Are steam engines pressure regulated?

[Brian Rupnow]

That is to say, is the steam pressure regulated, or is the speed simply regulated by controlling the volume of flow admitted to the cylinders, while the pressure remains constant? Something Bogstandard said the other day about self starting engines being able to self start without revving up got me wondering about this. On an air driven engine where the flow is more or less constant, we regulate the speed by turning the air pressure up or down. On a self starting engine, when we turn the air pressure low enough that the engine stops, then it won't self start untill we turn up the air pressure, meaning that as soon as the engine starts up, we have to turn down the air pressure to slow it down. On a steam engine, if the pressure is a constant, then yes, they probably should start at a low speed and maintain that slow speed without having to have this "start, rev up, then slow down to operating speed" cycle. I'm curious.---Brian

 

[Jasonb]

Controlled by the volume of air/steam. If I run my traction engines on air I can just set the compressor regulator and then vary the engine speed by using the regulator on the actual engine. Same with the stationary engines, most have governors which control the speed by a valve acting on teh air supply so no change in pressure.

Try putting a kink in your air hose and watch the engine slow down.

Following on from what Bogs was saying about not needing a large amount of air to start. Traction engines in the UK do not have a clutch unlike most US designs so you engage your chosen gear and then open the regulator on the engine. If you had to give it a lot of welly to start you would be jumping all over the place and poping wheelies, so you just ease the regulator open and the engine picks up slowly - no change in pressure just volume getting to teh piston.

J

 

[Brian Rupnow]

Jason---You contradict yourself. Don't you mean "Easing the flow control valve open." In my terminology, a "regulator" regulates pressure. A flow control valve regulates the volume of flow.

 

[Blogwitch]

Brian,

In a properly timed double acting twin cylinder engine, there should be at all times, on start up, at least one piston face on a power stroke. That is why they are self starting.

That is to say, if air, or steam, is fed to the engine, that piston face should start to push and so turn the engine, if it can overcome internal and external frictions, will start to run smoothly.

At low pressures, what you are seeing is friction stopping that single cylinder working, so a little bit more pressure must be admitted, just to overcome those friction forces. As soon it starts to move a little, another piston face will start to be acted upon, so giving you double the area being acted upon (in fact, a little less because of the cross section of the piston rod has to be subtracted) and so the engine (with static friction now no longer there) then starts to run up to speed, and you have to throttle back to maintain a very slow speed. At any one time, there are usually no more than two piston areas being acted upon by pressure, the others being on an exhaust stroke.

The smaller the value you can get that friction, the easier the engine will start from a stopped position.

Friction in a new engine usually comes from piston rings, gland packings and metal to metal contact.
That is why I stated that the more you can run it in under pressure, the better the engine will become at self starting.
One thing that must not be done is actually run the engine by turning the output shaft, say with a drill or something similar. The reason being, you will be wearing parts in an engine that don't normally wear, purely because the parts are being driven from a completely different direction than when running under it's own steam.

The larger the engine, the slower will be it's start up speeds, until you get to large full sized engines that only turn over at ten or twenty RPM max.
In model form, they just don't behave in the same way, purely because of the friction forces acting within an engine. Ratios of pressure and friction in smaller sizes are much more difficult to control, so that is also why I mentioned about the much longer tapers required in their regulation, they give you much better control at these tiny sizes, whereas in larger sizes, they are not absolutely necessary.

John

 

[Jasonb]

Jason---You contradict yourself. Don't you mean "Easing the flow control valve open." In my terminology, a "regulator" regulates pressure. A flow control valve regulates the volume of flow.

The usual term for a "flow control valve" on a steam engine is "the regulator" it regulates volume not pressure. As you can't instantly adjust the pressure of the boiler the pressure will remain the same so its the volume of steam getting to the engine that affects speed and the same controls are used whan running on air.

J

 

[GWRdriver]

Jason---You contradict yourself. . . . In my terminology, . . .

Yes in YOUR terminology but since Jason is British he will naturally tendency will be to use HIS terminology, which is just as valid as anyone's. If you understand that regulator = throttle the misunderstanding should be cleared up.

 

[Brian Rupnow]

Ah yes---The old "cross the pond" language thing. Bogs is right. Jason is right. I believe my question is answered. I will make a test with my donkey engine. I know that at low pressure it will run at a very low speed, but not self start. I also know that I can set my pressure regulator for a much higher pressure and control the speed of my engine by controlling the volume of flow. I suspect that if my engine is running very slowly at a high pressure but very little throttle, that it should then self start if the air supply is totally shut off at a seperate valve, then turned back on. Its the air pressure that supplies the motive force to overcome friction in the system, not the volume being admitted to the cylinder.----I think!!!

 

[Entropy455]

Pressure is only one aspect of steam. The temperature the steam is a significant variable also.

For example: a flow rate of one pound per minute of saturated steam at 100 psig will do significantly less work than one pound per minute of superheated steam at 100 psig. A steam engine is a heat engine, and the hotter the hot, the more efficient the engine will be.

Generally, the work you get out of a steam engine is equal to the difference in the steam’s Enthalpy between the inlet and outlet states. There will be irreversibility in the cycle, in which case engineers introduce Entropy - to determine the actual quality of power extraction from a given Enthalpy input.

 

[Don1966]

Can I get my two cents in. Entropy455 is correct. The definition for volume is a mass or quantity or amount or fullness omount of space measured in cubic units. Pressure is force per unit of area. Pressure is the work force. Pressure is controlled where as volume occupies a space.

