Maximum Cylinder Pressure

[JAKEFORD]

We do a compression test and measure maybe 110 psi at cranking speed.

What do you think the cylinder pressure would be when we run the engine?

Will it be higher under load at WOT and lower at idle?

A professional re-builder told me at a recent show that the maximum pressure is close to the compression test but I have a problem with this. I think for a few instants at TDC it will be much higher. As the piston moves down the bore it will reduce, maybe close to cranking pressure and maybe this is what he meant.

Does anyone know?

 

[flatheadmurre]

Are we talking compression stroke maximum or actual combustion ?
If the pressure at combustion wasnt higher the engine wouldnt work....

 

[JAKEFORD]

Answered my own question per attached diagram.

 

[Brian]

I think a compression test gives a comparison of pressures in all cylinders, as such, is a handy diagnostic tool, but certainly is not indicative of the absolute pressures a cylinder sees. Obviously, when ignition occurs, cylinder pressures are considerably increased. Bear in mind also, that using longer duration cams will indicate lower compression at cranking speed, but at higher revs that compression will also increase.

 

[oldford2]

We do a compression test and measure maybe 110 psi at cranking speed.

What do you think the cylinder pressure would be when we run the engine?

Will it be higher under load at WOT and lower at idle?

A professional re-builder told me at a recent show that the maximum pressure is close to the compression test but I have a problem with this. I think for a few instants at TDC it will be much higher. As the piston moves down the bore it will reduce, maybe close to cranking pressure and maybe this is what he meant.

Does anyone know?

Interesting.........
I also would think that at ignition the pressure would increase significantly. Any expert opinions?

 

[JSeery]

Cylinder pressure depends on what is going on within the cylinder during any given cycle or phase of a cycle. The fuel/air mixture in the cylinder is compressed on the compression stroke to something in the vicinity of the reading during a compression test. When the mixture is ignited it starts to burn and the burn rate determines the pressure rise within the cylinder. With most cams ignition occurs prior to TDC so the pressure is rising while the piston is still moving upward. Total cylinder pressure reached will depend in part on the burn rate of the fuel mixture. If the burn rate is to fast the pressures can go beyond the normal ranges, this is what is occurring during pinging and detonation.

This is called the Otto cycle, this link shows a fairly good example of how this can be graphed out. Click on the figure to see the phases of the cycle.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/otto.html#c6

 

[Binx]

We do a compression test and measure maybe 110 psi at cranking speed.

What do you think the cylinder pressure would be when we run the engine?

Will it be higher under load at WOT and lower at idle?

A professional re-builder told me at a recent show that the maximum pressure is close to the compression test but I have a problem with this. I think for a few instants at TDC it will be much higher. As the piston moves down the bore it will reduce, maybe close to cranking pressure and maybe this is what he meant.

Does anyone know?

It is a good question and hopefully JWL may answer the science facts, but air is like a foamy gelatin. Atmospheric pressure will push part of the air into a cylinder at high RPM but the remainder will be under partial vacuum and because of mechanical obstruction to laminar flow, the cylinder will not completely fill resulting in a lower compression. And that is why a flathead will run out of breath before it self-destructs.

Lonnie

 

[Tom Walker]

Answered my own question per attached diagram.

That graph is meaningless without a description of the process which produced the figures.
The pressure in the cylinder is directly related to the power output of the motor. Throttle shut at idle less pressure, WOT accelerating more pressure.

 

[JWL]

There are several ways to look at this question. In the original posting it was said the compression pressure is close to the actual cylinder pressure when the engine is running. Actually that is true---but, before you go ballistic, let me explain that pressure measurement would be what we call MEP(mean effective pressure). In other words, the AVERAGE pressure during the combustion cycle from the closing of the intake valve to the opening of the exhaust valve.

As many of you know, from reading my book Flathead Facts, the PEAK cylinder pressure for our Flatheads occurs at around 22 degrees ATDC(after top dead center). Many factors converge to vary this peak number from cycle to cycle within the same cylinder and from engine to engine. But, to put some numbers on it the peaks would probably be between 400 and 800 PSI depending on the engine. These numbers would occur at the RPM of maximum torque and at WOT(wide open throttle) probably between 2000 and 3000 RPM, again, depending on the engine.