Compression ratio

The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine.

The ratio is calculated by the following formula:

[itex]\mbox{CR} = \frac { ( \pi b^2 s) / 4 + V_c } {V_c} [itex], where
[itex]b[itex] = cylinder bore (diameter)
[itex]s[itex] = piston stroke length
[itex]V_c[itex] = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.

Due to pinging (detonation), the CR in a petrol or gas (LPG or CNG)-fueled engine will usually not be much higher than 10:1.
In engines with a ping sensor and an electronic control unit, the CR can be as high as 12.5:1 (2005 Audi A6 3.2)
In a turbo charged or super charged engine the CR will be around 8.5:1
In a diesel engine the CR will be 20:1 and higher.

Saab Variable Compression engine

Because cylinder bore diameter, piston stroke length, and combustion chamber volume are almost always constant, the compression ratio for a given engine is almost always constant.

One exception is the experimental Saab Variable Compression (SVC) engine. This engine uses a technique to dynamically alter the volume of the combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR).

To alter Vc, the SVC "lowers" the cylinder head closer to the crankshaft. It does this by replacing the typical one-part engine block with a two-part block, with the crankshaft in the lower block, and the cylinders in the upper portion. The two blocks are hinged together at one side (imagine a book, lying flat on a table, with the front cover held an inch or so above the title page). By pivotting the upper block around the hinge point, the Vc (imagine the air between the front cover of the book and the title page) can be modified. In practice, the SVC adjusts the upper block through a small range of motion, using a hydraulic actuator.

However, the SVC project was shelved by General Motors (the owner of Saab Automobile) due to cost, in favor of their GM global engines.

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