Page 242 - Handbook of Energy Engineering Calculations

FIGURE 12 Torque curves for typical air-cooled internal-
combustion engines. (a) Engine with good lugging quality will “hang
on” as load increases. (b) Performance curve for a high-speed engine
with maximum power output at top speed.

3. Determine the duty rating; draw a load curve for the engine
Refer to Table 11 for municipal service. There you will see that sanitary

pumps have a variable-load, heavy-duty rating. Figure 12 shows a plot of the
typical load variation in such an engine when driving a sanitary pump in
municipal service.

4. Select the type of drive for the engine

A variety of power take-offs are used for air-cooled gasoline engines, Fig. 13.
For a centrifugal pump driven by a gasoline engine, a flange coupling is
ideal. The same is true for engines driving electric generators. Both the pump
and generator run at engine speed. When a plain-flange coupling is used, the

correct alignment of the gasoline engine and driven machine is extremely
important. Flexible couplings and belt drives eliminate alignment problems.
In many instances a clutch is required between the engine and equipment
so that the power may be engaged or disengaged at will. A manually engaged

clutch is the most common type in use on agricultural and industrial
equipment.
Where automatic engagement and disengagement are desired, a centrifugal
clutch may be used. These clutches can be furnished to engage at any speed

between 500 and 1200 r/min and the load pickup is smooth and gradual.
Typical applications for such clutches are refrigerating machines with
thermostatic control for starting and stopping the engine.
Clutches also make starting of the engine easier. It is often impossible to

start an internal-combustion engine rigidly connected to the load.
There are many applications where a speed reduction between the engine
and machine is necessary. If the reduction is not too great, it may be
accomplished by belt drive. But often a gear reduction is preferable. Gear

reductions can be furnished in ratios up to 4 for larger engines, and up to 6
for smaller sizes. Many of these reductions can be furnished in either
enginewise or counter-enginewise rotation, and either with or without