Gas Engine Technology: Aermotor Speed Governor

Looking at Daniel R. Scholes’ patent for the governor mechanism on 8-cycle Aermotor engines.

Governor plate S

This drawing shows the governor plate ("s" and its eccentric bore ("t") on the crankshaft ("n"). The pivot weight locates the plate at "o" and "u" is the pick arm roller.

Illustration Courtesy Aermotor

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There are today very few companies still in operation with roots long enough to reach to the early days of gas engines and gas engine technology. One such company, however, is Aermotor.

Founded in 1888 in Chicago, Aermotor’s operations are now in Texas, where it manufactures windmills. Aermotor’s first product was the “mathematical” windmill, a new design windmill with more efficient blades and gearing for improved pumping power. Like many agriculturally oriented manufacturers of the era, Aermotor eventually branched into the burgeoning gas engine industry, manufacturing small engines suitable for farm work.

Aermotor produced a successful line of engines, including the 8-cycle Aermotor pumping engine (see collector Dave Irey’s story Aermotor Redux) and the unique fluted hopper engines, which featured a vertical, galvanized steel cooling hopper that was fluted to give it increased surface area for better cooling.

While the 8-cycle scheme (one ignition cycle in every four revolutions, ostensibly resulting in a cooler running engine that used less fuel) of the pumping engines is generally their most noted feature, an equally interesting feature is their unique hit-and-miss governor.

Designed by Aermotor engineer Daniel R. Scholes (later to become chief engineer and finally president of Aermotor) and awarded patent no. 987,177 on March 21, 1911, the governor design was, according to Scholes’ patent application, intended to “remove the influence of inertia from the governing mechanism.” Scholes claimed his governor ensured accurate, reliable operation by operating in response to centrifugal force only.

Although simple in design and construction, the Scholes governor is somewhat harder to describe. The heart of the governor mechanism is a metal plate sandwiched between the flywheel and the right crankshaft mount. The plate is faced on its outer edge. A roller that rides on the plate’s face is attached to a pick arm that latches and holds the exhaust rocker arm for governing. The plate’s mounting bore is slightly eccentric, allowing the plate limited perpendicular movement relative to the axis of the crankshaft. A weighted pivot on the flywheel’s inner hub is keyed to the plate and operates to move the plate into an eccentric position relative to the crankshaft. The action of the pivot is controlled by a spring attached to a rod radially located to the inside of the outer flywheel rim. With the obvious exception of the pick arm, all the governing mechanisms spin with the flywheel.

When the engine is running below a predetermined speed the spring pulls the pivot in such a way as to keep the governor plate concentric with the crankshaft so it has no input on the pick arm. When engine speed exceeds the predetermined speed the pivot weight overcomes the spring pressure. The pivot then pushes the governor plate into an eccentric position, causing the plate to lift the pick arm and latch out the exhaust valve.

This was the scheme originally sketched for the fluted hopper-style engines. Pumping engines like Dave Irey’s employed a modified version of Scholes’ design. Instead of a weighted pivot linked to the governor plate, a flat plate or strut is attached at the back or flywheel side of the rotating governor plate. From here an attached rod runs radially, engaging with the underside of the flywheel rim in a sliding fit. An adjustable weight is attached at the outer end of the rod with a spring seated in the flywheel rim pushing against the weight. At slow running speeds spring pressure against the weight holds the governor plate concentric to the crankshaft. When engine speed exceeds the desired operating speed, centrifugal force on the weight overcomes spring pressure, allowing the governor plate to move into an eccentric position on the crankshaft, thereby tripping the pick arm and locking out the exhaust. Governing speed is adjusted simply by moving the weight on the rod farther in or out.

It’s elegantly simple, and another interesting example of the different routes manufacturers took to distinguish themselves in the early days of gas engine technology.

Know of an interesting patent? Contact Gas Engine Magazine at 1503 S.W. 42nd St., Topeka, KS 66609-1265 •