Variable compression ratio

Variable compression ratio

Variable compression ratio is the technology to adjust internal combustion engine cylinder compression ratios on the fly. This is done to increase fuel efficiency while under varying loads. Higher loads require lower ratios to be more efficient and vice versa. Variable compression engines allow for the volume above the piston at 'Top dead centre' to be changed. For automotive use this needs to be done dynamically in response to the load and driving demands.

Why is it desirable

Petrol engines have a limit on the maximum pressure during the compression stroke, after which the fuel/air mixture detonates rather than burns. To achieve higher power outputs at the same speed, more fuel must be burnt and therefore more air is needed. To achieve this, turbochargers or superchargers are used to increase the inlet pressure. This would result in detonation of the fuel/air mixture unless the compression ratio was decreased, ie the volume above the piston made greater. This can be done to greater or lesser extent with massive increases in power being possible. The down side of this is that under light loading, the engine can lack power and torque. The solution is to be able to vary the inlet pressure and adjust the compression ratio to suit. This gives the best of both worlds, a small efficient engine that behaves exactly like a modern family car engine but turns into a highly tuned one on demand.

Variable compression ratio is becoming increasingly desirable as oil prices increase and car buyers have an increased interest in fuel economy.

The cylinder head can be altered by using a hydraulic system which is connected to the crank shaft and responds according to the load and acceleration required.


Variable compression engines have existed for decades but only in laboratories for the purposes of studying combustion processes. These designs usually have a second adjustable piston set in the head opposing the working piston. (very much like model aircraft 'Diesel' engines) Variable compression engines have been highly desirable but technically unobtainable for production vehicles due to the mechanical complexity and difficulty of controlling all of the parameters. The advances in low cost microcontrollers and a wealth of experience in their application to engine management now makes the control possible. SAAB introduced a variable compression to the world at the Geneva motor show in 2000 but has yet to reach production as of 2008 The design consisted of a monobloc head, which contained all of the valve gear, and the crankshaft/crankcase assembly. These parts were connected together by a pivot which allowed 4 degrees of movement controlled by a hydraulic actuator. This mechanism allows the distance between the crankshaft centre line and the cylinder crown to be varied.A supercharger was chosen in preference to a turbocharger to achieve the necessary response time and high boost pressure.

ee also

* Continuously variable transmission
* Variable valve timing
* Diesotto


* [ Saab press release]
* [ Automotive engineering international article]

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