GAS ENGINES 



5ii 



pressure in the cylinder. The negative pressure under which 

 the new charge is to be drawn into the cylinder must be 

 neutralized if the largest possible quantity of mixture is 

 to be secured. This is the reason that 

 the intake is left open to 570. • 



In a four-stroke cycle engine a power 

 impulse is delivered every fourth stroke, 

 and the motion of the engine must be 

 maintained by the momentum of the 

 flywheel covering the period of the other 

 three strokes (Fig. 405). When a more 

 even speed or more even torque (turning 

 motion on the crank shaft) is desired, it is 

 the practice to have more cylinders at- 

 tached to the same crank shaft. Each th^ G operation °of m a 

 one of the additional cyclinders has its two-stroke . cycle en- 



J gme. Fhts indicates 



own cycle and operates entirely inde- ihe overlapping of the 



1 , n .1 .1 1' i rrvt exhaust on the intake 



pendent 01 the other cylinders. The in the middle of the 

 smoothness of operation is most effectively tw °- stroke <** 

 produced by so setting the cycles of additional cylinders 

 that there is an even overlapping of one cycle onto the 

 next one. 



In a two-cylinder engine (Fig. 406),- with the impulse 

 evenly distributed, there is one power stroke 

 t for each half of the 720 , or 360 — one rev- 

 ^ olution. Two-cylinder crank shafts are 

 made in two forms, with the cranks 180 and 

 360 apart. Both types are used in both the 

 twin and the opposed engines. The crank 



Fig. 406. The , . rsr n 



simplest method of shait with the throws 180 apart is the 

 Uono^Shestrokl's standard used in two-cylinder opposed engines 

 in a four-stroke because the shaft is balanced naturally and 



cycle . J 



it is possible to get the impulse at regular 

 intervals of 360 of crank-shaft movement. Using a crank- 

 shaft with the throws 360 apart, it is necessary to use two 

 cylinders side by side in order to get regular impulse. The 



£ 



