"Two shafts are set at an angle of 165 deg. to each other and connected by a Hooke's joint; one serves as a pivot, the power being taken from the other. Four curved pistons are arranged on the cross-piece, two pointing towards one shaft and two towards the other, and on each shaft or jaw are formed two curved steam cylinders in which the curved pistons work. The steam enters and leaves the base of each cylinder through ports in the shaft, which forms a cylindrical valve working in the bearing as a seating.
"On the revolution of the shafts the pistons reciprocate in their cylinders in much the same way as in an ordinary engine, and the valve arrangement is such that while each piston is receding from its cylinder the steam pressure is driving it, and during the in-stroke of each, its cylinder is in communication with the exhaust. There are thus four single-acting cylinders making each a double stroke for one revolution of the driving-shaft. The engine has no dead centres, and has been at 1,000 revolutions per minute."
ROTARY ENGINES.
The three types of rotary engines here shown are similar in principle, and none of them is of great practical value, though the upper figure shows an engine that has met with a certain measure of commercial success.
It is not necessary to describe other of the rotary engines that have been made along more or less similar lines by numerous inventors, models of which are for the most part, as in the case of those just described, to be seen more commonly in museums than in practical workshops. Reference may be made, however, to a rotary engine which was invented by a Mr. Hoffman, of Buffalo, New York, about the beginning of the twentieth century, an example of which was put into actual operation in running the machinery of a shop in Buffalo, in 1905.
This engine consists of a solid elliptical shaft of steel, fastened to an axle at one side of its centre, which axis is also the shaft of the cylinder, which revolves about the central ellipse in such a way that at one part of the revolution the cylinder surface fits tightly against the ellipse, while the opposite side of the cylinder supplies a free chamber between the ellipse and the cylinder walls. Running the length of the cylinder are two curved pieces of steel, like longitudinal sections of a tube. These flanges are adjusted at opposite sides of the cylinder and so arranged that their sides at all times press against the ellipse, alternately retreating into the substance of the cylinder, and coming out into the free chamber. Steam is admitted to the free chamber through one end of the shaft of ellipse and cylinder and exhausted through the other end. The pressure of the steam against first one end and then the other of the flanges supplies the motive power. This pressure acts always in one direction, and the entire apparatus revolves, the cylinder, however, revolving more rapidly than the central ellipse.
For this engine the extravagant claim is made that there is no limit to its speed of revolution, within the limit of resistance of steel to centrifugal force. It has been estimated that a locomotive might be made to run two hundred or three hundred miles an hour without difficulty, with the Hoffman engine. Such estimates, however, are theoretical, and it remains to be seen what the engine can do in practise when applied to a variety of tasks, and what are its limitations. Certainly the apparatus is at once ingenious and simple in principle, and there is no obvious theoretical reason why it should not have an important future.