II. The method in which the steam is used in the engine is also a basis for a scheme of classifi cation. The fact that the same number of foot pounds of energy per minute may be secured by a small piston working at high speed or a larger piston working at a slower speed gives us our first classification, viz., high-speed. and low-speed en gines. The high-speed engine has the advantage of small dimensions and small weight for a given power. and, because the strokes are so frequent, of meeting variations in resistance more quickly than a slow-speed engine. Its disadvantages are its comparatively greater waste of steam, the greater wear of the moving parts, the greater danger of heating, and consequently the higher cost of construction and operation. Altogether, experience shows the slow or moderate speed en gine to he superior to the high-speed engine where circumstances permit its use. Between 600 and 800 feet per minute is regarded as a moderate piston speed, and over 000 feet per minute as a high piston speed. Referring to Fig. 2, it will he observed that the steam acts first against one side of the piston and then against the other side. Such an engine is called a double-acting engine. When the pressure of the steam is exerted against one side of the piston only the engine is called a single-acting engine. The single-acting prin ciple is utilized in the well-known Cornish engine for pumping and in certain high-speed rotative engines for electric light and power service. In the latter form of engine two cylinders are used which are coupled to separate cranks on the same shaft in order to secure continuous action. Two familiar forms are the Westinghouse and the Winans, and both are inverted vertical trunk engines. ()wing to their single action and the omission of the connecting rod, these engines escape many of the disadvantages of the double acting high-speed engine.
The Cornish single-acting pumping engine de serves particular notice because of its essentially peculiar steam-cylinder mechanism. It appears in two forms, the beam form and the direct-acting form. The beam Cornish engine has a vertical cylinder from whose top the piston rod extends, and has the usual connecting-rod connection with one end of a beam pivoted at the centre, to whose other end are attached the pump rods. The direct acting Cornish engine has the cylinder located directly over the mouth of the shaft and the pis ton rod passing out of its bottom connects directly with the pump rods. This form of Cornish engine is usually called the Bull Cornish, from the name of its first adopter. In both forms of Cornish engine the action of the steam is simply to raise the heavy pump rods, whose weight in falling displaces the water to be pumped. The cylinder of the Cornish engine has three valves, one for the admission of steam, one for the exhaust of the steam, and an equilib rium valve, these being shown in Fig. 3 at S. D. and E. respectively.
The cycle of operations is as follows: The pump rods being at the bottom of their stroke. the piston P of a beam Cornish engine will be at the top of the cylinder. The steam valve S and the exhaust valve D will be opened and the equilibrium valve E will be closed. The pressure of the steam drives the piston to the bottom of the cylinder, lifting the pump rods. When this operation has been completed, valves S and D are closed and valve E is opened. The opening
of valve E permits the steam above the piston to flow freely beneath it, equalizing the pressure on the two sides of the piston and leaving it free to return to the top of the cylinder under the pull of the heavy pump rods. The valves of the Cornish engine are worked by a special device called a ealaract. This consists of a weighted piston working in a cylinder, having a large in take valve and a small discharge valve, whose opening can be adjusted to various dimensions. During the working stroke of the pump this weighted plunger is lifted. drawing water into its cylinder through the large inlet valve. When the pump makes its return stroke the weighted plunger is released and gradually descends as its weight presses the water out of its cylinder through the small discharge valve. The return stroke of the weighted plunger actuates the valves of the steam cylinder of the pump so as to cause another working stroke.
The third subdivision of engines according to the method of using the steam comprises expan sive and non-expansive working engines. To understand the nature of this subdivision it will be found convenient to refer to the diagram Fig. 4. In this diagram the full lines represent the cylinder, piston, and piston rod. Now if steam be admitted behind the piston it will force it forward to the position which it occupies in the diagram. The steam may be made to perform this operation in two ways, non-expansively and expansively. When used non-expansively the steam enters the steam port at boiler pressure, and as this port remains open until the piston has completed its stroke, boiler pressure is main tained behind the piston during the whole stroke. If we assume the broken line ad to represent the steam pressure and the broken line de to repre sent the stroke, then the work done by the steam is represented by the (lotted rectangle abed; the pressure be at the end of the stroke is the same as the pressure ad at the beginning of the stroke, and a cylinderfnl of steam at full pressure has to be exhausted in order to make the return stroke. When using the steam expansively the steam valve is closed when the piston has reached some intermediate point, as e, called the point of cut-off, in its forward stroke, and no more steam is admitted into the cylinder. Full steam pressure is. therefore, maintained against the piston for the portion tie of its stroke, but after wards this pressure gradually decreases as the steam expands until at the end of the stroke it is represented by the line cf. The work done by the steam is represented by the area aefcd, which, as will be readily seen, is less than the area abed, representing the work done when using the steam non-expansively. In using the steam ex pansively, however, the amount at boiler pressure which is consumed at each stroke is represented by the rectangle whose base is ae and whose height is ad, as compared with the rect angle abed. representing the amount of steam at boiler pressure consumed at each stroke in non-expansive working. Evidently. from the dia gram, the amount of steam used in proportion to the work done is less in expansive working than in non-expansive working. This advan tage has made the expansive working engine practically• universal where circumstances will permit.