212 GENERAL SCIENCE 



and imparts its energy to the heavy fly wheel, which is 

 set in rapid motion. In No. 4 as the motion of the 

 fly wheel now drives the piston to the left, the exhaust 

 valve, D, is automatically opened, and the spent gas 

 escapes, making the characteristic noise of the gas engine. 

 The next movement of the piston is the same as in No. 

 i, and the same cycle of motions is repeated. 



143. Efficiency of Engines. The mechanical effi- 

 ciency of the gas engine is the highest of all heat engines, 

 becoming as much as 25 per cent, which is nearly twice 

 that of ordinary steam engines. The gas engine is free 

 from smoke (the smoke made by some automobiles is 

 due to the oxidation of an excess of lubricating oil in the 

 cylinders) and can be started without delay, but the fuel 

 is comparatively expensive. 



The efficiency of the best steam engines is about 17 

 per cent, while the efficiency of locomotives is about 

 8 per cent, that is, only 8 per cent of the heat energy of 

 the coal which is burned is transformed into mechanical 

 energy by the locomotive while it is running. The effi- 

 ciency of steam turbines is as high as that of the best 

 double-acting engines. The advantages of the turbines 

 are that they run smoothly, can- develop high speed, 

 and occupy only about one-tenth as much floor space as 

 ordinary engines of the same power. 



QUESTIONS AND EXERCISES 



1. Can steam be seen ? Is it heavier or lighter than ait ? 



2. Who invented the steam engine ? When ? 



3. Name the kinds of steam engines that you have seen. 



4. What is the difference between a gas engine and a steam 

 engine ? 



5. Where is the fire in a gas engine ? 



6. How does the gas engine get its power to run ? 



