Compression? What do you mean???

[djmercer1]

The pressure in the cylinder returns to zero (or really really close) each time - the exhaust port is open to the world.

The pressure as you read it "building" your gauge is built up by the valve at the gauge. In a perfect world, if the gauge and hose had NO volume, the reading would be instantaneous.


The correct compression gauge needs the valve at the tip, not at the gauge. None of this matters when the cylinder is very large in comparison to the hose volume - as in a car.

for this to be the case, you would have to crank slowly or have the piston stop with the port exposed. compression testing ive done(small 2-stroke to massive industrial diesel power plants) doesnt show the guage returning to 0psi every stroke. if what your suggesting is how a test works, what your are measuring is the compression build by one stroke, there wouldnt be any build up of pressure no matter how long you stroke the engine. the valve bieng at the tip, if only one stroke was building the pressure, would still render a lower reading as the valve wouldnt open until there was a pressure differential fromthe cylinder to the hose.

as for a vehicle this still isnt really the case, but id agree that the volume increase would mean less than on a small engine.

 

[Lou]

for this to be the case, you would have to crank slowly or have the piston stop with the port exposed. compression testing ive done(small 2-stroke to massive industrial diesel power plants) doesnt show the guage returning to 0psi every stroke. if what your suggesting is how a test works, what your are measuring is the compression build by one stroke, there wouldnt be any build up of pressure no matter how long you stroke the engine. the valve bieng at the tip, if only one stroke was building the pressure, would still render a lower reading as the valve wouldnt open until there was a pressure differential fromthe cylinder to the hose.

as for a vehicle this still isnt really the case, but id agree that the volume increase would mean less than on a small engine.

Remember that check valve that goes HISS when you release the pressure? You can completely remove the compression tester from the cylinder and the indication remains constant. The pressure in EVERY engine drops to near zero as the valves operate or the intake and exhaust ports are opened. With the check valve mounted on the gauge end of the hose you do not get a true picture of cylinder pressure on a small volume cylinder!

 

[djmercer1]

Remember that check valve that goes HISS when you release the pressure? You can completely remove the compression tester from the cylinder and the indication remains constant. The pressure in EVERY engine drops to near zero as the valves operate or the intake and exhaust ports are opened. With the check valve mounted on the gauge end of the hose you do not get a true picture of cylinder pressure on a small volume cylinder!

however, once there is a given amount of pressure built up to the guage side of the schrader valve, on the next cycle there wont be any build up until that trapped pressure is exceeded in the cylinder, this takes several cycles for the engine. else, the engine is effectively on a 'new start cycle' every ignition cycle. the ignition process in an engine is assisted by the latent pressure in the engine---this pressure is left when the engine is cycling at idle speed(ie the time when the exhaust port is open is too short to allow all pressure to escape).

 

[Lou]

however, once there is a given amount of pressure built up to the guage side of the schrader valve, on the next cycle there wont be any build up until that trapped pressure is exceeded in the cylinder, this takes several cycles for the engine. else, the engine is effectively on a 'new start cycle' every ignition cycle. the ignition process in an engine is assisted by the latent pressure in the engine---this pressure is left when the engine is cycling at idle speed(ie the time when the exhaust port is open is too short to allow all pressure to escape).

CHECK VALVE... Allows pressure to flow one way provided the pressure on the low pressure side exceeds the pressure on the high pressure side!!! Once equal no further exchange of pressure happens, EVEN if disconnected from the low pressure source!!!

 

[djmercer1]

CHECK VALVE... Allows pressure to flow one way provided the pressure on the low pressure side exceeds the pressure on the high pressure side!!! Once equal no further exchange of pressure happens, EVEN if disconnected fro the low pressure source!!!

exactly, for the pressure to reach the gauge on subsequent cycles, the pressure build up in the cylinder(by your theory, in a single cycle) must exceed the pressure being held back(in the gauge) by the valve(which was also build up by a single cycle). the idea that the pressure drops to 0psi every cycle reuires that the excess air causing the pressure in the cylinder can excape through the exhaust port in a couple tenths of a second, and this isnt possible if your stroking the engine at a normal start rate as you shold for a compression test.

 

[Lou]

Go out to your garage, hook up a tester, crank the engine HELL start it. Shut it off, disconnect the tester without actuating the release then tell me what it reads! If disconnected is the low pressure at zero??? Find another engineer maybe he can explain it to you!!!

