CALLAHAN ENGINE

Image Gallery Courtesy of John Wilcox, 47 Deland Ave., Columbus 14, Ohio John Wilcox

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Wilcox  47 Deland Ave., Columbus 14, Ohio

The accompanying picture is of an unusual 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 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 mechansim 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-stemed 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.

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