Two and Four Cycle Engines

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Two and Four Cycle Engines

• TRF 210

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2 Cycle Vs 4 Cycle

• Cycle a series of events that repeat themselves
• 2 cycle 2 piston strokes to complete one cycle
• TDC-BDC power stroke
• BDC-TDC compression stroke
• 4 cycle 4 piston strokes to produce one power
  stroke

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Advantages of a 2 cycle Engine

• Simpler in construction (only 3 major moving
  parts piston assembly, connecting rod, and crank
  shaft)
• Easier to maintain
• Lighter in weight
• Less expensive to manufacture

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Operation of 2 Cycle Engines

• 3 events must take place
• Burned gasses from previous stroke must be
  cleared from cylinder
• Fresh charge of air and gas must be brought in
• Air and fuel must be compressed for ignition
• This is all accomplished by the opening and
  closing of ports in the side of the cylinder by
  the piston

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Cylinder Ports

• Intake fresh air/fuel mixture flows from
  carburetor into crank case
• Transfer air/ fuel mixture enters cylinder
• Exhaust burned gasses leave cylinder

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Port Actions

• Power
• Exhaust
• Intake
• Compression
• One power stroke every time the piston reaches
  TDC

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Crank Case Compression

• Changes in crank case pressure is what allow the
  air/fuel mixture to enter and exit the crank case
• Piston as a valve
• Reed valve
• Rotary valve

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Cylinder Scavenging

• 2 cycle engines produce less power than 4 cycle
  engines because less fuel burned in the cylinder
• In a 2 cycle engine the fresh air/fuel mixture
  is used to clean out the exhaust gases
• This causes up to 25 of this fresh air/fuel
  mixture to be lost
• Cross-scavenging transfer and exhaust ports are
  opposite each other (requires a deflector)
• Loop-scavenging transfer and exhaust are 90
  degrees apart

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4 Cycle Spark Engines

• 4 cycle engines differ from two cycle engines in
  the way the air fuel mixture and exhaust enter
  and leave the cylinder
• There are also more major moving parts in 4 cycle
  engines

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The Four Stroke Cycle

• One out of every four strokes is a power stroke
• This engines requires two valves to open and to
  close to let the air/fuel mixture and exhaust in
  an out of the engine

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The Four Stroke Cycle

• Intake
• Compression
• Power
• Exhaust
• Valves are opened and closed at proper times by a
  valve train which includes a camshaft which is
  driven by the engine crankshaft

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Intake and Exhaust Valves

• Intake air/fuel mixture into cylinder
• Exhaust burned gases exit cylinder
• Valves can be located beside piston L-Block) or
  over the top over the piston (overhead valve)

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Operating the Valves

• Valves must be opened and closed at the correct
  times
• Each valve is kept closed by a valve spring
• Below the valve stem is a lifter which moves up
  and down in a hole in the cylinder block

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Operating the Valves Cont.

• The lifter rest on a cam which has a high spot or
  lobe
• The cam is located on the cam shaft there is one
  cam for each valve
• The camshaft is driven by a gear or chain on
  sprockets from the crank-shaft
• Cam shaft rotates with crank shaft to open and
  close valves

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Comparing 2 Cycle and 4 Cycle Engines

• Both use piston, cylinder, connecting rod crank,
  crank shaft, flywheel, and spark plug. The
  movement is also similar
• 2 cycle valve ports in cylinder wall 2 strokes
  for every one power stroke
• 4 cycle valves open and closed by a camshaft 4
  strokes for every one power stroke

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Identifying 2 cycle and 4 cycle engines

• 4 cycle
• Oil sump and oil plug
• Oil drain and refills required
• Muffler installed close to exhaust valve

• 2 cycle
• Not present
• Oil added in fuel
• Muffler in middle of cylinder