two stroke engine operation
DESCRIPTION
Two Stroke Engine Operation. Automotive Technology 1 Mr. Wasacz. Objectives. The student will be able to explain the operation of a 2 stroke engine The student will be able to identify the differences between a 2 and 4 stroke engine. - PowerPoint PPT PresentationTRANSCRIPT
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Two Stroke Engine Operation
Automotive Technology 1
Mr. Wasacz
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Objectives
The student will be able to explain the operation of a 2 stroke engine
The student will be able to identify the differences between a 2 and 4 stroke engine.
The students will be able to compare the lubrication systems of a 2 and 4 stroke engine
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Quick Reminder
Stroke is the movement of the piston from one end of the cylinder to the other
TDC to BDC BDC to TDC
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Two Stroke Engine
Performs the same tasks as a 4 strokeHowever, intake, compression, power, and exhaust take place simultaneously
The crankshaft only rotates ONCE, so the cycle is complete after only 2 strokes
The design of two strokes is much more simple!
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Variations in Design
Cross Scavenged Has contoured piston head to deflect air upward to prevent intake from flowing out through exhaust
Uses a reed valve or rotary valve to hold the incoming charge in the crankcase
The downward movement of the piston creates pressure in the crankcase
The Piston acts as a valve for the cylinder
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Cross Scavenged Diagram
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Variations in Design
Loop- Scavenged Does not deflect incoming gases Flat or Domed piston Ports are positioned and shaped so that incoming gases swirl in the cylinder
This swirl keeps the intake gases in, and forces the exhaust gases out
May have more than one transfer port to create this swirling effect
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Loop-Scavenged Diagram
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Intake into the crankcase
As the piston moves upwards in the cylinder, low pressure in the crankcase is created
This pressure allows the higher outside atmospheric to rush into the crankcase through the carburetor
When the air moves through the carburetor, it takes a charge of gasoline and oil with it.
This newly in took charge lubricates the bearings and moving parts in the crankcase
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Fuel Transfer
As the piston moves downwards, it compresses the air fuel and oil in the crankcase
When it travels far enough downward, it exposes the transfer port into the cylinder
The newly compressed air, fuel, and oil then rush upwards towards the cylinder
This new charge of air, fuel, and oil also cool the hot combustion area, and push the previous exhaust gasses outward
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Ignition-Power
The piston travels upwards, compressing the charge to approx 1/10th its original volume
Spark ignites charge when piston reaches TDC
The ignited gasses force the piston downward
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Exhaust
As the piston moves downwards, the exhaust port is exposed
This allows for the removal of gasesThe remainder of gases are forced out when the transfer ports are exposed
This completes one cycle of operation
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What it looks like all together
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Scavenging
When 2 strokes are properly designed, all exhaust gasses will be scavenged
This allows for a new charge to enter more rapidly for cleaner combustion
The charge needs to be held within the combustion chamber while the exhaust port is open
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Scavenging Con’t
Well engineered exhaust systems use the energy of sound waves to control this
As the exhaust is moving out, a megaphone shaped pipe allows the sound waves to be reflected back towards the cylinder
These reflected sound waves create back pressure for the exhaust system, and keeps the incoming charge held within the cylinder
Most 2 stroke engines will not use a straight exhaust pipe for this reason
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Rotary Disc Valves
The Rotary disc controls the flow of a new charge into the crankcase
Intake port is located directly in the crankcase (this allows for additional transfer ports in the engine block)
The crankshaft holds the rotary valve, and provides the rotation for it to open and close
The valve only has one cutout, allowing to to spin into position to allow for the transfer of a new charge into the cylinder
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Reed Valves
This also permits the intake of a charge directly into the crankcase
Reed is made of a thin flexible spring steel which is connected at one end
The Reed stop is made for thick inflexible steel. This prevents the reed from opening too far and becoming permanently bent
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Reed Valves Con’t
The reed is controlled by the outside atmospheric pressure
It closes due to the springiness of the steel, and the built pressure within the crankcase
The reed valve comes in multiple designs
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Advantages of 2 Strokes
Have less moving parts (less parts to fail)
Simpler DesignOperates at extreme anglesLighter constructionMore power strokes per rotation
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Disadvantages of 2 Strokes
Eratic OperationSmellsBurns oilMust have premixed oilVery Hot running
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Comparison of Lubrication systemsTwo stroke engines are lubricated by the charge Since the charge is what lubricates, it can operate at extreme angles
Four strokes use a pressure system or an oil slinger If the slinger is not touching the oil, lubrication will fail. This is why it does not operate at extreme angles
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Comparison to Four Strokes
Look in Book!