Unit 2: Introduction to Small Engines

Report
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Combustion – whether the engine is internal
or external combustion.
Ignition – compression versus spark ignition
Number of Strokes – 2 stroke or 4 stroke
Cylinder Design – vertical, horizontal, slant,
V, opposed, inline
Shaft Orientation - vertical or horizontal
Cooling System – liquid cooled or air cooled
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Two types of Engines based on Combustion
◦ External Combustion Engines
◦ Internal Combustion Engines
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External combustion engines separate the
heat source from the source of power.
The external heat source heats an internal
fluid, through a heat exchanger of some type
The heated fluid expands creating pressure
The pressure drives a turbine which provides
power for use.
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Steam engines
Stirling engines
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first invented by Thomas
Newcomen in 1705.
powered all early locomotives,
steam boats and factories
acted as the foundation of the
Industrial Revolution.
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Heat is obtained from fuel burnt in a closed
firebox
The heat is transferred to the water in a
pressurized boiler, boiling the water and
transforming it into saturated steam.
The steam is transferred to the motor unit
which uses it to push on a piston sliding
inside a cylinder to power machinery.
The used, cooler, lower pressure steam is
exhausted to atmosphere
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Invented by Robert Stirling in
1816,
Has the potential to be much
more efficient than a gasoline or
diesel engine.
Today, Stirling engines are used
only in some very specialized
applications, like in submarines
or auxiliary power generators
for yachts, where quiet
operation is important
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The Stirling cycle engine uses air as its liquid,
which resolves many of the issues with
steam. It is not as dangerous as steam and
does not lose as much energy in transition,
because it does not transition.
The gasses used inside a Stirling engine never
leave the engine. There are no exhaust valves
that vent high-pressure gasses, as in a
gasoline or diesel engine, and there are no
explosions taking place. Because of this,
Stirling engines are very quiet.
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The key principle of a Stirling engine is that a
fixed amount of a gas is sealed inside the
engine.
Stirling cycle involves a series of events that
change the pressure of the gas inside the
engine, causing it to do work.
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Heat is added to the gas
inside the heated
cylinder (top), causing
pressure to build. This
drives the hot piston in
its power stroke. This is
the part of the Stirling
cycle that does the work
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The heated gas expands
and pushes the hot
piston to the bottom of
its travel in the cylinder.
The expansion continues
in the cold cylinder,
which is 90° behind the
hot piston in its cycle,
extracting more work
from the hot gas.
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The gas is now at its
maximum volume. The
hot cylinder piston
begins to move most of
the gas into the cold
cylinder, where it cools
and the pressure drops
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Almost all the gas is now
in the cold cylinder and
cooling continues. The
cold piston, powered by
flywheel momentum (or
other piston pairs on the
same shaft) compresses
the remaining part of
the gas
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Because the heat source is external, it takes a
little while for the engine to respond to
changes in the amount of heat being applied
to the cylinder -- it takes time for the heat to
be conducted through the cylinder walls and
into the gas inside the engine. This means
that:
◦ The engine requires some time to warm up before it
can produce useful power.
◦ The engine can not change its power output
quickly.
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An internal combustion engine is one in
which:
◦ the combustion of a fuel is used to push a piston
within an cylinder
◦ the pistons movement turns a crankshaft that
provides mechanical power
◦ mechanical power moves the other parts of the
drive train
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2 stroke cycle
4 stroke cycle
Compression (diesel)
Rotary
Rocket
Gas Turbine
Jet (Hache)
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Fuel and air in the cylinder have
been compressed, and when
the spark plug fires the mixture
ignites.
The resulting explosion drives
the piston downward. Note that
as the piston moves downward,
it is compressing the air/fuel
mixture in the crankcase.
As the piston approaches the
bottom of its stroke, the
exhaust port is uncovered. The
pressure in the cylinder drives
most of the exhaust gases out
of cylinder.
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As the piston reaches the
bottom the intake port is
uncovered.
The piston's movement has
pressurized the mixture in
the crankcase, so it rushes
into the cylinder, displacing
the remaining exhaust gases
and filling the cylinder with a
fresh charge of fuel
The piston is shaped so that
incoming fuel doesn’t simply
flow right over the top of the
piston and out the exhaust.
