The Roper Hot Air Engine

Source: an article from Scientific American, Vol. XI, N°23.
Date: December 3, 1864
Title: Roper's Air Engine

Roper's Air Engine

In a previous article, we gave an illustration of Roper’s air engine, the first engine, we believe, either air or steam, which has proved practically successful in using the products of combustion to increase the pressure in the cylinder.

This achievement will permanently give to this invention a prominent place in the history of prime movers, as being one of the great steps in the progress of that foundation department of mechanics.

The great interest which attaches to this invention, and the fact that it has gone into practical use on a large scale, induces us to present to our readers another illustration of it, embracing some improvements which have been added since the first one appeared.

The engine is exceedingly simple. It may be regarded as a steam engine worked by air, with the furnace inside of the boiler.

Roper's hot air engine

Referring to the engraving, the large upright cylinder, A, is the chamber in which the air is heated, the fire being inside in direct contact with the air. The door through which the coal is introduced is on the opposite side of the air chamber, and is not shown in the cut. This door closes air-tight and is secured to withstand the pressure at which the air is worked.

As the air is expanded by heat it is let into the bottom of the working cylinder, B, through a valve opening, at the proper time, and forces up the piston, thus vibrating the beam at the top of the cylinder, and turning the wheel through the connections shown.

The two rods on the outer end of the beam operate the piston of the air-pump, C, by which the air is compressed and forced into the heating chamber, A. The air enters the heating chamber through two pipes, one above the grate and one below; the larger portion of the air entering above the grate.
This arrangement prevents a blast through the fire that would carry ashes and bits of unburned coal into the cylinder.

The general plan of this engine has been described by Professor Rankin and by Fairbairn as “Joule’s engine of constant pressure;” but without Roper’s device of placing the fire in the heating chamber, it would probably never have come into general use as an economical and practical motor.

This arrangement not only utilizes the pressure of the hot gases generated by the combustion of the fuel, but it has another advantage of far greater importance. One of the most serious difficulties in air engines has been the extreme slowness with which heat can be imparted to air through iron plates. When air is passed directly through the fire the oxygen that enters into combination with the carbon and hydrogen of the fuel, is, by the act of combination, heated to a temperature of some 3,000 degrees Fahr.

Then these white-hot gases are mingled with the cool air entering above the fire, and their temperature is brought down to the point at which they can be worked through the cylinder without destroying the packing. As the quantity introduced above and below the coal may be varied by a stop-cock upon the outside, the temperature of the working air can lie adjusted with the nicest precision.

A sufficient quantity of coal is introduced in the morning to last till noon, so the engine does not require to be stopped for feed any of oftener than the men must stop for the same purpose. The rapidity of the combustion is controlled perfectly by the quantity of air admitted below the grate.

Economy of air engines is claimed only in cases where small powers are required - from one to four horses - and in these cases the great saving is in dispensing with the services of an engineer. It is also claimed that two years experience has shown this engine to be less costly in interruptions and repairs than ordinary steam engines.

For further information in relation to this engine address Crosby, Butterfield & Haven, 47 Pearl street, Boston, or 22 Dey street, New York, where machines can be seen in operation. More than 200 of these engines are now in use, and not one, we are told, has ever been condemned.