Engine Operating Principles

1
Engine Operating Principles

• Small Engines

2
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.

3
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.

4
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 its valve
  spring.

5
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 closed by the valve spring.

6
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 the escape
  of the air-fuel mixture.

7
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 compressed air-fuel
  mixture.

8
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 piston up the cylinder.

9
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.
• One cycle of events has been completed.

10
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
  combustion chamber.

11
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.

12
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).