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the momentum in the
crankshaft starts driving the
piston back toward the spark
plug for the compression
stroke.
As the air/fuel mixture in the
piston is compressed, a
vacuum is created in the
crankcase.
This vacuum opens the reed
valve and sucks air/fuel/oil in
from the carburetor.
Once the piston makes it to the
end of the compression stroke,
the spark plug fires again to
repeat the cycle.
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It's called a twostoke engine
because there is a
compression stroke
and then a
combustion stroke.
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Two-stroke engines do not have valves, which
simplifies their construction and lowers their
weight.
Two-stroke engines fire once every revolution,
while four-stroke engines fire once every other
revolution. This gives two-stroke engines a
significant power boost.
Two-stroke engines can work in any orientation,
which can be important in something like a
chainsaw. A standard four-stroke engine may
have problems with oil flow unless it is upright,
and solving this problem can add complexity to
the engine.
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The lack of a dedicated lubrication system means that the
parts of a two-stroke engine wear a lot faster so engines
don’t last as long.
Two-stroke oil is expensive, and you need about 4 ounces
of it per gallon of gas
Two-stroke engines do not use fuel efficiently, so you
would get fewer miles per gallon.
Two-stroke engines produce a lot of pollution -- so much,
in fact, that it is likely that you won't see them around too
much longer. The pollution comes from two sources:
◦ The first is the combustion of the oil. The oil makes all two-stroke
engines smoky to some extent, and a badly worn two-stroke
engine can emit huge clouds of oily smoke.
◦ The second reason is that each time a new charge of air/fuel is
loaded into the combustion chamber, part of it leaks out through
the exhaust port. That's why you see a sheen of oil around any
two-stroke boat motor
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Chain saws
Lawn cutters
Snowmobiles
Outboard motors
Dirt bikes
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The cycle begins at top dead center (TDC),
when the piston is farthest away from the
axis of the crankshaft.
The intake valve opens.
The piston descends from the top of the
cylinder, reducing the pressure inside the
cylinder.
A mixture of fuel and air is forced (by
atmospheric or greater pressure) into the
cylinder through the intake (inlet) port.
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When the piston reaches the lower limit of its
travel, it begins to move upward.
The intake (inlet) valve (or valves) close
As the piston moves upward, the air/fuel
mixture is compressed
The compression stroke compresses the fuel–
air mixture.
The compression process also causes the
air/fuel mixture to increase in temperature.
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As the piston reaches the top of its travel on the
compression stroke, a high voltage electric spark
is produced at the spark plug.
The air–fuel mixture is then ignited near the end
of the compression stroke:
◦ by a spark plug (for a gasoline or Otto cycle engine)
◦ by the heat and pressure of compression (for a Diesel
cycle or compression ignition engine).
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The resulting pressure of burning gases pushes
the piston through the power stroke.
The power impulse is transmitted down through
the piston, through the piston rod (connecting
rod), and to the crankshaft. The crankshaft is
rotated due to the force.
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As the piston reaches the bottom of its travel,
the exhaust valve opens.
In the exhaust stroke, the piston pushes the
products of combustion from the cylinder
through an exhaust valve or valves.
When the piston reaches the top of its travel,
the exhaust valve closes, and the intake valve
opens.
The cycle repeats again with the intake
stroke.
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These four strokes require two revolutions of
the crankshaft. The process continuously
repeats itself during the operation of the
engine.
Thus the engine only fires once every four
strokes or every second time the piston reaches
the top of its travel.
http://www.youtube.com/watch?NR=1&v=GwFB
3RcVcHI
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use much less fuel than 2 strokes
produce less pollution
have a wider power band (the engine RPM
range over which the engine produces its
most power.
have a dedicated lubrication system which
means that
◦ They usually last longer
◦ They do not burn oil
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They are heavy, more complicated and more
expensive to build
They do not create as much power as a same
size 2 stroke.
They cannot operate in a non-vertical
position.
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Automobiles
ATV’s (4 wheelers)
Snowmobiles
Snowblowers
Lawnmowers
Motorcycles
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Rotary engines use the four-stroke combustion cycle,
which is the same cycle that four-stroke piston
engines use. But in a rotary engine, this is
accomplished in a completely different way.
