The commencement of the eighteenth century began the first steps towards the development of the modern form of engine. Savery and Newcomen made improvements, which were perfected by James Watt, who was born at Glasgow in 1737. Amongst other valuable improvements he first contrived to convert the reciprocating motion into a rotary one by means of the crank. In the year 1800 Watt retired from business, leaving the steam-engine in much the same condition as we find it now. The application of steam-power for locomotion on both land and water followed, and now stationary, locomotive, and marine engines, driven by steam, are distributed all over the civilised world.

The varieties of model engines are in many cases indicated by their names. Stationary engines are intended to be fixed, as those used for driving machinery. Locomotives are those which are intended to travel by steam, and are self-moving. Marine engines are those used to propel ships. Of these three classes we shall deal only with the first and third in the present chapter. Locomotives are much more complicated in their construction, and consequently are more difficult to make.

Horizontal engines are those having the cylinder lying with its axis in a horizontal position. Vertical engines have the cylinder upright; sometimes they are designated by the latter adjective. Beam engines have an oscillating beam; one end is connected to the piston and the other to a rod which drives the crank. Cylinders are single-acting when the steam is admitted only at one end, and consequently with these the crank is only propelled during half of its rotation. Double-acting cylinders are provided with valves which admit the steam at each end of the cylinder alternately. Oscillating cylinders are fitted to oscillate with the motion of the crank, and the steam-valves are usually contrived to act by this oscillating motion. Slide-valve cylinders have a sliding valve, worked by a rod connected to an eccentric on the crank shaft, which opens the steam ports to alternately admit live steam and exhaust at both ends of the cylinder. Slide-valve cylinders are invariably double-acting.

Boilers, which are the vessels in which water is converted into steam, are usually described by their shape and position. They may be cylindrical, spherical, etc., and horizontal or vertical. The construction also forms a distinguishing characteristic. Tubes are usually inserted in the boiler to convey the heat from the fire. These tubes—which are more properly called flues, especially in large boilers—vary in number from one of large gauge to scores of small ones, thus naming the respective boilers single-flue or multiflue. It may be advisable to mention here that tubular boilers are those in which the water circulates in the tubes, and the fire impinges on the outer surface. When the fire operates inside the tube it is called a flue. A tube carries water; a flue carries flame and the volatile products of combustion.

Boilers, or steam generators, that are used to contain the water which, when converted into steam, drives the engine, require to be sufficiently strong to withstand an internal or bursting pressure. This pressure is very great in high-pressure engines, but in models it is generally very low, and seldom exceeds twenty pounds to the square inch. The evaporating capacity of the boiler is according to the requirements of the engine it has to supply. The resistance of the piston to the steam shows the pressure at which it should be supplied. Boilers are generally tested, by means of a hydraulic pump, to stand a pressure at least double that at which it is intended to use them. It is unsafe to generate steam in any vessel that has not been properly tested. This fact cannot be too strongly impressed upon the mind of the reader.

Suppose a double-action cylinder, 1-inch bore and 2-inch stroke, is to make one hundred revolutions of the crank per minute, let us see how much steam will be wanted to drive it. The area of the piston is ·785 inch, and each revolution of the crank will require the cylinder to be filled twice—that is, one stroke in each direction. This will take a column of steam ·785 inch in diameter and 4 inches long for each revolution, or 314 cubic inches of steam per minute. If the speed is greater, the quantity of steam must be increased proportionately; and when running at the rate of one thousand revolutions per minute—a speed often attained—3,140 cubic inches of steam will be wanted to supply the cylinder. That is at the rate of about 100 cubic feet per hour.

The pressure of the steam has not yet been taken into account, but it obviously forms a most important factor in the calculation. Water in an open vessel boils at a temperature of 212° Fahr. Provided that the vessel allows the steam to escape freely, all the heat that can be applied will only generate steam at the same pressure, though it will escape faster. As the bubbles of steam ascend to the surface they escape, having only the pressure of the atmosphere to overcome. When water is confined in a closed vessel, like the boiler of a steam-engine, the temperature may be raised to considerably above the usual boiling-point. The heat is always proportionate to the pressure, and steam at a pressure of 120 lb. per square inch is equivalent to the heat represented by 345° Fahr.

A correct knowledge of the fact that pressure depends on temperature cannot be urged too strongly on the mind of the model engineer. In many model boilers it is quite impossible to raise the heat sufficiently to produce an adequate pressure. Boiling water at 212° Fahr. does not produce any available pressure of steam, it merely counterbalances the weight of the atmosphere, which is 15 lb. to the square inch. By increasing the heat, which can only be done in a closed vessel, available pressure is obtained. Thus 228° = 5 lb., 241° = 10 lb., 251° = 15 lb., and so on. The steam, and the water from which it is generated, and with which it remains in contact, have both the same temperature.

A cubic foot of water weighs 62·5 lb., and it will produce 882 cubic feet of steam, at a pressure of 15 lb. to the square inch above the normal atmospheric pressure; this is equal to a temperature of 251° Fahr. If the pressure is raised to 150 lb., which requires a temperature of 371°, only 187 cubic feet of steam will be produced. Steam is elastic, and hence the more it is compressed the greater will be its force. If one cubic inch of steam, at a pressure of 30 lb., is admitted into a cylinder, and the supply cut off when half filled, the steam will expand till it has filled the cavity, and in increasing its bulk twofold its force will diminish in inverse ratio. The pressure will therefore diminish to 15 lb. to the square inch. The expansive force of steam is always at work on the piston of the engine, and it varies in accordance with the arrangement of the valves.

Let us now trace the effect of the steam when admitted to the cylinder. When the governor valve is opened the steam flows along the pipe to the slide valve chest, and if one of the ports are open it reaches the cylinder. In traversing the pipes which conduct it to the cylinder the steam is cooled considerably and its force diminished. In course of time the parts become heated to a certain extent, and then the loss of power is less. When the steam enters the cylinder it at once exerts a certain force on the piston. This has the effect of turning the crank shaft, and in due course the slide valve closes the steam inlet. Now the steam within the cylinder acts expansively, and continues to drive the crank shaft to the end of the stroke. Then the exhaust port is opened, and allows the spent or dead steam to escape. At the same time the inlet at the other end is opened and the live steam rushes in and exerts its full pressure on the piston, causing it to travel in the opposite direction. The opening and shutting of the steam ports is effected by an eccentric on the crank shaft. In treating of the construction of these parts, the relative sizes will be given and the correct motion explained.