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Compressed air for initial engine test

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Mike Hurley28/05/2021 10:40:00
325 forum posts
87 photos

With any luck, hopefully in a short while I will be in position to test my refurbishment of the 3 x 5 Victorian workshop steam engine ( as detailed in the Work in progress forum ). To that end I'll initially run on compressed air just to check basic functionality ( or lack thereof! ).
My question to the assembled experts is 'how much' ? I shall need to hire, beg steal or borrow a suitable compressor or source but have no idea what capacity would be needed, having zero experience with such matters. I did a search on the forum but the few results were related to quite small models and I'm not sure if the info would scale up.
The 2 cylinders are 3" bore and about 5" long with a sweep length of 4" giving a volume of 46.6 cu in in total. They have a 90 degreee angular rotation difference. Would I be correct to assume a straightforward multiplication of total cylinder volume x theoretical target RPM would give me the required CFM? Sounds far too straightforward to be honest though.
But pressure at that capacity? - I have no idea what sort of figures we are talking about. Also need to factor in the fact that everything should be a bit tight at first.
Any pointers most gratefully received.
Regards Mike

Howard Lewis28/05/2021 10:53:11
6301 forum posts
15 photos

Two cylinders 3" bore and 4" stroke are likely to outrun the average 2 or 3 hp hobby compressor VERY quickly

The larger the reservoir, the longer your engine will run, before the compressor cuts in again and tries to keep up.

A 1/2" impact gun soon causes my compressor with 50L reservoir to cut in again.

With the regulator set to 5 psi, a 1/16" hole causes it to run on a 50% duty cycle which is a bit cruel (Floating a 1" Ali ball as a display item / crowd puller on the club stand .WHEN shows happen again! )

You may need a fairly large compressor, with a large capacity reservoir to run it any even a low speed.


Mike Hurley28/05/2021 11:03:36
325 forum posts
87 photos

Thanks Howard, I'm not expecting miracles from compressed air, just a basic 'does it go round at all' ? and no major leaks, rattles, bangs and bits dropping off etc. Speed wouldn't be much of an issue at this stage.

I know there is a big difference between c/air and steam in engines like this, but at present it's just one step at a time. I just don't want to go to the trouble of getting a source that will be too underpowering and then not know if its an issue with the engine.


not done it yet28/05/2021 11:18:41
6880 forum posts
20 photos

Apart from the OP under-estimating the swept volume by some 20% or so I would have thought that, while not keeping up with the flow for long, a 2-3HP compressor should be able to deliver air at, say 20psi, for sufficient time to test the engine? Depends on the receiver volume, to a greater or lesser extent, I would suggest? More of a problem if running at high rpm.

SillyOldDuffer28/05/2021 11:41:59
8863 forum posts
1995 photos

The calculation for cubic feet per minute needed to give a certain rpm is reasonable, but the pressure needed to turn the engine at all depends on it's internal friction, which is unknown,

However the minimum pressure can be estimated by winding a cord around a drum mounted on the output shaft and hanging weights on the end. The air pressure needed to move the piston can be calculated from the weight needed to spin the engine.

I think it's easier to guestimate it. A small workshop steam engine would have worked on steam at between 50 and 200psi, probably at the low pressure end. I'd be surprised if Mike's unloaded engine in good condition wouldn't turn on 10psi of air. And therein lies a problem: most compressors produce low volumes of high pressure air and are mismatched to anything needing large volumes of moderately pressured air. To run a steam engine on air for any length of time needs a big reservoir. If a short burst to prove it works is all that's needed, I think a 2HP compressor will do the job briefly, but their reservoirs are too small to maintain large airflows for long.

High-pressure low volume air pumps are easy to borrow or buy, as are low-pressure high-volume pumps (fans!). Anyone know of a readily available medium-pressure/medium volume air pump?


JasonB28/05/2021 12:11:47
23034 forum posts
2769 photos
1 articles

As they are double acting engines your calculation needs to be double, I make it 0.065cu.ft per rev

0.125 x 0.125 x 3.142 x 0.333 x 2 x 2

So if run at 100rpm you want a 6.5cfm FAD compressor which will be one being sold as approx 10cfm unless runs are kept short and you don't mind waiting for a tank to refill

Bazyle28/05/2021 18:16:00
6379 forum posts
222 photos

I think it might help to divide the test into two phases.

First to see if there is some major friction and the timing is reasonable you (may) need some pressure but only enough volume to turn it over haf a dozine times.

