The Advantages of Supercharging Coach Engines.
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TR"president and chief engineer of the: Schwitzer-Cummins Co..; Indianapolis, Ind., "U.S.A.. recently gave his views, . at a meeting ,of the S.A.E., on the subject of supereharging.
So far as motor !coaches are concerned,' he Pointed out that the advan
tages. can readily be ,seen. ExclusiVe • maintenanoe, cost and rum-dog
economy, the main facto in' the earning capacity! of the coach, in both urban.. and inter-urban services, is the ability to accelerate rapidly,.in which respect its performanee must be equal
to. that of a private ear. Experience with coaches has shown B30
that the engine speed shouldnot exceed 1,800 r.p.m. if satisfactory life is to be obtained. The sizes of power
• units have, of necessity, been cOntinuously increased to meet the -requirements of this service. A few years ago engines of from 400 pubic ins.-to 420 cubic ins, capacity were ample ; to-day a '550 cubic in, engine is inadequate. The former develop about 80 b.h.p. and the latter 110 b.h.p. at the 1,800 r.p.m. maximum.
As inertia forces increase in proportion to the product of the square of the engine speed and the mass, and since . mechanical. failures cannot be tolerated,
smaller engines should be used if possible.
The application of the supercharger will permit engine size to be reduced to 350 cubic ins. or 280 cubic ins. Such an engine, when supercharged, will develop b115 b.h.p. at 1,800 r.p.m. and, because of the lighter reciprocating parts, the speed could well be increased without injury, to 2,000 r.p.m., thus
giving 125 b.h.p. There is plenty of room on motor coaches for blowers of larger types running at higher speeds which would enable increased torque and horse-power to be obtained at vehicle speeds as low as 20 m.p.h.