Page 240 - Handbook of Energy Engineering Calculations

resulting—in some instances—in an engine having twice the power rating of
the electric motor.

2. Find the power required at the installed altitude and inlet-air
temperature
As noted above, altitude and inlet-air temperature both influence the required

rating of a gasoline engine for a given application. Since this engine will be
installed at an altitude of 8000 ft (2438 m), the power loss will be
(8000/1000)(3.5) = 28 percent. Further, the increased inlet-air temperature of
90°F (32.2°C) versus the standard of 60°F (15.6°C), or a 30° difference will

reduce the power output by (30/10)(1.0) = 3.0 percent. Thus, the total power
output reduction will be 28 + 3 = 31 percent. Therefore, the required rating of
this gasoline engine will be at least (1.31)(16.7) = 21.87 hp (16.3 kW).
Once the power requirements of a design are known, the next

consideration is engine rotative speed, which is closely related to the
horsepower and service life. Larger engines, with their increased bearing
surfaces and lower speeds, naturally require less frequent servicing. Such
engines give longer, more trouble-free life than the smaller, high-speed

engines of the same horsepower (kW) rating.
The initial cost of a larger engine is greater but more frequent servicing can
easily bring the cost of a smaller engine up to that of the larger one.
Conversely, the smaller, higher-speed engine has advantages where lighter

weight and smaller installation dimensions are important, along with a
relatively low first cost.
Torque is closely associated with engine rotative speed. For most
installations an engine with good lugging power is desirable, and in some

installations, essential. This is especially true in tractors, harvesters, and
hoists, where the load frequently increases considerably above normal.
If the characteristics of the engine output curve are such that the torque
will increase with reducing engine speed, the tendency for the increasing load

to reduce engine speed is resisted and the engine will “hang on.” In short, it
will have good lugging qualities, as shown in Fig. 12a. If the normal
operating speed of the engine is 2000 to 2200 r/min, the maximum lugging
qualities will result. Sanitary-pump drives do not—in general—require heavy

lugging.
If, however, with the same curve, Fig. 12a, the normal operating speed of