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Coolspring Spotlight: Circa-1893 10 hp Connelly

This Connelly gas engine at the Coolspring Power Museum has characteristics of different generations of the Connelly design.

| December/January 2018

  • Circa 1893 10 hp Connelly gas engine.
    Photo by the Gas Engine Magazine staff
  • This particular engine has characteristics of different iterations of Connelly’s evolving engine design.
    Photo by the Gas Engine Magazine staff
  • The pistons utilized a master-slave connecting rod connected to a single crank throw. The second cylinder is missing on this engine.
    Photo by the Gas Engine Magazine staff
  • At some point, this engine's secondary cylinder assembly was discarded, a conversion to 4-stroke Otto cycle operation was performed, and an oil-field clutch was installed.
    Photo by the Gas Engine Magazine staff

Circa-1893 10 hp Connelly

Manufacturer: Connelly Gas Engine Co., New Castle, PA
Horsepower: 10 hp 
Bore: 6-1/2 inch (primary cylinder); 10-inch (secondary cylinder) 
Stroke: 12-inch 
Owner: Paul Harvey

Born into an ambitious and prosperous family in Monongahela City, Pennsylvania, John Storer Connelly (along with his brother and uncle) was granted numerous patents related to the thriving petroleum and gas engine businesses. Beginning about 1885, John focused his creativity on developing a small, internal combustion engine powered locomotive (“motor”) to replace the horse in popular (but very polluting!) horse-drawn street trolleys. His “Connelly Gas Motor” was tested with some success in London and several U.S. cities including New York City, New York, and Chicago, Illinois, in the 1888-1894 time period, but was ultimately displaced by the new, more powerful, self-powered, electric street trolley.


Connelly’s unique 2-stroke stationary engine is operationally identical to his gas motor’s engine. During each cycle, combustion occurs in the smaller primary cylinder, which exhausts into the larger secondary cylinder. This secondary cylinder extracts additional power from each charge and then scavenges the primary cylinder. Both pistons utilize a master-slave connecting rod arrangement connected to a single crank throw. Ignition was by either electric igniter or hot-tube.

Engines were produced from about 1892 to 1898 in 5, 10, and 20 hp sizes. Uses included pumping oil wells and pumping water for railroads and municipal waterworks. One powered the New Castle News (Pennsylvania) newspaper printing press.

This particular engine embodies characteristics of the second or third iteration of Connelly’s evolving engine design. Its hot-tube ignition is not timed as in early illustrations, although there is evidence of provision for optional electric ignition. No sign of the original speed governor is evident. At some point, this engine’s secondary cylinder assembly was discarded, a conversion to 4-stroke Otto cycle operation was performed, and an oil field clutch was installed.


This engine was found powering a small oil field machine shop in western Pennsylvania. It is likely this was not its first working installation. It was brought to Coolspring in 1971.

Learn about this engine and 38 others in Coolspring: Discovering America’s Finest Antique Engine Museum, Vol. 2. Order online at


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