Gas Engine Technology: Aermotor Speed Governor

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


| August/September 2013



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

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.