Steam Engines Part 2

33,000 200 _A_ = ------------ = 192.4 square inches.

49 700

The nearest piston diameter of even inches is 16, which corresponds to an area of 201 square inches. a.s.sume a piston rod diameter of 2-1/2 inches, corresponding to an area of 4.9 square inches, from which the average or effective piston area is found to be ((2 201) - 4.9)/2 = 198.5 square inches.

Determining now the new piston speed, we have:

700 192.4 ----------- = 678.5 feet per minute.

198.5

a.s.suming the length of stroke to be 1.5 times the diameter of the cylinder, it will be 24 inches, or 2 feet.

This will call for 678.5 2 = 340 strokes per minute, approximately, or 340 2 = 170 revolutions per minute.

CHAPTER III

STEAM ENGINE DETAILS

Some of the most important details of a steam engine are those of its valve gear. The simplest form of valve is that known as the plain slide valve, and as nearly all others are a modification of this, it is essential that the designer should first familiarize himself with this particular type of valve in all its details of operation. After this has been done, a study of other forms of valves will be found a comparatively easy matter. The so called Corliss valve differs radically from the slide valve, but the results to be obtained and the terms used in its design are practically the same. The valve gear of a steam engine is made up of the valve or valves which admit steam to and exhaust it from the cylinder, and of the mechanism which governs the valve movements, the latter usually consisting of one or more eccentrics attached to the main shaft.

[Ill.u.s.tration: Fig. 8. Longitudinal Section of Slide Valve with Ports]

The Slide Valve

Fig. 8 shows a longitudinal section of a slide valve with the ports, bridges, etc. The valve is shown in mid-position in order that certain points relating to it may be more easily understood. The valve, _V_, consists of a hollow casting, with ends projecting beyond the ports as shown; the lower face is smoothly finished and fitted to the valve seat _AB_. In operation it slides back and forth, opening and closing the ports which connect the steam chest with the cylinder. Steam is admitted to the cylinder when either port _CD_ or _DC_ is opened, and is released when the ports are brought into communication with the exhaust port _MN_. This is accomplished by the movement of the valve, which brings one of the cylinder ports and the exhaust port both under the hollow arch _K_. The portions _DM_ and _ND_ of the valve seat are called the bridges.

It will be seen by reference to Fig. 8 that the portions _OI_ and _IO_ are wider than the ports which they cover. This extra width is called the _lap_, _OC_ being the outside lap and _DI_ the inside or exhaust lap. The object of outside lap is that the steam may be shut off after the piston has moved forward a certain distance, and be expanded during the remainder of the stroke. If there were no outside lap, steam would be admitted throughout the entire stroke and there would be no expansion. If there were no inside lap, exhaust would take place throughout the whole stroke, and the advantages of premature release and compression would be lost. Hence, outside lap affects the cut-off, and inside lap affects release and compression. A valve has _lead_ when it begins to uncover the steam port before the end of the return stroke of the piston. This is shown in Fig. 9, where the piston _P_ is just ready to start on its forward stroke as indicated by the arrow. The valve has already opened a distance equal to the lead, and the steam has had an opportunity to enter and fill the clearance s.p.a.ce before the beginning of the stroke. The lead varies in different engines, being greater in high-speed than in low-speed types.

[Ill.u.s.tration: Fig. 9. Ill.u.s.tration showing Lead of Slide Valve]

[Ill.u.s.tration: Fig. 10. Diagrammatical View of Eccentric]

The Eccentric

The slide valve is usually driven by an eccentric attached to the main shaft. A diagram of an eccentric is shown in Fig. 10. An eccentric is, in reality, a short crank with a crank-pin of such size that it surrounds the shaft. The arm of a crank is the distance between the center of the shaft, and the center of the crank-pin. The throw of an eccentric corresponds to this, and is the distance between the center of the shaft and the center of the eccentric disk, as shown at _a_ in Fig.

10. The disk is keyed to the shaft, and as the shaft revolves, the center of the disk rotates about it as shown by the dotted line, and gives a forward and backward movement to the valve rod equal to twice the throw _a_.

[Ill.u.s.tration: Fig. 11. Relations of Crank and Eccentric]

In Fig. 11 let _A_ represent the center of the main shaft, _B_ the crank-pin to which the connecting-rod is attached (see _H_, Fig. 1), and the dotted circle through _B_ the path of the crank-pin around the shaft. For simplicity, let the eccentric be represented in a similar manner by the crank _Ab_, and its path by the dotted circle through _b_.

Fig. 12 shows a similar diagram with the piston _P_ and the valve in the positions corresponding to the positions of the crank and eccentric in Fig. 11, and in the diagram at the right in Fig. 12. The piston is at the extreme left, ready to start on its forward stroke toward the right.

The crank-pin _B_ is at its extreme inner position. When the valve is at its mid-position, as in Fig. 8, the eccentric arm _Ab_ will coincide with the line _AC_, Fig. 11. If the eccentric is turned on the shaft sufficiently to bring the left-hand edge _O_, Fig. 8, of the valve in line with the edge _C_ of the port, the arm of the eccentric will have moved from its vertical position to that shown by the line _Ab'_ in Fig.

11. The angle through which the eccentric has been turned from the vertical to bring about this result is called the _angular advance_, and is shown by angle _CAb'_ in Fig. 11. The angular advance evidently depends upon the amount of lap.

If the valve is to be given a lead, as indicated in Fig. 12, the eccentric must be turned still further on the shaft to open the valve slightly before the piston starts on its forward movement. This brings the eccentric to the position _Ab_ shown in Fig. 11. The angle through which the eccentric is turned to give the necessary lead opening to the valve is called the _angle of lead_, and is shown by angle _b'Ab_. By reference to Fig. 11, it is seen that the total angle between the crank and the eccentric is 90 degrees, plus the angular advance, plus the angle of lead. This is the total angle of advance.

