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ch.5.notebook
1
February 20, 2014
Feb 207:43 AM
Ch. 5
TwoCycle and
FourCycle Engines
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February 20, 2014
Feb 207:44 AM
Stroke of the piston is its movement in the cylinder from one end of its travel to the other
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February 20, 2014
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Fourstroke cycle engine
4 strokes
1.Intake
2.Compression
3.Power
4.Exhaust
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February 20, 2014
Feb 207:47 AM
Intake Stroke
Piston traveling downward
Volume of space in cylinder increases
Creates a vacuum, draws in air/fuel mixture
Intake valve open, atmospheric pressure forces air through the carburetor
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February 20, 2014
Feb 207:49 AM
Intake Valve Functions1. Must open at correct instant to permit intake of airfuel mixture
2. Must close at correct time and seal during compression
3. Its shape must be streamlined, so the flow of gases into combustion chamber will not be obstructed
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February 20, 2014
Feb 207:50 AM
Compression StrokeCreated by piston moving upward in the cylinder
Squeezing action while both valves are closed
Valves sealed, piston rings prevent leakage past the piston
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February 20, 2014
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As the piston moves upward, the airfuel mixture is compressed into a smaller space. This increases the force of combustion for two reasons:1. When the tiny molecules of air and fuel are squeezed together, heat energy is creaed. The air/fuel mixture is heated close to its flashpoint(will ignite spontaneously) When it ignites, it is instaneous and complete for all of air/fuel mixture
2. The force of combustion is increased because tightly packed molecules(air/fuel mixture) are highly activated and are striving to move apart. This energy, combined with the expanding energy of combustion, provides tremendous force against the piston
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February 20, 2014
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Power Stroke
both valves closed
As the piston compresses the charge and reaches the top of the cylinder, an electrical spark jumps the gap and ignites the airfuel mixture
Force of explosion forces the piston downward
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February 20, 2014
Feb 207:57 AM
Power Stroke
Fuel charge must ignite and expand in short time
Amount of power produced by power stroke depends on the volume of air/fuel mixture in the cylinder and the compression ratio of the engine
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February 20, 2014
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Compression Ratio
proportionate difference in volume of cylinder and combustion chamber at bottom dead center (BDC) and at top dead center (TDC)
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February 20, 2014
Feb 208:10 AM
After the piston has completed the power stroke, the burned gases must be removed from cylinder before introducing a fresh air/fuel mixture
exhaust valve opens and rising piston pushes gases from cylinder
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February 20, 2014
Feb 208:33 AM
Exhaust Valve
When closed, it must seal
When open, it must allow streamlined flow of exhaust gases through the port
Scavengingremoval of gases from cylinder
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February 20, 2014
Feb 208:38 AM
Heat Transfer
Heat must be controlled or the valve will deteriorate rapidly
The hottest part of the valve, the valve head, transfers heat through the valve seat to the cylinder block
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February 20, 2014
Feb 208:39 AM
Valve Timing
Top Dead Center (TDC)
Bottom Dead Center(TDC)
If timing marks are aligned on camshaft and crankshaft, the valve timing will take care of itself
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February 20, 2014
Feb 208:41 AM
Valve Overlapoccurs when both valves are open at the same time
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LubricationPlace correct quantity and grade of engine oil in the crankcase
Splash and pump are two most common methods
Must be drained and and replaced periodically
Must be operated in upright position to lubricate properly
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February 20, 2014
Feb 208:53 AM
TwoStroke CycleIntake, compression, power and exhaust strokes take place during only two strokes of the piston
Only takes one revolution of the shaft to complete one twostroke cycle
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February 20, 2014
Feb 208:56 AM
TwostrokeSmaller, lighter
Will get adequate lubrication at extreme angles because it recieves lubrication as fuel is mixed with oil and is passed through the engine
Must install correct mixture of fuel and oil
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February 20, 2014
Feb 208:57 AM
Variations in Design of TwoCycle
Crossscavenged
Loopscavenged
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February 20, 2014
Feb 208:58 AM
CrossScavengedspecial contour on piston head, which acts as a baffle to deflect airfuel charge upward in cylinder
Usually have reed valveshold incoming charge in crankcase so it can be compressed
Piston acts as a valve in opening and closing intake
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February 20, 2014
Feb 209:00 AM
LoopScavengedFlat or domed piston.
The fuel transfer ports in loopscavenged engines are shaped so that the incoming airfuel mixture swirls
Forces gases out and permits new air in
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February 20, 2014
Feb 209:03 AM
Principles of Operation
Location of ports is essential to correct timing of intake, transfer, and exhaust functions
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Intake into CrankcaseAs piston moves upward into the cylinder, crankcase pressure drops and intake port is exposed
Atmospheric pressure is greater than crankcase pressure, air rushes through the carburetor and into the crankcase
While passing through the carburetor, the intake air pulls a charge of fuel and oil along
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February 20, 2014
Feb 209:07 AM
Fuel Transfer
Piston moves downward
Transfer port opens, new charge cools combustion area and pushes out exhaust gases
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Feb 209:08 AM
Ignition Power
Piston travels upward, compresses airfuel charge in cylinder to about 1/10 of volume
Spark ignites at TDC
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February 20, 2014
Feb 209:09 AM
Exhaust
Piston exposes exhaust port, most of burned gases are expelled
Other gases escape when new airfuel mixture rushes in
COMPLETES ONE CYCLE
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February 20, 2014
Feb 209:13 AM
SCAVENGING and TUNING
Best efficiency fuel charge must be held in the cylinder momentarily while the exhaust port is open
Well engineered exhaust systems use the energy of sound waves from the exhaust gases for proper tuning
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February 20, 2014
Feb 209:15 AM
Rotary Disc valve Engine
Intake port located directly in the crankcase, allowing room for additional transfer ports that promote better fuel transfer and scavenging
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February 20, 2014
Feb 209:17 AM
Reed Valve Engine
permits fuel intake directly into the crankcase
Reed is made of thin, flexible spring steel
Reed stopstops reed valve from opening too far, thick and inflexible