The Ravel Rotary–A Free-Piston Engine from Way Back!

By Staff
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115 S. Spring Valley, Road, Wilmington, Delaware 19807. e-mail:

In 1878 a certain Monsieur Ravel designed an engine which for
its combination of curious and interesting features, would be hard
to beat. For a start, it was a rotary motor: rotary, that is, in
the sense of the WWI Gnome aircraft engines, where the whole device
went round and round, not the modern Wankel where the insides go
round and round. Next, it was a free-piston engine, as was the Otto
& Langen of 1867, but with a few major differences, as you will
see. More like the free-piston engines which were the up and coming
new power plants of the 1950s. Finally it was a half-stroke like
the Lenoir, but with high expansion like the Otto, and flame

Let’s see what it was like, how it was supposed to work and
what the inventor claimed for it. Look at the first diagram. There
are two ‘motor cylinders’ E, and two ‘impulse
cylinders’ A, although the piston C is sitting in the lower
impulsion cylinder in the drawing, and so it is not labeled. The
piston F moves in the cylinder E as the engine rotates, operated by
the cam H, and the lower one has, at this point, just pulled an
air/gas mixture in from somewhere. It’s hard to see exactly
where, but in the second drawing are two pipes, i and k, which
supply gas. The second one is the fuel line, and we’ll come
back to the first one. From there somehow the gas finds it way to
the slide valve G, first through a double hollow shaft, the center
of which handles the exhaust. The internal passages that take it
from there are no clearer than the slide valves, but you get the
idea. It looks as if the air comes in directly at the slide valve.
M. Ravel says that the proportions of gas and air were regulated
‘by a special disposition of the slide valves, and the
detonating mixture is perfectly homogeneous.’ Now that we have
this excellent flammable mixture in the lower cylinder E, ignition
is called for. The gas line is the feed for the gas flames, but
maybe M. Ravel wanted to keep to himself the secret of how the
flames were kept alight while the cylinders whizzed through the
air. The jets appear to have been located, as many such igniters
were1, in a cavity of the slide valves G, but no detail
is to be seen. On the other hand, he was keen to point out that
unlike M. Hugon’s engine of 1860, this one did not have that
objectionable electric ignition with all those difficult batteries
and coils and stuff.

Now, ignited, the contents of E explode and force their way into
the lower cylinder A, where the throw the heavy piston C to the
other end of the cylinder. The item at D is a rubber bumper to
prevent the piston too strongly massaging the opposite cylinder
head. It does sound as if M. Ravel may have done some
experimentation here, as this was one of the areas that, as I
remember, gave a lot of trouble in the 1950 period, when quite
complex gas cushion systems were used. The exhaust gases then
discharge, maybe through ports which I can’t find either, to
the central exhaust pipe h, eventually exiting from the left hand
end of the machine. At the same time, the whole engine is rotating
on its bearings B, and the other piston and cylinder E F are
preparing to loft the heavy piston C back to its initial spot in
the cylinder. The business of throwing a heavy piston, which is not
physically connected to anything at all, to and fro inside a
rotating cylinder, is not one that I can easily relate to, but I am
told that moving the center of mass in this manner could indeed
drive such an engine. You can get some sort of a feel for what is
going on by remembering that you could as a kid certainly swing
your swing higher and higher and higher by appropriate body
English, without ever touching the ground.

There were, it was said, many advantages to this design. It was
‘the silent machine par excellence’ and did not make all
the dreadful noise that the Otto & Langen engines made. (And
they were noisy.) Its dimensions were small, and so installation
space was no problem. (The Otto machines were very tall indeed.) As
we have seen, it used gas jet ignition and avoided the problems of
electricity. (Lenoir did indeed have many, many problems with his
electric ignition.) It was said to be a most efficient user of the
heat developed by its internal combustion, and so there was no need
for cooling water. (Like the Rhone, it could also be claimed to
have useful cooling from the air passing the moving cylinders.) And
finally was claimed its low gas consumption, 600 liters of gas per
horsepower-hour. Since Hugon was using 2,500 to 3,000 liters, and
the then remarkably efficient Otto & Langen under test at the
1867 Paris Exposition used some 1,200 liters, these were strong
claims indeed.

But it is just as clear that there were a few little
disadvantages too! M. Richard, the author of an excellent book on
gas engines, said in 1892, that ‘this most curious motor, based
on the free-piston principles of Otto & Langen, were never able
to function in a practical manner, the absence of preliminary
compression resulting shock loading on the cylinder heads which
nothing was able to resist.’ Clearly, despite the rubber buggy
bumpers, the cylinder heads were well and truly massaged. Although,
like the Rhone, it could never have been a very convenient engine,
it is a pity that it enjoyed no success, for it is certainly an
interesting engine. I wonder, is it possible that with modern
materials such a rotary motor could be made to work?

1. Sparks & Flames, MacKeand J.C.B., Tyndar Press,
Montchanin, DE 1987.

2. Les Nouveaux Moteurs a Gaz et a Petrole, Richard G.,
Dunod Editeur, Paris 1892.

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