<div class="htmlview paragraph"><b>E</b>NGINEERS have different ideas regarding highly efficient and moderately efficient engines, but designers dare not ignore the fact that the public requires today a small very high-speed engine, with good torque at low speeds, and capable of revolving efficiently at very high speeds. These two characteristics are difficult to attain, since in practice one is really opposed to the other. To obtain high speeds with power, the valve areas, valve parts, carbureter, etc., should not be restricted in any way, while to get a good mixture at low speed with heavy torque means a different valve-setting and more or less restricted port and valve areas, etc., to secure high gas velocities.</div> <div class="htmlview paragraph">The author states that the fundamentals of high-speed engines are high volumetric efficiency; high compression, to aid in obtaining rapid combustion at high speeds, and light reciprocating and rotating parts, to secure high mechanical efficiency. This is followed by a close comparative study of details of design and construction. In the course of the discussion attention is given to materials and other details of general manufacturing. The application of the engine under discussion to such uses as searchlights, captive balloons, tanks and tractors, is illustrated. Many photographs and charts are used. One interesting table shows the influence of the amount of lubrication on the brake-horsepower developed by engines.</div>