The heart of a rotary engine is the rotor. This is
roughly the equivalent of the pistons in a piston
engine. The rotor is mounted on a large circular lobe
on the output shaft. This lobe is offset from the
centerline of the shaft and acts like the crank handle
on a winch, giving the rotor the leverage it needs to
turn the output shaft. As the rotor orbits inside the
housing, it pushes the lobe around in tight circles,
turning three times for every one revolution of the
rotor.
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The cycle starts when the tip of the rotor passes
the intake port.
When the intake port is exposed to the chamber,
the volume of that chamber is close to its
minimum.
As the rotor moves past the intake port, the
volume of the chamber expands, drawing air/fuel
mixture into the chamber.
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As the rotor continues its motion around the
housing, the volume of the chamber gets
smaller and the air/fuel mixture gets
compressed.
By the time the face of the rotor has made it
around to the spark plugs, the volume of the
chamber is again close to its minimum. This
is when combustion starts.
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When the spark plugs ignite the air/fuel
mixture, pressure quickly builds, forcing the
rotor to move.
The pressure of combustion forces the rotor
to move in the direction that makes the
chamber grow in volume. The combustion
gases continue to expand, moving the rotor
and creating power.
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Once the peak of the rotor passes the
exhaust port, the high-pressure combustion
gases are free to flow out the exhaust.
The rotor continues to move forcing the
remaining exhaust out of the port.
When the rotor passes the intake port and the
whole cycle starts again.
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http://www.youtube.com/watch?v=QRiPSlx8P
xA
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A small engine is a Spark Ignition or
Compression Ignition engine based on how
the fuel is ignited.
Spark ignition
◦ the fuel mixture is ignited with an electrical spark
◦ Commonly use gasoline
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Compression
◦ The fuel mixture is ignited by compressing the fuel
mixture under pressure and heat
◦ Commonly use diesel fuel
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A stroke is one complete travel of the piston
from top dead center to bottom dead center or
vice versa.
Two (2) Stroke
◦ Utilizes two strokes to complete the intake,
compression, power (ignition) and exhaust cycle
◦ Fuel intake and compression on one stroke
◦ Power (ignition) and exhaust on the other stroke
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Four (4) Stroke
◦ Utilizes four strokes to complete the intake,
compression, power (ignition) and exhaust cycle
◦ Intake, compression, power (ignition) and exhaust
occurs on a different stroke.
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Cylinder Design
◦ Vertical – pistons travel up and down vertically
◦ Horizontal - pistons travel back and forth in the
horizontal plane
◦ Slant – pistons are oriented at an angle to the
vertical
◦ V – pistons are divided into two banks at an angle
to the vertical forming a v-shape
◦ Inline – pistons are all oriented in the same
direction
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the cylinders are arranged inline in a single
bank that move vertically:
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also known as horizontally opposed or a
boxer
the cylinders are arranged in two banks on
opposite sides of the engine:
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the cylinders are
arranged inline and
specifically designed
such that the cylinders
are inclined at a 30degree angle from
vertical.
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V - the cylinders are arranged in two banks
set at an angle to one another:
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the cylinders are arranged in a line in a single
bank. Can be arranged vertically or slanted.
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Vertical
◦ The shaft extends from the bottom of the engine.
◦ common applications of a vertical engine include:
 walk-behind rotary lawnmowers
 yard tractors
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Horizontal
◦ The shaft extends from one side of the engine and
rotates parallel to the ground.
◦ common applications of a vertical engine include:
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Generators
snow throwers
water pumps
pressure washers.
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Vertical
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Horizontal
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Air Cooled
◦ Air is circulated around the cylinder block and
cylinder head to maintain the desired temperature
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Liquid Cooled
◦ Liquid is circulated through cavities in the cylinder
block and cylinder head to maintain the desired
temperature
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Heat is also removed through the exhaust
system and radiant heat from the engine
components.
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Most small, single-cylinder engines are
cooled by a stream of air developed by fan
blades on the flywheel.
The air stream is deflected around the
cylinder and cylinder head by a metal or
plastic cover called a shroud.
