Want to know what your static and dynamic compression are? The static and dynamic compression ratio calculator can do it for you. As with other calculators, equations are provided as well. The dynamic compression is always lower than the static compression if the intake valve closes after bottom dead center (ABDC). Don’t know your intake valve closing angle? Calculate it here.
Calculating static compression is simply the ratio of the total volume in the cylinder at bottom dead center (BDC) to the compressed volume at top dead center (TDC). The dynamic compression ratio is similar as it is the ratio of volumes. The compressed volume is the same in both static and dynamic compression. However, the uncompressed volumes are different. This is because the intake valve is open for some time while the piston is moving up the bore in the compression stroke.
To calculate the dynamic compression, we must find out how far the piston has moved up the bore by the time the intake valve closes, sealing the cylinder. The intake closing event is given in degrees of crankshaft rotation, θ , after BDC. The loss in compression from the open intake valve can be accounted for by finding an expression for the effective stroke, lstroke,eff. To do so, we must find the vertical movement, lloss, the wrist pin makes from BDC to when the intake valve closes, which is the loss of stroke. This can be done using basic trigonometry. The figure below quantizes all values that relate to lloss. The cross in the red circle marks the crank centerline, and the cross in the blue circle marks the rod centerline. As the intake closing angle increases, the value lloss increases to a maximum of the total stroke length.
The equations above show the expression for the loss of stroke, lloss, and the effective stroke length, lloss. Once the effective stroke length is known, the dynamic compression ratio may be calculated the same way as the static compression ratio.