All About Engines

How a Modern Engine Works 15 the last one is called the “angle of advance,” so we get the rule: angle required to allow for lap + angle required to produce lead = angle of advance. Since the lap on the outside of the valve had to be allowed for in the angle of advance, why, it may be asked, is it put on the valve at all ? The reason will be clear if the reader considers what happens when the valve is moving backwards—that is, from right to left. The piston is now moving to the right, and the effect of the outside lap is to shut off steam before the piston has completed its stroke, and this saves steam. When water is converted into vapour, this vapour becomes more and more elastic as the temperature rises, and it is not the full force of the boiler pressure but the expansive force of the steam which is used in the engine. If only a small portion is admitted and then the admission port is closed, it will exert a force on the piston right up to the end of the stroke in its effort to expand. It is, in fact, usual to cut off at three-quarters, one-half, one-third, one-quarter, or even one-fifth stroke in order to utilise this expansive property to the full. Of course, the pressure and temperature both fall during expansion, so that the average force acting on the piston is less than that which would be produced by the boiler pressure acting throughout the whole stroke. For example, if steam is admitted at 150 lb. on the square inch and cut off at one-fifth stroke, it expands to five times its original volume, and the pressure falls to 30 lb. on the square inch. As we shall see better later, even this is wasteful, because steam