Additional engine heat is dissipated through
cooling fins around the cylinder.
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The cooling system on liquid-cooled cars
circulates a fluid through pipes and
passageways in the engine.
As this liquid passes through the hot engine
it absorbs heat, cooling the engine.
After the fluid leaves the engine, it passes
through a heat exchanger, or radiator, which
transfers the heat from the fluid to the air
blowing through the exchanger.
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A machine that converts a form of energy into
mechanical force.
Combustion engines generates heat from an
internal or external source and converts that
energy into rotation force on the crankshaft.
A small engine is an internal combustion
engine that converts heat energy from the
combustion of a fuel into mechanical energy
generally rated up to 25 horsepower.
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Energy is the resource that provides the
capacity to do work
Two forms of energy are:
◦ Potential energy
 Stored energy due to its position, chemical state or
condition
 Water behind a dam, because of its position
 Gasoline based on its chemical state
◦ Kinetic energy
 Is energy of motion (released potential energy)
 Water falling over a dam
 A speeding automobile
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Small gasoline engines convert the stored
potential energy of gasoline into kinetic
energy of the rotating shaft
The rotating shaft of the engine is used to do
work required by the engine application ie.
Mow grass, throw snow etc.
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All internal combustion engines operate by
utilizing basic principles of :
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Heat
Force
Pressure
Torque
Work
Power
Chemistry
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All matter is composed of atoms that are in
constant motion.
Heat is the kinetic energy caused by atoms
and molecules in motion within a substance
◦ Heat added to a substance causes the particles
velocity to increase resulting in a higher internal
energy
◦ Heat removed from a substance causes particle
velocity to decrease resulting in decreased internal
energy.
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A substance can be a liquid, solid or gas
The state of a substance depends upon the
intensity of vibration of the molecules
To change to or from one state to another
requires the addition or removal of heat
energy.
◦ When heat is added to ice it changes to water, when
heat is added to water it changes to steam (vapor)
◦ As the compression in a cylinder increases, heat
increases, the liquid gasoline droplets in the fuel
mixture change to a gaseous state preparing the
fuel for more efficient combustion.
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Heat is transferred (flows) from one
substance to another when a temperature
difference exists.
Heat is always transferred from a substance
with a higher temperature to a substance with
a lower temperature
Heat transfer rates are proportional to the
temperature difference between the two
substances
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There are three methods of heat transfer:
◦ Conduction
 Occurs when particles of a substance come in direct
contact with each other
◦ Convection
 Occurs when heat is transferred by currents in a fluid
◦ Radiation
 Occurs when radiant heat travels without a material
carrier
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Transfer of heat using direct contact
Heat one end of a metal rod, kinetic energy is
passed from one to another, heat is
transferred from one end to the other.
In a small engine:
◦ Engine oil is in direct contact with the hot engine
parts.
◦ Heat is transferred from the hotter parts to the oil
◦ As the oil moves to the oil reserve the cooler
crankcase assemble conducts the heat from the oil
to the air contacting the outside of the engine
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Heat is transferred by currents in a fluid
As air is warmed by a fire, the warm air rises and
is replaced by cooler air. The movement of air
continues as long as the air is heated by the fire.
In a Small engine convection occurs in a liquid
cooled radiator:
◦ A radiator is a multi-channeled container that allows air
to pass around the channels to remove heat from the
liquid within
◦ Warm liquid is pumped into the top of the radiator
cooled in the radiator and moves back into the engine
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Occurs when radiant heat travels without a
material carrier
Radiant energy travels through space without
producing heat. Heat is produced when the
waves strike an opaque object.