Once you have got those things sorted it would hopefully turn over at only a few pounds pressure, after all 3in bore gives a lot of force per pound pressure. To get volume at minimal pressure find something like an airbed and a bunch of children who will love the idea of all piling on to it to get the air out. Another angle on this is an oil barell that you fill with water (mains provides a fair pressure) pushing the air out. Don't forget alos that such an engine might also work with assistance of a vacuum from a condenser.

Zan28/05/2021 18:49:50
313 forum posts
20 photos

I think in addition, you will need a big diameter feeder pipe to minimise throttling

Mike Hurley28/05/2021 20:12:08
325 forum posts
87 photos

Thanks for the suggestions guys - all very useful info.

I had overlooked that it is in fact double acting, so needed to figure that in my volumes! I think I now have some indication of the sort of values I need to work with, and useful methods on determining necessary pressures / air volumes that may be involved.

Bazyle's idea of oil barrels and water is quite interesting and may well be something worth examining just out of interest, particulary after having had a quick look at the daily cost of hiring compressors of the sort of sizes suggested in the thread! However, after the time and effort (and not inconsiderate cost in materials and tooling) involved in the restoration over the last 3 years, I might just have to grin and bare it.

It's primarily a case of determining that its basically sound and operational before investing more time and effort in this project.

As usual, thanks again to all for taking your precious time to read and respond. All the best. Mike

SillyOldDuffer29/05/2021 11:54:36
8863 forum posts
1995 photos
Posted by Bazyle on 28/05/2021 18:16:00:
...Another angle on this is an oil barell that you fill with water (mains provides a fair pressure) pushing the air out. ...

By coincidence I've just read something similar. In 1717 Dr Halley built a diving bell and replenished the air by alternating lowering a pair of weighted casks to the divers, each time compressing fresh air in the barrel and releasing it inside the bell. 'I caused a couple of barrels of about 30 gallons each to be cased with lead, so as to sink empty, each of them having a bunghole in its lowest parts to let in the water, as the air in them condensed on their descent , and to let it out again...'


UK minimum water pressure is supposed to be 7 metres of head, about 10psi, so filling an oil drum would do the trick. Air pressure will be low until enough water enters the drum to compress it. I'd half fill the barrel before tapping off any air.

Is it dangerous? Maybe! There's a lot of energy stored in compressed air and releasing it with a bang can do a lot of damage, especially in a confined space.  There are a lot of unknowns, always bad news in engineering:

  • Water pressure varies and most properties get 1 bar (14psi) or more. Unless the pressure is measured (which can be done by estimating from the flow rate), how stressed a barrel will be is unknown. And unlike the diving bell example, the outside isn't supported by sea-water, only by the atmosphere. Not good.
  • I've no idea how much internal pressure an oil drum will take before a seam fails, and because oil drums aren't designed or tested as pressure vessels this is high risk.
  • As oil drums are made of relatively thin metal, they will inflate like a balloon putting severe stress on the seams, implying high risk of fatigue failure. How good, or bad the seams are at resisting alternating pressure is also unknown.
  • The mode of failure is unknown too. Big difference between a seam that acts as a safety valve by opening up slowly, and a seam that ruptures all at once.

Some interesting guidance from this thread:

0.2 PSI - Breaks Windows
0.5-1.0 PSI - Shatters windows with body penetrating velocity.
1.0-2.0 PSI - Destroys typical wood frame structure
2.0-3.0 PSI - Blows in brick facing of steel frame building

5 PSI - Eardrum rupture
8 PSI - Fatal head injury (due to being propelled into stationary objects)
10 PSI - Serious lung damage
11-15 PSI - Fatal bodily injury (lung rupture or internal organ displacement)

Note: these are overpressures, i.e. the pulse of energy released in the instant that a pressure vessel fails. It depends on how much energy is stored (big boilers much more dangerous than small ones); how far away the pressure vessel is (stood next to is much more dangerous than 10 metres away), how confined (in the same room is much more dangerous than in the open air), and how quickly the rupture occurs (fast failures are far more dangerous than slow deflations.)

and, Ex : for vessel with 1 m^3 , diameter of 1m, under 10 bar, thk 5 mm, the energy is :
-compressed air : 2 300 000 J
(The original thread gives formula.)

