Page "Compression ratio" Paragraph 37

The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length ( measured between centers ).

The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio.

This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present.

This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder.

Under ideal ( adiabatic ) conditions, the exponent would be 1. 4, but a lower value, generally between 1. 2 and 1. 3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison.

For example, if the static compression ratio is 10: 1, and the dynamic compression ratio is 7. 5: 1, a useful value for cylinder pressure would be ( 7. 5 )^ 1. 3 × atmospheric pressure, or 13. 7 bar.

(× 14. 7 psi at sea level = 201. 8 psi.

The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187. 1 psi.