Page 234 - Handbook of Energy Engineering Calculations

output for the equivalent hp rating. Other advantages of these two speed
ranges include longer service life and, in some instances, lower maintenance
costs.
Usually an application will have its own requirements, such as allowable
engine weight, available space, output torque, load speed, and type of service.

These requirements will often indicate that a particular speed classification
must be used. Where an application has no special speed requirements, the
speed selection can be made on the basis of cost (initial, installation,

maintenance, and operating costs), type of parts service available, and other
local conditions.

5. Analyze the engine output torque required
In some installations, an engine with good lugging power is necessary,
especially in tractors, harvesters, and hoists, where the load frequently

increases above normal. For good lugging power, the engine should have the
inherent characteristic of increasing torque with drooping speed. The engine
can then resist the tendency for increased load to reduce the output speed,
giving the engine good lugging qualities.

One way to increase the torque delivered to the load is to use a variable-
ratio hydraulic transmission. The transmission will amplify the torque so that
the engine will not be forced into the lugging range.
Other types of loads, such as generators, centrifugal pumps, air

conditioners, and marine drives, may not require this lugging ability. So be
certain to consult the engine power curves and torque characteristic curve to
determine the speed at which the maximum torque is available.

6. Evaluate the environmental conditions

Internal-combustion engines are required to operate under a variety of
environmental conditions. The usual environmental conditions critical in
engine selection are altitude, ambient temperature, dust or dirt, and special or
abnormal service. Each of these, except the last, is considered in previous
calculation procedures.

Special or abnormal service includes such applications as fire fighting,
emergency flood pumps and generators, and hospital standby service. In
these applications, an engine must start and pick up a full load without

warmup.