Two Stroke Engine Operation

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Two Stroke Engine Operation

• Automotive Technology 1
• Mr. Wasacz

2
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 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 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 gases
• The 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 Cont

• 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 Cont

• 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 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
  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!