SOME COMMON MECHANICAL MOTORS 



579 



FIG. 369. Fourth 

 cycle, diagram. Ex- 

 haust stroke. 



opened and as the pis- 

 ton returns to the 

 closed end of the cylin- 

 der it forces the prod- 

 ucts of combustion 

 out through the ex- 

 haust port (Figs. 369 

 and 370). 



The four strokes are 

 (1) suction stroke, (2) 

 compression stroke, 



(3) working stroke, 



(4) exhaust stroke. 

 These four strokes 

 constitute one com- 



plete Cycle Or round of cycle, 



37 o._Fourth 



engine. Ex- 



haust stroke. 



action. 



694. Purpose of Compression. The purpose of compressing 

 the gas before igniting it is to secure the most rapid com- 

 bustion possible. The gas in the cylinder is much like brush 

 in a brush heap. It we wish the brush to burn rapidly we 

 must tramp it down into a compact mass. The more compact 

 the brush, the more rapidly it burns and the hotter the fire. 

 In like manner the compression of the gas in the cylinder 

 produces more rapid combustion, and hence a higher tem- 

 perature. Greater pressure on the piston results. 



695. Compression must not be too Great. Whenever a 

 gas is compressed heat is produced. The pump becomes 

 hot when we "pump up" a bicycle or automobile tire. In 

 compressing the gas in the cylinder of a gas engine, care must 

 be taken that the temperature of the gas is not raised to the 

 kindling temperature (see Art. 95) before the end of the 

 compression stroke. What would be the result if the gas 

 were ignited during the" second stroke? How would this 

 affect the power of the engine? 



696. Compression for Different Gases. Different fuels 



