Subject:
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Re: Engine Speed Regulation
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Newsgroups:
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lugnet.off-topic.geek, lugnet.trains
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Date:
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Fri, 29 Oct 1999 12:54:17 GMT
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Reply-To:
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lpieniazek@novera.com=nospam=
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Viewed:
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90 times
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Hao-yang Wang wrote:
> And a related question: How do you control the speed of a steam locomotive?
Working from memory and I could be wrong. But I doubt it.
Review. Reciprocating steam engines function by admitting pressurised
steam into a chamber (the cylinder) where it expands, doing work against
the walls. One of the walls (the piston face) can move, and the volume
of the chamber expands. The piston transmits linear motion which is
converted to circular motion(in rod engines) via the piston rod. (In
geared engines it typically is via a crankshaft). After the steam
expands it is exhausted and after the piston travels back the cycle
repeats.
Now, for efficiency most railway reciprocating engines have a cylinder
with the piston in the middle. Steam can be admitted on either side of
the piston so work can be done against it in both directions. This
requires that the piston rod exit the cylinder in a pressure tight way.
The steam is regulated via a valve. This valve is actually a block of
metal with passages in it that slides linearly inside the "steam chest"
which is on adjacent to the cylinder. The steam chest is supplied with
high pressure steam from the boiler. The valve covers and uncovers
various ports to admit steam through them and into one or the other side
of the cylinder.
In a two cylinder engine, the pistons will be connected in such a way
that they are a quarter turn of the drivers off. This ensures that no
matter how the engine comes to rest there will always be at least one
piston face that is in an appropriate position for steam to be admitted
and work to be done. (1)
Now, how to regulate speed? You use a thing called the throttle. This is
essentially a bar that, via linkages that adjust the valve gear(2),
regulates the proportion of the total stroke cycle that steam is
admitted to the cylinder. The longer, proportionally, that the steam is
admitted, the more expansion you get and the more work done against the
piston face. As work increases beyond that required to maintain steady
state you get acceleration.
Now, actually it's not THAT simple. You have to take timing into
account. There is another bar called the "reverse gear(1)" or "Johnson
bar" that regulates WHEN in the cycle the steam is admitted. This is
sometimes called the "cutoff". As the cycle speed increases you need to
let the steam in earlier and earlier or it doesn't have time to expand.
(this is analogous to advancing/retarding ignition timing in an otto or
wankel cycle internal combustion engine).
It's called the reverse gear because if you adjust it far enough you are
admitting steam at the "wrong" point for forward travel which eventually
slows the engine down, and if you leave it there, starts working the
engine in reverse. Unlike an otto or wankel, a railway reciprocating
steam engine is just as happy running in reverse as forward. It HAS to
be, there is no transmission per se, the piston rod connects to the
connecting rod at the main driver and there is no gearing to change
direction of travel.
1 - this is true even if you've "thrown a rod" unless you are very very
unlucky. (Throwing a rod means that you've lost a rod bearing and either
the piston rod or the connecting rod had to be removed on that side, so
you're only running on one cylinder).
2 - gear in the sense of apparatus, it's actually an assembly of levers
and pivots... the throttle and reverse gears are levers that transmit
linear adjustment motion to the valve gear, mounted on or near the
cylinder/valve assembly.
--
Larry Pieniazek larryp@novera.com http://my.voyager.net/lar
- - - Web Application Integration! http://www.novera.com
fund Lugnet(tm): http://www.ebates.com/ ref: lar, 1/2 $$ to lugnet.
NOTE: Soon to be lpieniazek@tsisoft.com :-)
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Message has 2 Replies:
 | | Re: Engine Speed Regulation
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| (...) Sorry Larry, not true. The throttle controls the pressure in the steam chest (Ideally), which controls the speed. (in fact, it does regulate pressure, but also regulates volume) What you are suggesting is done on some engines (corliss mill (...) (25 years ago, 29-Oct-99, to lugnet.off-topic.geek, lugnet.trains)
| | | Re: Engine Speed Regulation
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| Thanks for the explanation! (Wow, so it is really possible to explain how the steam engine works without using a single picture!) (...) This looks like the PWM (pulse width modulation) method RCX uses to control its motor powers. Why the steam (...) (25 years ago, 2-Nov-99, to lugnet.off-topic.geek, lugnet.trains)
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Message is in Reply To:
| | Engine Speed Regulation
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| Here are some of the questions that have been nagging me since I was a kid. How does the press of the throttle affect the RPM and the torque of the engine? By looking at the R/C model engines, it seems to me that the throttle controls the amount of (...) (25 years ago, 28-Oct-99, to lugnet.off-topic.geek)
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