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Steam & Excursion > “Compounding “ Comments

Date: 01/09/22 10:12
“Compounding “ Comments
Author: wcamp1472

A reader recently sent me a PM inquiring about articulated compounds.
I thought others might wonder in a similar fashion.
So, I've posted my ideas below......

"That's a wonderful question, with a multifaceted answer. 
The lesson is that you get nothing for free, there are always 
costs to trying to use steam pressure twice... 

Let's start at the beginning.   
'Compounding' with reciprocating engines, simply means that  
the steam used in the larger, low pressure pistons comes from  
the steam released from the first use in the high pressure cylinders. 

Any PRESSURE that is in the supply pipe to the Low Pressure pistons, 
comes from crowded ( used) steam being compressed against the L.P. pistons. 

Thus, steam pressure 'builds up' , down stream from the HP exhaust ( equally),
and acts AGAINST the High Pressure 
pistons ---- in order to drive the LP engine.

The boiler may supply steam at ( hypothetical example) 200 psi to the H.P. pistons,
but the lower pressure in  
the receiver pipe to the L.P. pistons also acts AGAINST the pistons
in the H.P. cylinders, as well as powering the LP pistons.
( Archemedes' principle that pressure acts equally, in all directions...).

If the receiver pressure ( 'used' steam)  is 100 psi, the H.P. pistons, are  
moved by the differential pressures acting on the piston' s 2 faces.... so that 
means that the effective, useful application for the new people are rrrpressure on the H.P. pistons
is only 100 psi 
delivered to the rear engine... that "differential pressure" is what drives the rear axles.*

The expanded steam from the HP pistons, is pressurized in the receiver pipe by the
steam pumped out of the 
H.P. pistons.  So, the Low Pressure pistons are now supplied
with steam pumped, forward, from 
the HP pistons....at a delivery pressure in the
receiver pipe of, typically, 100 psi. 

The L.P pistons exhaust their steam directly up the stack, at a 'back pressure' of  
10 to 25 psi.  That ordinary back pressure subtracts power from the 100 psi in the
pipe and acting on the front pistons.

So, 'compounding ' in actuality, has the HP pistons also shoving against the 
L.P pistons, while trying to pull a train. 

Next.... At starting in 'compound', the LP pistons receive no pressure ---- they're waiting
for the steam 
pressure in the receiver pipe to build-up, before they get any pressure to work with.  
So, if 
starting in compound, the rear engine does all the 'work'.  

if starting in compound, a typical 2-6-6-2 effectively starts as an 0-0-6-0. 

There is no steam supplied to the LP pistons until several hundred feet
of distance 
moved in order to build up sufficient pressure in the receiver pipe
to the front 'engine'. 

Thus, from the earliest days of compounding, the 1880s, designers have used 
'pressure-limiting'  valves to admit live steam from the boiler to supply the receiver 
pipe feeding the LP pistons.   The pressure-limiting valves allow lower, limited pressure
to run the LP pistons.   The greater diameter of the LP pistons means that,
if at full 
BP, the LP cylinders and wheels would spin wildly, out of control,  
on compound articulateds. 

So, the 'starting' valves used on the articulated compounds, re-direct the live steam 
in two different paths. 

One 're-direction' allows the steam exhausted from the HP pistons to be fed directly 
to the base of the smokestack.   At the exhsust standpipe in the smoke box, where  
the steam exhausted from the LP pistons flows up the stack, the exhausted steam  
from the HP pistons is fed to a circular ring at the top of the standpipe.  Thus, there 
are two, concentric, streams of exhaust steam up the stack. 

The diverted "live-steam" path admits boiler pressure into the steam ( receiver) pipe
steam to the larger diameter LP pistons; but, the pressure in that delivery pipe 
is limited ---- in order to prevent the front engine from wildly spinning.. 

Using steam expansively was first improved by British inventor Parsons, 
who invented the steam turbine ---- about 1880.

Parsons used a shaft, with several stacked discs equipped with stubby turbine blades. 
and between each disc were fixed blades redirecting the steam flow to the next stage 
of expansion. 

Each stage of expansion disc was increased in diameter, to keep the leverage,
& rotative force applied 
to the shaft approximately the same as the previous
stage using the higher pressure.  Each successive stage uses larger diameter discs &
works with lower pressure and, by definition, lower temperature steam

Thus, the turbine discs get larger further down the shaft.....which is  'compounded'
use of the steam. 

In the 1930s, the Germans perfected the use of fuel oil burned directly, and flames
acting on the compound-arranged 
turbine blades--- the invention of today's jet engine'!

In todays world, jet engines now power 'propellers' at the input of the engines. 
Actually, the earlier variants using propellers, were called "turbo-props"....
variable pitch propellers driven by the jet engine's central shaft. 

Later, turbo-props were replaced by multiple, variable-pitch fan blades. 
Now called TurboFan engines.

Now, when I stare, transfixed, at the con-trails forming in the sky, I admire
Parsons for 
his imagination and work that went into his understanding about how 
all heat engines actually work.   He had wondered exactly the SAME questions that you 
have been pondering ...  Congratulations ".

Thank you,

Wes Camp

When starting in 'simple', the HP cylinders get the benefit  of direct pressure
    from the boiler... [ less any back pressure from the exhaust ring].  So that when starting
   in 'simple', not only do the front pistons get direct pressure, but the rear pistons get the
   benefit of nearly full-boiler pressure, per square inch. ... as opposed to 'compounding'
   with the pressure building in the receiver pipe ---- that receiver pressure acts against
    the HP pistons, as they stuff the "once-used steam" into the LP pistons..)


