From: msb@sq.sq.com (Mark Brader)
Subject: Re: compound locos (was Re: High Speed Rail Query)
Organization: SoftQuad Inc., Toronto, Canada
Date: Fri, 6 Nov 92 06:48:45 GMT

Many different arrangements of compound steam locomotives were built.
From the books I've read, it appears that compounding was most popular
in various parts of continental Europe, though it was certainly used
in Britain and the US and elsewhere.  (Actually, I haven't read much
about steam locomotives "elsewhere", other than in Canada of course.)

I think that in continental Europe the most common arrangement was
3 or 4 cylinders, i.e. 1 or 2 high-pressure and 2 low-pressure.
Generally the cylinders would all be out of phase with each other.
These would just be described as a 3- or 4-cylinder compound without
any special descriptive term.

(In Britain there were also quite a number of 3- or 4-cylinder *simple*
locomotives; they started more smoothly than 2-cylinder ones, and if
only 2 cylinders were outside, the overall width was reduced as compared
to an otherwise equivalent 2-cylinder engine, which was important due
to the narrow loading gauge there.  The world speed record holder for
steam locomotives, Mallard, is a 3-cylinder simple.)

As has been noted, another arrangement was a 2-cylinder compound, with
the high- and low-pressure cylinders on opposite sides.  The correct name
for these is cross compounds.  They were the simplest of all types of
compound, but probably had the least smooth action since steam was
only drawn from the boiler twice per revolution of the wheels.

There were two types of compounds where the high- and low-pressure
cylinders acted synchronously so that some parts could be shared
between them; again, smoothness was traded for reduced complexity.
One kind, already mentioned, had the two cylinders' pistons on a
common piston rod, so the cylinders were one behind the other;
this was called a tandem compound.  The other kind had the cylinders
one above the other, with two piston rods driving onto a common
crosshead.  *This* was the Vauclain compound, named for Sam Vauclain.
In the Vauclain compound, and I expect the tandem also, a single
casting would used for the pair of cylinders.

The other most common type, again already described, was the Mallet
articulated compound, with the low-pressure cylinders driving a
separate set of wheels on a pivoted truck or bogie.  In later US
usage, of course, "Mallet" was commonly used for all engines with this
type of articulation whether compound or simple.  "Mallet" is pronounced
Mallay in French, and something like that in English.

Once there was also a "Mallet" with a different sort of compounding.
A railway in New Zealand was rebuilding a simple Mallet, if I recall
correctly, and wanted to put new cylinders on it.  However, it happened
that while no suitable cylinders were at hand, a couple of pairs of
Vauclain compound cylinders were.  So the engine emerged as a *Vauclain
compound Mallet articulated*, with 8 cylinders, 4 high- and 4 low-pressure!
I believe it ran in this form for a number of years.

The other common type of articulated engine was the Garratt, where the
boiler rides mostly over empty space in between two bogies each with
a set of driving wheels and cylinders.  Garratts were mostly simple,
but I believe a compound version was tried as well, with low-pressure
cylinders on one bogie.  This would involve *three* traversals of flexible
steam joints between the boiler and the low-pressure cylinders -- if the
idea was going to be tried, one might think that making each bogie a
cross compound might be better.

In Britain around the turn of the century, the London and North Western
Railway built a number of unusual compounds.  Some of them had 2 high-
pressure cylinders and only 1 low-pressure, which therefore had to be very
large.  Some of them had the high-pressure and low-pressure cylinders
driving onto separate wheels, which (to reduce friction) were not coupled
-- unfortunately the reversing mechanism chosen was one that would be
reliable only if the wheels *were* coupled, because it relied on the
back pressure of the wheels on the cylinders.  The result was that, if
the rails were slippery, when the train attempted to start forward after
reversing into a terminal station, it would sometimes sit there with
one pair of wheels turning in each direction!  This went on for *years*
until the autocratic chief engineer of the railway finally retired.

Finally, there were some experiments with triple-expansion compounding,
i.e., with low-, medium-, and high-pressure cylinders.  This was a very
common practice on marine steam engines; some of them even had quadruple
expansion.  On locomotives, though, triple expansion was never economical.

Everything in this article is from memory, and therefore there may be
some errors.  I have tried to indicate where I feel not quite certain.
Most of it may be taken as accurate.
-- 
Mark Brader	"What can be more palpably absurd than the prospect held out
utzoo!sq!msb	 of locomotives travelling twice as fast as stagecoaches?"
msb@sq.com		 -- The Quarterly Review (England), March 1825

This article is in the public domain.