TYPES OF RECIPROCATING ENGINES In classifying engines with regard to their general arrangement of parts and mode of working, account has to be taken of a con siderable number of independent characteristics. We have first a general division into condensing and non-condensing engines, with a subdivision of the condensing class into those which act by sur face condensation and those which use injection. Next there is the division into compound and non-compound, with a further classification of the former as double-, triple-, or quadruple-expan sion engines. Again, engines may be classed as single or double acting, according as the steam acts on one or alternately on both sides of the piston. Again, a few engines—such as steam hammers and certain kinds of steam pumps—are non-rotative, that is to say, the reciprocating motion of the piston does work simply on a reciprocating piece; but generally an engine does work on a continuously revolving shaft. In most cases the crank-pin of the revolving shaft is connected directly with the piston-rod by a con necting-rod, and the engine is then said to be direct-acting; in other cases, of which the beam engine is an historical example, a lever is interposed between the piston and the connecting-rod. The same distinction applies to non-rotative pumping engines, in some of which the piston acts directly on the pump-rod, while in others it acts through a beam. The position of the cylinder is another element of classification, giving horizontal, vertical and inclined cylinder engines. In most vertical engines the cylinder is above the connecting-rod and crank. In oscillating cylinder en gines (a type still found in some river boats) the connecting-rod is dispensed with ; the piston-rod works on the crank-pin, and the cylinder oscillates on trunnions to allow the piston-rod to follow the crank-pin round its circular path. In rotary engines there is no piston in the ordinary sense ; the steam does work on a revolv ing piece, and the necessity is thus avoided of afterwards con verting reciprocating into rotary motion.
Beam Engines.—In the single-acting atmospheric engine of Newcomen the beam was a necessary feature ; the use of water packing for the piston required that the piston should move down in the working stroke, and a beam was needed to let the counter poise pull the piston up. Watt's improvements made the beam no longer necessary; and in one of the forms he designed it was discarded—namely, in the form of pumping engine known as the Bull engine, in which a vertical inverted cylinder stands over and acts directly on the pump-rod. However, the beam formed a convenient driver for pump-rods and valve-rods; and the parallel motion invented by Watt as a means of guiding the piston-rod, which could easily be applied to a beam engine, was, in the early days of engine-building, an easier thing to construct than the plane surfaces which are the natural guides of the piston-rod in a direct-acting engine. In modern practice the direct-acting type has almost wholly displaced the beam engine.