STEAM ENGINES 191 



HORSEPOWER AND STEAM CONSUMPTION 



Indicated Hors'epower. The indicator furnishes the most 

 ready method of measuring the pressures on the piston of a 

 steam engine and, in consequence, of determining the amount 

 of work done in the cylinder and the corresponding horsepower. 

 The power measured by the use of the indicator is called the 

 indicated horsepower. It is the total power developed by the 

 action of the net pressures of the steam on the two sides of the 

 moving piston. The indicated horsepower is generally repre- 

 sented by the initials I. H. P. 



Friction Horsepower. The part of the indicated horsepower 

 that is absorbed in overcoming the frictional resistances of 

 the moving parts of the engine is termed the friction horse- 

 power. If the engine is running light, or with no load, all the 

 power developed in the cylinder is absorbed in keeping the 

 engine in motion, and the friction 'horsepower is equal to 

 the indicated horsepower. This principle furnishes a simple 

 approximate method of finding the friction horsepower of a 

 given engine; as, however, the friction between the surfaces 

 increases with the pressure, the power absorbed in over- 

 coming the engine will be greater as the load on the engine is 

 increased. 



Net Horsepower. The difference between the indicated 

 horsepower and the friction horsepower is the net horsepower. 

 It is the power that the engine delivers through the flywheel 

 or shaft to the belt or the machine driven by it, and is sometimes 

 called the delivered horsepower. Since the power that an engine 

 is capable of delivering when working under certain conditions 

 is often measured by a device known as a Prony brake, the net 

 horsepower is frequently called the brake horsepower, abbrevi- 

 ated B. H. P. 



Mechanical Efficiency. The mechanical efficiency of an 

 engine is the ratio of the net horsepower to the indicated horse- 

 power; or it is the percentage of the mechanical energy devel- 

 oped in the cylinder that is utilized in doing useful work. To 

 find the efficiency of an engine, when the indicated and net 

 horsepowers are known, divide the net horsepower by the 

 indicated horsepower. 



