42O FARM MOTORS 



563. Dynamos and magnetos. Since the battery is ex- 

 pensive and short lived, other provisions are made for 

 supplying electric currents. One of the most satisfactory 

 of these is by connecting the engine to a form of magneto 

 or dynamo. The amount of power needed to drive a 

 dynamo is exceedingly small, but at all times sufficient 

 current is provided to give reliable ignition. A magneto 

 differs from a dynamo in that the pole pieces of the mag- 

 netos are made of permanent magnets, while those of the 

 dynamo are electromagnets. 



It is often easier to start an engine with a magneto 

 than with a dynamo. However, after speed is reached, 

 the dynamo, as a rule, is a little more satisfactory than 

 the magneto. These small dynamos are usually provided 

 with a self-governing device which will regulate the 

 speed and in this way obtain the proper voltage foi 

 ignition. 



564. Cooling of gasoline engines. There are three 

 methods of carrying the excess heat away from the gaso- 

 line engine cylinder, namely: (i) air cooled; (2) water 

 cooled ; and (3) oil cooled. 



The air-cooled engine (Fig. 310) is provided with ribs 

 or flanges extending from the cylinder, which gives up a 

 certain amount of heat to the air. This may be assisted 

 by a draft of air blown upon the cylinder by a fan, bring- 

 ing more air in contact with the flanges. Air-cooled en- 

 gines are necessarily of small units, but where the engine 

 is small and exposed to freezing weather it is preferable 

 to any other. 



Water-cooled engines are the type in most general use, 

 and water is perhaps the best means of carrying the ex- 

 cess heat from the cylinder. There are three general 

 plans in use for cooling with water. One is to have a 

 large tank sitting near and connected to the engine (Fig. 



