THE INTERNAL-COMBUSTION TRACTOR 



THE third great source of power for direct traction 

 plowing is the internal-combustion tractor, which 

 differs from the steam tractor in the kind of fuel 

 used and the method of transforming the chemical 

 energy in that fuel into mechanical energy for useful work. 

 We have seen that in the steam engine the heat is applied 

 externally to a vessel containing some liquid medium, ordi- 

 narily water. The temperature of the water is thus raised above 

 the vaporizing point. The resulting gas is admitted to the 

 cylinder under pressure and performs work by its expansion 

 and cooling. The internal-combustion engine admits the 

 fuel directly to the working cylinder, where it is vaporized. 

 It is compressed to a density of four or more atmospheres and 

 ignited. The combustion is practically instantaneous; hence 

 the pressure rises with the force of an explosion behind the pis- 

 ton. The action may be compared to that of a gun, the piston 

 taking the place of the bullet, but constantly returning to be 

 acted upon again. The gases work directly upon the piston; 

 hence there is little chance for loss of heat by radiation, as is 

 the case at every step in the steam engine cycle, and much 

 greater efficiency is the result. 



The essential parts of the internal-combustion engine con- 

 sist of a cylinder and movable piston, with means for vapor- 

 izing the fuel and delivering it to the cylinder; for regulating 

 the power of the engine; for igniting the charge; for allowing 

 the charge to enter and the burned gases to escape; for cleaning 



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