Mechanical Governors - Working Principle, Types And Function

Most engine-powered applications, from lawn mowers to tractors to standby power systems, even your cars must be controlled, automatically, by a “governor” . The governor system is like a cruse control system. It keeps the engine running at the speed you select, regardless of changes in the load. Since there is always a continuous load change as time passes with frequency and suddenness that would require the full attention of-and probably overwhelm- a human operator, the governor handles the load change.


You can think of the load as the amount of work the engine must perform: for a mower, the height of the grass; for a tiller, the depth of the tines; for a chipper, the thickness of the branches. Without a governor, you would need to adjust the throttle manually each time your lawn mower ran across a dense patch of grass. A governor does the job for you by detecting changes in the load and adjusting the throttle to compensate.
Method of Operation of Governors

The governor system behaves like an unending tug of war between one of two governor springs, which pull the throttle toward the open position, and a spinning crankshaft, which tries to close the throttle. (see diagram).

 When the load on the engine increases - a typical example is when you move your running lawn mower from the driveway to the grass - crankshaft revolutions (speed) drop. this thus reduces the centrifugal force on the balls of the governor this makes then move closer to the  center of rotation. Since the governor spring is still tugging, it causing the throttle plate to open.In response, a larger volume of air-fuel mixture enters the carburetor, increasing engine speed to compensate for the increased load. The crankshaft speeds up due to larger energy from the engines flywheel , and the tug of war resumes, until a new equilibrium is achieved. With each change in load, the tension between the governor spring and the load brings about a new equilibrium, known as the engine's governed speed.

Neither side wins until the engine is shut off. At that point, without the crankshaft spinning, the governor spring pulls the throttle to the wide-open position.

Types of Governors

Two types of governor are common on small engines - mechanical and pneumatic.

• Mechanical Governor

A mechanical governor uses gears and flyweights inside the crankcase as a speed-sensing device that detects changes in the load and adjusts the throttle accordingly. If you're operating your small engine under a light load, the carburetor needs to deliver a relatively small amount of air-fuel mixture to the combustion chamber. As the crankshaft spins, centrifugal force causes the flyweights to open. As they open, they apply pressure to the governor cup and governor crank, which are linked to the throttle. The throttle is pulled toward the closed position.

As the load on the crankshaft increases, the flyweights spin more slowly. The reduced centrifugal force on the flyweights results in less pull on the throttle toward the closed position. But since the governor spring tension remains, the throttle reopens until the desired governed speed is achieved.

• Pneumatic Governor

A pneumatic governor uses a movable air vane, made of metal or plastic, as a speed-sensing device by registering the change in air pressure around the spinning flywheel. The pneumatic governor design is simpler and parts are easier to access. It is also a slightly less reliable design, since small particles of debris can interfere with the pneumatic governor's operation.

The pneumatic governor also relies on one or two springs to pull the throttle toward the open position. As the load lessens and engine speed increases, air blown by the flywheel also begins to increase, causing the governor blade to pull the throttle plate toward the closed position in its effort to maintain a steady engine speed.

picture Credit ;fromthedepthsgame.com

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