How does a Pneumatic Governor work?

The pneumatic governor is the most successfully used in small and medium-sized engines. They are sensitive to variations in toque loading and ensure stable idling control. In this article, I will discuss the Pneumatic Governor.


Pneumatic Governor


Generally, the pneumatic governor has been designed to provide sensitive speed control within the entire operating range of an engine. It is, therefore, suitable for commercial vehicles, marine, and industrial engines and since it is a small compact unit, it is particularly suitable for engines of small capacity where space is created and weight saving is of primary importance.

The MICO/BOSCH, as well as C.A.V. The pneumatic governor, consists of a venturi flow control unit located between the induction pipe on the engine, the air filter, and a diaphragm unit mounted directly on the fuel injection pump. Referring to Fig, it will be seen that the body of the venturi unit is flanged at one end so that it can be secured to the induction pipe on the engine, and is a fixture at the other end to accommodate the air filter. The diameter of the throat will vary according to the capacity of the engine to which it is fitted.

Airflow through the throat of the venturi is regulated by a butterfly valve mounted on a spindle carried in bushes pressed into the venturi body. The Control lever is secured to the butterfly valve spindle and is connected by a suitable linkage to the accelerator pedal. Maximum speed and idling stops provide a means of adjusting the butterfly valve’s movement limits.

A small venturi situated within the main venturi may be secured to, or cast integrally with the body of the venturi unit. Projecting into the auxiliary venturi at right angles to the airflow is a pilot tube that is connected to the diaphragm unit by the flexible pipe.

The flexible leather diaphragm is clamped between the governor’s housing and the governor’s cover. It is connected to the control rod of the fuel injection pump and the spring tends to move the diaphragm and control rod toward the maximum fuel stop.

Control rod movement is limited by adjustable stops. Auxiliary idling stops may be fitted to governors with pitot venturi units. It consists of the plunger which is spring loaded by the spring. The adjusting screw permits adjustment of the spring pressure acting on the plunger.

Maximum fuel stops are normally fitted at the end of the pump housing away from the governor to provide a means of adjusting the maximum fuelling and may be scaled after setting, to prevent unauthorized adjustment. The lever is connected to the stop control in the driving cab. It is spring-loaded and remains inoperative when the engine is running. When the stop control is operated, the control rod is moved to the ‘no fuel’ position.

An excess fuel device of the type illustrated in Fig. may be fitted to permit overriding of the maximum fuel stop to provide excess fuel for starting. The maximum fuel stop is carried on a spring-loaded plunger. When the control rod is in the normal maximum fuel position the end of the control rod bears against the stop screw.

Depression of the plunger aligns the stop screw with a hole drilled at the end of the control rod, thus permitting the control rod to move beyond the normal maximum fuel position. When the engine has started and the accelerator pedal has been released, the control rod moves toward the idling stop and the plunger and the maximum fuel stop are returned automatically to the normal working position to the spring.

Pneumatic Governor Diagram

(A) Venturi valve control, (B) Venturi throat, (C) Venturi butterfly valve, (D) Vacuum pipe union, (E) Vacuum pipe, (F) Diaphragm housing union, (G) Diaphragm housing, (I) Pump housing, (J) Diaphragm, (K) Main diaphragm spring, (L) Stop lever, (M) Oil cap, (N) Control rod stop, (P) Auxiliary idling spring, (Q) Auxiliary idling set-screw, (S) Auxiliary idling plunger

Pneumatic Governor System

All air used by the engine passes through the venturi control unit. As the speed of the air flowing through the venturi increases, air pressure fails.


Two things cause the speed of the air following the venturi to change-

  1. Change in speed.
  2. Change in position of the butterfly valve.

Changes in air pressure in the venturi (due to changes in the speed of the air passing over the pitot tube in the venturi) are transmitted to the diaphragm chamber of the vacuum pipe. When the engine is at rest, the control rod is held by the diaphragm spring in the maximum fuel position and the air pressure is the same on both sides of the diaphragm.

When the engine is running, the air pressure on the spring side of the governor falls. The driver’s accelerator is connected to the butterfly valve. Pressing the accelerator down opens the butterfly valve. The idling stop on the venturi limits the amount the butterfly valve can close. The maximum speed stop on the venturi limits the amount of the butterfly valve that can open.

When the engine is started the control rod will be in the maximum fuel position at normal air temperature. As the accelerator is pressed down and opens the butterfly valve, the engine speed increases. For each accelerator position, there is a corresponding engine speed, for example, if the accelerator is kept halfway down then the engine speed will remain midway between idling and maximum.

Suppose that the driver keeps the accelerator half down and an uphill gradient is reached. The increased load will cause the engine r.p.m. to fall. A fall in engine rpm will reduce the speed of the air in the venturi. A reduction of air speed in the venturi will cause the air pressure in the venturi to rise.

The rising air pressure in the venturi will be transmitted to the spring side of the diaphragm and cause the diaphragm with the pump control rod to move to increase the fuel supplied to the engine. With more fuel, the engine will develop more power and so increase engine r.p.m. to the speed as it was before the hill was reached.

