Q What is the effective compression ratio of an engine? I
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know that the ratio is calculated by dividing the clearance volume plus the swept volume by the clearance volume. Is effective compression ratio dependent on speed?
AEffective compression ratio is based on
the volume swept by the piston above the point at which the inlet valve closes on the compression stroke. Valve closing is delayed until well past bottom dead centre to make use of the inertia of the air of a diesel (or mixture of a petrol engine) which, at higher speeds, continues to flow into the cylinder until the valve closes despite the fact that the volume above the piston is being progressively reduced with piston movement. Normally the higher the rpm of the engine the greater is the delay before the valve is fully closed.
It is pertinent to point out that near bottom dead centre, the angular displacement of the crankshaft is relatively large compared with the linear displacement of the piston, the speed of which increases to a maximum when the connecting rod and crank are at right angles.