Calculating (actual) compression pressure in automotive engines is not a straightforward process of comparing the cylinder's maximum and minimum dimensional volumes as the piston travels through its full cycle. That comparison merely defines mechanical compression ratio. Most engines never achieve a final compression pressure that is purely a result of compression ratio. Other factors such as valve timing, ambient conditions, throttle position and engine load can affect actual compression pressure significantly, and a usable calculation becomes an educated estimate that must consider the other terms.
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Estimate the dynamic volumetric compression ratios at different engine speeds. Most engines don't close the intake valve at the bottom dead centre (BDC) point of the piston's travel. At higher engine speeds the inward flow of air does not stop when the piston reaches the BDC point due to the air's high momentum through the intake manifold. Keeping the intake valve open past BDC, as the piston starts to rise, effectively lowers the compression ratio at low engine speeds to prevent detonation (knocking) and results in raising it at higher speeds when more horsepower is sought.
If the intake valve closes 60 degrees after BDC, the starting compression volume is only about 80 per cent (can vary) of what it would be at BDC, so in this example of a basic mechanical compression ratio of 9:1, the effective ratio may only be about 9.0 x 0.813 = 7.32:1. At higher engine speed, with the throttle wide open, the effective starting volume may approach the cylinder's actual volume, less just a few per cent of flow loss. So the high-speed effective compression ratio may be approximated by multiplying the base 9.0:1 mechanical ratio by 0.95 to get 8.55:1
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Calculate basic compression pressure from the two low- and high-speed effective compression ratios of 7.32:1 and 8.55:1, respectively, assuming standard sea-level atmospheric pressure of 6.67 Kilogram per square inch absolute (psi). Multiplying the low-speed effective compression ratio of 7.32:1 x 14.7 would yield a compression pressure of 49.4 Kilogram per square inch gauge (psia). The high-speed value would be the 8.55:1 effective compression ratio x 14.7 psia, or 125.69-psia.
Correct the pressure for the specific heat effect factor. When the air is compressed, some of its heat is extracted because of its lower volume, but it has nowhere to go, so the heat results in a higher temperature, which in turn increases the pressure above what it would have been under ideal conditions. (It is actually this higher temperature that spontaneously ignites the fuel in diesel engines with their much higher compression ratios of about 18:1.)
For air, this factor is about 1.4:1. So the estimated compression pressures in this example would be 108.84 psi x 1.4 = 152.37 psi in the low-speed case, and 125.69 psi x 1.4 = 175.97 psi in the higher-running-speed case.
- Performance buffs use online compression calculators to approximate what engine modifications might do to affect compression pressure, and can be useful for verifying your own estimates.
- Measuring compression pressure with a gauge using spark plug holes is a low-speed method that will read pressures lower than what your engine does at speed, and should therefore only be used for comparing cylinders.
- Detonation from too-high compression can do significant damage to engines.