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7/29/2019 Two Stroke Operation
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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 thedifferences 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 stroke
However, 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 moresimple!
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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 incominggases swirl in the cylinder
This swirl keeps the intake gases in, and forcesthe exhaust gases out
May have more than one transfer port to createthis 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 outsideatmospheric to rush into the crankcasethrough the carburetor
When the air moves through the carburetor, ittakes a charge of gasoline and oil with it.
This newly in took charge lubricates thebearings and moving parts in the crankcase
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Fuel Transfer
As the piston moves downwards, itcompresses the air fuel and oil in the
crankcaseWhen it travels far enough downward, it
exposes the transfer port into the cylinder
The newly compressed air, fuel, and oil thenrush upwards towards the cylinder
This new charge of air, fuel, and oil also coolthe 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 volumeSpark ignites charge when piston
reaches TDC
The ignited gasses force the pistondownward
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Exhaust
As the piston moves downwards, the
exhaust port is exposed
This allows for the removal of gases
The remainder of gases are forced out
when the transfer ports are exposedThis 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 entermore rapidly for cleaner combustion
The charge needs to be held within the
combustion chamber while the exhaustport is open
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Scavenging Con’t
Well engineered exhaust systems use theenergy of sound waves to control this
As the exhaust is moving out, a megaphoneshaped pipe allows the sound waves to bereflected back towards the cylinder
These reflected sound waves create backpressure for the exhaust system, and keepsthe 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 newcharge into the crankcase
Intake port is located directly in the crankcase(this allows for additional transfer ports in theengine 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 tospin 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 chargedirectly into the crankcase
Reed is made of a thin flexible spring steelwhich is connected at one end
The Reed stop is made for thick inflexiblesteel. 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 outsideatmospheric pressure
It closes due to the springiness of thesteel, and the built pressure within thecrankcase
The reed valve comes in multipledesigns
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Advantages of 2 Strokes
Have less moving parts (less parts to
fail)
Simpler Design
Operates at extreme angles
Lighter construction
More power strokes per rotation
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Disadvantages of 2 Strokes
Eratic Operation
Smells
Burns oil
Must have premixed oil
Very Hot running
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Comparison of Lubrication
systems Two stroke engines are lubricated by the
charge
Since the charge is what lubricates, it can operateat extreme angles
Four strokes use a pressure system or an oilslinger
If the slinger is not touching the oil, lubrication willfail. This is why it does not operate at extremeangles