[Ill.u.s.tration: Fig. 12. Piston just beginning Forward Stroke]

The relative positions of the piston and valve at different periods of the stroke are ill.u.s.trated in Figs. 12 to 16. Fig. 12 shows the piston just beginning the forward stroke, the valve having uncovered the admission port an amount equal to the lead. The crank is in a horizontal position, and the eccentric has moved from the vertical an amount sufficient to move the valve toward the right a distance equal to the outside lap plus the lead. The arrows show that steam is entering the left-hand port and is being exhausted through the right-hand port.

[Ill.u.s.tration: Fig. 13. Steam Port fully Opened]

In Fig. 13 it is seen that the valve has traveled forward sufficiently to open the steam port to its fullest extent, and the piston has moved to the point indicated. The exhaust port is still wide open, and the relative positions of the crank and eccentric are shown in the diagram at the right. In Fig. 14 the eccentric has pa.s.sed the horizontal position and the valve has started on its backward stroke, while the piston is still moving forward. The admission port is closed, cut-off having taken place, and the steam is expanding. The exhaust port is still partially open.

[Ill.u.s.tration: Fig. 14. Valve has started on Backward Stroke]

[Ill.u.s.tration: Fig. 15. Both Steam Ports Closed]

In Fig. 15 both ports are closed and compression is taking place in front of the piston while expansion continues back of it. Release occurs in Fig. 16 just before the piston reaches the end of its stroke. The eccentric crank is now in a vertical position, pointing downward, and exhaust is just beginning to take place through the left-hand port.

This completes the different stages of a single stroke, the same features being repeated upon the return of the piston to its original position. The conditions of lap, lead, angular advance, etc., pertain to practically all valves, whatever their design.

[Ill.u.s.tration: Fig. 16. Exhaust Begins]

Different Types of Valves

In the following are shown some of the valves in common use, being, with the exception of the Corliss, modifications of the plain slide valve, and similar in their action.

[Ill.u.s.tration: Fig. 17. Engine with Piston Valve]

_Double-Ported Balanced Valve._--A valve of this type has already been shown in Fig. 2. This valve is flat in form, with two finished surfaces, and works between the valve-seat and a plate, the latter being prevented from pressing against the valve by special bearing surfaces which hold it about 0.002 inch away. The construction of the valve is such that when open the steam reaches the port through two openings as indicated by the arrows at the left. The object of this is to reduce the motion of the valve and quicken its action in admitting and cutting off steam.

[Ill.u.s.tration: Fig. 18. Section through Cylinder of Engine of the Four-valve Type]

[Ill.u.s.tration: Fig. 19. Different Types of Corliss Valves]

_Piston Valve._--The piston valve shown in Fig. 17 is identical in its action with the plain slide valve shown in Fig. 8, except that it is circular in section instead of being flat or rectangular. The advantage claimed for this type of valve is the greater ease in fitting cylindrical surfaces as compared with flat ones. The valve slides in special bushings which may be renewed when worn. Piston valves are also made with double ports.

[Ill.u.s.tration: Fig. 20. Longitudinal Section through Corliss Engine]

[Ill.u.s.tration: Fig. 21. The Gridiron Valve]

_Four-Valve Type._--Fig. 18 shows a horizontal section through the cylinder and valves of an engine of the four-valve type. The admission valves are shown at the top of the ill.u.s.tration and the exhaust valves at the bottom, although, in reality, they are at the sides of the cylinder. The advantage of an arrangement of this kind is that the valves may be set independently of each other and the work done by the two ends of the cylinder equalized. The various events, such as cut-off, compression, etc., may be adjusted without regard to each other, and in such a manner as to give the best results, a condition which is not possible with a single valve.

_Gridiron Valve._--One of the princ.i.p.al objects sought in the design of a valve is quick action at the points of admission and cut-off. This requires the uncovering of a large port opening with a comparatively small travel of the valve. The gridiron valve shown in Fig. 21 is constructed especially for this purpose. This valve is of the four-valve type, one steam valve and one exhaust valve being shown in the section.

Both the valve and its seat contain a number of narrow openings or ports, so that a short movement of the valve will open or close a comparatively large opening. For example, the steam valve in the ill.u.s.tration has 12 openings, so that if they are 1/4 inch in width each, a movement of 1/4 inch of the valve will open a s.p.a.ce 12 1/4 = 3 inches in length.

[Ill.u.s.tration: Fig. 22. The Monarch Engine with Corliss Valve Gear.--A, Rod to Eccentric; B, Governor; C, Reach Rod; D, Radial Arm; E, Steam Valve; F, Bell-crank; G, Wrist Plate; H, Exhaust Valve; K, Dash-pot]

_Corliss Valve._--A section through an engine cylinder equipped with Corliss valves is shown in Fig. 20. There are four cylindrical valves in this type of engine, two steam valves at the top and two exhaust valves at the bottom. This arrangement is used to secure proper drainage. The action of the admission and exhaust valves is indicated by the arrows, the upper left-hand and the lower right-hand valve being open and the other two closed.

Side and sectional views of different forms of this type of valve are shown in Fig. 19. They are operated by means of short crank-arms which are attached to a wrist-plate by means of radial arms or rods, as shown in Fig. 22. The wrist-plate, in turn, is given a partial backward and forward rotation by means of an eccentric attached to the main shaft and connected to the upper part of the wrist-plate by a rod as indicated.

The exhaust valves are both opened and closed by the action of the wrist-plate and connecting rods. The steam valves are opened in this manner, but are closed by the suction of dash pots attached to the drop levers on the valve stems by means of vertical rods, as shown.