Author Topic: Exhaust Valve Timing  (Read 333 times)

Offline Captain Jerry

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Exhaust Valve Timing
« on: July 19, 2019, 01:37:34 PM »
I don't know if itis worth discussing but I have been wondering about the negative effect of steam pressure in the cylinder after completion of the power stroke. the normal method of opening the exhaust valve with an eccentric produces a slow release on a sinusoidal curve. I think I understand the function of a condenser to cool the exhaust steam and reduce back pressure.  Has there ever been a linkage or release mechanism that provides a quick opening of the exhaust valve?


Jerry
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Offline crueby

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Re: Exhaust Valve Timing
« Reply #1 on: July 19, 2019, 01:59:09 PM »
I think thats part of the benefit of the Corliss valve setup, a quick wide open/close on the valves. On compound engines, they often had a receiver space between the cylinders to collect the steam too. Also as you mentioned the condensor at the end to draw off the exhaust at low pressure.

Offline Zephyrin

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Re: Exhaust Valve Timing
« Reply #2 on: July 19, 2019, 03:29:06 PM »
several type of valve gear with poppets, Caprotti, Franklin, Chapelon, were used on rapid locos with fast opening of the steam ports, but it was at the end of the steam engine era...
the compression at the end of the return stroke is a part of the cycle and is not that detrimental to the function.

Offline ChuckKey

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Re: Exhaust Valve Timing
« Reply #3 on: July 19, 2019, 03:39:49 PM »
Poppet or drop valve gears also perform the same function, though all these gears are more concerned with sharp, early cut-off than with exhaust events.

With conventional harmonic (eccentric driven) valve gears and slide or piston valves, release occurs when the valve is at (or near) mid position, when it is moving at its fastest, so that release happens fairly smartly anyway.

Much the bigger problem with harmonic gears when set to produce early cut-off, is that you also get early compression (exhaust closing). In full size, compression is useful in decelerating the reciprocating masses, so reducing bearing loads. In full size or model, compression also raises the pressure and temperature in the cylinder, ideally close to inlet pressure, so that there is no sudden (and therefore wasteful) inrush of live steam when the valve opens.

The ideal compression point depends on the clearance (dead) volume of the cylinder. With slide and piston valves the clearance volume is relatively large and so relatively early compression is desirable. Corliss and poppet valves allow much smaller clearance volumes, so the compression point can be later, as there is less space to compress the remaining exhaust steam into. Less energy is put into compression, so it is more efficient.

Offline gbritnell

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Re: Exhaust Valve Timing
« Reply #4 on: July 19, 2019, 03:48:08 PM »
As Zephyrin stated, with any type of valved steam engine, D, piston, rotary, part of the function of the valve/timing is to close the valve prior to the piston reaching the end of it's stroke. This builds up pressure which has a twofold function, 1 to slow down the inertia of the piston rod assembly so there isn't as much stress while changing direction and 2. to increase the cylinder pressure so that the incoming steam charge more closely matches the cylinder pressure. A condenser on a conventional steam engine has mainly one function, to condense the steam back to water for economy. On a steam turbine the condenser does this plus creates a negative pressure to help scavenge the exhaust steam.
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Offline ChuckKey

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Re: Exhaust Valve Timing
« Reply #5 on: July 19, 2019, 04:05:01 PM »
A condenser on a conventional steam engine has mainly one function, to condense the steam back to water for economy.
The vacuum also improves efficiency by allowing considerably more energy to be extracted from the steam by the low pressure cylinder.

Offline crueby

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Re: Exhaust Valve Timing
« Reply #6 on: July 19, 2019, 04:40:41 PM »
A condenser on a conventional steam engine has mainly one function, to condense the steam back to water for economy.
The vacuum also improves efficiency by allowing considerably more energy to be extracted from the steam by the low pressure cylinder.
Agreed, on some of the very large pumping compound engines I have been researching, they only had a few psi on the third cylinder, which was 96 inch diameter, and they relied on the condenser to get the most work from it.

Offline Captain Jerry

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Re: Exhaust Valve Timing
« Reply #7 on: July 19, 2019, 10:29:27 PM »
Wow, that got a lot of good detailed answers. I asked the question in a very general way to encourage participation so now I would like to narrow it down some and take away some of the more esoteric considerations.  Lets consider only simple, non-compound double acting steam cylinders.  As I see it, early cutoff of steam is intended to get the maximum benefit of the expansion of high pressure steam, but at the very end of the power stroke, in the instant that the piston reverses travel, any remaining steam pressure on that side of the piston which is now an exhaust chamber, becomes a resistance to the cycle. Even if the exhaust valve begins to open at that very instant, it is a gradual event, still retaining some pressure which begins to bleed of, slowly but increasing as the valve opens more completely. Not only must it bleed of any residual steam pressure, it must bleed off the pressure created by the returning piston.  My intended question was 'why not bang the exhaust valve wide open at this point' using some kind of release mechanism.


This is just the exact opposite of Corliss, or Greene. or other quick cut-off valves which act to close the steam valve quickly.  It would of course require that the steam and exhaust valves be independently controlled and would still allow closing of the exhaust valve slightly before the end of the stroke to provide cushioning.


It would not be quite as complex as the steam cut-off valves, because it would not require governor control. Its timing could be fixed.  A quick opening release is not exactly the same as a quick closing release and has some special design challenges.