 

[djmercer1]

Go out to your garage, hook up a tester, crank the engine HELL start it. Shut it off, disconnect the tester without actuating the release then tell me what it reads! If disconnected is the low pressure at zero??? Find another engineer maybe he can explain it to you!!!

ok, if i disconnect the the tester after a compression test without bleeding the schrader, it will maintain the highest pressure the gauge has seen. this exercise still doesnt prove your theory of an engine building full pressure in a single stroke(when you finish a compression test, disconnect the tester from cylinder in mid stroke and you will often get the 'hiss' of escaping pressure from the cylinder, though hard to prove). typically auomobile engines require 3-5 cycles to build full pressure, with the relatively small difference between volumes in tester and cylinder this may increase to 8-10 for a chain saw, there is no basis to say that a tester will read low depending on where the schrader valve is located.

cylinder pressure readings in a running engine typically run at from max pressure at ignition(similar to a compression test) to about 75% when begining the compresion stroke of that value(a minimum value), otherwise the engine is shot and runs like crap.

 

[Lou]

If the cylinder is 2 milliliters in volume and the hose between the cylinder and schrader is 1 milliliter that will not effect on the overall compression ratio of the cylinder?
Where do you work? I want quit buying their products!!!

 

[djmercer1]

If the cylinder is 2 milliliters in volume and the hose between the cylinder and schrader is 1 milliliter that will not effect on the overall compression ratio of the cylinder?
Where do you work? I want quit buying their products!!!

well, the physical properties of the tester wont affect the compression ratio of an engine. the compression ratio is fixed considering the max and min volumes of the engine(mostly set by design and manufacture). the maximum pressure that can be built in the engine does change----typically its lower due to wear/abuse).

if the volume of the tester is comparable to the cylinder, then a large volume reservoir in the tester means you will have to cycle the engine many more times you would for a small volume tester, the location of the schrader valve is immaterial to the test result. the other consideration is the size and number of fittings between the gauge and the cylinder---there is a pressure loss associated with each fitting and a unit-length loss for tubing.

 

[Lou]

The volume compressed remained the same but we increased the total volume by 50%. You contend this will have no effect on the total pressure created? You understand we are compressing a gas not a fluid?

 

[djmercer1]

The volume compressed remained the same but we increased the total volume by 50%. You contend this will have no effect on the total pressure created? You understand we are compressing a gas not a fluid?

im guessing that we still dont agree on the fact that there is latent pressure in the cylinder after a cycle? what im saying is that, the additional volume of the compression gauge can overcome by additional cycles of the engine.

 

[Lakeside53]

Not sure about Lou , but I certainly disagree.

1+1=2

You just compressed air on a piston upstoke, and got back the exact same volume on a downstroke... (not combustion in a compression reading).

Pressure and volume are a constant (ignoring temperature)- Boyles law...

http://en.wikipedia.org/wiki/Boyle's_law

 

[sperho]

There is a ram effect when an engine is at working speed. How is that accounted for? Or is it? Or does it need to be?

Now, rather than buying something to solve a real or perceived problem: It's easy to measure the volume of a hose. That measured volum could be used to correct for the hose perturbing the measurement by increasing the displacement. A 70cc saw measured with a tester with a 5cc hose and a check valve at the gauge would equal a 5/70*100% = 7.1% error. Multiply your gauge pressure by 1.071 and be done with it. Problem solved, eh?

 

[ol'homey]

djmercer,
Any volume in the hose exposed to the combustion chamber before the schrader valve has the same effect on a compression reading as additional combustion chamber volume. Greater cc volume=lower max psi
I know the above doesn't make any sense to you so here's something you can try to prove who's right. Take a long air hose say 25' or so and attach one end to the spark plug hole and the other to your compression tester. Start pulling. Without a schrader valve in the line the effective combustion chamber volume will be so large it won't even move the needle on the gauge no matter how many times you pull it. If the volume of hose didn't matter you would still be able to obtain a reading, but it does matter, so you won't. Now put a schrader valve in the spark plug fitting. Your effective cc volume is now equal to true cc volume. Pressure will begin to build in the line and eventually will top out at the correct psi reading even though you've got a 25' line attached to your gauge. Now move the schrader valve say 2' down the hose away from the cc. Start pulling. Your psi reading will top out at a much lower reading than the one taken with the schrader valve in the plug fitting. Keep moving the schrader valve back and soon you won't get any reading at all.