   Compound operation delivers the sum of acting piston pressures & reduced HP piston thrusts to the
   rear driving axles.  Another factor in calculating "Sarting Tractive Effort",  for articulated compounds...).

Edited 13 time(s). Last edit at 01/10/22 03:24 by wcamp1472.

Date: 01/10/22 06:07
Re: “Compounding “ Comments
Author: PlyWoody

You saw the rear drivers spinning.  It is not a slippery engine.  They learned from their mistake of using cut-off as that deprived the front cylinder of enoght steam pressure to make the lead driver give some pull.  Lesson learned, no cut-off, and the front engine help pull so the rear driver don't spin.  All working fine and just needs to get out and run onto the TT and do its thing.

Date: 01/10/22 07:38
Re: “Compounding “ Comments
Author: wcamp1472

What you say makes sense, I hadn't expected that effect.
The LP pistons need effective steam pressure to contribute
beneficial traction.

I wonder if staying in 'simple' until higher speeds are reached
would work better?

Its also the result of NO substantial trailing weight behind the tender..
( see also 4014 experience).

The engines are designed, from experience & the drawing board, 
to perform at the maximum capacity, pulling the designed train-weight....
like 50 or more loaded coal cars.
They' re not built built to be entertaining, "circus performers"...

Pulling a few coaches takes different, less 'efficient', operating 
conditions.....like longer cutoffs, much lighter throttle, etc...

Hooking-up implies full-thrittle openings...as in fighting a heavy train
Don't try 'hooking-up', unless you've got a good heavy train...and ability
to have an open throttle .... or, you could drop the boiler pressure...to
low enough to keep good "Main Resevoir" air pressure...


Edited 2 time(s). Last edit at 01/10/22 07:48 by wcamp1472.

Date: 01/10/22 09:27
Re: “Compounding “ Comments
Author: holiwood

Thanks for the great explanation, a few more questions 
So, in compound operation the High Pressure cylinders do work to turn rear drivers and provide push to the stream going to the Low Pressure cyliders, is that righjt?
Does the pressure in the pipe between HP and LP cylinders go up and down a lot?
Does the push provided by the HP cylinders raise the pressure in the pipe to the LP cylinders much? 

Date: 01/10/22 09:41
Re: “Compounding “ Comments
Author: wcamp1472

Q #1. = YES
Q#2.  = If starting in 'compound', you get a bump in pressure,
             in compound operation, pulling a load, pressure remains 
             steady, at whatever the design ratio of the volumes between 
              two sets of cylinders.
Q#3.   It depends,..


I call the pipe supplying steam to the LP cylinders: the receiver...
The pressure in receiver pipe can come from one of two sources: live steam from the boiler,
at a limited upper pressure, or 'used' steam pumped from the HP cylinders into the 
receiver pipe.  The engineer has an air-pressure operated mechanism that directs the
steam paths according to the position of the 'starting' control valve selected by the engineer 

If in compound operation, the steam pressure in the receiver pipe fluctuates up untill
the impulses and the expanding ( exhausted) HP steam fills the receiver.  
once filled,it then "evens-out".

The volume in the receiver is large enough that the individual impulses from the steam sent
from the HP cylinders is too small to affect the total expanded steam that has filled the
receiver volume.

If you had a gauge on the receiver, AND a heavy train, you would see a steady pressure,
'averaged-out' to approximate the pressure exhausted by the HP cylinders...
Again, a massive train-weight affects the performance.  You're operating an engine
designed for steam production commensurate for a heavy, steady demand.

Trying to design such a Loco to be efficient at pulling around 'a caboose',
is quite a challenge when you're powering 12 drivers and full boiler pressure.

Remember that the pressure in the receiver is determined by both the exhausted steam
volume of the HP cylinders, and the track speed.  If you had a gauge on the receiver,
it would show, if under loaded operation, a steady pressure to the LP cylinders of a pressure
range between 85 psi and 135psi, approximately....assuming a boiler pressure of 200 psi..
"Your mileage may vary".

As, we've noted, the operators are discovering the realities of operating 1309.
1309 is the teacher, the rest of us are her students --- she tells us when we're doing
it right, and if we're doing it wrong!

Who knows for sure, what the compounding pressures will be with only the light excursions
in 1309's future.  One excursion operator in the Dakotas, operates their compound articulateds
in compound service exclusively.


Edited 4 time(s). Last edit at 01/10/22 15:34 by wcamp1472.

Date: 01/11/22 10:13
Re: “Compounding “ Comments
Author: Earlk

The logging malleys the Black Hill Central run don't have all the extra simpling features.  The Simpling Valve is simply a valve which routes steam at boiler pressure to the low pressure cylinders with no regulation.  It seems the piping is rather small in diameter so it doesn't have much volume, so that if the front engine slips, it quickly runs out of steam feeding the cylinders, or the pressure drops to the point where traction is recovered.

There is a serious hill on the BHC which puts the malleys on their knees, and I have heard/seen video where the engine bogs down, then the exhaust gets louder and sharper briefly when the engineer opens the simpling valve, then shuts it off once a bit of speed is restored.  It happens a couple of times in the runby. I imagine one has to open and close the valve to keep from slipping the front engine.
I think Mary McP posted the video a couple of months back. Maybe she can repost it for us.

I did a search, here is the video.....


Edited 2 time(s). Last edit at 01/11/22 10:21 by Earlk.

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