Chapter 22. -DIESEL AND GASOLINE ENGINES 



passing TDC during the latter part of an upstroke 

 (exhaust and the early part of a downstroke (in- 

 take). The intake and exhaust openings are both 

 open during this interval of time. The overlap 

 of intake and exhaust permits the air from the 

 blower to pass through the cylinder into the 

 exhaust manifold, cleaning out the exhaust gases 

 from the cylinder and, at the same time, cool- 

 ing the hot engine parts. 



Scavenging air must be so directed, when it 

 enters the cylinder of an engine, that the waste 

 gases are removed from the remote parts of 

 the cylinder. The two principal methods by 

 which this is accomplished are sometimes re- 

 ferred to as port scavenging and valve scaveng- 

 ing. Port scavenging may be of the direct (or 

 cross-flow) loop (or return), or uniflow type. 

 (See fig. 22-21.) 



An increase in air flow into cylinders of an 

 engine can be used to increase power output, 

 Ln addition to being used for scavenging. Since 

 the power of an engine is developed by the burn- 

 ing of fuel, an increase of power requires more 

 fuel; the increased fuel, in turn, requires more 

 air, since each pound of fuel requires a certain 

 amount of air for combustion. Supplying more air 

 to the combustion spaces that can be supplied 

 through the action of atmospheric pressure and 

 piston action (in 4-stroke cycle engines) or 



scavenging air (in 2-stroke cycle engines) is 

 called supercharging . 



In some 2-stroke cycle diesel engines, the 

 cylinders are supercharged during the air in- 

 take simply by increasing the amount and pres- 

 sure of scavenge air. The same blower is used 

 for supercharging and scavenging. Whereas 

 scavenging is accomplished by admitting air 

 under low pressure into the cylinder while the 

 exhaust valves or ports are open, supercharging 

 is done with the exhaust ports or valves closed. 

 This latter arrangement enables the blower to 

 force air under pressure into the cylinder and 

 thereby increase the amount of air available 

 for combustion. The increase in pressure re- 

 sulting from the compressing action of the 

 blower will depend upon the engine involved, 

 but it is usually low, ranging from 1 to 5 psi. 

 With this increase in pressure, and the amount 

 of air available for combustion, there is a cor- 

 responding increase in the air-fuel ratio and in 

 combustion efficiency within the cylinder. In 

 other words, a given size engine which is super- 

 charged can develop more power than the same 

 size engine which is not supercharged. 



Supercharging a 4-stroke cycle diesel engine 

 requires the addition of a blower to the intake 

 system since the operations of exhaust and in- 

 take in an unsupercharged engine are performed 



PORT DIRECT SCAVENGING 



VALVE UNIFLOW SCAVENGING 



UNIFLOW PORT SCAVENGING 



75.152 



Figure 22-21,— Methods of scavenging— diesel engines. 



567 



