the movement of a body against an opposing force. In the mechanical sense of the term,
this is done when resistance is overcome by a force acting through a measured distance.
Work is measured in units of foot-pounds. One foot-pound of work is equivalent to lifting
a l-pound weight a distance of 1 foot (fig. 2-17). Work is always the force exerted over
a distance. When there is no movement of an object, there is no work, regardless of how
much force is exerted
the ability to do work. Energy takes many forms, such as heat, light, sound, stored energy
(potential), or as an object in motion (kinetic energy).
performs work by changing from one form to another. Take the operation of an automobile
for example; it does the following:
a car is sitting still and not running, it has potential energy stored in the gasoline.
a car is set in motion, the gasoline is burned, changing its potential energy into heat
energy. The engine then transforms the heat energy into kinetic energy by forcing the car
action of stopping the car is accomplished by brakes. By the action of friction, the
brakes transform kinetic energy back to heat energy. When all the kinetic energy is
transformed into heat energy, the car stops.
the rate at which work is done. It takes more power to work rapidly than to work slowly.
Engines are rated by the amount of work they can do per minute. An engine that does more
work per minute than another is more powerful.
capacity of an engine is measured in horsepower (hp). Through testing, it
was determined that an average horse can lift a 200-pound weight to a height of 165 feet
in 1 minute. The equivalent of one horsepower can be reached by multiplying 165 feet by
200 pounds (work formula) for a total of 33,000 foot-pounds
per minute (fig. 2-18). The formula for
horsepower is the following:
Hp = (ft-lb. per
min)/ 33,000 = (L x W)/(33,000
L = length, in
feet, through which W is moved
W = force, in
pounds, that is exerted through distance L
t = time, in
minutes, required to move W through L
A number of
devices are used to measure the hp of an engine. The most common device is the
dynamometer (fig. 2-19) may be used to bench test an engine
that has been removed from a vehicle. If the engine does not develop the recommended
horsepower and torque of the manufacturer, you know further adjustments and/or repairs on
the engine are required.
dynamometer (fig. 2-19) is used for
automotive service, since it can provide a quick report on engine conditions by measuring
output at various speeds and loads. This type of machine is useful in shop testing and
adjusting an automatic transmission.
On a chassis
dynamometer, the driving wheels of a vehicle are placed on rollers. By loading the rollers
in varying amounts and by running the engine at different speeds, you can simulate many
and checks are made without interference by other noises, such as those that occur when
you check the vehicle while driving on the road.
that measures the actual usable horsepower of an engine is the prony brake (fig.
2-20). It is used very little today, but is
simple to understand. It is useful for learning the concept of horsepower-measuring tools.
It consists of a flywheel surrounded by a large braking device. One end of an arm is
attached to the braking device, while the other end exerts pressure on a scale. In
operation, the engine is attached to, and drives, the flywheel. The braking device is
tightened until the engine is slowed to a predetermined rpm. As the braking device slows
the engine, the arm attached to it exerts pressure on a scale. Based on the reading at the
scale and engine rpm, a brake horsepower valve is calculated by using the following
(6.28 x length of
engine rpm x
scale reading) / 33,000
It must be
noted that 6.28 and 33,000 are constants in the formula, meaning they never change. For
example, a given engine exerts a force of 300 pounds on a scale through a 2-foot-long arm
when the brake device holds the speed of the engine at 3,000 rpm. By using the formula,
calculate the brake horsepower as follows:
(6.28 x 2 x 3000 x 300) / 33,000 = 342.55 brake horsepower
a force that, when applied, tends to result in twisting an object, rather than its
physical movement. When the torque is being measured, the force that is applied must be
multiplied by the distance from the axis of the object. Torque is measured in pound-feet
(not to be confused with work which is measured in foot-pounds). When torque is applied to
an object, the force and distance from the axis depends on each other. For example, when
100 foot-pounds of torque is applied to a nut, it is equivalent to a 100-pound 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
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.
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.
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.
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 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 the amount of
air-fuel mixture, the greater the power produced by the engine.
volumetric efficiency increases engine performance. Volumetric efficiency can be
increasedin the following ways:
the intake mixture cool by ducting intake air from outside the engine compartment. By
keeping the fuel cool, you can keep the intake mixture cooler. The cooler the mixture, the
higher the volumetric efficiency. This is because a cool mixture is denser or more tightly
the intake passages (fig. 2-23). Changes to the
intake passages that make it easier for the mixture to flow through will increase the
volumetric efficiency. Other changes include reshaping ports to smooth bends, reshaping
the back of the valve heads, or polishing the inside of the ports.
the time that the valves open or how far they open can increase volumetric efficiency.
supercharging and turbocharging, you can bring the volumetric efficiency figures to over
EFFICIENCY is the relationship between the actual power produced in the engine
(indicated horsepower) and the actual power delivered at the crankshaft (brake
horsepower). The actual power is always less than the power produced within the engine.
This is due to the following:
losses between the many moving parts of the engine.
a four-stroke-cycle engine, a considerable amount of horsepower is used to drive the valve
mechanical efficiency standpoint, you can tell what percentage of power developed in the
cylinder is actually delivered by the engine. The remaining percentage of power is
consumed by friction, and it is computed as frictional horsepower (fhp).
EFFICIENCY is the relationship between actual heat energy stored within the fuel and
power produced in the engine (indicated horsepower). The thermal efficiency figure
indicates the amount of potential energy contained in the fuel that is actually used by
the engine to produce power and what amount of energy is actually lost through heat. A
large amount of energy from the fuel is lost through heat and not used in an internal
combustion engine. This unused heat is of no value to the engine and must be removed from
it. Heat is dissipated in the following ways:
cooling system removes heat from the engine to control engine operating temperature.
major portion of the heat produced by the engine exits through the exhaust system.
engine radiates a portion of the heat to the atmosphere.
portion of this waste heat may be channeled to the passenger compartment to heat it.
lubricating oil in the engine removes a portion of the waste heat.
addition to energy lost through waste heat, there are the following inherent losses in the
energy is consumed when the piston must compress the mixture on the compression stroke.
from the fuel is consumed to pull the intake mixture into the cylinder.
from the fuel is consumed to push the exhaust gases out of the cylinder.
combination of all these factors in a piston engine that uses and wastes energy leaves the
average engine approximately 20 to 25 percent thermally efficient.