Development of the Franklin Direct Air-Cooled Engine 310004
FEATURES of the design of the various cylinders built by the Franklin organization in its development program leading up to the present design are discussed in this paper. The relation of waste heat to cooling-fin areas and cooling-blast velocities is shown and discussed for cylinders up to 3½-in. bore.
Characteristics of the cooling system, including fan, fan housing and air housings, are discussed at length, and the authors contend that no more power, if as much, will be absorbed in the cooling system as in that of the indirect air-cooled engine. Results of tests showing the ability of the engine to cool under the severest conditions of load and temperature are given.
Since the quietness of any engine is dependent upon constant valve-clearances, the authors describe in detail the method followed in the Franklin design to maintain at less than 0.003 in. any variation in clearance. A careful analysis is made for each part in the valve-gear mechanism that is affected by expansion.
Power curves of earlier engines built by the company are compared with the present-production power-plant. Without a change in piston displacement, the brake horsepower at 3500 r.p.m. has been increased by 76 per cent. This increase has resulted from (a) better cooling characteristics of the cylinder, (b) increased valve area, (c) decreased absorption of power by the fan and (d) increased compression-ratio.