Page 243 - Handbook of Energy Engineering Calculations

clutches.

Related Calculations. Table 11 shows 54 different applications and duty
ratings for small air-cooled gasoline engines. With this information the
engineer has a powerful way to make a sensible choice of engine, drive,
speed, torque, and duty cycle.

Important factors to keep in mind when choosing small internal-
combustion engines for any of the 54 applications shown are: (1) engines
should have sufficient capacity to ensure a factor of safety of 20 to 25 percent
for the power output. (2) Between high- and low-speed engines, the latter

have longer life, but first cost is higher. (3) In take-off couplings, the flexible
types are preferred. (4) A clutch is desirable, especially in heavier equipment,
to disconnect the load and to make engine starting easier. (5) In operations
where the intake air is dusty or contains chaff, intake screens should be used.

(6) An oil-bath type air cleaner should always be used ahead of the
carburetor. (7) Design engine mountings carefully and locate them to avoid
vibration. (8) Provide free flow of cooling air to the flywheel fan inlet and
also to the hot-air outlet from the engine. Carefully avoid recirculation of the

hot air by the flywheel. (9) If the engine operation is continuous and heavy,
Stellite exhaust valves and valve-seat inserts should be used to ensure long
life.
Valve rotators are also of considerable value in prolonging valve life, and

with Stellite valves, constitute an excellent combination for heavy service.
Exclusive of aircraft, air-cooled engines are usually applied in size ranges
from 1 to 30 hp (0.75 to 22.4 kW). Larger engines are being built and,
depending on the inherent cooling characteristics of the system, performing

satisfactorily. However, the bulk of applications are on equipment requiring
about 30 hp (22.4 kW), or less. The smaller engines up to about 8 or 9 hp (5.9
to 6.7 kW) are usually single-cylinder types; from 8 to 15 hp (5.9 to 11.2 kW)
two-cylinder engines are prevalent, while above 15 hp (11.2 kW), four-

cylinder models are commonly used.
Within these ranges, air-cooled engines have several inherent advantages:
they are lightweight, with weight varying from about 14 to 20 lb/hp (8.5 to
12.2 kg/kW) for a typical single-cylinder engine operating at 2600 r/min to

about 12 to 15 lb/h (7.3 to 9.1 kg/kW) for a typical four-cylinder unit running
at 1800 r/min. Auxiliary power requirements for these engines are low since