Author Topic: Pressure versus controlled flow on air powered engines  (Read 2345 times)

Offline Brian Rupnow

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Pressure versus controlled flow on air powered engines
« on: November 26, 2017, 02:33:13 PM »
Many home model machinists build steam engines and run them on compressed air. I run my engines thru a regulator to govern the speed of the engine. This works fine to slow the engine down or speed the engine up---but---At a very low pressure the engine is pretty wimpy and puts out very little power. I also have a number of flow controls available to me. I'm thinking of getting rid of the regulator and running  high pressure air to a flow control, then high pressure controlled flow air to the engine from the flow control. The fact that I am controlling the volume of air entering the engine but the air is still at high pressure should allow me to slow the engine down, but still maintain  a powerful torque capability at the engine. Comments please.---Brian

Offline Brian Rupnow

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Re: Pressure versus controlled flow on air powered engines
« Reply #1 on: November 26, 2017, 02:47:02 PM »
I have just answered my own question. If I take the regulator out of the system and supply high pressure air thru a flow control valve, the engine is much more powerful at low rpm.

Offline Stuart

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Re: Pressure versus controlled flow on air powered engines
« Reply #2 on: November 26, 2017, 03:21:34 PM »
Brian

For fun put a pressure gauge in the feed to the engine that yellow/green pipe so we have no misunderstanding also what is the HP air pressure

Results in psi will be OK not so good at K pascal,s


Stuart
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Offline Gas_mantle

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Re: Pressure versus controlled flow on air powered engines
« Reply #3 on: November 26, 2017, 03:45:14 PM »
Hi Brian,

When you made the engine I seem to remember reading that you didn't want to fit piston rings or gland packing. If that is the case I'd expect you to get a lot of improvement by fitting them.

Even a bit of graphite string wrapped round a piston groove will help  :)

Offline Jasonb

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Re: Pressure versus controlled flow on air powered engines
« Reply #4 on: November 26, 2017, 04:35:44 PM »
The 10 series never had piston rings, just a couple of oil grooves. If teh fit is right that is all they need.

I do similar Brian though still keep my regulator fair low, then just the flow valves to set individual speeds

<a href="http://www.youtube.com/watch?v=x4X3Mu2x5UA" target="_blank">http://www.youtube.com/watch?v=x4X3Mu2x5UA</a>

Offline Brian Rupnow

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Re: Pressure versus controlled flow on air powered engines
« Reply #5 on: November 26, 2017, 04:38:52 PM »
The air supply feeding that manifold of flow control valves is 60 psi.

Offline Jasonb

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Re: Pressure versus controlled flow on air powered engines
« Reply #6 on: November 26, 2017, 05:17:04 PM »
I think Stuart wanted to know the pressure on the other side of the Manifold

Offline Gas_mantle

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Re: Pressure versus controlled flow on air powered engines
« Reply #7 on: November 26, 2017, 05:35:18 PM »
The 10 series never had piston rings, just a couple of oil grooves.

Jason, that raises an interesting point - I don't have a set of Stuart drawings for the engine so wasn't aware they were made without piston rings. In my 'Ladybird' book of steam engines the drawings show the option of either rings or oil grooves.

I bought a Stuart 10h a while back, it looks to have been made by a skilled machinist and it does have a graphite ring - for it's size it's an incredibly powerful engine  :)

Although I do have Viton rings for my Muncaster engine if I'm not happy with the fit I'll use graphite string.  :)

Offline Stuart

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Re: Pressure versus controlled flow on air powered engines
« Reply #8 on: November 26, 2017, 05:52:48 PM »
Thanks Jason
I thought I had made it plain with the colour of the pipe maybe I should have said color  :stir:
My aim is for a accurate part with a good finish

Offline Brian Rupnow

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Re: Pressure versus controlled flow on air powered engines
« Reply #9 on: November 26, 2017, 07:11:00 PM »
Jason--Pressure is a constant. If it is 60 psi going into a flow control, it is 60 coming out the other side. All a flow control does is limit the volume of air going thru it, but has no effect on the pressure.

Offline dieselpilot

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Re: Pressure versus controlled flow on air powered engines
« Reply #10 on: November 26, 2017, 07:48:42 PM »
You can be certain pressure downstream of your needle valve is not 60PSI until volume is reduced to almost nothing.

Offline Stuart

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Re: Pressure versus controlled flow on air powered engines
« Reply #11 on: November 26, 2017, 07:57:18 PM »
Brian

But have you actually measured it ?

I used to work with pneumatic temp sensors for HVAC controls by Johnsonís controls

All it was was a 20 psi input to a tee with a pressure gauge in one leg calibrated to read temp instead of psi this fed a accurate orifice which was covered by a bimetallic strip

That why I asked about the pressure after your controlled valve , with the pipe stopped up yes you get line pressure but allow some leakage ( ie your engine ) then the pressure must drop as you now do not have the flow to maintain the line pressure after the control

With respect Stuart
My aim is for a accurate part with a good finish

Offline Jasonb

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Re: Pressure versus controlled flow on air powered engines
« Reply #12 on: November 26, 2017, 08:35:34 PM »
And the pressure in the yellow hose will go up as the engine is put under load to a point where working hard the valve chest will be at almost 60psi though a lot less when just freewheeling.

