The Porter-Allen was one of the first high-speed steam engines. High shaft speeds were desirable from three points of view. They enabled more horsepower to be generated in a small engine, they were compatible with the shaft speeds required for electrical dynamos, and a smaller flywheel would suffice to remove vibration from the crankshaft. The Porter-Allen operated at speeds about four times as great as the speed of a Corliss engine. One feature which enabled the engine was the governor which was patented by Charles T. Porter, in 1858. The distinctive feature of the governor was the weight mounted on the governor shaft. The weight loaded down the flying balls without changing the centrifugal force on them. The governor could therefore be rotated at a high rate, and still be sensitive to variations in engine speed.
The Porter-Allen, like the Corliss, is an automatic cut-off engine since speed control is exercised by varying the point of closure of the steam admission valves rather than by operating a throttle valve. The action of the governor is shown on this model. The admission valve operating rods, on the near side of the crosshead guides, derive their motion from a link whose total travel during a power cycle can be varied by the governor. At low speeds the travel is greatest, and so the valves open furthest, and more steam is admitted into the cylinder. As the speed increases the travel of the rod decreases, and the valves open less, and also are cut off earlier in the cycle. The normal range of valve operation of the Porter-Allen engine is between one quarter and one-half of the stroke.