DIESEL ENGINESDOE-HDBK-1018/1-93Diesel Engine Fundamentalsengine. When the piston travels below the level of the ports, the ports are "opened" andfresh air or exhaust gasses are able to enter or leave, depending on the type of port.The ports are then "closed" when theFigure 8 Diesel Engine Valvepiston travels back above the level ofthe ports. Valves(refer to figure 8)are mechanically opened and closed toadmit or exhaust the gasses as needed.The valves are located in the headcasting of the engine. The point atwhich the valve seals against the headis called the valve seat. Mostmedium-sized diesels have eitherintake ports or exhaust valves or bothintake and exhaust valves. TimingGears,Camshaft,andValveMechanismIn order for a diesel engine tooperate, all of its components mustperform their functions at very precise intervals in relation to the motion of the piston.To accomplish this, a component called a camshaft is used. Figure 9 illustrates acamshaft and camshaft drive gear. Figure 2 and Figure 3 illustrate the location of acamshaft in a large overhead cam diesel engine.Figure 9 Diesel Engine Camshaft and Drive GearA camshaft is a longbar with egg-shapedeccentric lobes, onelobe for each valve andfuel injector (discussedlater). Each lobe has afollower as shown onFigure 10. As thecamshaft is rotated, thefollower is forced upand down as it followsthe profile of the camlobe. The followers areconnected to theengine's valves and fueli n j e c t o r s t h r o u g hvarious types oflinkages called pushrodsand rocker arms. TheME-01Rev. 0Page 10
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