 

[sperho]

djmercer,
Any volume in the hose exposed to the combustion chamber before the schrader valve has the same effect on a compression reading as additional combustion chamber volume. Greater cc volume=lower max psi
I know the above doesn't make any sense to you so here's something you can try to prove who's right. Take a long air hose say 25' or so and attach one end to the spark plug hole and the other to your compression tester. Start pulling. Without a schrader valve in the line the effective combustion chamber volume will be so large it won't even move the needle on the gauge no matter how many times you pull it. If the volume of hose didn't matter you would still be able to obtain a reading, but it does matter, so you won't. Now put a schrader valve in the spark plug fitting. Your effective cc volume is now equal to true cc volume. Pressure will begin to build in the line and eventually will top out at the correct psi reading even though you've got a 25' line attached to your gauge. Now move the schrader valve say 2' down the hose away from the cc. Start pulling. Your psi reading will top out at a much lower reading than the one taken with the schrader valve in the plug fitting. Keep moving the schrader valve back and soon you won't get any reading at all.

In theory, yes. It falls apart when you begin to consider the hose as a column of air that has momentum. How fast you crank it makes a difference in that case. Truth is, the argument is somewhere between these two extremes. That being said, any question can be eliminated by using a tester with the check valve as close to the cylinder as possible.

This is a serious question: what do the readers of this thread think the point of a compression test is? A followup question may occur.

Andy: Define "average compression". By my determination, I would say that average compression for any engine would be zero psig.

 

[ropensaddle]

my lisle has valve at gauge end and I can get 175 does that mean I actually have better compression? I think I could adjust my values to know if cylinder is in good shape but that is the way I c it

 

[pgg]

In reality reading compression figures on a chainsaw is pretty much a meaningless exercise, you know the compression's f##*d because the saw's f##*d, you know power is lacking, you've gone thru the carb a million times, when the compression's f##*d, the saw's basically a worn out piece of rattling junk, time for an overhaul or a new saw..

 

[djmercer1]

Not sure about Lou , but I certainly disagree.

1+1=2

You just compressed air on a piston upstoke, and got back the exact same volume on a downstroke... (not combustion in a compression reading).

Pressure and volume are a constant (ignoring temperature)- Boyles law...

http://en.wikipedia.org/wiki/Boyle's_law

well. that just it, the pressure isnt really the same as it was at the initial state of the cycle(remember the air trapped behind the schrader valve), and thus boyles doesnt apply because we arent dealing with constant volumes. infact if air is prevented from entering the cylinder when the piston returns to the orginal position there will in fact be negative pressure. this has the effect of increasing the volume to compensate for the excess volume. every cycle of the engine there is an incease in air volumedue to what has ben trapped behind the schrader.


added:
in doing a compression test, the engine is acting as an air pump and is doing work, and has work done on it.

 

[Crofter]

Here is a couple of things to muddy the water. The speed of cranking has a fair bit of effect on the observed guage reading and it can be seen to increase after a number of repeated cycles because of the heat of compression affecting the reading. It is really obvious with diesels where the compressed temperature is upwards of 750 deg. F. I do not have the figures at hand but it is quite considerable. Because of the speed of cranking variables on observed pressure a lot of diagnostics use a leak down test to assess ring sealing. Even if there were ZERO leakage past the rings and the compression guage had ZERO internal volume it would still take a number of cycles to see the maximum because of the heat affect. This is in addition to filling the volume of the guage apparatus.

With the shrader valve at the cylinder end of the hose it takes a number of additional cycles to fill the additional hose volume to equalize pressure before the guage will rise to potential cylinder pressure. Without the shrader valve the reading will drop to near zero between every compression stroke; I have experienced a faulty valve and it is very obvious. You also cannot replace it with one from a tire valve stem as the check valve spring is way much heavier and wonks readings lower by quite a bit. When you see the readings drop every stroke, you know cylinder pressure is dropping between compression strokes. When you realize that the exhaust port must be able to unload cylinder pressure in .002 of a second or less at operating RPM you can be sure it does so at cranking rpm.

 

[Lakeside53]

well. that just it, the pressure isnt really the same as it was at the initial state of the cycle(remember the air trapped behind the schrader valve), and thus boyles doesnt apply because we arent dealing with constant volumes. infact if air is prevented from entering the cylinder when the piston returns to the orginal position there will in fact be negative pressure. this has the effect of increasing the volume to compensate for the excess volume. every cycle of the engine there is an incease in air volumedue to what has ben trapped behind the schrader.


added:
in doing a compression test, the engine is acting as an air pump and is doing work, and has work done on it.

Boyles laws was to illustrate a point - when NOT using a schrader valve.


Obviously you don't agree with me, Lou or others, so read the last part of Crofters post again... The pressure is effectly zero in the cylinder by the bottom of each stroke...


Read also Henry's laws and Charles laws...