Principles of Engine Operation, Two- and Four-Stroke Engines
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Transcript of Principles of Engine Operation, Two- and Four-Stroke Engines
C H A P T E R 5
Principles of Engine Operation, Two- and Four-Stroke Engines
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• 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.
Learning Objectives
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• Describe two-stroke engine operation and explain the principles of two-cycle operation.
• List the advantages and disadvantages of two-stroke and four-stroke engines.
Learning Objectives
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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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• Piston travels downward
• Volume of space above piston increases– Creates partial
vacuum
• Intake valve open and exhaust valve closed
Intake Stroke
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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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• Piston moves upward• Both valves closed• Mixture is
compressed– Force of combustion is
increased
Compression Stroke
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• Both valves closed• Air-fuel mixture
ignited• Burning action forces
piston downward
Power Stroke
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• Intake valve closed and exhaust valve open
• Rising piston pushes exhaust gases from engine
Exhaust Stroke
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• Exhaust valve– Allow a streamlined
flow of exhaust gases– Heat must be
controlled
Exhaust Stroke
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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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• Piston moving upward• Crankcase pressure
drops• Intake port exposed• Intake air through
carburetor pulls fuel and oil into crankcase
Intake into Crankcase
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• Piston moving upward• Compresses air-fuel
charge• Spark ignites air-fuel
mixture• Piston driven
downward– Creates crankshaft
motion
Ignition-Power
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• Piston moving downward
• Exhaust port exposed• Exhaust gases
expelled• Complete exhausting
occurs when transfer ports are opened– New charge rushes in
Exhaust
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• Piston moving downward
• Air-fuel charge in crankcase compressed
• Transfer port opened– Compressed charge
rushes through port
Fuel Transfer
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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