The Callahan Camstopper Gas Engine

A detailed history about the Callahan Camstopper gas engine and how the engine works.

Hit and miss governor

Callahan # 1939, 5x9, 36 foot wheels, 4-cycle, natural gas, make and break ignition, hit and miss governor holding exhaust open by stopping side shaft. One of John P. Wilcox's collection.


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Learn about the Callahan Camstopper gas engine. 

The accompanying picture is of an unusual antique gas engine by W. P. Callahan & Co. of Dayton, Ohio. Like the Columbus engine shown in the March-April issue of GEM, the Callahan governs by stopping the rotation of the cams. In this case, however, the entire shaft is brought to a standstill by means of a clutch in the hub of the timing gear. To the best of my knowledge, the Callahan Camstopper gas engine is the only American engine to stop the whole shaft. The idea was to reduce wear in the side shaft bearings and cam followers, and an additional advantage was gained in that the engine idles very quietly. Although the shaft is impacted back to speed, the jar is not noticeable, at least in engines of this small size, and the mechanism has held up well. It was, of course, necessary to drive the governor head directly from the crankshaft, and to provide means for keeping the side shaft in place so as to hold the exhaust valve open while it is stopped. The latter was achieved by use of a slightly double-lobed cam profile, so that the exhaust follower rests between the lobes and positions the shaft while still holding the valve well open.

The short-stemmed valves are located in the lower near side of the cylinder, with their stems pointing directly toward the side shaft, and both are power operated. One suspects that the volume of the chamber they open into was overlooked when the head-to-piston clearance was chosen, as the compression ratio is extremely low. Main bearings are oiled by chains dipping into oil reservoirs case into the base. Gasoline models have forced injected into the intake pipe from a pump mounted on the boss under the side shaft.

Exhaust gasses leave the cylinder through a port uncovered by the piston, as well as through the exhaust valve. This arrangement, known as an "auxiliary exhaust port," was popular among the engine builders of Ohio and Western Pennsylvania, and serves a couple of purposes. Since the port opens first, pressure in the cylinder is relieved by the time the valve opens, and a much lower force is required to unseat it. Also, the initial blow-down through the port expands and cools the gasses which later pass out through the valve, allowing it to operate at a substantially lower temperature than would otherwise be the case.

The basic patent on the Callahan governor was granted to Peter T. Coffield in March of 1897 and assigned to Callahan, who was then building steam-driven ice and refrigerating machinery. The original model was built in sizes up to 50 HP, and a good percentage of them, including the one pictured, were sold by the Fairbanks Co. of New York City under their own label. This outfit was (and still is) a jobber of machine shop equipment and never built an engine of their own, although they are responsible for quite a few Bates and Edmonds engines floating around which say "Fairbanks" on their hoppers. They sold the Buckeye vaporizing oil engine and the St. Marys Hvid-injection Diesel as well, but apparently with more regard for the manufacturer's name plate. The Callahan was also sold in Los Angeles by Frank H. Howe as "The Coffield."

In 1912, the company came under new management and the engine was redesigned to a more modern appearance and a more conventional valve placement. It is not clear whether the original governing principle was retained. Although they continued in business in the old buildings which still are standing, few of the newer model seem to have been built, as I am not aware that any exist today. One other old model has turned up, an 18 Hp. in the collection of Harold Ottaway of Wichita, Kansas. Anybody know of more?