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How does a speed controller work in a small motor?

The control system is like a cruise control. It keeps the speed of your lawnmower or your outdoor power products constant. When properly set, Briggs & Stratton governors will keep your speed constant regardless of engine load - the amount of work the engine has to do.

When driving a lawn mower, the engine load can be influenced by the hills and the height of the grass. In the case of a motor hoe, the load may depend on the depth of the tines, while the load on a chopper depends on the thickness of the branches.

Without a regulator, you would have to manually adjust the throttle every time your mower drives over a dense area of ​​grass. A regulator does this work for you by noticing changes in load and adjusting the throttle to compensate.

Your small motor contains either a pneumatic or mechanical controller. The main difference between the two lies in the way they sense speed.

How mechanical engine governors work

To control engine speed, a mechanical governor uses gears and centrifugal weights in the crankcase to detect changes in load and adjust throttling accordingly.

When running your small engine on a low load, the carburetor needs to deliver a relatively small amount of the air-fuel mixture to the combustion chamber. This is all controlled by the crankshaft, which rotates quickly under light load and which rotates more slowly when the engine is working harder.

When the crankshaft rotates rapidly, the flyweights open and put pressure on the governor pot and the crankshaft. This closes the throttle valve, thereby limiting the air-fuel supply to the engine.

As the engine load increases, the crankshaft rotates more slowly. This leads to a loosening of the flyweights and a renewed opening of the throttle valve. Regulator springs keep the throttle valve at the desired maximum speed.

How pneumatic motor controllers work

The speed sensor on the pneumatic controller is a movable metal or plastic flap. This small engine part registers the change in air pressure around the rotating flywheel.

The pneumatic regulator also relies on a spring or two to pull the throttle towards the open position. As the load decreases and the engine speed increases, the air blown by the flywheel also increases and this causes the regulator vane to pull the throttle towards the closed position, trying to maintain a steady engine speed. The pneumatic regulator design is simpler and the parts are more accessible.

Do you have any questions about regulators in small motors for lawn mowers & outdoor power products?
To find out whether your lawnmower or outdoor power product uses a mechanical or pneumatic governor, please refer to your engine manual.

Contact a Briggs & Stratton dealer near you for personal assistance!