Ch 14- Small Gas Engines

Report
Chapter 14
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Identify differences between internal and
external combustion engines
Understand 2-stroke vs. 4-stroke engines
Understand subsystems of small gas engines
Discuss procedures for assembling and
disassembling small gas engines
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External combustion engines: produce heat
outside of the cylinder containing the piston
◦ Often used boilers to create steam
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Internal combustion engines: produce heat
Inside of the cylinder containing the piston
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More reliable than ECE
Produce more power than similar size ECE
Used to power MOST vehicles in the USA
Used in agriculture and construction industries
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Cylinder ( aka. cylinder bore): is a hole in the
block that directs the piston during
movement
The ICE began replacing the ECE about 100
years ago.
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All ICEs convert chemical energy into
mechanical power and share common
mechanical elements
Two main types of engines
◦ Two Stroke
◦ Four Stroke
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Can be any number of cylinders
(1,2,3,4,6,8,10,12) and all are coupled to a
single crank-shaft
Crank-shaft: converts the reciprocal motion
of the pistons into rotary motion and powers
the load
Piston: a cylindrical engine component that
slides back and forth in the cylinder when
propelled by the force of combustion.
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Stroke: the movement of the piston from the
bottom limit of its travel to the top limit of its
travel in the cylinder bore.
Require 4 strokes of the piston to complete
one cycle
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Intake Stroke
Compression Stroke
Power Stroke (combustion)
Exhaust Stroke
Intake
Compression
Power
Exhaust
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Intake Stroke: (downward) creates a partial
vacuum drawing air into the cylinder through
the carburetor where liquid fuel is atomized
and mixed with the air (called a fuel-air
charge).
Intake valve is open
Exhaust valve is closed
4-stroke
graphic
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Compression Stroke: (upward) Fuel-air charge is
squeezed to about 1/10th of its original volume
Bottom Dead Center (BDC) when the piston is at
its lowest point (crankshaft is rounding the
bottom of its travel)
Top Dead Center (TDC) when the piston is at its
highest point (crankshaft is rounding the top of
its travel)
Compression ratio is mathematical relationship
between BDC and TDC (ie: 10:1 compression)
Intake and exhaust valves are closed
4-stroke
graphic
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Power Stroke: (downward) With piston near
TDC the compressed fuel-air charge is
detonated (by the spark plug)
Combusting gasses expand pushing down
piston.
The connecting rod pushes down on the
crank shaft causing it to rotate
Intake and exhaust valves are closed
4-stroke
graphic
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Exhaust: (upward) Piston moves from BDC to
TDC pushing the spent fuel-air mixture out
of the cylinder
Piston is moved up by momentum or by
power stroke of another piston pushing on
the crank shaft
Intake valve is closed
Exhaust valves is open
4-stroke
graphic
http://upload.wikimedia.org/wikipedia/commo
ns/a/a6/4-Stroke-Engine.gif
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Every upward stroke is a compression stroke
Every downward stroke is a power stroke
Intake and Exhaust stroke occur during the
compression and power strokes
Every revolution of the crankshaft produces
power
◦ On a 4-stroke engine, it takes 2 revolutions
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2-stroke engines are more powerful for their size
Good at high RPM (revolutions per minute)
applications
Simpler design than 4-stroke (less parts)
◦ No valve train
◦ No cam-shaft
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Lighter than 4-stroke engines of comprable
power
◦ No oil reservoir
◦ No valve train, cam, etc.
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Can be operated at any angle (no oil reservoir)
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Intake and exhaust occur through ports on
the side of the cylinder.
Oil is mixed with the fuel and burned in the
combustion chamber.
Pressure from the moving piston pushes
gas/air/oil where it needs to go.
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Exhaust is dirtier than 4-stroke because oil is
burned
They wear more quickly than 4-stroke
because every other stroke is a power stroke
◦ They don’t last as long
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Mixing oil with fuel is inconvenient and if
forgotten it will destroy the engine
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http://www.animatedengines.com/twostroke.
html
http://fr.wikipedia.org/wiki/Fichier:2Stroke_Engine_ani.gif
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Many of them on all engines
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All must perform properly for peak performance
Cooling subsystem
Electrical subsystem
Lubrication subsystem
Mechanical subsystem
Governing subsystem
Fuel subsystem
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Can be cooled by air or liquid
Air cooled systems
◦ Cooling fins increase surface area
◦ Flywheel blades direct air across engine fins
◦ Sheet metal shrouds direct the air
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Liquid cooled systems
◦ Water jackets surround cylinder walls
◦ Water pumps move water through jackets to
radiator
◦ Radiator expose surface area to surrounding air
◦ Thermostat allows/impedes flow of water to
radiator
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Oil distribution mechanism
Oil seals
Piston rings
Oil
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ALL moving parts must be lubricated
Splash lubrication method
◦ Better for small gas engines
◦ “Oil dipper” attached to bottom of connecting rod
flings oil up on bottom of pistons
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Piston Rings
◦ Oil ring: (bottom ring) limits the amount of oil that
squeezes past the piston into the combustion
chamber
◦ Compression ring(s): (upper ring(s)) contain
combustion, scrape oil off of cyl. walls back into
crankcase.
