How much pressure needed to run on air

[Michael CallaghanParticipant @michaelcallaghan68621]

[Michael CallaghanParticipant @michaelcallaghan68621]

Hi, can anyone let me know how much pressure is needed to test run a locomotive to check timing. Thanks

[noel shelleyParticipant @noelshelley55608]

20PSI should do, if things are free. Noel

[Dennis RaynerParticipant @dennisrayner52782]

I think you'll find that adequate CFM is a significant requirement but I've no idea how much is needed.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Dennis Rayner on 13/02/2023 15:08:14:

I think you'll find that adequate CFM is a significant requirement …

That's my experience with small horizontal engines. They turn over OK at surprisingly low pressure, say 5psi, but need lots of volume. A small high-pressure pump won't cut it; what's needed is moderate pressure and a big reservoir tank.

The air pressure needed to turn an engine is proportional to how much friction is in the bearings. motion and valve gear. If it turns over easily by hand, low pressure. If hard to turn, more pressure.

Once the engine is running, it's the volume of air consumed by the piston moving in the cylinder space that matters. The engine won't run If the volume of air output by the pump can't fill the space inside the cylinder fast enough. The amount of air needed depends on the size of the cylinders and how many strokes per second the piston does. Not sure to guesstimate it but more than 4 x cylinder volume per stroke. Say the cylinder is 2 cubic inches and the wheels are turning at 60rpm, then 8 cubic inches per second, about 1/4 cubic feet per minute, As a garage size compressor usually has a tank containing a bit under 1 cubic foot of air, something like a few minutes run time. Mid range compressors often come with the same sized tank but with more-or- less powerful motors. A 3HP motor will run the engine for longer than a 1HP compressor.

So many variables make it hard to answer the question accurately, but lowish pressure backed up by a big reserve tank,

Be interesting to find out what actual performance people who've done it report.

By the by, a compressor not being able to run an engine for long doesn't mean compressed air is safe in a boiler that hasn't passed a hydraulic pressure test. Unlike water, compressed air acts like a giant spring. It stores a lot of energy that's released instantly if the boiler lets go. An enormous number of watts are released in a sharp pulse, essentially a flameless explosion with shrapnel and ruptured eardrums etc…

Dave

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