CLASSIFICATION. Steam engines may be classi fied (I.) according to the nature of their mecha nism, (II.) according to the manner in which the steam is used, and (III.) according to the use to which the engine is put.
I. Reciprocating engines may be grouped ac cording to the position of the axis of the piston into horizontal engines, vertical engines, and in clined or diagonal engines. Fig. 2 shows the posi tion of the parts of a horizontal engine, that is, an engine with the axis of the cylinder and piston horizontal. The advantages of such an engine are convenient access from the ground to all parts of the mechanism, distribution of the weight over a large area, and the location of the centre of gravity of the machine close to the founda tion; its disadvantages are greater friction and wear of the cylinder owing to the fact that the piston is supported upon the bottom of the cyl inder.
In the vertical engine the axis of the cylinder and piston is vertical ; if the cylinder is carried by a frame above the crank shaft so that the pis ton rod extends downward, the engine is called an inverted rcrtical engine; and if the cylinder is below, with the piston rod extending upivard to a crank shaft above, the engine is called a direct rertical engine. The advantages of the vertical engine are that the cylinder friction and consequent cylinder wear of the horizontal en gine are largely avoided and that a comparatively small area of ground is occupied by the engine. Its disadvantages are that the weight of the piston, piston rod, crosshead, and connecting rod acts with the steam in one direction and against the steam in the other direction, thus producing unequal effort on the crank on alternate strokes, which has to be counteracted by special construe tions; that the different parts of the mechanism are at different heights, requiring more attend ants: that the centre of gravity of the machine is high, reducing its stability. The limited foun dation area required for the vertical engine espe cially adapts it to places where room is scarce or expensive, as in steamships and city power houses. See STEAM NAVIGATION.
Inclined engines possess in a measure the ad vantages and disadvantages of both the horizontal and the vertical engine. In such engines the axis of the cylinder is at an inclination between the vertical and horizontal, and the cylinder is usu ally set below with the piston rod projecting diag onally upward to the crank shaft. They are used chiefly in side-wheel steamboats of shallow draught and in ferry-boats.
An engine in which the motion of the piston is transmitted by a connecting rod direct to the crank, as shown by Fig. 2, is called a dircet acting engine. As applied to pumps this term has a somewhat different meaning. (See l'imirs ANu Pun PING MACHINERY.) In contradistinction to direct-acting engines we have beam engines, of which the engine shown by Fig. 1 is an early example. In the beam engine the connecting rod is journaled to one end of a beam rocking on a horizontal axis. To the other end of the beam is journaled a- pitman rod which connects with the crank shalt. The engines for side-wheel steam boats are usually beam engines; and they are also used for pumping engines.
Besides the forms of engines mentioned there are various special forms, a few of which are sufficiently important to merit mention. The oscillating engine dispenses with the connecting rod, the piston rod connecting directly to the crank, and the cylinder is mounted upon suitable trunnions so that it can oscillate in conformation to the swaying of the piston rod in operating the crank. The trunk engine dispenses with the pis ton rod, the connecting rod being attached direct to the piston, which takes the form of a hollow cylinder closed at one end. The back-acting engine has the connecting rod extending back from the crosshead to a crank shaft back of the engine instead of extending forward as shown by Fig. 2.