According to their use, engines may also be designated as stationary, semi-portable, portable, locomotive, locomobile (traction), marine, hoist ing, or pumping engines, and steam fire-engines.
Engines whose pistons travel lengthwise in the cylinders, and which have a reciprocating motion, are called "rotative" or reciprocating engines, rotative being an arbitrary term practically meaning the same thing as "rotary," which term is applied to those engines wherein the piston (often called the " follower ") rotates in the cylinder about its own axis and about the axis of the cylinder.
Engines may be further distinguished as "single-acting" if the steam works only on one side of their pistons, and "double-acting" if the steam is admitted first on one side and then on the other. They may be "sin gle" or "duplex" respectively; single if there is but one cylinder working npon the shaft, and duplex if there are two cylinders which have exactly the same function. They may be " compound " if the exhaust from one or more of the cylinders enters one or more other cylinders, and " non-com pound " if the exhaust discharges into the open air or into a condenser. If the exhaust is condensed by contact with a jet of cold water or with metallic surfaces cooled by a current of water, they are "condensing" engines, in contradistinction to those in which the exhaust is not con densed and which are known as " non-condensing" engines. They may have " fixed " or " variable" ent-off. If variable, they may be variable only by hand or " automatic," which variation may be effected by changes in the load or in the steam-pressure, or as this change must be made by hand. If adjustable, they may be variable while the engines are running or only when they are stopped. If automatic, they may be so by reason of the point of cut-off being changed, or by merely choking off the steam supply so as to lessen it in case of decreased load or initial pressure. They may or may not have a beam. The cylinders may be fixed or oscillating.
When the piston consists, instead of a piston-rod acting upon a con necting-rod or directly upon a plunger or piston, of a " trunk " or hollow cylinder in which vibrates the connecting-rod attached directly to the piston, the construction is known as a " trunk " engine. If the trunk passes through a stuffing-box so that steam may be used upon the full area of the piston upon one side and upon a smaller area upon the other, it is a " half-trunk " engine. When the diameter of the cylinder equals the stroke of the piston, it is known as a " square " engine.
When the connecting-rod of a horizontal reciprocating-engine on the out-stroke passes under the line of the main shaft, the engine is said to be " mider-running;" if it passes over the line of the main shaft on the out-stroke, it is " over-running." (A locomotive is under-running when moving ahead, and over-running when moving backward, because the cylinders are arranged with the piston-rod and cross-head behind them.) In an over-running horizontal engine, the top of the fly-wheel, when there is one, runs from the cylinder; in an under-running engine it runs toward the cylinder.
That end of the cylinder which is next the cross-head is called the "inner" end of any engine, the " front" end of any horizontal station ary engine, and the " crank " end if there be a The end farthest from the cross-head is the "out" end in any engine, and the "back" end in any but a locomotive. The heads of a vertical engine having a crank may be designated either as the " crank " and the " out " end, or as the " upper " and the " lower " end.
afnilions of the various parts of the steam-engine it may be well to name its principal pieces and functions in a running form. The steam from the boiler passes into and through the in which there is placed a and in which there should also be a back slop valve. In an engine which throttles the steam or cuts off its supply in
the pi112 there is a worked by a governor or regu/ator. From the supply-pipe the steam passes in many engines into the steal/I ckes!, in which there is placed a valve, generally in these cases a slide valve, which may or may not be balanced for the purpose of making it work with a maximum of pressure and friction upon its seat. In sonic slide-valve engines there is a separate valve. working upon the back of the main or upon a partition in the steam-chest. Many engines have no steam-chest proper, and many have no slide-valve, its place being taken (1) by a rock-valve, which is a slide-valve bent to a curve around an axis at right angles to its motion, instead of being flat upon its working side; (2) by a which is equivalent to a slide-valve wrapped around an axis parallel to its direction of motion; (3) by a plug-•a/ve, which is practically a rock-valve having control of only one port and only one function instead of two ports and two functions; or (4) by a which is a disc or a pair of discs fixed upon a stein, having motion parallel to that stein, and opening or closing a circular aperture for the passage of steam. The steam is admitted by thevalves that open and control the ports, which are the mouths of the (also called into the cylinder, where it acts upon the piston. The ports and passages which admit the steam are called and pas sages, the term eduction being applied to those used only to discharge steam which has been in the cylinder. The steam which has been used in the cylinder is termed waste or exhaust steam, and is discharged through the exhaust port and exhaust pipe either into the open air, or into another cylinder, or into a condenser, where it is condensed into hot water by the action of cold or constantly renewed by the or by a head from a tank, reservoir, or other source of pressure and flow. (In a locomotive the exhaust goes through a blast pipe, whose end is called the The ends of the cylinder are called heads or covers, in one of which there is a through which passes the The lubricator discharges into the steam cylinder, or into the steam-chest, or into the steam-pipe near the steam chest, its object being to lubricate the valves and piston. At each end of the cylinder there may be an automatic or cock (also called for the discharge of water which may collect in the cylinder. The cylinder is usually covered with a lagging, purposely a poor conductor of heat. Sometimes its walls are double, and the space between, through which there is a circulation of live steam, or exhaust steam, or hot air, is called the jacket. Where there is a condenser the excess of air, which would otherwise mar the vacuum, is removed by the air-pump at the same time that the water of condensation is taken away, the reservoir which contains this water being called the hot well. Through the valves live steam is forced through pipes, chest, and cylinder (escaping by the before the engine is started. The pressure in the condenser is measured by the The is usually attached to a generally working in or upon guides, which give a straight-line motion to the piston-rod and to the piston. The cross-head may, however, be given a true straight-line motion by or In those engines in which the cylinders oscillate they swing upon trunnions. The reciprocations of the piston are communicated to the crank and by a in most other than beam-engines; sometimes a beam is interposed before the acting-rod. On the crank-shaft there is a which by its momen tum equalizes the motion and tends to keep the engine steady, although the load and the pressure of the steam may vary. The valves are worked by the or valve mechanism.