580 MACHINES, WORK, AND ENERGY 



require different amounts of compression to produce the 

 largest amount of power. In practice, the different gases 

 are compressed about as follows: 



Kerosene, compressed to about y to H of its original volume, 

 Gasoline, compressed to about Y$ to ^ of its original volume, 

 Alcohol, compressed to about Ho to Hs f its original volume, 

 Natural gas, compressed to about M to Ho of its original volume, 

 Coal gas, compressed to about % to Ho of its original volume, 

 Producer gas, compressed to about Ho to H 5 of its original volume 



697. Keeping the Cylinder Cool. We saw that Watt, 

 when developing the steam engine, adopted the motto, 

 "Keep the cylinder as hot as possible." With gas engines, 

 however, the danger is that the cylinder will get too hot 

 hot enough to ignite the gas too soon. The cylinders of 

 small gas engines are sometimes cooled by air, a fan forcing 

 the air past the cylinder. Such engines are called AIR-COOLED 

 ENGINES. All large gas engines are WATER-COOLED ENGINES. 

 The cylinders of water-cooled engines are surrounded by 

 jackets similar to the jackets Watt used on his steam engines. 

 But in this case the jacket contains flowing water to keep the 

 cylinder cool instead of steam to keep the cylinder hot. 



698. Need of the Heavy Flywheel. We have seen that 

 in the four-cycle engine, force is being exerted upon the 

 piston but one-fourth of the time, i.e., during the third stroke. 

 But the engine must continue to do work all the time. We 

 should therefore expect the engine to run with an unsteady 

 motion. It would do so were it not for the heavy flywheels. 

 The heavy flywheels absorb a large amount of energy dur- 

 ing the working stroke and give it up again during the other 

 three strokes. Being massive they have great capacity for 

 holding energy and therefore vary but little in speed during 

 the cycle. 



699. Many-cylindered Engines on Automobiles. It is 

 undesirable to load an automobile with heavy flywheels, 

 although a steady motion is very desirable. For this reason 

 nearly all gasoline engines used on cars are constructed with 



