Types of Reciprocating Engines

Coupled Engines.—When uniformity of driving effort or the absence of dead points is important, two independent cylinders often work on the same shaft by cranks at right angles to each other. Such engines, which are called "coupled," can start readily from any position ; the ordinary locomotive engine is an example. In some engines three cylinders, set to work on cranks i 2o° apart, co-operate in giving a still more uniform drive. Winding engines for mines and collieries, in which ease of starting, stopping and reversing is essential, are very generally made by coupling a pair of cylinders on opposite sides of the winding drum.

Compound Engines, Coupled or Tandem.—Large direct acting engines are usually compounded either by having a high and a low-pressure cylinder side by side, with cranks at right angles, or by putting one cylinder behind the other with a common centre line. The latter is called a tandem arrangement. In a tandem engine, since the pistons, agree in phase, the steam may expand directly from the small into the large cylinder. But the connecting-pipe and steam chest form a receiver of considerable size, and it is preferable to cut off the supply of steam to the large cylinder at an earlier stage of the stroke. For mill engines the compound tandem and compound coupled types of engine are very usual. The high-pressure cylinder is often fitted with some form of trip-gear.

The chief advantage of compounding, as was pointed out in the historical section, is that it reduces the condensation which hot steam undergoes in the cylinder through contact with metal which has been chilled by the exhaust of the previous stroke. Compound working acts beneficially by narrowing the range through which the temperature periodically fluctuates in the metallic surfaces exposed to contact with the operative steam. The amount of steam that is at once condensed on admission depends on the extent to which the surface with which it comes in contact has been chilled during the previous exhaust. For this reason there is less loss through initial condensation in a compound engine giving the same total ratio of expansion.

Uniflow Engine.—The periodic give and take of heat between the steam and the metallic surfaces, which is a serious source of loss in any reciprocating engine, can be reduced by having separate valves for admission and exhaust, so that the same port-surfaces and valve-surfaces are not brought into contact with hot and comparatively cold steam. This idea is carried further in what is called the "uniflow" engine, by an ingenious arrangement which secures that the ends of the cylinder, where the steam is admitted, shall always be kept hot, the exhaust taking place at the middle of the cylinder through ports in the circumference which are un covered by the motion of the piston.

The uniflow design is a recent but conspicuously successful de parture from earlier practice. It allows high pressure steam to be expanded in a single cylinder without the drawbacks ordinarily entailed in non-compound working. There is, however, still enough exchange of heat between the metal and the steam to make super heating highly advantageous.

Condensation.

In land engines a jet form of condenser is common, but surface condensation is resorted to when the avail able water-supply is unsuited for boiler feed. When there is no large supply of condensing water a very fair vacuum can be ob tained by using an evaporative condenser, consisting of a stack of pipes into which the exhaust steam is admitted and over which a small amount of cooling water is allowed to drip. This water is evaporated by the heat which the condensing steam gives up. Such a condenser is placed in the open, generally on a roof where the air has free access. The amount of water it uses need not exceed the amount of steam that is condensed, and is a small fraction of the amount that would be required in a jet or surface condenser.

High-speed Direct-acting Engines.

Prior to the develop ment of the steam turbine the demand for engines suitable for driving electric generators without the intervention of a belt led to the introduction of various forms of direct-acting engine adapted to run at a high speed. Some of these were single-acting, steam being admitted to one side of the piston only, generally the back, with the result that the rods could be kept in a state of thrust throughout the revolution, and alternations of stress in them and at the joints thereby avoided, together with the knocking and wear of the bearing brasses which it is apt to cause. To secure, however, that the connecting-rod should always push and never pull against the crank-pin there had to be much cushioning during the out stroke on account of the fact that from about the middle of that stroke to the end the reciprocating mass was being re tarded. In engines of this class designed by P. W. Willans, which were at one time much used, but are now displaced by turbines, the cushioning was provided by means of a supplementary piston which compressed air during the out stroke ; the energy which the reciprocating masses had to part with in losing their motion during the second half of the out stroke was stored in this air and was re stored in the succeeding down stroke. The engine was entirely enclosed in a casing the bottom of which formed an oil bath in which the cranks splashed to ensure ample lubrication. This fea ture is retained in many modern high-speed engines.