Compression? What do you mean???

[Lakeside53]

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.

.

yes...

Take a volume of air at any one temperature, reduce the volume by compression which increase the temperature and the pressure, wait for it to cool and it's back the ratio of the volumes.

It's a good point, but if this was a big effect on 10:1 engines, in perfect world you'd see MORE than "volume compression pressure" as the air heated, then it would settle back to "unheated compression pressure" as it cooled. O.K., so the rings aren't perfect and it's not a linear equation. The cylinder isn't materially warm by hand cranking, the inlet air is roughly ambient temp so the compressed volume will settle to roughly that of the air at ambient.

I see the temperature effect on a stroke when cranking, but not a huge amount. You can see the gauge move back as the air cools.

I agree leakdown is a more reliable measure..

DJMercer1 - this is not what you're talking about... and it's why I said "ignoring temperature" in an eariler post.

 

[Tzed250]

PV=nRT


The guage is telling you the result of cranking the engine (cranking speed) to compress a gas. Te result will go up or down depending on the effectiveness of the pumping apparatus (ring seal on a two-stroke), and the compression ratio ([Swept volume + final volume]/final volume).

 

[djmercer1]

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...

ok, after some deeper thought, i going to agree with you that the schrader needs to be at the connection port of the engine for the most acurate result. in examining the power plants we have installed at work the major difference is that they are operating under signifcant back pressure and are of significant size, unlike chainsaw motors etc.

also, i dont think that the thermo laws like charles, boyles or the idl etc are being properly applied here since we are not dealing with either fixed volumes, or monoatomic gases(important here due to gasolines low vapour pressure/boiling point). the only method of analyzing the test to my mind is first principles and to do it in discrete steps, not the test as a whole. i would think temperature is effectively irrelevant when manually cranking the engine, but temp could be important if you have warmed up the engine properly prior to testing.

edit add:
as to the gas vapoour pressure issue, id think that at 0psi(test start) the there is a gas/air mist input into cylinder and at the peak pressure say 150psi(test end) that some of the gas vapour would be condensed into a liquid thus giving a seemingly variable compression ratio due to the lost volume of the condensed gas vapour. im not sure if this would be a signifcant volume though, but dealing with small volume cylinders it could be.

 

[Lakeside53]

The idea behind Henry, Charles and Boyles laws is that to model a real world sitution they often all need to be applied simulataneously... they interact.

As for the fuel vapor idea... try it without any gasoline mix. If the vapor "condensed" materially, it would not be good for combustion.

 

[sperho]

With the Schrader valve at the wrong end... I have an Actron with the valve at the wrong end too. Argh.

I retract my statement above regarding the Bailey's compression tester. Truth is, I don't know where it's check valve is. I saw the relief valve up near the gauge and may have thought that's where it was. I used my Actron gauge tonight and I had to take the long reach adapter off of it. I've always used the long reach adapter for my motorcycles. It had been so long since I'd had it off that I had forgotten that there was in fact a Schrader valve at the threaded end of the hose, so I should be good to go with my compression readings using this gauge. There is also a relief valve near the gauge so you can release the pressure and then do another test for replicate data. That's likely how the Bailey's tester is set up; perhaps Grande Dog can confirm.

For what it's worth, here is my gauge at a good price: http://www.actron.com/product_detail.php?pid=16171 You can see in the instruction manual where the check valve and release valves are. A very versatile tester for not too many bucks.

 

[maccall]

I retract my statement above regarding the Bailey's compression tester. Truth is, I don't know where it's check valve is. I saw the relief valve up near the gauge and may have thought that's where it was. I used my Actron gauge tonight and I had to take the long reach adapter off of it. I've always used the long reach adapter for my motorcycles. It had been so long since I'd had it off that I had forgotten that there was in fact a Schrader valve at the threaded end of the hose, so I should be good to go with my compression readings using this gauge. There is also a relief valve near the gauge so you can release the pressure and then do another test for replicate data. That's likely how the Bailey's tester is set up; perhaps Grande Dog can confirm.

For what it's worth, here is my gauge at a good price: http://www.actron.com/product_detail.php?pid=16171 You can see in the instruction manual where the check valve and release valves are. A very versatile tester for not too many bucks.

I bought mine elsewhere, but it looks identical to the one from Baileys, and it has the valve at the screw-in tip.

 

[sperho]

I followed up with Grande Dog of Bailey's and he confirmed that their tester has a check valve at the cylinder end of the hose... Good deal.

 

[Crofter]

Some guages have a pressure release near the guage but that is separate from the shrader type check valve that is at the cyl. end of the hose. Some types that are ok for large cylinders do not have the one at the cyl. end. You got to check them out to see what you have and if it will give proper readings on small cc cylinders.