Compression ratio Guide, Meaning , Facts, Information and Description

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

To calculate the compression ratio;
b = cylinder bore (diameter)
s = piston stroke length
V_c = 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.

CR= (π/4 × b² × s + V_c) / V_c

Due to pinging the CR in a petrol or gas (LPG or CNG)-fueled engine will not exceed 10:1
In a turbo charged engine the CR wil 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 Saab Variable Compression (SVC) engine. This engine uses a technique to dynamically alter the volume of the combustion chamber (V_c), which, via the above equation, changes the compression ratio (CR).

To alter V_c, the SVC "lowers" the cylinder heads closer to the crankshaft. It does this by replacing the typical one-part engine block with a two-part block, with the crank shaft 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 V_c (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.

External links

• SVC

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