, thus changing the combustion chamber volume at piston top dead center. Because of this, the top and bottom parts of the engine were separated and each required its own cooling system. Thanks to turbocharging and SVC, the 1.6 litre inline-5 engine produced 225hp and 300Nm of torque using 2.8 bars of boost.
In 2003, Nissan took a different approach to implementing a variable compression ratio system. Nissan showcased its Variable Compression Ratio Piston-Crank System, or VCR for short. It uses a multi-link piston and crank mechanism to vary the compression ratio. Like the Saab SVC, it is also based on a turbocharged engine concept. Compression ratio varies from 8.0:1 to 14.0:1, and the highest compression ratio is used during slow and steady city driving when the car runs mostly off-boost. During hard acceleration when the turbocharger kicks in, the compression ratio is reduced up to 8.0:1. During medium engine loads and low to medium levels of boost, a middle ground such as 11.0:1 is used.
The conventional conrod is replaced by a multilink system that consists of three links An upper link is connected to the piston pin, a lower link revolves freely while connected to a crank pin, and a control link connects the lower link to a control shaft. The control shaft has bearing that has a certain amount of eccentricity to it. When the control shaft rotates, the fulcrum of the control link bearing changes, and this causes the top dead center position of the piston to be either moved up or down vertically. This modifies the combustion chamber volume at piston top dead center, this altering the cylinder’s compression ratio. Basically, the system is designed in such a way that the angle of the control shaft causes the top dead center position to move, so to vary the compression ratio you adjust the angle of the control shaft.
Nissan is also developing a similiar VCR system concept for diesel engines. I’m not sure if a production engine using this technology is in the works or not. VCR has probably been made outdated by direct injection – the Volkswagen Golf GTI’s 2.0 litre TFSI engine is turbocharged yet has a high compression ratio of 10.5:1!