Offline Stuart

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Re: Pressure versus controlled flow on air powered engines
« Reply #13 on: November 26, 2017, 08:58:38 PM »
Yes Jason that will indeed happen ,for that was the principle I out lined bimetallic on the orifice full pressure away the pressure drops

This also is the case on 12 inch to the foot locoís etc. Hence the use of boiler pressure gauges and steam pressure gauges so the drive can assess the load because the regulator is in fact a flow control

My aim is for a accurate part with a good finish

Offline MJM460

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Re: Pressure versus controlled flow on air powered engines
« Reply #14 on: November 26, 2017, 10:54:53 PM »
Hi Brian,

The world turns and we are now waking Tuesday morning in this part of the world, well time for morning tea really.

While simpler schemes work, you have indeed uncovered the best way to control your engine on air, or even steam for that matter.  To understand why, it is necessary to understand how the engine, the flow control valve and the pressure controller work together.

The engine uses gas pressure on the piston face to produce torque, and if that torque is higher than the resistance to turning, the engine starts rotating and accelerates.  As it accelerates, the torque produced reduced due to pressure losses in the supply pipe and valve gear due to flow.  When the engine torque equals the resisting torque steady speed is reached.  Just that Newton's law about equal and opposite forces, but applied to a rotating system in this case.

A pressure control valve opens and closes in response to the pressure on the outlet side of the valve in a manner which maintains constant pressure at the valve outlet.  If the engine goes faster, it uses more air, and the pressure control valve opens to allow more air flow.

That flow control valve, like the locomotive regulator, is technically described as an adjustable flow orifice.  Unlike a fixed orifice, simply a round hole, the flow area is the annulus between the needle valve and its seat.  The flow through this annulus is determined by the pressure difference across it.  With 60 psig upstream, about 75 psi absolute, if the downstream pressure (at your engine) is less than about 40 psia or 25 psig, there will be sonic flow through the orifice and the flow will be proportional to the upstream pressure, regardless of how low the downstream pressure goes.  Open the needle valve a little, gives more flow, and the engine being a volumetric machine goes faster.  If the engine is using less air than this, the engine side pressure rises, the engine produces more torque and unless the load is changing, goes faster.  Similarly if you close the needle valve, a little, the area reduces, same sonic velocity in that annulus, so less flow.  The engine will use enough air to lower the pressure at its inlet, thus less torque, and the load makes the engine go slower.

If the engine is loaded to go even slower, constant flow through the needle valve, means the engine inlet pressure rises, to above that 25 psig, flow through the annulus is now dependent on upstream and downstream pressure, but still still controlled, just not exactly proportional to the annulus upstream pressure alone. 

The upstream pressure is dependent on your compressor and the connecting piping.  If you include a pressure controller, it controls the pressure at its outlet, but has no control over the pressure beyond that needle valve.  That is controlled purely by the balance between the needle valve flow and the engine consumption.

However, now you have constant pressure at the inlet of the needle valve the flow is directly proportional to that pressure and the open area of that annulus between the needle and its seat.

Of course simpler schemes work.  If you have only a pressure control, then, as you have found, speed is controlled only by engine load, and at low pressure on the piston, the torque is low.

If you have only the flow valve, the engine speed is determined by the needle valve opening and the upstream pressure.  If someone changes the upstream pressure by starting many other engines or using a cleaning nozzle, that will affect your engine speed.  More importantly if they increase the pressure for any number of valid reasons, your engine will go faster, possibly faster than you want.

I know, too much information.  The takeaway is that you have a come up with a solid control system for your engine, to overcome the observed problem.  However, the air pressure at the engine manifold is no longer controlled by your pressure controller, which only influences the pressure to the inlet side of your flow valve, but will rise and fall as Stuart says, depending on the engine load and speed and the flow valve opening.

MJM460

The more I learn, the more I find that I still have to learn!

Offline crueby

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Re: Pressure versus controlled flow on air powered engines
« Reply #15 on: November 26, 2017, 10:59:56 PM »
I've been using needle valves on my engines for a while, work quite well on the lower speed side of things, much better than cutting the pressure at the compressor. As MJM says (but much cut down), needle valve flow control good!

 :cheers:

Online Ian S C

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Re: Pressure versus controlled flow on air powered engines
« Reply #16 on: November 27, 2017, 01:41:28 AM »
I presume that the butterfly valve opperated by the govenor on thelve likes of a Stuart Turner S-9 has a similar effect to a needle valve, and this seemed to work ok, throttling the air supply while still maintaining adequate power on an inlet pressure of 30 to 60 psi.  The engine would opperate down to a pressure that went off the gauge, probably less that , as long as the volume of air was enough, put the engine on a smaller compessor with insufficient air volume, and no way would the engine run and keep running.
Ian S C