Q. When is this likely to occur?

A. In cutting off very short where there is much midgear lead or inside lap, or both.

Q. How much compression should an engine have in order to make her pass her centers?

A. That depends on the amount of clearance. There should be enough to fill the clearance space at stroke end at boiler pressure. With 15 to 18 per cent clearance you must have high compression.

Q. As regards clearance space and compression, what is the essential difference between locomotive and stationary engine practice?

A. In the stationary engine the exhaust passages are of fixed dimensions, while in the locomotive they are likely to be clogged up by gum from the fuel, or have to be changed in size to accommodate some new fuel; and often if the exhaust nozzle could be kept of uniform size, the back pressure could not be so regulated as to give

constant volume of steam in the cylinder with the valve closed for compression.

Q. How is over-compression best obviated?

A. By making the ports long, giving plenty of valve travel and outside lap, and some negative inside lap (inside clearance).

Q. What is gained by the increased travel and outside lap?

A. Good port opening at short cut-offs; later compression.

Q. What by the inside clearance?

A. Delayed exhaust closure, hence reduced compression.

Q. In compounds, which cylinder needs the most inside clearance?

A. The H.P.

Q. Is the beneficial effect of inside clearance greater with slow or with high speeds?

A. With slow.

Q. Can inside clearance be a source of waste?

A. Yes, on very slow-speed engines.

Q. Does inside clearance affect the compression after admission?

A. No.

Q. Has it the same value in freight as in passenger service, on the same engine?

A. No; less. It is, as a rule, of advantage only on high-speed engines.

Q. What is the principal cause of back pressure?

A. The exhaust nozzle, not the valve.

Q. What will back pressure average in non-compounds?

A. About four pounds per square inch.

Q. Can a table be made grouping the effects, upon each of the elements: admission, cut-off, release, and compression of increase in angular advance, valve travel, inside lap and outside lap?

A. Yes; such a table is here given:

Q. What is the most economical point of cut-off?

A. The earliest at which the condensation and reevaporation in the cylinder and other causes, such as wire drawing, do not neutralize the gain by expansion. Q. How can this point be found?

A. Only by actual experiment with each engine and each set of conditions of train, grade, piston speed, boiler pressure, external temperature, wind, etc.

CHAPTER XLIX

THE SLIDE VALVE

Q. By what means is the steam admitted to and allowed to escape from cylinders?

A. By a device called a slide valve, playing parallel with the cylinder length on a ported seat, driven by suitable mechanism, and controlling steam admission to and exhaust from both ends of the cylinder.

Q. What are the functions of a slide valve?

A. To let steam into only one end of the cylinder at a time; to cut off the supply at a certain point of the stroke; to let it escape from one cylinder end into the exhaust pipes as soon as it commences to enter the other (in some cases to close the exhaust before all the waste steam has been released); to prevent steam passing directly from the entrance ports into the exhaust passages.

Q. When and by whom was the slide valve invented? A. Toward the end of the eighteenth century, by Matthew Murray, of Leeds, but in a crude form.

Q. In what forms does the slide valve exist?

A. In two principal types; the plain short D and the piston valve.

Q. Who invented the simple long D slide?

A. Murdock, Watt's assistant.

Q. What are the principal parts of the plain slide valve?

A. The body or arch, and the legs.

Q. What is the most simple and usual slide valve used for an American standard locomotive?

A. The valve consists in effect of a plate or block, as shown in section Fig. 128, having in its under surface a cavity which extends at right angles to the direction of the valve, and parallel with the ports in the valve-seat.

Crosswise projections from the top of the valve enable the valve-rod to be attached either by screws and nuts or by a collar or frame surrounding the projections, in such a manner that the valve is free to change its position with respect to the valve-rod, as its face and that of its seat wear away.

Q. Describe the seat upon which this type of plain slide valve or short D valve is placed?

A. As shown in Fig. 149, and in Figs. 150 and 151, it consists of a plain surface having three narrow ports, with parallel edges, all of which are at right angles to the direction of motion of the piston and of the valve. The central (and larger) one communicates only with the exhaust-passage, the end ones with the cylinder, at the counterbore, each serving alternately for admission and exhaust for its own cylinder end. There are usually shoulders at each end, so that the valve may in its travel extend beyond them, instead of cutting away material and wearing a low place in the seat.

Q. What would be the effect of omitting the shoulders in the seat?

A. If the valve were given short travel and wore itself a low place in the seat, there would, if the travel was increased or the valve adjusted so as to be brought nearer to or further from the crosshead end of the cylinder, be either a smash-up, or a leak between the steam-chest and cylinder.

Q. Why are the bridges made narrow?

A. To decrease friction, hence reduce the force needed to move the valve.

Q. What is the usual bridge width in American engines?

A. From 15/16 to 11/4 inch.

Q. What about the length of the valve seat for ordinary D valves?

A. Generally enough to leave the width of one bridge at each end with greatest valve travel-unless this would