Page:The New International Encyclopædia 1st ed. v. 18.djvu/611

* f STEAM ENGINE. 525 STEAM ENGINE. condenser. In a compound or multiple-expansion engine the steam, after having done its work in one cylinder, is exhausted into a succeeding cylin- der of larger size, where it continues to perform work. The operation may be explained by the diagram Fig. 5. Steam enters the cylinder HP, which is connected with cylinder LP by two passages a and 6 closed by valves, and here per- forms the work of driving piston Pi to the front end of the cylinder. This steam, instead of being exhausted into the air or into a condenser to allow the return stroke of the piston P,. is ex- hausted through the passage a into cylinder LP, where it acts to drive forward piston P„. Were the size of the cylinder LP the same as that of the cylinder HP, it is evident that the propulsive effect of the steam on piston V^ would be exactly counterbalanced by the back pressure on piston Pi, and, therefore, that there would be no in- crease, but rather a loss, in the work done. If, however, cylinder LP is made twice the diameter of cylinder HP. then the forward pressure on piston P; is greater than the liack pressure on piston P, and a position or working effect is ob- tained by the second use of the steam. A two- cylinder engine like Fig. 5 using steam as de- scribed is called a compound engine. Were the exhaust steam from the second cylinder to enter a third and larger cylinder and continue working we should have a triple expansion engine. If a fourth cylinder were added to the set we should have a quadruple expansion engine. The e.xpan- FlG. 5. sive working of steam has not been successfully carried beyond four expansions. The cylinders of compound engines are generalh' arranged parallel to each other or side liy side, or else one behind the other, or tandem. A tandem com- pound with the cylinders set vertical is called a steeple engine. When the cylinders are placed side by side, but some distance apart so as to allow space for a fly wheel between them, the engine is commonly called a cross-compound en- gine. Modern marine engines for screw vessels are nearly always inverted vertical compound or multiple expansion engines. (See Plates un- der Steam Navig.vtion.) The arrangement of the cylinders of triple and quadruple expansion engines varies considerably. The great practical advantage of the compound or multiple expansion engine is the economy resulting from the expan- sive use of steam, as indicated in discussing ex- pansive and non-expansive working engines. It is obviously possible to use the steam according to any of the methods discussed with any of the arrangements of mechanism previously dis- cussed and thus make a great number of com- binations. 111. A very usual classification of steam en' gines is based upon the service or use to which the engine is to be put. It includes the following groups: Stationary engines, locomotive engines, and portable engines. The first group or station- ary engines include mill engines, power-house en- gines, pumping engines, blowing engines, air com- pressors, hoisting engines, and others. Locomo- tive engines include locomotives, marine engines, traction engines, steam ruail rollers, self-pnjpelled fire engines, and others. Portable engines in- clude a wide variety of engines, such as agricul- tural engines, so designed that the engine and boiler are self-contained and do not require a foundation or permanent stricture of any kind. Structural Details. Structurally the steam engine consists of the working parts and the fixed structure or frame by which the working parts are supported and maintained in their relative positions. The supporting frame is usually a structure of east or forged iron or steel varying in shape to suit the form of engine of which it is a part. The working parts of an engine comprise the cylinder and piston, the valves and valve gear, the governor, the piston rod, crosshead, and connecting rod, and the crank shaft and fly wheel. The function of the engine frame is to constitute a fixed link in a kinetic chain, and to perform this function it must be essentially rigid and strung. In a horizontal engine, as shown in Fig. 2, it will be evident that the duties of the frame are to support the cylinder C at one end, and the fly wheel W at tiie other end, and to steady and a'lign the trans- forming liiechanism connecting these two parts. The frame of a vertical engine performs an ex- actly similar duty, but of course differs in con- struction to suit the different arrangement of the moving parts. Thus in an inverted vertical engine there is a broad base plate, on which the crank shaft is directly supported. From this base plate rises a columnar structure support- ing the cylinders and guiding the reciprocating parts. Cast iron and cast steel are particularly adapted to furnish the rigidity and massiveness desired, and are, therefore, the materials com- monly employed for engine frames. In certain classes of engines, such as the locomotive and the marine engine, where lightness combined with great strength is an essential, forged steel frames are used. Turning now to the working parts, the cyl- inder and piston as the motor elements naturally call first for consideration. The engine cijUnder is a barrel-shaped or box-shaped casting, with its interior bored out into the form of a perfect e.vlinder. The ends of this cylinder are closed by cover-like castings fastened by studs or bolts to the bodv. The front head has a circular ori- fice at the centre to allow for the exit of the piston rod, and where a tail rod is used a simi- lar orifice is necessary in the back head. The barrel or body of the cvlinder has one or more orifices for the admission and exhaust of the steam, and these orifices are covered by a box- like chamber called the steam chest, inside of which work the vah-cs, as will be described immediately. The body and often the ends of the cylinder casting are usually covered with a lagging or envelope of some material which is a poor conductor of heat and whose purpose is