In this article we will discuss about:- 1. Introduction to Governor 2. Principle of Governor 3. Classification of Governing System 4. Regulation 5. Hunting.
Introduction to Governor:
Governor is a mechanical device, designed to control the speed of an engine within specified limit, used on tractor or stationary engines.
It is used for:
(i) Maintaining a nearly constant speed of engine under different load conditions,
(ii) Protecting the engine and the attached equipments against high speeds, when the load is reduced or removed.
Tractor engines are always fitted with governor. There is an important difference in principle between the controls of a tractor engine and that of a motor car. In case of motor car, the fuel supply is under direct control of the accelerator pedal, but in tractor engine, the fuel supply is controlled by the governor. The operator changes the engine speed by moving the governor control lever.
A governor is essential on a tractor engine for the reason that load on the tractor engine is subjected to rapid variation in the field and the operator cannot control the rapid change of the engine speed without any automatic device. For example, if the load on the tractor is reduced, the engine would tend to race suddenly. If the load is increased, the engine would tend to slow down abruptly.
Under these circumstances, it becomes difficult for the operator to regulate always the throttle lever to meet the temporary changes in the engine load. A governor automatically regulates the engine speed on varying load condition and thus the operator is relieved of the duty of constant regulating the throttle lever to suit different load conditions.
Principle of Governor:
Engine Governor is used for automatically controlling the speed of an engine by regulating the intake of fuel or injection of fuel, so that engine speed is maintained at the desired level under all conditions of loading.
During operation, the load on the tractor engine frequently varies depending upon condition of ground, soil and attached implements.
These load variations cause the engine speed to change accordingly when the load on the engine speed may rise beyond safe limit causing- (1) Chances of accident, (2) Excessive wear of the engine parts and (3) Increased fuel consumption.
For better field performance, the moving speed and power take-off speed for implements and machines should not very much. This can only be done if the amount of air-fuel mixture or fuel delivered to the engine cylinder is varied according to changes in the engine speed. Governor used on tractor engine is called Variable speed governor and the one used on stationary engine is called Constant speed governor.
Classification of Governing System:
Governing system is classified as:
1. Hit and miss system and
2. Throttle system
1. Hit and Miss System:
In this system, the frequency of explosions or power strokes of engine are regulated. When the engine speed exceeds the rated speed, the exhaust valve gets open, which prevents fresh air-fuel mixture or fuel to enter into the engine cylinder during suction stroke.
The main principle in this method is to keep all explosions alike at maximum intensity but to vary the number of explosions per unit time, depending upon the power requirement. A detent or a latch is provided on the exhaust valve push rod which prevents the valve from closing when the speed is too high.
The inlet valve remains closed as long as the exhaust valve is held open. Consequently no charge is drawn into the cylinder on the suction stroke and no power is developed in the engine cylinder due to complete absence of the fuel and as a result, the speed falls. Thus, it is found that explosions are missed intermittently but every charge is of normal strength. This method is mostly used on gas engines, it is not common on other engines.
2. Throttle System:
The system consists in controlling the amount of fuel mixture or fuel during suction stroke and thereby changing the explosion intensity in the cylinder. Here the number of explosions is not reduced, only intensity of explosion is changed. There is uniform firing in the cylinder throughout the period of operation.
A suitable butterfly valve is provided in the air passage between the inlet manifold and the carburetor. If the speed decreases, the throttle remains wide open and more charge goes to the engine cylinder. When the speed goes up, the throttle comes to nearly closed position and less charge goes inside the engine cylinder.
Throttle system of governor can be divided into three types:
(а) Centrifugal governor
(b) Pneumatic governor and
(c) Hydraulic governor
(a) Centrifugal Governor:
The main principle of a centrifugal governor lies in the fact that when a weight rotates about a point, it tends to fly away in tangential direction from the centre of rotation. If the weight is constrained by a linkage, it can be made to operate a control lever.
Centrifugal governor consists of:
(i) Spring loaded centrifugal weights
(ii) Sliding collar
(iii) Spring
(iv) Throttle valve
(v) Connecting rod.
The connecting rod controls the throttle valve, provided in the air-fuel passage. The centrifugal weights may be either mounted on the engine crankshaft or on another shaft driven by the crankshaft. The sliding collar moves along the axis of the shaft, depending upon the speed of centrifugal weights.
