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Governor

Due to higher gas pressures in the cylinder of a CI engine, components are made stronger, than those used in a

SI engine, by increasing their dimensions, which of course increases weight. This leads to engine damage if the

speed exceeds as given value, depending upon the strength / weight ratio of the components.

The injection pump has a rising output characteristic that means for a given control rod setting the output

increases with speed and compression pressures, which are practically the same at idling as well as at maximum

speed, making it difficult to obtain a steady idling speed and therefore causing the engine either to race or to

stall.

To control the engine at idling speed and to limit the maximum speed are the two main duties performed by the

governor. If the governor attends to these two duties only, it is known as an idling and maximum speed

governor, i.e. two speed governors. If a unit regulates throughout the speed range is termed an all speed

governor. In some instances, a torque control requirement may be added; most frequently to provide torque

back up, by arranging for the torque to increase as the speed falls from the maximum to a preset level. However,

torque control may also be applied at the other end of the speed range.

The general functions of a governor are as follows: (a) To supply extra fuel for starting the engine at various temperatures experienced during service.

(b) To maintain the idling speed constant at a predetermined rpm regardless of temperature and load.

(c) To prevent the engine from over­speeding if the accelerator is depressed suddenly at light load or the load is

suddenly reduced.

id) An all­speed governor maintains, within pre­set limits for full and zero loads, a constant speed appropriate to

the accelerator pedal angle, regardless of variations in load.

Three main types of governor used for in­line pumps are mechanical, pneumatic, and hydraulic.

10.10.1.

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Mechanical Governors

An idling and maximum speed governor, shown diagrammatically in Fig. 10.17, is commonly used on heavy

vehicle engines. It is mounted at the end of the injector pump, and contains two weights, rotated by the pump

camshaft. A force in an inward direction in exerted by the springs on the weights. The bell cranks link each

weight to the bottom end of a floating lever. The top end of the lever is connected to the control rod. The centre

of the lever is mounted on an accentric, which is rotated by the accelerator pedal.

The governor in the engine­stationary position is shown in Fig. 10.17A, and in this condition the weights are

fully retracted, so that the control rod is held in the maximum fuel position required for starting the engine. Once

the engine is started, rotation of the weights causes the centrifugal effect to overcome the spring force and

the weights then take thi position (Fig. 10.17B) where the control rod is withdrawn to the idling setting. In this

position the weights are controlled only by the outer (weaker) springs, permitting sensitive control.

Considering the accelerator pedal stationery, any increase in speed causes a slight outward movement of the

weights so that the control rod is moved in the direction to reduce the fuel delivery. Similarly, if the weight are

moved in the inward direction, the control rod increases the fuel delivery to prevent stalling of the engine.

Between idling and maximum speeds, the weights maintain the same position, i.e. appear to be locked together.

During this period, downward movement of the accelerator pedal rotates the eccentric and moves the control rod

in a direction to increase the fuel delivery. At the maximum speed, in the range of 1800­2000 rpm, the high

centrifugal force acting on the weights overcomes the strong force of outer springs causing the outward

movement of the weights (Fig. 10.17C). Consequently, the control rod decreases the quantity of fuel delivered,

and thereby reduces the engine power, irrespective of the position of the accelerator pedal.

A small quantity of engine oil, stored in the governor housing, lubricates the moving parts. The fuel level is to

be checked at regular intervals of every 3000 km run. A number of adjusting nuts, screws and stops are

provided with this governor. The settings should not be disturbed, as it requires special tools and the specific

knowledge.

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Fig. 10.17. Idling and maximum speed mechanical governor.

10.10.2.

Pneumatic Governors

Often all speed type of pneumatic governor (Fig. 10.18) is used with in­line pumps fitted to light CI engines.

When the accelerator pedal is held to a set position, the governor maintains a constant speed till the load on the

engine is too high. A spring­loaded diaphragm (Fig. 10.18), connected to the control rod, seals a chamber and is

connected by a pipe to a venturi control unit in the inlet manifold. A butterfly valve, fitted in the waist of the

venturi, is linked directly to the accelerator pedal.

Fig. 10.18. Pneumatic governor system. When the engine stationery, the diaphragm spring positions the control rod at the maximum fuel or excess­fuel

condition. This is a manually operated control installed at the end of the pump, which is fitted with an excess­

fuel device. This enables extra fuel to be delivered for cold starting, and cannot be operated from the driving

position.

After starting of the engine, operation of the accelerator pedal produces a depression in the venturi and

diaphragm chamber so that atmospheric pressure forces the diaphragm and control rod to the idle setting. With

the pedal in any set position, venturi depression increases with engine speed, causing reduction in the control

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rod opening. Similarly a decrease in speed causes an increase in the control rod opening. As the accelerator pedal is depressed, the butterfly valve is opened so that the venturi depression is decreased. This allows the spring

to open the control rod due to which the engine speed increases until a balance is established between the spring

thrust and venturi depression.

A stop screw, acting on the lever and controlling the butterfly valve, limits the maximum speed. When the valve

reaches its stop, any tendency for the engine to increase speed intensities the venturi depression and reduces the

control rod opening. A stop control on the dash board enables the driver to override the governor and move the

control to the non­fuel position. When any part of the governor system is disconnected, the engine should not be

started.

Hydraulic Governors

The hydraulic system is used with in­line pumps where smooth, slow idling speeds are desired. A gear­type oil

pump is driven from the end of the pump camshaft, and supplies oil through various valves to operate a piston

connected to the control rod. High cost of this system limits its use.