All About Engines

320 All About Engines be remembered that the lower line represents the rise of pressure during the compression stroke. In the case of very high-speed petrol engines, such as are used on motor-cars and aeroplanes, an indicator diagram is very difficult to obtain. At 1,200 revolu- tions a minute only one-tenth of a second is avail- able for drawing the diagram, and the mechanism of the ordinary steam engine indicator is incapable of recording changes which occur with such rapidity. But a very beautiful instrument has been devised in which the changes of pressure are communicated to a tiny mirror, which reflects a spot of light upon a piece of sensitised paper fixed on a revolving drum. In this way the indicator diagram is photographed with far greater accuracy than is obtainable in a pencil drawing. It will be clear that the indicator cannot be used for turbines. Brake Horse-power The indicated horse-power, or i.h.p., as it is called, merely tells you at what rate the steam or explosive mixture is doing work in the cylinder, and gives no information as to the rate at which the engine will perform useful work outside. Obviously, this will be less than the i.h.p., because some of the work done by the working fluid, i.e. steam or explo- sive gases, will be absorbed in driving the engine itself. Thus work is required to start or stop or vary the speed of the reciprocating parts, to vary the speed of the rotating parts, and to overcome the