The distance between the weights is directly proportional to the centrifugal force and inversely proportional to the spring tension. At low loads, the engine speed tends to go high, the centrifugal weights tend to fly outward against spring tension, thereby closing the throttle valve and reducing the entry of charge inside the engine.
Similarly at high loads, the speed tends to become slower, and the centrifugal weights come closer due to spring tension. The contraction of the weight causes the throttle to open fully, allowing more fuel to come in the engine cylinder to increase the speed of the engine. Centrifugal governor is very common on tractors and stationary engines.
(b) Pneumatic Governor:
Pneumatic governor is used on both carburetor engine and diesel engine.
It consists of:
(i) Diaphragm
(ii) Thrust lever
(iii) Control rod
(iv) Flexible tube
(v) Venturi valves
(vi) Control lever
(vii) Pull button and
(viii) Accelerator pedal.
In such governors, a pressure depression is created in the engine intake manifold, which causes the diaphragm to operate a lever connected to the control rod of the fuel injection pump in diesel engine or to the connecting lever of the throttle valve in carburetor engine.
There are two chambers in the diaphragm unit:
(a) Atmospheric chamber, which is connected to the atmosphere and,
(b) Vacuum chamber, which is connected to the venturi unit.
A leather diaphragm separates these compartments. Difference of pressure in the venturi tube is communicated to the diaphragm chamber to operate the spring loaded diaphragm and pump control rod. There is a vacuum pipe which connects the air tight chamber on one side of the diaphragm and the venturi tube in the intake manifold. There is butterfly valve at the throat of the venturi.
The butterfly valve is actuated by a throttle lever. When the throttle lever is operated, the butterfly valve is moved and a change is brought in the pressure at the venturi throat. This pressure change is transmitted to the diaphragm through the vacuum pipe.
The diaphragm either moves back against the spring or is moved outwards by the spring. The movement of diaphragm actuates the control rod of fuel injection pump or the throttle lever of the carburetor to increase or decrease the fuel supply, thus controlling the engine speed. This type of governor is popular on small engines.
(c) Hydraulic Governor:
This governor is based on the principle that the change of pressure in one part of the system, produces an opposite change of much greater magnitude in another part. A pump is used to deliver the oil from the fuel injection pump into a chamber called amplifier under pressure. Pressure drop depends upon the amount of oil flowing i.e. upon pump speed which is in turn dependent upon engine speed.
Movement of the piston against spring pressure moves the control rod to open position, increasing pump delivery. End thrust on the amplifier piston tends to open the amplifier valve, but is resisted by the control spring.
Depression of the accelerator compresses the spring and increases the load on the amplifier valve which opens at a pressure that depends upon the control position, selected by the driver. An idling valve is provided to give greater sensitivity under idling conditions when the rate of oil flow is low. This type of governor is not in common use for tractors and small stationary engines.
Governor Regulation:
The governor is fitted on an engine for maintaining a constant speed, even then some variation in speed is observed at full load and no load conditions. In normal working, a variation of about 100 rev/min is observed between full load and no load condition for a good governor.
Hence it is possible to regulate the governor to maintain a higher or lower speed by changing the tension of the spring. The extent of regulation done is expressed in terms of percentage called Percentage regulation. This is also called speed droop. It is the variation in the engine speed between full load and no load condition. It is usually expressed as percentage of rated speed.
This is given as –
Where,
R = % regulation
N1 = speed at no load
N2 = speed at full load
Precautions should be taken that governor seal should not be broken carelessly. Seal should be broken by a technician, who knows about governor adjustment.
Governor Hunting:
Governor hunting is the erratic variation of the speed of governor when it over compensates for speed changes.
When the governor produces a periodic effect on the engine speed like too fast and then too slow, then too fast and so on, it is sign of Governor hunting. In such cases, it is observed that when the engine speeds up quickly, the governor suddenly responds, the speed drops quickly, the governor again responds and this process is repeated.
The reason for the governor hunting may be due to incorrect adjustment of fuel pump or carburetor, improper adjustment of the idling screw and excessive friction. Hunting may be due to governor being too stiff or due to some obstruction in free movement of governor components.