When the load is taken off the engine, for example, when a vehicle starts to go down a hill, engine speed rises, air speed through the venturi increases, air pressure falls in the diaphragm spring chamber and the injection pump control rod is moved to reduce fuel output and so prevent engine speed continuing to rise. If the load on the vehicle is too great, the engine stalls. In practice, of course, a vehicle driver changes to a lower gear to prevent stalling. The pneumatic governor is controlled by engine r.p.m.

When the driver takes his foot off the accelerator, the butterfly valve closes to its idling stop. In the idling position, the governor’s diaphragm contacts a spring-loaded idling damper. This damper is fitted to give uniform idling speed by causing the pump control rod to move quickly when a correction to r.p.m. is needed. At speed above idling, the damper has no effect. When the shut-off lever is operated, a cam pushes the control rod to the no-fuel position and the engine stops.

Pneumatic Governor Maintenance


The diaphragm is made of specially prepared leather and should give lasting service. It will be kept flexible by the addition of one tablespoon full of castor oil through the oil cap or non-detergent lubricating oil at weekly intervals.

On dust-proofed governors (lubricator cap fitted), the diaphragm is preserved by fuel oil mist. No additional lubrication is necessary. The diaphragm unit may be checked for leaks in the following manner :

  • Discontinue the flexible pipe from the diaphragm.
  • Operate the stop control, thus moving the diaphragm and control rod to the “no-fuel” position.
  • Blank off the pipe union on the diaphragm unit. A finger placed over the union will suffice.
  • Release the stop control.

After a small initial movement the control rod will remain stationary, providing there are no air leaks in the diaphragm unit, should the control rod move when the stop control is released, check the joint between the governor housing and the governor cover and the union on the governor cover. If these joints are right, the diaphragm is at fault and must be renewed.

Keep the air filter and venturi clean. The stop control rod should be checked for freedom of movement at frequent intervals.

Adjustment on the bench-

The maximum fuel stop is adjusted on the power test bench.

Adjustment on the engine-

The maximum speed, the idling speed, and the idling damper stops must be adjusted when the engine has reached its normal operating temperature.  The shut-off lever should be adjusted to give stopping without interference.

Pneumatic Governor in Diesel Engine

All Diesel-engine injection pumps operate in conjunction with a governor, which is required to control the injected quantity of fuel so that the engine neither stalls when idling nor exceeds the maximum speed for which it is designed. An engine is also frequently required to operate steadily at certain Pacific speeds between idling and maximum speed regardless of load.

A governor capable of holding any speed steady between idling and maximum speed is called a variable speed governor. Its special feature is its ability to hold the engine speed constant within close limits, with changing load, and at any point within its range to which it has been pre-set by the control lever.

Various types of variable speed governors are manufactured. The distinctive feature of the EP/RSV governor, (manufactured by MICO under a BOSCH license) apart from their compactness, is the simplicity with which their governing range and speed drop can be altered by employing adjusting screws.

Similarly, C.A.V has developed d the BRV governor, which is a conventional centrifugal-operated spring-opposed fly-weight type.

Suitable linkage converts the changing position of the weights to a longitudinal movement operating the rod controlling the fuel output of the injection pump. The movement of this fuel control rod is also affected by the spring-loaded throttle or control lever, and it is the division of the fuel control between the centrifugal governor and the throttle control lever that gives the BRV its special characteristic.

Principle of Operation of the Pneumatic Governor in Diesel Engine


The RSV governor is a mechanical governor, mounted directly on the fuel in the injection pump and driven from its camshaft. The description of the principle of operat10n may be divided into two sections: The transfer of forces and the linkage components.

Transfer of forces-

Attached to the camshaft of the pump is the flyweight assembly. The two flyweights are pivoted on pins fixed in the coupling flange. On being thrown out of the centrifugal force, the weights through their bell crank extension will push the guide sleeve assembly against the tensioning lever. The tensioning lever has its fulcrum at the top of the governor cover. The main governor spring pulls the tensioning lever against the guide sleeve assembly.

The spring tension changes depending on the position of the swiveling lever. The swiveling lever and the control lever, therefore, determine the tension of the governor spring. The travel of the tensioning lever toward the flyweights is limited by the full load stop screw.

As long as the governor’s spring tension exceeds the flyweight force at the guide sleeve assembly, the tensioning lever stays on the full load stop. at a certain speed, the flyweight forces overcome the governor spring tension, and the tensioning lever is pushed away from the full load stop.

Linkage components-

The guide lever and the tensioning lever have a common fulcrum at the top of the governor cover. The other end of the guide lever is hooked onto the guide sleeve assembly. The fulcrum of the floating lever is at the bottom of the governor cover. The guide lever is connected to its middle by a pin.

At the upper end, the floating lever carries a link by which it is connected to the control rod of the pump. The auxiliary starting spring is attached to the floating lever on the end of the governor housing on the other end.

It can be seen that every movement of the guide sleeve assembly will be transferred by the guide lever to the floating lever and over the link to the control rod.

Note- The stop position with MICO or BOSSCH governors is always with control rod motion toward the governor cover.


The pneumatic governor is the device used to increase the efficiency of the engine. This is a small device but has the biggest impact on the engine power and performance. So, maintaining the pneumatic governor is very important and also this increases the performance of the engine.

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