The piston 1 is accurately fitted to the cylinder, and made to move in it steam tight by packing, with which it is sur rounded. This piston divides the cylin der into two compartments, between which there is no communication by which steam or any other elastic fluid can pass. A case B placed beside the cylinder, contains the valves by means of which the steam which impels the piston is admitted and withdrawn, and the pis ton commences its motion in each direc tion. The upper steam-box B is divided into three compartments by two valves : above the upper steam-valve V is a com partment communicating with the steam pipe S. Below the exhausting valve E is another compartment communicating with a pipe called the eduction pope, witch leads downwards from the cylinder to a vessel called the condenser, which we shall presently describe. By this educ tion pipe the steam is withdrawn from the cylinder after it has driven the pis ton. By the valve V a communication may be opened or closed between the boiler and the top of the cylinder, so as to admit or intercept the supply of steam from the one to the other. By the valve E a communication may be opened or closed between the top of the cylinder and the condenser, so that tta, steam in the top of the cylinder may either be per mitted to escape into the condenser or confined in the cylinder. The continua tion 8' of the steam-pipe leads to the lower steam-box B, which, like the upper, is divided into three compartments by two valves V' and E'. The upper com partment communicates with the steam pipe S', and the lower with the eduction pipe. By means of the valve V' steam may be admitted from the steam-pipe S' to the bottom of the cylinder, and by means of the valve E' this steam may be permitted to escape to the condenser.
The four valves V, E, V', and Eg are in the engine represented in the figure connected by a system of levers with a single handle or spanner m, which being pressed upwards or downwards opens and closes the valves in pairs. Thus when it is pressed down, the levers connected with it raise the upper exhausting valve E and the lower steam valve V', and close the upper steam valve V and the lower exhausting valve P. On the other hand, when the spanner m is pressed up it opens the upper steam valve V and the lower exhausting valve E', and at the same time closes the upper exhausting valve E and the lower steam valve V'.
Mr. Piment has improved the mode of condensing the steam after it has done its duty in the cylinder. The steam on leaving passes into tubes surrounded by water intended for the boiler, which con denses it quickly, and is itself warmed. One great advantage is the great purity of the water so obtained thereby pre venting incrustation, and the water itself being heated up to 95°, there is a saving of fuel. It is now extensively used.
Perkins's High-Pressure and Safety Engine is made applicable to all purposes of steam navigation, and consists of a steam-pipe from the generators, convey ing the steam to the admission-valve, lying horizontally at the back of the cylinder, from whence it acts on the un derside of the piston at a pressure of 2000 lbs. on the inch. The cylinder is about a 15-horse-power ; the piston only six inches in diameter ; and the length of stroke only 20 inches.
Palmer's apparatus, as well as Brun ton's, consumes the smoke, or supplies sufficient air for the purpose, either by condensation or exhaustion, turning from right to left or left to right.
By placing vane-wheels within the chimney of the boiler of a small steam engine, the smoke and hot air is return ed through the fire and passed down wards, in a current, towards the ground.
Sometimes an air-pump has been used instead of a fan, and no chimney is re quisite; hence steam-earriages emit no smoke.
Non-condensing Steam-Engines. The form and structure of non-condensing engines differ in nothing from that of double-acting condensing engines, except in the absence of the condensing appara tus ; that is to say, the condenser, the air pump, and the cold water and hot water pumps. The steam, after it has impelled the piston, instead of being conducted to a cold vessel to be condensed, is sim ply allowed to escape into the atmosphere, and is commonly ejected into the chim ney of the furnace.
The operation of such a machine is ex tremely simple. The valves by which the steam is admitted to, and allowed to escape from the cylinder, are exactly simi lar to those of the double acting engine. In the down stroke of the piston, the up per steam valve being open, admits steam from the boiler above the piston, and the lower exhausting valve allows the steam below to escape through a tube which leads to the chimney, up which it rushes. In the up stroke, the lower steam valve being open admits steam from the boiler below the piston, and the upper exhaust ing valve being open allows the steam above the piston to escape to the chim ney.
It is evident, in such a machine, that the piston is always resisted by the pres sure of the steam escaping to the chim ney. As such escape cannot be effected except by steam of greater pressure than that of the atmosphere, it follows that the piston is always resisted by a force somewhat greater than the atmospheric pressure. The steam which urges the piston is, therefore, only effective by the excess of its pressure above that of the escaping vapor, which may be taken at about 16 lbs. per inch; but which varies in different engines.
As the steam used in non-condensing engines must of necessity have a pres sure considerably exceeding that of the atmosphere, such machines have been generally called high-pressure engine* ; while those which condense the steam have been, on the other hand, called law-pressure engines. These terms are not, however, correctly expressive of the nature of these engines respectively. Since the pumps of the non-condensing engine are dispensed with in this, the beam may be so likewise, and a still further simplification results from an oscillating movement given to the cylin der; such are termed vibrating engines, and are successfully used where space is limited as in marine engines. Many en gines in which condensation is used, especially those in which the expansive principle is applied with much effect, are worked with steam of a high pressure, not unfrequently with a pressure amount ing to from two to three atmospheres. It is, therefore, not correct to call such machines low pressure engines. It is, however, true that engines worked with out condensation must, of necessity, be worked by steam of a pressure which is generally called high pressure.
i An improvement oscillating engines has been introduced, which consists in the substitution of a circular valve for the ordinary slide valve, thus dispensing with eccentric guides, which renders the engine less complicated, more easily managed, as one lever suffices for use. It is especially applicable to small river boats.
All locomotive engines, without excep tion, used on railways or common roads, arc high-pressure non-condensing en gines. (See LOCOMOTIVE ENGINE, and