Engine Cranking Pressure
Copyright © 2003 Eric Fahlgren
Last updated 2007-09-18 11:17 PDT

Calculator

This calculator computes the maximum ideal cylinder gauge pressure you should expect to see from a four-stroke cycle internal combustion engine. It assumes ideal conditions in that there will be no cylinder leakage and no heat loss into the engine from the compressed gasses. It also assumes that the engine is turning slowly enough that there will be no resonance effects in the intake or exhaust tract.

Engine & Environmental Parameters
Compression Ratio:1 
StrokeSame units as rod length
Rod lengthSame units as stroke
Cam durationdegrees
Altitudemeters
Temperature° Fahrenheit
 
Results
Percent of stroke%
Effective CR:1 
Temperature ratio:1 
Chamber temperature° Fahrenheit
Chamber pressurepsig
 

Notes

Compression Ratio

You must know the mechanical compression ratio of your engine to get reasonable results from this calculator. CR and intake valve closing are the two most significant factors in the calculation of the combustion chamber pressure.

Most modern normally aspirated (NA) engines have a CR in the range of 9:1 to 11:1. NA racing engines may be as high as 23:1, but are more typically from 12.5:1 to 15:1. A Diesel engine usually has a CR over 20:1; turbo- and supercharged motors are on the other end of the spectrum with CRs from 7:1 to 9.5:1.

Stroke & Rod Length

The ratio of stroke to rod length (often called L) determines how closely the piston tracks the vertical motion of the crank pin. With an infinitely long rod, crank pin vertical motion is translated into piston motion directly. With shorter rods, the piston accelerates more slowly when coming away from TDC and BDC, resulting in a greater "pause" in motion at the ends of the stroke (this is accompanied by greater acceleration in the middle of the stroke, and by higher piston-to-cylinder-wall pressures).

If you don't know the dimensions of your engine, use 1.0 for the stroke and 1.5 for the rod length. The ratio is what is important, and 1.5:1 is probably close enough.

Camshaft Duration

The camshaft duration number is used to determine when the intake valve closes, therefore you should use the zero-lash timing (not the 1mm or 0.050" specification). It is assumed that intake cams open 30° before top dead center (BTDC) and the remainder of the duration extends the closing time. For example, if the given intake valve duration is 250°, then the crank angle over which the intake valve is open goes from 30° before top dead center to 250-30-180 = 40° after bottom dead center (ABDC).

If you know the actual ABDC closing time of the intake valve use closing+210 for the cam duration value. Camshafts designed for use with superchargers or long-duration racing camshafts are very likely to have earlier opening timing.

If you can't find a zero-lash specification for your cam, you can estimate it. Hydraulic cams usually have about 20° ramps on either side of a 0.050" measurement, so a 230° @ 0.050" specification is about 270° @ 0". Solid cams usually have much longer ramps, but if the engine is cold then it won't be operating with zero lash, so the 20° figure is probably good enough here also.

Altitude

The altitude at which you are running compression tests has an effect on the resulting gauge numbers. This is easy to see by imagining that you are in the dead vacuum of space and running a compression test; the result will obviously be zero.

This has a relatively small effect on the results, so you can safely leave it at zero (i.e., sea level) unless you live in a high mountainous area. At my locale, Ann Arbor, MI, the elevation is about 250 meters ASL, which reduces calculated pressures on the order of 5 psi (about 2%). In Boulder, CO, with an elevation of 1650 meters you will see calculated pressures about 40 psi lower than at sea level. (Maybe I should make this local barometric pressure, as that's what is actually going on here...)

Temperature

The ambient temperature has no bearing on pressure, it is only shown since it's interesting to see how it affects the combustion chamber temperature.

Two results are given related to temperature: temperature ratio (the absolute increase in temperature over ambient, i.e., the one that makes a diesel fire) and chamber temperature. The chamber temperature is computed by multiplying the temperature ratio by the absolute Rankine temperature then converting back to Fahrenheit.

 
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