Over Speed Trip in Diesel Engines & Types of Over Speed Trips

An over speed trip is a safety feature provided on the diesel engine of the ship to restrict uncontrolled acceleration of the engine, leading to mechanical failure or untoward accidents. In order to prevent the speed of a diesel engine to go beyond the pre-set speed range, an over speed trip is used in the diesel engines.

How Over Speeding can be Harmful?

A Diesel engine is designed for the mechanical stress associated with the centripetal and centrifugal forces of the moving parts inside it in a specified operational range. Centripetal force is directly proportional to the square of the rotational speed, stress increases rapidly with increase in speed. Mechanical connection strength can be overcome by the exceeding stresses due to the increase in operational speed. These can result in breaking of rotating parts or damage to the machinery itself. Over speed is thus a serious safety hazard and can lead to a fatal situation.

ship engine
Representation Image

What does Over Speed Trip Does?

Due to sudden changes in the load on the diesel engine, the speed of the engine may vary. Though a governor is provided to control the speed of the diesel engine, the speed might go out of control, damaging the engine. Thus, for this reason over speed trips are used.

No matter what type of the over speed trip the engine uses, the main aim of the over speed trip is to cut the fuel supply to the engine cylinders in case the engine speed rises above a specific level.

Preventing Over Speeding of Engine

Reducing the likelihood of an uncontrolled and catastrophic over speed is essential and can be done by two methods:

a)      Mechanical over speed trip

b)      Electronic over speed trip

In this article we will have a look at the electronic over speed trip.

Electronic Over Speed Trip

To understand the electronic over speed trip, a normal lay out of the system is described below. The electronic over speed trip consists of

a) Fly wheel mounted speed sensor

Magnetic speed sensor is preferred in generator engines. Due to the discontinuity of actuator surface (gear tooth of flywheel) voltage is excited in the pick off coil of sensor, producing an electric analog wave. This cyclic wave created by the flywheel is read by the sensor.

b) Signal condition unit

This unit act as a receiver to the speed sensor. Basic function of the signal conditioner is to convert one type of electronic signal which may be difficult to read into another type into a more easily read format. This can be achieved by amplification, excitation and linearization of an electrical signal.

c) Detection and Comparison unit

There is a set value which is normally 10 % above the rated speed and acts as base value for this unit. Signal condition unit output is continuously detected and compared with the set value.

d) Trip signal unit

If the difference between the set value and detected value is above the limit, then this unit gives a trip signal which in turn shuts down the generator.



  1. hello sir, i want to know , how main engine overspeed trip is tested & what is the time duration of testing

  2. Dear Pundari,

    It is better to test the functioning of the over speed trip rather testing it at its real set speed. The set point is lowered then the required speed. once the generator reaches this lowered set point, it will trip. To run the generator in the actual overspeed condition is dangerous and long run in such speed can lead to problem in the bottom end bolts of the con rod.

  3. anish, good day sir. im working in a ship, our aux generator is running but its not getting a load. the generator is running fast in 5sec then it will go slowly, what is the problem of this. tnx

  4. Dear Sir,

    With thanks for all above information , kindly note recently we faced with aux engine over speed on one o our fleet vessel an led to total failure , explosion and fire in engine room.

    Same aux engine was repaired/overhauled just 20 days back of accident in repair yards.
    Over speed trip were not activated and cause explosion.

    Kindly advise possible factors which to be consider for such big accident.

    With many thanks in advance.

  5. i am working on CAT engine its many time show over speed alarm i change relay but the problem still there let me know what is the main problem and i can diagnose i

  6. Its good to finally find a site like this. THUMBS UP!!!

    Please assist me with this issue, my 5 cylinder Sulzer Harbor Generator is starting, only to be tripped by the over speed device when attempting to attain its rated RPM of 760. I suggested that the UG 8 governor might be faulty. Please what seems to b the problem?

  7. Please can you kindly help me, there is a cummins gen, and it has been tripping on over speed. Please can you kindly assist/give me a better idear to sort out this problem..

  8. Please Sir, I need possible remedy or solutions to overspeed shutdown of Cummins power generator.What are the curses of that over speed?

  9. Reasons have already been listed in the article. Overspeeding of engine is not healthy for rotational part of the engine (bearing housing bolts, crankshaft and its bolts etc.)

  10. A nice site, I’m working on CT6 with overspeed issue… Have adjusted the Woodward the problem still persist, what really the possible causes?

  11. You can contact any time related with all type of Woodward governing system and caterpillar engines related Quires, My contact Number is +91 9987396573

  12. @Alan: in Many engine circuit, a test knob is provided. You can use that to test the overspeed trip.

  13. 8 DK28 daihatsu generator engine .how to try out overspeed trip.i have a sensor in in flywheel so it may be a electronic type .how to carry outtests can somebody help sharing information?

  14. @Benasher: Usually, the value of the speed sensor controller is reduced so that as the generator starts and reeatrip will operatoe ched the speed, the

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