Steam engine on compressed air

[GordonL]

I am trying to run a Corliss engine on compressed air using an airbrush compressor. My question is what am I missing in my calculations? The engine has 1.125 bore x 1.5 stroke. This calculates to 1.49 ci/stroke. Since it is powered both directions it should use 2.9 ci per revolution of the flywheel. The compressor is rated for 1 cubic feet per minute with 40 psi to 58 psi set points. 1 cf = 1728 ci. 1728/2.9 should result in 595 revolutions. It is turning the flywheel at probably 25 RPM and it shows no pressure on the gage. It runs on my shop compressor with at about 5-7 PSI. I realize that there are probably some leakage around the piston and the valves but this is far from the expected result. Other folks are running similar engines on airbrush compressors. Is there a problem with my calculations or is the compressor just not putting out the volume that is is rated for?

[crueby]

Math looks right to me. Corliss rotary valves are very hard to make fit with no leaks, so that may be where a lot is going - I know mine leaked around the valves. Its amazing how much volume a seamingly small leak can let escape, but I have no data on how that would calculate out. Also may be leakage around the piston?   One thing to try would be to hold the flywheel at different points in the stroke so it cannot rotate, and listen to hear how much hissing there is and where.

And compressors not making rated numbers? No surprise there! For that type of compressor, I would not be surprised if the rated volume is at a very low pressure, not while maintaining the 40+ psi. I used to work in consumer products, and saw how many games companies played with rating numbers - often many paragraphs of fine print.

If its able to run that large and engine at 25 rpm, I'd call it a win. Any type of portable small compressor I've seen struggles to run anything more than a small oscillator engine.

[GordonL]

I did try turning it around by hand and could not hear any obvious leaks at any point but my hearing is poor so it may be leaking but I cannot hear it. Running at the 25 RPM it has barely enough power to keep the flywheel turning. I get about the same results using a Tailgater 120 volt tire inflator with a whole lot more noise.

[crueby]

Does the airbrush compressor or its manual say 1 CFM at which pressure? Usually they do, but not always. If not, its most likely they are measuring the CFM at little to no pressure. Shop type ones will give that rating, the little compressors often don't, since the number is so low.

[GordonL]

It does not specify volume at pressure. I think that you are probably correct that it is rated at 1 CFM at 2 PSI. I will try putting at about 25 PSI with the shop compressor and see if I can hear any leaks at that pressure.

[Kim]

My experience matches what Chris said.

I have an airbrush compressor and it will not run anything more than the smallest of steam engines meaningfully.  If you let the compressor run till it fills up the tank so up to pressure, it will run a small engine for a few seconds till the compressor kicks on again (almost immediately) and then just slow down to almost nothing.

Those little compressors just can't keep any pressure if you're taking any volume of air.

If you can get an engine as large as your Corrilis to run at 25 rmp, that's pretty impressive, I'd say.

Kim

[Jasonb]

Smaler compressors usually quote the DISPLACEMENT not the FAD ( Free Air Delivery) so you may only be getting 40% of the quoted figure due to losses and that will drop as pressure goes up.

[BillTodd]

As an aside : Adding a heater down stream of the tank will improve the performance considerably. The compressed air needs heat to expand and , unlike steam, does not have much heat capacity so needs to steal heat from your engine (an engine designed for 100'C plus operation probably wont like the cold)

 An air heater was a standard fitting on commercial  pneumatic vehicles.

http://www.douglas-self.com/MUSEUM/TRANSPORT/comprair/comprair.htm

[internal_fire]

Serious compressors are typically rated by volume at the specified pressure. (Or multiple pressures.)

Small hobby-type compressors are typically rated by input volume, which is essentially the same as the displacement volume.

Therefore the volume rating is for zero output pressure.

Gene

[GordonL]

Fortunately I got the airbrush compressor from Amazon and I can return it for a full refund. On to plan B.