Heat is produced on earth by radiant energy
from the sun
In a small engine radiant energy is produced
by the heated metal that radiates heat away
from the parts where there is little air flow
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The primary function of the radiator
is to transfer waste heat
The processes that accomplish this
are:
◦ Convection
◦ Conduction
◦ Radiation
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These processes are dependent upon
3 variables:
◦ The existence of temperature differences
between liquid and air
◦ The existence of temperature differences
between coolant and air flow
◦ The design of the heat transfer surfaces
to maximize their potential
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Temperature is the
measurement of the degree or
intensity of heat
Temperature can be measured
by a glass thermometer
◦ A glass tube is filled with alcohol
or mercury
◦ The tube has a scale
◦ Based on the known expansion
rate of the material
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The quantity of heat is the amount of heat
required to produce an accepted standard of
physical change in matter
A British Thermal Unit (BTU) is the amount of
heat energy required to raise the temperature
of one pound (lb) of Water 1oF (Fahrenheit)
A Calorie is the amount of heat energy
required to raise the temperature of one
gram (g) of Water 1oC (Celsius)
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Force is anything that changes or tends to
change the state of rest or motion of a body
Force is measured in pounds (lb) in the
English system and newtons (N) in the metric
system
One or more forces can act on body
Force acting on a body does not always
produce motion
Force applied in different ways can produce
pressure, torque or work
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Pressure is a force acting on a unit of area
Area is the number of unit squares equal to
the surface of the object
In an internal combustion engine force is
exerted by combustion pressure applied to
the area of the piston head
P = F/A
Where
◦ P – pressure (in lb/sq. in.)
◦ F – Force (in lb)
◦ A – Area (in sq. in.)
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What is the pressure exerted on the top of a
4.91sq. in. piston if the combustion exerts a
force of 2000 lb ?
P = F/A
P = 2000 lb/ 4.91sq. in.
P = 407.33 lb/sq. in.
P = 407.33 psi
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Torque is a force acting on a perpendicular radial
distance from a point of rotation
The result is a twisting or turning force
expressed in pound-feet (lb-ft) or newtonmeters (Nm) that may or may not result in motion
Torque is found by applying the formula:
T=Fxr
Where
◦ T – torque (in lb-ft or Nm)
◦ F - force (in lb or N)
◦ r – radius (in ft or m)
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How much torque is produced by a 60 lb
force pushing on a 2’ lever arm ?
T=Fxr
T = 60 lb x 2 ft
T = 120 lb-ft
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Work is the force applied through a parallel
distance causing linear motion.
Work is measured in lb-ft or Nm
Torque and work are very similar. The real
difference is that torque may not produce
motion.
Work is found by applying the formula
W=FxD
Where
◦ W – work (in lb-ft or Nm)
◦ F - force (in lb or N)
◦ D – distance (in ft or m)
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What is the amount of work performed if a
horse pulled a container that weighed 330 lb
a distance of 100’ ?
W=FxD
W= 330 lb x 100 ft
W= 33,000 lb-ft
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Power is the rate at which work is done
Power ratings include horsepower, the watt
(W) or kilowatt (kW)
Power is found applying the formula:
P = W/T
Where
◦ P – power (in lb-ft/min or W)
◦ W – work (in lb-ft or Nm)
◦ T - time (in min)
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What is the power output of an engine that
produces 100,000 lb-ft of work in 6 min. ?
P = W/T
P = 100,000 lb-ft / 6 min.
P = 16,666.67 lb-ft/ min.
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Horsepower (HP) is a unit of power equal to:
◦ 746 watts (W)
◦ 33,000 lb-ft/min
◦ 550 lb-ft/s
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Horsepower is commonly used to rate power
produced by and engines at a finite speed
The formula for Horsepower is:
HP = W/(T x 33,000)
Where
◦ HP – horsepower (in HP)
◦ W – work (in lb-ft/min)
◦ T - time (in min)
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What is the horsepower rating of an engine
that produces 412,500 lb-ft in 2.5 minutes ?
HP
HP
HP
HP
=
=
=
=
W/(T x 33,000)
412,500 lb-ft /(2.5 min x 33,000)
412,500 lb-ft /(82,500 min)
5 HP
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All internal combustion engines utilize some
form of fossil fuel (hydrocarbon)
Combustion chemistry involves the
combining of hydrocarbon fuel with oxygen
from the atmosphere.
When ignition occurs in the engine a chemical
reaction between the hydrocarbon molecule
and atmospheric oxygen causes an exchange
of elements which releases heat energy
2 C8H18 + 25 O2 + 94 N2
Fuel Mixture
16 CO2 + 18 H2O + 94 N2
Exhaust Gasses

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