The general design problem is considered by reference to the principle of similarity.
The principle is briefly reviewed and applied to lubrication, propeller drives, cooling and detonation.
It is shown that while small engines should be capable of a larger output per square inch of piston area, in practice the reverse is the case. It is also shown that small engines should, in general, operate at a higher efficiency than large ones. A promising line of attack for improving the lubrication of small engines is indicated.