Diesel Engine FundamentalsDOE-HDBK-1018/1-93DIESEL ENGINE SPEED,FUEL CONTROLS, AND PROTECTIONUnder these conditions, equal oil pressures are maintained on both sides of the bufferpiston and tension on the two buffer springs is equal. Also, the oil pressure is equal onboth sides of the receiving compensating land of the pilot valve plunger due to oil passingthrough the compensating needle valve. Thus, the hydraulic system is in balance, and theengine speed remains constant.When the engine load increases, the engine starts to slow down in speed. The reductionin engine speed will be sensed by the governor flyweights. The flyweights are forcedinward (by the spring), thus lowering the pilot valve plunger (again, due to the downwardspring force). Oil under pressure will be admitted under the servo-motor piston (topsideof the buffer piston) causing it to rise. This upward motion of the servo-motor piston willbe transmitted through the terminal lever to the fuel racks, thus increasing the amount offuel injected into the engine. The oil that forces the servo-motor piston upward alsoforces the buffer piston upward because the oil pressure on each side of the piston isunequal. This upward motion of the piston compresses the upper buffer spring andrelieves the pressure on the lower buffer spring.The oil cavities above and below the buffer piston are common to the receivingcompensating land on the pilot valve plunger. Because the higher pressure is below thecompensating land, the pilot valve plunger is forced upward, recentering the flyweightsand causing the control land of the pilot valve to close off the regulating port. Thus, theupward movement of the servo-motor piston stops when it has moved far enough to makethe necessary fuel correction.Oil passing through the compensating needle valve slowly equalizes the pressures aboveand below the buffer piston, thus allowing the buffer piston to return to the centerposition, which in turn equalizes the pressure above and below the receivingcompensating land. The pilot valve plunger then moves to its central position and theengine speed returns to its original setting because there is no longer any excessiveoutward force on the flyweights.The action of the flyweights and the hydraulic feedback mechanism produces stableengine operation by permitting the governor to move instantaneously in response to theload change and to make the necessary fuel adjustment to maintain the initial enginespeed.Rev. 0ME-01Page 37
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