GAS ENGINES 



361 



two, three, four, six, and sometimes eight cylinders. The six- 

 cylinder engines are the most popular for touring cars. 



407. Operation of Engine. The operation of a four-cycle 

 machine may be understood by studying the four different 

 steps in the working of the engine. 

 There are two openings or valves in the 

 cylinder an inlet valve for the mixture 

 to enter, and an exhaust valve for the 

 disposal of the gases. When the piston 

 is at its highest position, the valves are 

 closed. As soon as the engine is run- 

 ning, the motion of the fly-wheel carries 

 the piston down, and the partial vacuum 

 created behind causes the inlet valve to 

 open because the outside atmospheric 

 pressure is greater than the inside pres- 

 sure. Many up-to-date engines have a 

 mechanical inlet, and do not depend 

 upon atmospheric pressure to open the 

 inlet valve. The explosive mixture of 

 air and gas enters and fills the cylinder. 

 The momentum of the fly-wheel is suffi- 

 cient to keep the piston moving. The 

 greatest power is derived from an engine when the gas 

 explodes just before the piston reaches the highest point, 

 because the speed of the piston makes it necessary to 

 ignite the gas at the top of the stroke in order to have 

 complete combustion. The spark-plug, screwed into the 

 opening, gives off a spark which explodes the mixture.. As 

 the piston rises again, the exhaust valve opens mechani- 

 cally and the burnt gases, still very hot, escape through the 



FIG. 177. The Cylin- 

 der and Water 

 Jacket (in section) 

 ofaGasEngine. De- 

 signed to prevent 

 premature ignition 

 of the mixture and 

 to assist in lubricat- 

 ing the piston. The 

 water surrounds 

 the cylinder in the 

 section marked 

 "Water." 