Note: 10bar is 145psi, 5mm thick steel is much stronger than an ordinary oil drum, and there are no weedy seams! 2300000J is the energy needed to deliver 0.63kW for an hour, or 1.7M ft-lbs. The example stores considerably more energy than an water filled oil-drum! Nonetheless, I wouldn't want to be anywhere near an oil-drum full of compressed air if it went pop.

Definitely approach with caution!



Edited By SillyOldDuffer on 29/05/2021 11:56:41

Redsetter29/05/2021 12:13:27
205 forum posts
3 photos

Not sure how portable the engine in question is, but presumably it will go in the back of your car.

Why don't you speak nicely to your local garage or tyre fitter, or even just connect it to the air line at the petrol station?

Mike Hurley30/05/2021 09:51:15
325 forum posts
87 photos

Thanks SoD for the warnings, I shall definately take them on board and probably avoid this experiment all together!

Redsetter - had comsidered this but its about 70Kg and quite an arkward shape to heave around. It's currently on a simple stand so that I can work on it, one plan is to eventually make a HD wheeled truck so I can move it easily if I want to take it any Steam rallies etc ( if and when they happen again)

Anyway, thanks for the input both. Take care


Paul Lousick30/05/2021 11:00:06
2071 forum posts
725 photos

My traction engine has a 1.75" bore and a stroke of 4.25". I have an 8 cfm air compressor. When I pressurise the compressor reservoir and the boiler (45 litres) to 90 psi, the engine will run for about 30 seconds. The compressor cannot deliver the volume of air required to keep the engine running. (Air does not have the expansion rate of steam). A friend owns a full size, 4NHP Fowler traction engine and has moved it, although very slowly with a large diesel powered compressor that they use for running jack hammers.

(I'm not sure but I think that an engine with  2 cylinders are 3" bore and about 5" long with a sweep length of 4" giving a volume of 46.6 cu in in total  will not fit in the back of your car and run from the air line at the petrol station)


Edited By Paul Lousick on 30/05/2021 11:08:04

Paul Kemp30/05/2021 15:24:05
726 forum posts
27 photos

Most petrol station air lines these days seem to have the output of a fish tank pump and would struggle to inflate a balloon! Many are self contained electric units without seemingly any reservoir. All the places where I live the forecourt units take the whole 50p (5 minutes maybe?) to inflate a LR tyre from near flat to 35psi, that won't give you many slow revolutions!


Paul Lousick31/05/2021 00:30:32
2071 forum posts
725 photos

Paul K

Do you have to pay 50p to inflate your tyres in UK ? All of our service stations, here in Aust. are free. (and no comments please about being full of hot air)

Paul L.

Paul Kemp31/05/2021 01:40:40
726 forum posts
27 photos

Paul L

Depends where you go, most Tesco stations and a lot of the independents charge 20 - 50p for air. The local sainsbury is free but they are all these pedestal mounted self contained units with the compressor buried inside. Fine if you have a small car with low profile tyres but something with decent size wheels they struggle with! You don't know how well off you are! I spent a bit of time round Sydney and worked for a while at Fourjacks, Newcastle really enjoyed it (but not the flight home!). Have folks in Thiroul and my uncle used to be a manager at Port Kembla steel works so I know the area a little, not keen on your creepy crawlies though! Not sure about being full of hot air - most of your fellow countrymen I have worked with have been full of beer lol. Some the welders would put in a 12hr shift, spend 8 hrs in the pub and be back at 06.00 next morning ready for another day at the office! Great bunch to work with, how they kept up the pace I don't know!


Paul Lousick31/05/2021 05:45:36
2071 forum posts
725 photos

Hi Paul,

I know the Fourjacks company. But accusing my countrymen of being full of beer is a bit much !!!! Many of our customs and standard are taken from our British ancestors. I have also noticed that most of the Youtube videos of model and full size traction engines seem to start and end at a pub. (I had assumed to fill up with water and purchase a sandwhich). And that most of the police shows on British TV like Mid Summer Murders, Vera, Lewis, etc all end up at the pub. (not that I am saying there is anything wrong with that).

Cheers, Paul.

not done it yet31/05/2021 06:47:03
6880 forum posts
20 photos

Haha, Paul L.

One also needs to remember that a lot of those ancestors were not of a higher class/standard at the time of entry.🙂

I try to carry a portable tyre inflator when on a distant run. Has to be electric these days (weight and ease of use). Current one is what I call a ‘skeleton’ pump - completely open - as I haven’t printed a suitable container for it yet.

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