Page 239 - Handbook of Energy Engineering Calculations

altitude.
Also, in keeping with good industrial practice, a gasoline engine is not
generally operated continuously at maximum output. This practice provides a
factor of safety in the form of reserve power. Most engine manufacturers
recommend that this factor of safety be 20 to 25 percent below rated power.

This means that the engine will be normally operated at 75 to 80 percent of
its standard rated output. The duty cycle, however, can vary with different
applications, as Table 11 shows.

TABLE 11 Duty Ratings for Combustion Engine Application

For the 10-hp (7.46-kW) electric motor we are replacing with a gasoline

engine, the motor can deliver—as discussed—25 percent more than its rating,
or in this instance, 12.5 hp (9.3 kW) for short periods. On the basis that the
gasoline engine is to operate at not over 75 percent of its rating, the
replacement engine should have a rating of 12.5/0.75 = 16.7 hp (12.4 kW).

In summary, the gasoline engine should have a rating at least 67 percent
greater than the electric motor it replaces. This applies to both air- and liquid-
cooled engines for sea-level operation under standard atmospheric conditions.
If the engine is to operate at altitude, a further allowance must be made,