Ch05 Principles of 2

Report
CHAPTER 5
Principles of Engine
Operation, Two- and
Four-Stroke Engines
Learning Objectives
• Explain simple engine operation.
• Explain why gasoline is atomized in the small
engine.
• Describe four-stroke engine operation and explain
the purpose of each stroke.
• Explain the concept of valve timing.
• Compare the lubrication system in a four-cycle
engine to the system in a two-stroke engine.
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Learning Objectives
• Describe two-stroke engine operation and explain
the principles of two-cycle operation.
• List the advantages and disadvantages of twostroke and four-stroke engines.
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Principles of Engine Operation
•
•
•
•
Converts chemical energy into mechanical energy
Gasoline engine is an internal combustion engine
Gasoline must ignite easily and burn quickly
Energy produced by burning gasoline must be
controlled
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Gasoline Must Be Atomized
• The more surface area exposed to air, the more
vapor will be given off
• More vapor leads to faster burning
• Gasoline must be atomized
• Atomization
– Increased burning area
– Explosive release of heat energy
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Elementary Engine
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Two- and Four-Stroke Engines
• Engines identified by number of piston strokes
required to complete one operating cycle
• Each stroke is either toward the rotating
crankshaft or away from it
– Bottom dead center
– Top dead center
• Strokes identified by job they perform
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Four-Stroke Engine
• Four strokes needed to complete operating cycle
–
–
–
–
Intake stroke
Compression stroke
Power stroke
Exhaust stroke
• Two strokes occur during each crankshaft
revolution
• Two crankshaft revolutions complete one
operating cycle
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Four-Stroke Engine
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Intake Stroke
• Piston travels
downward
• Volume of space
above piston
increases
– Creates partial
vacuum
• Intake valve open and
exhaust valve closed
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Intake Stroke
• Atmospheric pressure forces air through
carburetor, through intake valve port, and into
cylinder
• Intake valve must open and close at the correct
time
– Incoming air-fuel mixture cools valve during engine
operation
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Compression Stroke
• Piston moves upward
• Both valves closed
• Mixture is
compressed
– Force of combustion is
increased
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Power Stroke
• Both valves closed
• Air-fuel mixture
ignited
• Burning action forces
piston downward
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Exhaust Stroke
• Intake valve closed
and exhaust valve
open
• Rising piston pushes
exhaust gases from
engine
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Exhaust Stroke
• Exhaust valve
– Allow a streamlined
flow of exhaust gases
– Heat must be
controlled
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Four-Stroke Engines
• Valve Timing
– Measured in degrees of crankshaft rotation
– Varies with different engines
– Valve overlap
• Lubrication
– Provided by oil in the crankcase
• Splash and pump systems
– Oil must be drained and replaced periodically
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Two-Stroke Engine
• Two strokes occur during each revolution of
crankshaft
• Advantages over four-stroke engine
–
–
–
–
Simpler in design
Smaller
Lighter
Adequate lubrication at extreme angles
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Two-Stroke Engine
• Cross-scavenged
– Contoured piston head prevents air-fuel charge
from going out exhaust port
– Use reed valves or rotary valve
• Loop-scavenged
– Flat or slightly domed piston head
– Transfer ports cause incoming air to swirl
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Two-Stroke Engine
(Kohler Co.)
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Two-Stroke Engine Operation
(Rupp Industries, Inc.)
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Intake into Crankcase
• Piston moving upward
• Crankcase pressure
drops
• Intake port exposed
• Intake air through
carburetor pulls fuel
and oil into crankcase
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Ignition-Power
• Piston moving upward
• Compresses air-fuel
charge
• Spark ignites air-fuel
mixture
• Piston driven
downward
– Creates crankshaft
motion
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Exhaust
• Piston moving
downward
• Exhaust port exposed
• Exhaust gases
expelled
• Complete exhausting
occurs when transfer
ports are opened
– New charge rushes in
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Fuel Transfer
• Piston moving
downward
• Air-fuel charge in
crankcase
compressed
• Transfer port opened
– Compressed charge
rushes through port
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Four-Stoke Engine vs. Two-Stroke Engine
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Review
Why is a gasoline engine considered an internal
combustion engine?
Because gasoline is combined with air and burned
inside the engine.
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Review
What is atomization and why is gasoline atomized?
Atomization involves breaking gasoline up into tiny
droplets and mixing it with air. Gasoline is atomized to
produce the rapid burning required in an engine.
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Review
What are the four-strokes in a four-stroke cycle?
Intake stroke
Power stroke
Compression stroke
Exhaust stroke
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Review
What is valve overlap?
Valve overlap is a condition in which both valves are
open at the same time.
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Review
Why do two-stroke engines receive adequate
lubrication even when operated at extreme angles?
Because it receives its lubrication as fuel mixed with
oil is passed through the engine.
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Review
How many crankshaft revolutions are required to
complete a two-stroke cycle?
One
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Review
What advantages does a two-stroke engine have
over a four-stroke engine?
A two-stroke engine:
…is simpler in design than a four-stroke engine
…is smaller and lighter than a four-stroke engine of
equivalent horsepower
…will receive adequate lubrication even when
operated at extreme angles
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Permission granted to reproduce for educational use only.

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