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

| September/October 2001

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 ignition.

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?


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