STEAM ENGINE. 524 STEAM ENGINE. H peculiar steani-cylincier 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 usuall}' 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 (lumped. 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, respectiveh'. 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 nf 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 cataract. 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 Tinderstand 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 rfc to repre- sent the stroke, then the work done by the steam is represented by the dotted rectangle abed; the pressure be at the end of the stroke is the same Fig. 4. as the pressure ad at the beginning of the stroke, and a cylinderful 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 ae 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 aefed, which, as will be readilj' seen, is less than the area abed, representing the work done when using the steam non-expansively. In using the steam ex- pansivelj', 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. Evidentlj-, from the dia- graifl. 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. The fourth subdivision of steam engines ac- cording to the method of using the steam com- prises condensing and non-condensing engines. In a non-condensing engine the waste steam from the cylinder is exhausted into the air at air pressure, or 14.7 pounds per square inch. In the condens- ing engine it exhausts into a vacuum and is con- densed into water. Thus, in a non-condensing engine the steam has to force the piston again.st a back pressure of 14.7 pounds per square inch, while in a condensing engine this back pressure is obliterated, with a consequent gain of 14.7 pounds per square inch in the eflfective working pressure of the steam. The practical advantage of the condensing engine may, therefore, be ex- pressed by saying that it will receive the same power with a smaller cylinder and greater power with the same cylinder, as compared with a non- condensing engine. ( See Co?<denser. ) A fifth subdivision of steam engines according to the metliod in which steam is used comprises simple engines, compound engines, and multiple expan- sion engines. In a simple engine the steam, after having done its work in forcing the piston through its stroke, is exhau.sted into the air or into a
