Engine Operating Principles

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					Engine Operating
Principles

Small Engines



                   1
Internal Combustion Engine

   Internal Combustion Engine
    – Is an engine where air and fuel are burned
      with the release of this energy taking place
      within the engine.
    – Their operation can be divided in
      compression, ignition, and carburetion.



                                                 2
Four Stroke Cycle

   Four Stroke
    – The four events in the operation of the
      four-stroke engine are:
      • Intake, compression, power, and exhaust.
    – Each event occurs with a linear movement
      of the piston which is called a stroke.
    – Each cycle of events is completed after two
      revolutions of the crankshaft.

                                                   3
    Four Stroke Cycle
   The first event in the operation of the
    four stroke is the intake stroke.
    – At the beginning of the intake stroke, as
      the piston moves downward, the intake
      valve will start to open.
    – This allows the air-fuel mixture to flow into
      the combustion chamber above the piston.
    – The intake valve cam lobe high point on
      the camshaft has turned to open the intake
      valve and the exhaust valve is held shut by
                                                      4
      its valve spring.
Four Stroke Cycle
– The downward movement of the piston
  creates a partial vacuum in the combustion
  chamber.
– The air-fuel mixture from the carburetor
  rushes into the combustion chamber as
  long as the intake valve is open and the
  pressure in the cylinder is lower than the
  pressure at the carburetor outlet.
– At the bottom of the piston stroke, bottom
  dead center (BDC) the camshaft has
  turned until the cam lobe no longer holds
  the intake valve open and the valve is       5
  closed by the valve spring.
    Four Stroke Cycle
   The second event is the compression
    stroke.
    – During the compression stroke both the
      intake and exhaust valves are closed and
      the piston moves upward compressing the
      air-fuel mixture.
    – The top of the stroke is called top dead
      center (TDC).
    – The piston rings form a seal between the
      piston and the cylinder wall which prevents
                                                    6
      the escape of the air-fuel mixture.
Four Stroke Cycle

    – The compressing of the air-fuel mixture in
      the combustion chamber increases the
      temperature of the mixture making it easier
      to ignite.
   The third event is the power stroke.
    – The intake and exhaust valves are both
      closed.
    – When the piston approaches or is at the
      top of the stroke a spark jumps the spark
      plug gap igniting the heated and            7
Four Stroke Cycle

    – The fuel mixture burns rapidly and the
      expanding gases create pressure in all
      directions.
    – The piston is propelled downward which in
      turn turn the crankshaft producing rotary
      motion.
   The forth event is the exhaust stroke.
    – When the piston reaches the bottom of the
      power stroke the momentum of the
      flywheel and the crankshaft moves the     8
Four Stroke Cycle
– The exhaust valve is opened by the high
  point of the exhaust valve cam lobe on the
  camshaft.
– The intake valve is held shut by the valve
  spring.
– Burned or exhaust gases are forced out of
  the combustion chamber past the exhaust
  valve into the exhaust passage.
– When the piston is near the top of its
  stroke in the cylinder the exhaust valve
  closes.
                                               9
– One cycle of events has been completed.
Two Stroke Cycle

   Two Stroke
    – The four events in the operation of the two
      stroke engine are:
       • Intake, compression, power, and exhaust.
    – All these events must occur but more than
      one event takes place during each piston
      stroke.
   The First event is:
    – The air-fuel mixture is compressed in the
                                                    10
      combustion chamber.
Two Stroke Cycle

 – The spark jumps the plug gap igniting the
   air-fuel mixture.
 – This again forces the piston downward in
   the cylinder.
 – The downward travel of the piston
   uncovers the exhaust port first and than
   the intake port.
 – The incoming air-fuel mixture forces the
   exhaust gases out the exhaust port.
 – This is called scavenging.                  11
Two Stroke Cycle
– During the downward movement of the
  piston all the power event has been
  completed and part of the intake and
  exhaust event has been completed.
– The piston starts its upward movement in
  the cylinder covering the intake and
  exhaust ports thus trapping the air-fuel
  mixture in the combustion chamber.
– The four events are completed in tow
  strokes and the crankshaft has made one
  revolution (360).                          12

				
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