DIESEL ENGINE SPEED,DOE-HDBK-1018/1-93Diesel Engine FundamentalsFUEL CONTROLS, AND PROTECTIONGovernorDiesel engine speed is controlled solely by the amount of fuel injected into the engine by theinjectors. Because a diesel engine is not self-speed-limiting, it requires not only a means ofchanging engine speed (throttle control) but also a means of maintaining the desired speed. Thegovernor provides the engine with the feedback mechanism to change speed as needed and tomaintain a speed once reached. A governor is essentially a speed-sensitive device, designed to maintain a constant engine speedregardless of load variation. Since all governors used on diesel engines control engine speedthrough the regulation of the quantity of fuel delivered to the cylinders, these governors may beclassified as speed-regulating governors. As with the engines themselves there are many typesand variations of governors. In this module, only the common mechanical-hydraulic typegovernor will be reviewed.The major function of the governor is determined by the application of the engine. In an enginethat is required to come up and run at only a single speed regardless of load, the governor iscalled a constant-speed type governor. If the engine is manually controlled, or controlled by anoutside device with engine speed being controlled over a range, the governor is called a variable-speed type governor. If the engine governor is designed to keep the engine speed above aminimum and below a maximum, then the governor is a speed-limiting type. The last categoryof governor is the load limiting type. This type of governor limits fuel to ensure that the engineis not loaded above a specified limit. Note that many governors act to perform several of thesefunctions simultaneously.Operationof a GovernorThe following is an explanation of the operation of a constant speed, hydraulically compensatedgovernor using the Woodward brand governor as an example. The principles involved arecommon in any mechanical and hydraulic governor.The Woodward speed governor operates the diesel engine fuel racks to ensure a constant enginespeed is maintained at any load. The governor is a mechanical-hydraulic type governor andreceives its supply of oil from the engine lubricating system. This means that a loss of lube oilpressure will cut off the supply of oil to the governor and cause the governor to shut down theengine. This provides the engine with a built-in shutdown device to protect the engine in theevent of loss of lubricating oil pressure.ME-01Rev. 0Page 34
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