STEAM ENGINES 369 



steam engine goes into useful work. In multiple-expand- 

 ing steam engines this percentage is often raised as high 

 as 20. The rest of the energy is lost by radiation, con- 

 densation in the cylinder, and the amount carried away 

 to exhaust. The temperature of the walls of the cylinder 

 rises and falls as live steam enters and expands to the 

 pressure of exhaust; in other words, the cylinder walls 

 have practically the same temperature as the exhaust 

 steam, so when the live steam enters it heats the walls 

 to a temperature nearly equal its own. This then is the 

 loss due to radiation. As the steam expands in the cylin- 

 der there is a great deal of it which condenses. Due to 

 this condensation, the latent heat of the steam is thrown 

 off, doing no work. Not only is all the heat left in that 

 part of the steam which entered the cylinder to fill the 

 piston displacement lost when release takes place, but 

 about one-third of the steam which enters the clearance 

 space is a total loss. Hence the smaller the clearance 

 volume the more economical the engine. 



513. Slide valve. The slide valve is the most common 

 method for regulating the admission of steam to and ex- 

 haust of the steam from a steam engine cylinder. Its 

 functions are: (i) admission of the steam to the cylinder 

 to give the piston an impulse ; (2) to cut off the supply 

 of steam at the proper point; (3) to open a passage for 

 the escape or exhaust of the steam from the cylinder; 

 (4) to close the exhaust port at the proper time to 

 retain enough steam in the cylinder to give the piston a 

 cushion. 



514. Lap of valve. When the valve is in mid position 

 (Fig. 247) the amount it laps over the edges of the steam 

 port is known as lap. The amount which the valve laps 

 over the outside is outside lap, and that which it laps 

 over the inside is inside lap. 



