Question

(a) With the aid of a block diagram describe the operation of an electronic governor fitted to an auxiliary diesel engine. (8)

(b) Engines fitted with an electronic governor may behave erratically during load changes. Explain the possible causes. (8)

Answer 1

Part (a)

The block diagram shows the general arrangement. An Electronic governor provides engine speed adjustment from no-load condition to full load. It consists of a Controller, an Electro-Magnetic Pickup (MPU) and an actuator to carry out the necessary speed control and regulation. The speed sensor consists of a set of gear teeth that rotate at engine speed and a Magnetic Pick Up (MPU) that has a slight air gap. The MPU has a permanent magnet with a pole piece surrounded by a coil, the MPU is installed above the flywheel teeth and depending upon its distance from the gear teeth or slot, the magnetic field of the MPU varies from a maximum to a minimum respectively. The permanent magnet creates its own magnetic field. During running as each ferrous gear tooth passes the core, the reluctance path is decreased and the flux lines increase. The change in flux lines produces an ac sine wave voltage in the coil the frequency of which represents the engine speed.

The AC voltage is amplified and rectified to a DC voltage which is proportional to the engine RPM. This DC voltage is compared with the desired set voltage at the controller (corresponding to the desired RPM) and an appropriate electric signal is sent to an electro-hydraulic converter. The electro-hydraulic converter processes the signal and operates an actuator (hydraulic cylinder and piston) to increase or reduce the fuel supply as required. An actuator position feedback to the controller is provided as shown.

The electronic controller has different modes of operation to implement various functions. These include;

Detecting the starting of an engine and subsequently directing the fuel supply.
Suppressing the smoke generated by the engine as its speed increases.
Adjusting the droop percentage.
Remote speed control.
Idle speed operation: It provides fixed speed control over the entire torque capacity of the engine.
Maximum speed control: It is used to eliminate over speeding of the engine.

Part (b)

The problem may be with the governor or prime mover. To ascertain which, the governor actuator may be disconnected from the fuel pump control and the control lever held manually firm at a position that will maintain the required RPM. This should be done carefully when the ship is in open waters so that sudden tripping of the alternator due to low voltage does not expose the ship to a hazardous situation.

The electric load may be increased slightly when the RPM will drop which may be restored by operating the control lever manually. Next, the electric load is reduced slightly when the RPM will rise which again may be restored manually. If the operation is still erratic, the problem is with the engine. If the operation is normal then the problem is with the governor.

If the problem is with the engine, the following may be the causes:

Fuel pump racks sticking
Air lock in the fuel system
Water in fuel
Fuel pump plungers occasionally sticking

If the problem is with the governor, the erratic operation may be due to following causes:

Actuator linkage sticking
The Magnetic Pick Up unit (MPU) not adjusted properly, slack and moving thus the air gap varying
Defective MPU
Governor not adjusted properly; too high a gain may cause hunting; gain should be reduced in such case
Loose electric connection
Other problems in the electronic circuitry, PCB

✓ Verified Model Answer (Text Solution)