Orville and Wilbur's Hit and Miss Engine

Orville and Wilbur Wright built their own hit and miss engine and used it to machine parts for their airplane


| January 2000



Operation of the exhaust valve cam in a "homegrown" engine dating to 1901.

Operation of the exhaust valve cam in a "homegrown" engine dating to 1901.

In 1901, two men in Dayton, Ohio, needed a small engine to power a lathe and a drill press in their shop. Whether what they wanted was unavailable or simply beyond their means is unknown: In any case, they decided to build their own. 

The men who built the engine were named Wright: Brothers Orville and Wilbur. Their shop engine powered the lathe and drill press which were the only machines used to build the parts and engine they designed and built for their first powered airplane. That plane made the first controlled power flight in history on Dec. 16, 1903 at Kitty Hawk, N.C. The Wrights also used the shop engine for tests, to see how much horsepower the plane would need, how big an engine to build, and how to make the right size propellers.

As was their habit, they conducted research before building. Their engine was mostly made of cast iron. They likely had the parts cast, as they did not appear to have any way of casting parts themselves.

Their 3 hp engine looked like many others of the day. It was a horizontal cylinder type, with two flywheels about two feet in diameter, set on a cast iron base. The engine had a six-inch bore and seven-inch stroke, and developed just over 3 hp at 447 rpm. The engine was air cooled with several thin fins on the cylinder, but it had no fan, counting instead on radiation and natural air circulation. The engine ran on gas from the city gas main, and used a flame-heated copper hot tube for ignition. The intake valve was operated by vacuum, and had a linkage connected to the gas intake valve so that it opened to allow gas to flow when the main intake valve was pulled open.

Speed and timing were controlled by an unusual exhaust valve operating system. Like most hit-and-miss engines, it controlled the rpm. It did so with a long rocker arm which ran from the crankshaft to the exhaust valve on the side of the cylinder at the other end of the engine. The arm had a roller on the crankshaft end, which ran on a crescent-shaped cam which rocked. As the crank shaft turned, the roller rode over the top of the cam, causing the rocker arm to open the exhaust valve. As the roller passed the center of the cam, the cam pivoted so that on the next revolution, the roller went under the cam, leaving the valve closed. The result? Perfect four-stroke operation without a geared cam. You could see hundreds of hit-and-miss engines, and never see one with this system.

The governor also was unusual. A small rod was fastened along a spoke of one of the flywheels. A weight slid back and forth on the rod, where it was held toward the center of the wheel by a spring. After each time it operated the exhaust valve, the long rocker arm held the valve in the open position by a spring catch. As the fly wheel speed slowed, the weight moved inward until it struck and released the catch that had been holding the arm off the cam. The valve closed and the engine fired again.