244 THE POPULAR SCIENCE MONTHLY 



mixture of air and gas immediately after the explosion and it may be 

 from 1,200° to 1,700° C. on the absolute scale; and the temperature is 

 reduced by expansion to perhaps half this value. The temperature and 

 pressure of the steam which is supplied to a steam engine is seldom 

 higher than 190° C. (463° on the absolute scale) and about 175 pounds 

 per square inch, respectively. Any higher temperature and pressure 

 involves a great deal of danger in the boiler. The lowest condenser 

 temperature in steam engine practise is about 50° C. (323° on the 

 absolute scale). 



The sweeping processes which take place in a steam engine are 

 as follows : 



(a) Friction between the moving parts of the engine. This fric- 

 tion results in the immediate reconversion into heat of a portion of the 

 mechanical energy developed by the engine. 



(b) Wire drawing. If the pipes and passages traversed by the 

 steam through the boiler to the engine are small, the pressure in the 

 cylinder with open ports will be lower than boiler pressure, so that 

 the entering steam passes from a region of high pressure into a region 

 of low pressure. Also as the cut-off valve closes, steam will rush into 

 the cylinder through a narrowing aperture. This effect is called wire 

 drawing, and to provide against loss of efficiency from this cause the 

 pipes must be of ample size and the cut-off valve must operate very 

 quickly. 



(c) Eadiation. The cooling of pipes and cylinders by the giving 

 of heat to surrounding cooler bodies is a sweeping process, and is to 

 be obviated as much as possible by covering the pipes and the cylinder 

 with a thick coating of porous insulating material. 



(d) Cylinder condensation. As a charge of steam in the cylinder 

 expands it cools and cools the cylinder and piston, so that when steam 

 is next admitted it heats cylinder and piston up again and is itself 

 cooled. This effect can not be eliminated, but it can be largely 

 reduced by providing separate passages for the ingress and egress of 

 steam; and by using a series of cylinders of increasing size of which 

 the smallest cylinder takes steam directly from the boiler and exhausts 

 into the next large cylinder, which in turn exhausts into a still larger 

 cylinder, and so on. In this way the range of temperature in each 

 cylinder is small and the effects of cylinder condensation are greatly 

 reduced. A steam engine in which expansion of the steam takes 

 place in two stages (in two cylinders) is called a compound engine. 

 An engine in which the expansion of the steam takes place in three 

 stages (in three cylinders) is called a triple expansion engine. 



The loss of efficiency due to cylinder condensation is greatly reduced 

 by the use of superheated steam because the exchange of heat between 

 the steam and the cylinder walls is very greatly reduced when the steam 