Jerry
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Offline ChuckKey

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Re: Exhaust Valve Timing
« Reply #8 on: July 19, 2019, 11:25:42 PM »
Exhaust valves are opened before dead centre to minimise back pressure. If you look at an indicator diagram, actual or notional, you will see that the area (= work) lost is not that much
and in practical terms may not justify extra complication. Poppet valve exhausts do open pretty sharply. This thing is noted for chucking the exhaust out in square blocks:
 

Offline Captain Jerry

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Re: Exhaust Valve Timing
« Reply #9 on: July 20, 2019, 03:32:55 AM »
Thanks for the nice video. Although I have never been involved with live steam model locos, I have a strong interest.  I am old enough to remember the late days of steam on the rails.  As a young boy (6-8) I lived in Ft Worth, two blocks from the Prairie and the Prairie and from there, the Prairie stretched out of sight. only broken by a railroad track headed northeast. I spent more than a few afternoons watching strings of 100 cattle cars, pulled by 4 steam engines (4-6-2 I believe) on a slight upgrade. Wonderful sound.  Shortly after that, we moved to Virginia where the most common load was steam coal, big engines and heavy loads made a wonderful sound.  I have heard big steam working hard.

My more recent interest has been large stationary mill engines but I have never had the opportunity to see or hear one actually working hard. Museum engines sound like a library with a big pendulum clock and someone pressing pants. Tick, tick, ssshh, tick, tick, ssshh.  But the engineering still fascinates me.

Capprotti valve gear and other poppet valve designs seem to have come along in the later years in the railroad engines but seem to have little if any impact on the industrial market.  Poppet valves are usually quick opening and quick closing valves driven by cams of various shapes and heights with much more control of the steam pressure curve than the common slip eccentric mechanisms on stationary engines. Cams can be designed to produce steep curves with flat peaks of varying duration.  Eccentrics are round and generally produce a smooth sinusoidal curve although the use of a 4-bar linkage can shape the curve to some degree.  Early cut-off on the steam inlet is used to increase efficiency and speed control but the exhaust valves on a Corliss engine are generally left to open and close in a smooth, regular motion.  As ChuckKey has said, Caprotti exhaust valves are opened sharply slightly before BDC to eliminate backpressure on the opposing power stroke.


It seems to me that a similar quick opening exhaust would be an advantage on a stationary engine.  I will think about it some more and then I will take a nap.


Jerry
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Offline MJM460

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Re: Exhaust Valve Timing
« Reply #10 on: July 20, 2019, 07:28:54 AM »
Hi Captain Jerry, overnight (for me) others have mentioned many of the alternatives to the eccentric, and you know about those anyway, so now my time zone gives me a chance to reply, I would like to go back to your original questions.

First, it is true that any elevation of the exhaust pressure reduces the differential pressure and hence the force on the piston so reduces the work done.  However, that is not the whole story.

The sinusoidal opening characteristic of the eccentric does mean that the exhaust valve initially opens slowly, so in reality, very little is lost if the valve is set to start opening a bit early.  Also, as the piston is moving slower in that last bit of the stroke, the power output is rapidly reducing to zero at the end of the stroke before the other side of the piston starts contributing.  The slight reduction in output is most likely outweighed by the simplicity of the eccentric motion, especially in small size engines.

In addition, while I am not totally sure about the cushioning theory, it is so widely mentioned that I would trust the observation of the engineers that a little cushioning, by reducing the load on the bearings, reduces the bearing wear and tear, and is worth the slightly lower output.  Throw in an extra shovel full of coal and save the bearings.  I think that is what I would do anyway.  As already mentioned, the cushioning effect by opening the inlet valve a bit early gives some compression which reduces the thermodynamic losses if inlet pressure steam is admitted to a cylinder at exhaust pressure as well as providing that cushioning.

You will notice my use of that well known technical term, a “bit”.  (Nothing to do with a byte).  It is where an experienced ear, finely in tune with what the engine is telling you, beats complex theory of valve setting every time for the final tuning.

The engineers who developed steam powered machines were an ingenious lot, and I suspect if there was a better way of operating the steam and exhaust valves, they would have tried it a long time ago.

On the condenser, if the condenser is open to atmosphere, it operates at atmospheric pressure, so the piston sees atmospheric pressure plus the contribution of the resistance of the valve ports and passages to the steam flow, the same as without a condenser.  So the condenser mainly conserves water, though it will be oily water, not good for immediate return to the boiler, but in a model, saves the oil slick on the water surface or bench top.

In the condenser, the steam condenses at the pressure determined by the cooling water temperature and heat transfer area, so a bit above the cooling water temperature.  This forms the partial pressure of water vapour in the condenser, and the partial pressure of air makes up the rest of atmospheric pressure.  So no advantage to engine power output.

If the condenser is maintained separate from the atmosphere, things get  more complicated.  The steam temperature in the condenser will mean a pressure significantly below atmospheric.  However, there is always some air, partly dissolved in the boiler feed water, and partly due to the inevitable inwards leakage through rod packings etc when the exhaust is under part vacuum.  So there is condensate and air at pressure lower than atmospheric, and accumulation of air and water soon mean no vacuum as the pressure builds until water and air can get out.  This is where air and condensate pumps come in.  The air pump and condensate pump, either separate devices or a combined unit, do the work necessary to move the air and condensate against the prevailing pressure difference.  In an industrial plant, steam jet ejectors remove the air in one or two stages, while a centrifugal pump removes the water for treatment before reuse.

A nice exercise to make a model with working air and condensate pumps, but I suspect in model sizes the friction losses in the pumps might exceed the additional engine output from the lower exhaust pressure.

I am sure that in my attempt at brevity, if that is the word, I have raised as many questions as I have attempted to answer, so don’t be shy to continue the conversation.

MJM460
The more I learn, the more I find that I still have to learn!