What is the governor system? How does it function?
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. 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.
For information regarding governor adjustment for your engine, please visit our Adjusting the Governor FAQ.
How A Governor Functions
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. 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 drop. But the governor spring is still tugging, 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, 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.
Two types of governor are common on small engines - mechanical and pneumatic.
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.
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.
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