Editing Compression ratio (section)

==Dynamic compression ratio==
{{Unreferenced section|date=September 2022}}
The ''static compression ratio'' discussed above — calculated solely based on the cylinder and combustion chamber volumes — does not take into account any gases entering or exiting the cylinder during the compression phase. In most automotive engines, the intake valve closure (which seals the cylinder) takes place during the compression phase (i.e. after [[bottom dead centre]], BDC), which can cause some of the gases to be pushed back out through the intake valve. On the other hand, intake port tuning and [[scavenging (automotive)|scavenging]] can cause a greater amount of gas to be trapped in the cylinder than the static volume would suggest. The ''dynamic compression ratio'' accounts for these factors.

The dynamic compression ratio is higher with more conservative intake [[camshaft#Timing|camshaft timing]] (i.e. soon after BDC), and lower with more radical intake camshaft timing (i.e. later after BDC).<ref>{{cite web |title=Cam Timing vs. Compression Analysis |website=victorylibrary.com |url=https://victorylibrary.com/mopar/cam-tech-c.htm |access-date=14 July 2019}}</ref> Regardless, the dynamic compression ratio is always lower than the static compression ratio.

Absolute cylinder pressure is used to calculate the dynamic compression ratio, using the following formula:
<math display="block">P_\text{cylinder} = P_\text{atmospheric} \times \text{CR}^\gamma</math>
where <math>\gamma</math> is a [[polytropic process|polytropic]] value for the [[ratio of specific heats]] for the combustion 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 ratio of specific heats 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. 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<sup>1.3</sup> × atmospheric pressure, or 13.7&nbsp;[[bar (unit)|bar]] (relative to atmospheric pressure).

The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a dynamic compression ratio similar to an engine with lower compression but earlier intake valve closure.