Question on compression..

DYORD · December 28th, 2007, 07:46

Just re-lube my TM Aug gearbox. Test fired it and warm air is comming out. Is that a sign of good compression or.. lack of lube?

Jimski · December 28th, 2007, 11:44

warm? how warm ?
it's weird !

Danke · December 28th, 2007, 12:21

Warm air = Expansion
Cold air = Compression

m102404 · December 28th, 2007, 12:59

Don't want to get into a debate or anything, but from what I understand, air that is being compressed will heat up (e.g. similar to a diesel engine). Air will also heat up as it traveling/flowing at high speed and meets resistance or changes direction (i.e. when a large cylinder of air is forced through a narrow cylinder).

Cylinders in some high powered air rifles can reach internal peak temperatures temporarily hot enough to detonate petroleum lubricants that may be present (i.e. exactly like a diesel engine).

Now, not sure how much of that is due to the friction of the piston head and the cylinder walls vs. the actual compression of the air captured in the cylinder.

On a physical observation note, when I'm testing mechboxes and place my finger over the nozzle to provide some cushioning of the piston impact, the air is warm (not hot).

To qualify the above...I almost failed university physics...so take my comments with a grain of salt.

Jimski · December 28th, 2007, 13:03

makes sense!

Danke · December 28th, 2007, 13:06

Two real world compression = cooled air examples are carburetor icing (if you're familiar with carbed engines) and when you dump the air from the tank of a compressor. It'll scream out and then stop as the moisture in it freezes from the sudden drop in temp an blocks the outlet. It'll melt quickly and the release more air and repeat.

Here's a semi comprehensible chart of temp vs. density.

http://www.engineeringtoolbox.com/ai...ies-d_156.html

The Saint · December 28th, 2007, 13:12

You really should be checking your compression by test pushing your piston through your cylinder with the cylinder head in, or using a chrony and comparing the actual fps with the spring rating. Testing temperature is not reliable in any way.

m102404 · December 29th, 2007, 15:22

Saint is completely right about compression testing before you assemble.

On another note, I spoke with a physics oriented person and hopefully I can relay the jist of what he said. He refered to Boyles (might not have an "e") law, which I won't butcher by trying to restate it here (check google for the exact details of how pressure, volume and temperature of gasses are related).

If you take a fixed cylinder of air and sqeeze it into a smaller volume, you are in-putting energy into the "system" and it will result in the air temperature rising. The common bicycle pump is a good example, where the hose will heat up (where as the cylinder will also heat up, but the effects of the cylinder wall/piston friction are removed by looking/feeling the hose vs. the cylinder), as air is pushed through it.

Referring to your cooling observation...as the compressed air expands into a greater volume (i.e. the atmosphere), Boyles law calculates that the temperature will drop as the pressure decreases...causing it's surrounding (the carb) to cool/chill/freeze.

Further, as you compress the air into a tank (or solid vessel, such as a CO2 cartridge), the air temperature is raised. This heat will dissipate into/through the vessel and into it's surroundings and the system will typically return to room temperature. Thus when the gas is released, the system will drop in temperature.

So...I think both explanations are right, but referring to two different systems.

But I put my finger over the nozzle of a body/mechbox build I did last night and it was warm...so whether or not it was due to the cylinder friction/compression/expansion/etc...it was still warm (but not hot)!

DYORD · December 30th, 2007, 16:27

^ that'll make sense... I'm starting to get it..

So I don't have anything to worry about.

Kos-Mos · December 30th, 2007, 18:26

checked with my Star G36K mechbox... warm also, so I guess it is normal.

As stated before a few times, check the compression BEFORE assembling your mechbox. Also check friction at the same time. It should be free enought so that the piston slide by itself when there is no gear and the mechbox is pointed down AND still have a nice compression.

