P. A. Holliday, in the _Engineer_, derives a new formula for computing the horse-power of the four stroke cycle, single-acting engine. For each horse-power developed by these engines about 21,000 cubic inches of displacement is necessary, per minute. D = Cylinder bore in inches. S = Stroke in inches. M.P.S. = Mean piston speed in feet per minute. R = Ratio of stroke to bore. N = Revolutions per minute, then √(B.H.P. × 2220) D = ———————————————— M.P.S. 6 M.P.S. Knowing the value of D, N = ———————— S For high speed, low ratio (R), four stroke cycle engines, approximately 22,000 cubic inches displacement per minute is required. √(2,330 B.H.P.) D = ——————————————— M.P.S. In both formulae, the air compressor for fuel injection is included. (32) Semi-Diesel Type Engine. In the “Semi-Diesel” Type Engine the oil is injected into the cylinder at the point of greatest compression in the same manner as in the Diesel engine, and like the Diesel it compresses only pure air. In regard to the compression pressure, however, it stands midway between the pressure of the Diesel engine and that of the ordinary “aspirating” type oil engine, as the compression averages about 150 pounds per square inch. While this is a much higher pressure than that carried by the ordinary kerosene engine which compresses a mixture of kerosene vapor and air, it is not sufficiently high to ignite the oil spray by the increase in temperature due to the compression, but ignites the charge by means of a red hot bulb or plate placed in the combustion chamber. This type of engine is built both in the two stroke and four stroke cycle types, the events occurring in the same order as in the two stroke and four stroke Diesel types, that is, pure air is drawn into the cylinder on the suction stroke (four stroke cycle) or is forced in at the beginning of the compression stroke (two stroke cycle), and is compressed in the combustion chamber. At the end of the compression stroke, the fuel is injected against the red hot bulb or plate by which the charge is ignited. Expansion follows on the working stroke after the fuel is cut off, and release occurs at the end of the stroke.