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Figure 2-20.—Prony brake. | Up
Construction Mechanic Basic Volume 01 - Construction methods and practices | Next
Thermal Efficiency |
force being applied from a wrench that is l-foot long.
When a 2-foot-long wrench is used, only a 50-pound
force is required. An illustration of a torque wrench in
use is shown in figure 2-21.
DO NOT confuse torque with work or with power.
Both work and power indicate motion, but torque does
not. It is merely a turning effort the engine applies to the
wheels through gears and shafts.
ENGINE TORQUE is a rating of the turning force
at the engine crankshaft. When combustion pressure
pushes the piston down, a strong rotating force is
applied to the crankshaft. This turning force is sent to
the transmission or transaxle, drive line or drive lines,
and drive wheels, moving the vehicle. Engine torque
specifications are provided in a shop manual for a
particular vehicle. One example, 78 pound-feet @
3,000 (at 3,000) rpm is given for one particular engine.
This engine is capable of producing 78 pound-feet of
torque when operating at 3,000 revolutions per minute.
FRICTION is the resistance to motion between
two objects in contact with each other. The reason a sled
does not slide on bare earth is because of friction It
slides on snow because snow offers little resistance,
while the bare earth offers a great deal of resistance.
Friction is both desirable and undesirable in an
automobile or any other vehicle. Friction in an engine is
undesirable because it decreases the power output; in
other words, it dissipates some of the energy the engine
2-16
Figure 2-21.—Torque wrench in use, tightening main bearing stud
of an engine.
produces. This is overcome by using oil, so moving
components in the engine slide or roll over each other
smoothly. Frictional horsepower (fhp) is the power
needed to overcome engine friction. It is a measure of
resistance to movement between engine parts.
Frictional horsepower is POWER LOST to friction. It
reduces the amount of power left to propel a vehicle.
Friction, however, is desirable in clutches and brakes,
since friction is exactly what is needed for them to
perform their function properly.
One other term you often encounter is INERTIA.
Inertia is a characteristic of all material objects. It
causes them to resist change in speed or direction of
travel. A motionless object tends to remain at rest, and a
moving object tends to keep moving at the same speed
and in the same direction. A good example of inertia is
the tendency of your automobile to keep moving even
after you have removed your foot from the accelerator.
You apply the brake to overcome the inertia of the
automobile or its tendency to keep moving.
The term efficiency means the relationship between
the actual and theoretical power output. Volumetric
efficiency (fig. 2-22) is the ratio between the amount of
air-fuel mixture that actually enters the cylinder and the
amount that could enter under ideal conditions. The
greater volumetric efficiency, the greater the amount of
air-fuel mixture entering the cylinder; and the greater
Fire 2-22.—Demonstrating volumetric efficiency.
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