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December 28th, 2007, 07:46	#1
DYORD Trader rating Join Date: Jan 2007 Location: Philippines	Question on compression.. Just re-lube my TM Aug gearbox. Test fired it and warm air is comming out. Is that a sign of good compression or.. lack of lube? __________________ MORE FPS :banghead:

December 28th, 2007, 11:44	#2
Jimski Trader rating Join Date: Jun 2005 Location: Montreal	warm? how warm ? it's weird ! __________________

December 28th, 2007, 13:03	#5
Jimski Trader rating Join Date: Jun 2005 Location: Montreal	makes sense! __________________

December 28th, 2007, 12:21	#3
Danke Prancercise Guru Trader rating Join Date: Oct 2007 Location: Danger Zone	Warm air = Expansion Cold air = Compression

December 28th, 2007, 12:59	#4
m102404 Tys Trader rating Join Date: Apr 2006 Location: Toronto	Don't want to get into a debate or anything, but from what I understand, air that is being compressed will heat up (e.g. similar to a diesel engine). Air will also heat up as it traveling/flowing at high speed and meets resistance or changes direction (i.e. when a large cylinder of air is forced through a narrow cylinder). Cylinders in some high powered air rifles can reach internal peak temperatures temporarily hot enough to detonate petroleum lubricants that may be present (i.e. exactly like a diesel engine). Now, not sure how much of that is due to the friction of the piston head and the cylinder walls vs. the actual compression of the air captured in the cylinder. On a physical observation note, when I'm testing mechboxes and place my finger over the nozzle to provide some cushioning of the piston impact, the air is warm (not hot). To qualify the above...I almost failed university physics...so take my comments with a grain of salt.

December 28th, 2007, 13:06	#6
Danke Prancercise Guru Trader rating Join Date: Oct 2007 Location: Danger Zone	Two real world compression = cooled air examples are carburetor icing (if you're familiar with carbed engines) and when you dump the air from the tank of a compressor. It'll scream out and then stop as the moisture in it freezes from the sudden drop in temp an blocks the outlet. It'll melt quickly and the release more air and repeat. Here's a semi comprehensible chart of temp vs. density. http://www.engineeringtoolbox.com/ai...ies-d_156.html

December 28th, 2007, 13:12	#7
The Saint Trader rating Join Date: Jan 2005 Location: Quebec	You really should be checking your compression by test pushing your piston through your cylinder with the cylinder head in, or using a chrony and comparing the actual fps with the spring rating. Testing temperature is not reliable in any way. __________________ "The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."

December 29th, 2007, 15:22	#8
m102404 Tys Trader rating Join Date: Apr 2006 Location: Toronto	Saint is completely right about compression testing before you assemble. On another note, I spoke with a physics oriented person and hopefully I can relay the jist of what he said. He refered to Boyles (might not have an "e") law, which I won't butcher by trying to restate it here (check google for the exact details of how pressure, volume and temperature of gasses are related). If you take a fixed cylinder of air and sqeeze it into a smaller volume, you are in-putting energy into the "system" and it will result in the air temperature rising. The common bicycle pump is a good example, where the hose will heat up (where as the cylinder will also heat up, but the effects of the cylinder wall/piston friction are removed by looking/feeling the hose vs. the cylinder), as air is pushed through it. Referring to your cooling observation...as the compressed air expands into a greater volume (i.e. the atmosphere), Boyles law calculates that the temperature will drop as the pressure decreases...causing it's surrounding (the carb) to cool/chill/freeze. Further, as you compress the air into a tank (or solid vessel, such as a CO2 cartridge), the air temperature is raised. This heat will dissipate into/through the vessel and into it's surroundings and the system will typically return to room temperature. Thus when the gas is released, the system will drop in temperature. So...I think both explanations are right, but referring to two different systems. But I put my finger over the nozzle of a body/mechbox build I did last night and it was warm...so whether or not it was due to the cylinder friction/compression/expansion/etc...it was still warm (but not hot)!

December 30th, 2007, 16:27	#9
DYORD Trader rating Join Date: Jan 2007 Location: Philippines	^ that'll make sense... I'm starting to get it.. So I don't have anything to worry about. __________________ MORE FPS :banghead:

December 30th, 2007, 18:26	#10
Kos-Mos Trader rating Join Date: Nov 2005 Location: Lévis (QC)	checked with my Star G36K mechbox... warm also, so I guess it is normal. As stated before a few times, check the compression BEFORE assembling your mechbox. Also check friction at the same time. It should be free enought so that the piston slide by itself when there is no gear and the mechbox is pointed down AND still have a nice compression.

Question on compression..

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