518 ENGINEERING ON THE FARM 



the gradual lowering of the quality of gasoline and the desire 

 of the engine builders to eliminate the intermittent thrust 

 upon the crank shaft and bearings. With this an impulse 

 is received regularly every cycle. The governor operates 

 by reducing the quantity of mixture or by reducing the 

 quality of the mixture — generally the first. The governing 

 is accomplished almost entirely by the use of a butterfly 

 valve attached to the centrifugal mechanism. 



BEARINGS 



The bearings used on a gasoline engine crank shaft are 

 usually of hard babbit. The better engines are equipped 

 with a very hard babbit with a bronze shell between the 

 babbit and the yoke. The function of the bronze is to 

 increase the extraction of heat developed in the babbit of the 

 bearing. Ball and roller bearings are sometimes used on the 

 ends of a crank shaft. All ball bearings are provided with 

 special hardened and ground raceways. 



COOLING SYSTEMS 



Temperature. The temperature developed in the combus- 

 tion chamber of a gas engine may run as high as 3000 F. 

 A great deal of this heat is absorbed by the walls of the 

 cylinder and the head of the piston. The temperature of igni- 

 tion of gasoline is about 900 F. This temperature exists 

 for a very small part of the working stroke, which makes 

 engine cooling a problem of much less seriousness than if it 

 were maintained for a longer period. The heat is taken 

 from the cylinder walls by circulation of air, water, or oil. 



Air cooling. Air cooling is the most simple, since it 

 involves only the provision of fins or lugs on the external 

 surface of the cylinder in order to increase the surface 

 exposed and powerful fans to increase the quantity of air 

 moved by the heated surface. A jacket surrounding the 

 cylinder conserves the air movement from the fan and 

 permits the removal of more heat than where the air is 

 blown or sucked by without the aid of this jacket. 



