Understanding Engine Compression Ratio CC

Female Speaker: An engine's Compression Ratio can be a guide to the power it can generate. It's not always obvious with the one engine is bigger than the other. The size of the engine block can be misleading. Two blocks can be the same size, but one has cylinders board out to larger volumes. The standard measure of size is called Displacement. Displacement is the volume a piston displaces in the cylinder as it moves from its lowest point or bottom dead centre to its highest point, top dead centre. This is also called swept volume. Notice that displacement does not include the volume above top dead centre. Engine size is then the sum of the displacements of all the cylinders of the engine. It is called total engine displacement. For this engine it is 2 liters. Another guide to engine power is compression ratio. It compares two volumes in the cylinder; one is swept volume plus clearance volume. That's the volume above top dead centre, the other is the clearance volume only, putting these volumes into a ratio gives us the compression ratio 6 to 1. The larger the first volume and the smaller the second, the higher the engines compression ratio and the more powerful the engine.


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