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Oil
◦ Protects internal parts from corrosion
◦ Cleans engine for foreign matter and allowing it to
settle into the oil reservoir (crankcase or oil pan)
◦ Seal the engine by filling small spaces between
moving parts (ie: piston rings and moving parts)
◦ Cushion moving parts from the power stroke
◦ Improve fuel economy by reducing friction
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Viscosity: measures resistance to flow
(thickness)
◦ Developed by the
◦ Society of Automotive Engineers (SAE)
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Converts the force of the expanding gasses
during combustion into mechanical power
and delivers it to the crankshaft
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Engine block (housing for all components)
Piston
Piston pin (aka: Wrist pin)
Connecting rod
Crankshaft (crankpin journal)
In a 4-stroke engine the crankshaft also
powers the camshaft and valvetrain.
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Opens and closes valves by pushing on rods
called lifters (some are adjustable for cam
wear)
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Heavy metal disk attached to the Crankshaft
◦ Inertia of the rotating engine created by power
stroke helps the engine coast through the exhaust,
intake and compression stroke
◦ Smoothes out the power produced by the engine so
it does not continually speed up and slow down
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This system takes the most wear (usually not
visible)
◦ Measurements are made in critical areas for wear
and for warpage
◦ Micrometers
◦ Feeler gauges (AKA: thickness Gauge)
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Produces the current
that fires the sparkplug
◦ Permanent magnet in the
flywheel
◦ Magnet passes the
armature as flywheel
spins creating low voltage
◦ Converted to high voltage
in the ignition coil
◦ Spark jumps the gap in
the spark plug to ignite
fuel/air charge
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Timing
◦ Shear pin (key) keeps
flywheel aligned on the
crankshaft so spark is
produced before TDC
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Spacing of armature
◦ Too close will rub on
flywheel
◦ Too far produces weak
spark
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Sparkplug
◦ Must be “gapped”
properly using feeler
guage
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Work in conjunction with one another
Governing system is designed to keep the
engine running at the desired speed
regardless of load
Fuel subsystem is responsible for creating the
fuel/air mix used to power the engine and
deliver it to the combustion chamber
◦ Carburetor
◦ Fuel injectors
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Fuel is pressurized and
sprayed into the
cylinder before TDC
Very common on cars
and trucks with gas or
diesel engines
Regulated by
computers in modern
cars to achieve
maximum performance
with minimum
emissions
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Very common on small
engines and older cars
Fuel vapor is drawn
through the carb by
the air that rushes past
it (by the intake stroke)
This occurs in the
venturi.
Venturi Effect states
that pressure
decreases as velocity
increases.
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Venturi: Narrow restricting section of
carburetor where air speeds up and drafts the
fuel vapor along with it into Cylinder
Choke: Plate-like device (usually) that varies
the amount of air that can enter the carb.
Throttle: plate-like device located in back of
venturi that regulates amount of fuel air mix
entering the cylinders.
Load: condition under which an engine runs
when it does work
◦ Choke plate and Throttle are open
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Idle: the condition an engine will run under
when it is warmed up to temperature and
NOT under load
◦ Choke is open
◦ Throttle is closed
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Idle Bypass Circuit: small passageway that
allows some air/fuel mix to escape around
the throttle plate to keep engine running
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All complex machines need maintenance,
periodic testing and troubleshooting to run
their best
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Emissions testing
Temperature regulation
Tune-ups
Air filter changes
Oil changes
Etc.
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Volumetric Efficiency: measures how well the
engine “breathes.” Measure of how much fuel
air mixture is drawn into cylinders with the
amount that could be drawn in.
Mechanical efficiency: Percentage of power
developed in the cylinder compared to the
power that is actually delivered to the
crankshaft
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Thermal Efficiency: (aka heat efficiency)
measure of how much heat is actually used to
drive the pistons downward.
◦ Only about 25% is used to drive the piston
downward, the rest is lost.
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Practical Efficiency: simple measure of how
efficiently an engine uses its fuel supply
◦ If used for motive power it is measured in MPG
◦ Takes into account all losses of efficiency
 friction
 Drag
 Thermal loss, etc
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Developed as a means of comparing the
power produced by James Watt’s steam
engine to the amount of work a horse could
do.
◦ 550 foot-pounds per second
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Horsepower capability is affected by
◦ Bore: diameter of the piston
◦ Stroke: Distance from TDC to BDC
◦ Frictional loss: within the engine (frictional vs nonfrictional bearings)
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Brake Horsepower
(bhp): the hp available
for use at the
crankshaft. Increases
with engine rpm then
decrease when engine
is revved to high
Indicated horsepower
(ihp): Theoretical term.
Measure of the power
developed by the fuel
air charge upon
ignition
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Frictional Horsepower
(fhp): represents the
part of the potential hp
lost due to friction
within the engine
ihp-bhp=fhp
Rated horsepower
(rhp): usually
represents about 80%
of the engines bhp
because engines should
not be run at full
capability all the time
(the sticker rating)

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