The Danger of Run Away Engines

High speed operation is dangerous and can lead to a run away engine


| July/August 1984



Recently at a local show in New York State, an unattended engine suddenly increased in speed very quickly. Before there was time to react, the machine was joyfully spraying parts over a 25-foot radius. The run away engine went on its merry way until a daring soul removed the spark plug wire and terminated the madness. This little episode raised my interest, and some research on the topic was quite revealing. Engineering books and machining handbooks have a great deal of information concerning cast iron machinery and flywheels. After satisfying my curiosity about the strength of old engines, I became very cautious and aware of the danger relating to a "run away engine."

Many "Ole Tyme" engines were run faster than their rated RPM. This short article gives reasons why this high speed operation should be avoided, offers a way to calculate RPM limits for any particular cast iron flywheeled engine.

Antique gasoline engines had very few problems. One was weight factor. Engines ran slowly because of the stress limitations of large, heavy, bulky parts. Speed of most "Ole Tyme" engines was controlled by means of a governor. The governor maintained a constant speed while at the same time preventing the RPM's from exceeding the maximum rating. Many times operators "tied down" governor weights, thereby allowing operation at the fastest possible speed. Although a highly dangerous practice, it did increase horsepower of the engine. On occasion, increased centrifugal force in the flywheel caused fractured cast iron flywheel parts to sail in all directions inflicting death and injury. (Velocity of the flying parts can easily reach a discharge speed of several hundred miles per hour.) Changes in force within a spoked flywheel rim are proportional by the square of change in the engine RPM. For example: If the RPM of an engine is 200 and the centrifugal force in the flywheel rim is 50 pounds per square inch, and the RPM is raised to 400, the force will then increase to 2500 pounds per square inch. That is a tremendous increase in force and should be kept in mind by all gas engine buffs.

The following is an easy formula to determine the maximum safe RPM for any particular cast iron flywheel:

N=   C x A x M x E x K/D

              WHERE