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The Cause Of Overspeed In A Diesel Generator

Apr. 25, 2019

Diesel generator sets are mainly used in power supply of various prime, standby and emergency power sources, also can be used in electric power supply for electric propulsion system. Its importance is self-evident. When we use diesel generator set, it maybe a overspeed problem, this is a special fault, it will not only cause serious damage to spare parts, but also may endanger personal safety. What is the cause of overspeed and how to resolve the problem?


1. Fault phenomenon of diesel generator overspeed


Under normal operation, the engine speed suddenly rises, so that the engine speeds up beyond the rated speed and loses control, and emits a huge roar, the exhaust pipe emits a lot of black smoke or blue smoke.


2. Cause of failure


The fundamental cause of diesel engine overspeed is that diesel engine loses or changes its speed regulation characteristics. Faults of fuel injection pump and governor, such as stagnation or looseness of relevant components, will cause excessive oil supply and loss of normal speed regulation characteristics, while the combustion chamber entering additional diesel or oil will change the speed regulation characteristics of diesel engine.


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1) Faults of injection pump and speed regulator


A. When assembling and repairing, impact the plunger, oil outlet valve seat of fuel injection pump, too much tightening torque will cause the deformation of plunger sleeve, impurities entering into the clearance of plunger pair, too thin gasket on the positioning bolt of plunger sleeve or too long bolt, which will cause the plunger of fuel injection pump to rotate inadequately or be stuck in the position of large oil supply;


B. Loosening of fixing screw for adjusting gear ring of fuel injection pump plunger which causes injection pump plunger loss control;


C. The ball head of the plunger adjusting arm or the gear rod adjusting arm of the fuel injection pump does not enter the adjusting fork groove, so that the plunger is in the position of large oil supply;


D. Accelerating pedal pull rod or oil supply adjusting gear rod and the movable part of governor are stuck, and oil quantity adjusting gear rod and speed adjusting pull rod are out of joint;


E. The excessive lubrication oil, too viscous or too dirty in the governor cause the weight of the governor can not be thrown out in time when the speed rises, which causes the pull rod or the gear rod to lose control of the governor;


F. Full speed governor can not be thrown away because of skew of flying ball seat or abrasion of inclined sliding groove of thrust plate transmission plate, which increases the sliding resistance of flying ball;


G. The springs of injectors are too weak or broken, which results in too low injection pressure and excessive fuel consumption in cylinders;


H. Flying ball Pin Shaft of governor broken off and flying ball flying off.


The above reasons make the speed of diesel engine increase, while the "speed characteristic" of plunger fuel injection pump increases the fuel supply with the increase of the speed, which leads to the vicious circle between the speed and the fuel supply, resulting in the occurrence of overspeed accidents.


2) Additional fuel enters the combustion chamber


A. Engine oil level too high causes excessive oil to rush into the cylinder then enter the combustion chamber to burn;


B. Serious wear of piston rings and excessive clearance of cylinder wall lead to a large amount of oil entering combustion chamber to burn;


C. Excessive gap between air door and its catheter coordination is too large, which cases oil for valve distribution chamber enter into combustion chamber to burn;


D. After cleaned air cleaner, the excessive residual gasoline makes the higher concentration gasoline vapor enter the combustion chamber to burn;


E. A diesel engine with a supercharger enters the combustion chamber to burn because of the damage to the oil seal of the supercharger;


F. The solenoid valve of the cryogenic starter leaks oil, which causes excess diesel oil to enter the combustion chamber to burn;

G. Horizontal diesel engine inclines seriously, causing oil to flow into the valve chamber and be sucked into the combustion chamber to burn.


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3. Solution to overspeed fault of diesel generator set


1) Forced stoppage

Once the diesel engine has overspeed, the following measures should be taken promptly to force the diesel engine to stop immediately

A. Quickly retrieve the accelerating pedal to the stop position;


B. For fuel injection pumps with exposed rods or toothed rods, the rods should be quickly pulled back to the stop position;


C. For diesel engines equipped with decompression devices, the decompression handle should be quickly pulled to the decompression position, so that the diesel engine shuts down because of the uncompressed gas;


D. If a closing device is installed on the intake pipe, the extinguishing device shall be closed immediately or the intake port of the air filter shall be blocked quickly with cloth, and the fuel tank switch shall be closed to cut off the gas and oil circuit, stop the oil supply and gas supply, and cause the cylinder to be suffocated and shut down;


E. Put the transmission into the high gear in time, while trampling the brake pedal to the bottom, and slowly lift the clutch pedal, so that the engine is forced to extinguish.


2)After the generator set stop working, to make a definite diagnose


A. If the acceleration pedal is raised rapidly, the engine will not continue to rise after reaching the maximum speed, the fault is the acceleration pedal rod or arm is jammed;


B. If the acceleration pedal is raised rapidly, the engine speed can not be reduced or extinguished immediately, usually the governor is out of order;


C. If the acceleration pedal is picked up, the engine speed continues to rise, indicating that the fuel injection pump has a fault;

D. If the fuel supply system is good, it means that there is extra fuel or oil entering the cylinder.

All above problems should be maintained by professional personnel. 


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