Regards Don

 

[Brian Rupnow]

My test was inconclusive. I never did get the engine to self start at higher pressure, lower volume. I increased the air pressure to 50 psi, at which pressure it blew all my plastic air lines off there barbs.

 

[gbritnell]

There's a bunch of yeses and no's in what has been said. Yes the speed of a steam engine is controlled by volume as a governor or control valve will limit the volume over a given time period. The pressure from the source is constant, without a pressure regulator, so if the cylinder is given enough time to fill then at a certain point the pressure will be the same as the source.
Brian, you said that you increased your pressure to 50lbs. and the engine never did start then I suspect that your valve timing is off or you have a tremendous amount of friction in your components. If the engine turns over somewhat freely by hand then I would suspect the former. With quartered cranks there will always be pressure on one of the pistons and with a bore of .75 and a pressure of 25 lbs. then the pressure on a piston will be 11 lbs. I would think that if you put a 10 lb. bag of potatoes onto one of your pistons it would surely push the connecting rod down.
You also state that once running it will run slowly at a much lower pressure. That is because once started the inertia of the parts is enough to rotate the engine until the valve opens and puts pressure on the piston. I would check to see that your valve timing is correct and equal at both ends of the stroke.
gbritnell

 

[Brian Rupnow]

The only timing I have ever done on either engine/cylinder is per Elmer Verbourg.--Set piston at top or bottom dead center and set valve at mid travel. That works so well that I have never tried to adjust it "after the fact".

 

[tel]

With the piston on TDC and just starting to come down the inlet port should just be showing a hairline gap.

 

[gbritnell]

Ok fine, but that also depends on several things, the lap of the valve and the travel of the valve are just a couple. It your piston is at TDC and the valve is in mid travel that doesn't mean that it's starting to uncover the ports. If the lap of the valve over the ports is large than the piston will travel quite a distance before the port starts to open. Likewise if the port is closer to the center of the cylinder then the valve has to travel farther to start the opening. The proper way to set the valve is first make sure it is traveling the same distance at both ends of it's travel by adjusting the valve rod link. Once done put the piston at TDC and turn the eccentric until the valve (outside opening D valve) has just started to clear the port (faint gap). This will give you close to the proper valve timing for your engine.
gbritnell

 

[Blogwitch]

Brian,

The only timing I have ever done on either engine/cylinder is per Elmer Verbourg.--Set piston at top or bottom dead center and set valve at mid travel. That works so well that I have never tried to adjust it "after the fact".

We all have to start learning about things somewhere, this is why we are trying to explain the necessities of getting things correct. By the time this lot has sunk in, you will start to realise that just by following a few basic rules with steam (or air) engines, you can get your little motors running a lot better.

George and Tel have told you how the cylinder timing should be, but then you have to get the two cylinders into the correct timing relationship to each other.

This is where the 90 degrees between each cylinder comes into it's own.

It doesn't matter which direction you do it, one cylinder leading or trailing the other, you just need to swap over to the opposite way to give you the correct direction of rotation. But one thing that MUST be done is to have that lead or trail exactly 90 degrees. It is that which gives you the correct self starting position for a double acting twin cylinder engine.

See C-o-C at the bottom, and the better you can set this angle, the smoother and better the engine will run.The sleeve method is the easiest way to allow adjustment of the angle when the cylinders are a fair distance apart, as in your engine, or actually cut two flats 90 degs apart under your crank disk screws (if they are both drilled in exactly the same position), but do make sure you have the engine running in the correct direction first.


John

 

[Jasonb]

Yes as the others have said your eccentric needs to lead the crank by 90 degrees plus another 8-10 degrees. You will get a better and smoother running engine due to steam/air being admitted before the end of the stroke which has a cusioning effect on the piston.

This will mean that your engine will only run in one direction so if you want to reverse it you will need a way of reversing the timing be that a simple slip eccentric or one of the many types of valve gear like Stephensons, firth radial, etc

 

[picclock]

First of all, I don't know a lot about engines, however I do know a bit about gas flow. The trick to getting your engine to start and run at very low revs may be to use a pressure regulator, rather than a flow restriction valve. An adjustable pressure regulator will supply air at a constant pressure regardless of the amount of air consumed by the engine, whereas a flow restrictor will supply air at differing pressures depending on the amount of air consumed.

In the UK low cost pressure regulators for propane use (0-3bar) are readily available and may prove an attractive alternate solution, provided the maximum flow rate of the regulator is not exceeded.

I think that for tricky to control engines this solution would give stable and precise control, but its just an idea from reading this thread. As I said, I don't know a lot about engines .. .

Best Regards

picclock

 

[Jasonb]

Brian IS using a pressure regulator at the moment, its the one on his compressor which works just like a screw down gas bottle regulator.

The problem is he needs to use a high pressure to get the engine going and then because it runs too fast has to turn the pressure reg down.

As he has no volume control the engine starts and then there is nothing to stop a large amount of air being admitted so it races away. Better to stay with a higher pressure set on the compressor regulator and vary the volume of air/steam with a flow valve.

J

 

[tel]

With twin engines I always time them as I would two singles - that way being a couple of degrees off the 90° difference won't matter so much. No more than a couple of degrees tho'.

 

[chuck foster]

brian i have nothing useful to add to this thread other than to say thanks for asking these questions and thanks guys for the answers.
i have never timed a valve and i don't for see doing one, but this is all very interesting nun the less.

this is one of the many reasons i like this forum. ask a question and receive many many answers and various ways to reach you goal.

chuck watching with great interest