Lesson 3: Reciprocating Engine Theory Of Operation

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Lesson 3 Reciprocating Engine Theory Of Operation
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Reciprocating Engine Theory Of Operation

• Engine Operating Principles

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Reciprocating Engine Theory Of Operation

• Operating Cycles
• Two-Stroke
• Requires only one upstroke and one downstroke of
  the piston to complete the required series of
  events in the cylinders.
• Completes the operating cycle in one revolution
  of the crankshaft.
• What are the sequence of events that take place
  to convert the chemical energy into mechanical
  energy?

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Reciprocating Engine Theory Of Operation

• Intake
• Compression
• Ignition
• Power
• Exhaust

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Reciprocating Engine Theory Of Operation

• Two-Stroke

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Reciprocating Engine Theory Of Operation

• Four-stroke
• Four strokes required
• to complete the events.
• Two revolutions of the
• crankshaft (720) are
• required to complete
• the four strokes.

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Reciprocating Engine Theory Of Operation

• Engine Power And Efficiency
• Work Force x Distance
• Horsepower ft-lb per min or ft-lb per sec
• 33,000
  550

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Reciprocating Engine Theory Of Operation

• Piston Displacement
• The greater the piston displacement the greater
  the horsepower an engine will develop.
• The volume displaced by a piston is its
  displacement.
• Expressed in cubic inches.

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Reciprocating Engine Theory Of Operation

• Piston Displacement
• PD Area of cross section of cylinder multiplied
  by total distance the piston moves.
• V A x H

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Reciprocating Engine Theory Of Operation

• Compression Ratio
• Comparison of the volume of space in a cylinder
  when the piston is at the bottom of the stroke to
  the volume of space when the piston is at the top
  of the stroke.
• The higher the engine compression ratio, the
  higher the engine efficiency (horsepower output)
  will be.

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Reciprocating Engine Theory Of Operation

• Manifold Absolute Pressure (MAP)
• Average absolute pressure of the fuel/air charge
  in the intake manifold.
• Measured in units of inches of mercury (Hg).
• Dependent mostly on ambient air pressure, engine
  speed, and throttle setting.
• Supercharging increases MAP.

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Reciprocating Engine Theory Of Operation

• Compression Ratio Manifold Pressure.
• Determines the pressure in the cylinder when both
  valves are closed.
• Pressure of charge before compression is
  determined by the manifold pressure.
• Pressure at the height of compression is
  determined by manifold pressure times the
  compression ratio.

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Reciprocating Engine Theory Of Operation

• Absolute Gauge Pressure.
• Absolute Pressure
• Identifies the pressure measurement as one that
  is based on a comparison of the pressure in the
  manifold with pressure at absolute zero.
• Gauge Pressure
• Compares the pressure being measured against
  ambient pressure.

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Reciprocating Engine Theory Of Operation

• Indicated Horsepower
• Horsepower calculated from the indicated mean
  effective pressure and the other factors which
  affect the power output of an engine.
• The power developed in the combustion chamber
  without reference to the friction losses within
  the engine.

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Reciprocating Engine Theory Of Operation

• Brake Horsepower (BHP)
• The power delivered from the engine to the
  propeller for useful work.
• Total horsepower lost due to friction is
  subtracted from indicated horsepower.
• The measurement of an engines BHP involves the
  measurement of torque.
• Torque Force times distance

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Reciprocating Engine Theory Of Operation

• Friction Horsepower
• Indicated horsepower minus brake horsepower.

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Reciprocating Engine Theory Of Operation

• Friction And Brake Mean Effective Pressure
• Indicated Mean Effective Pressure
• Pressure used to create frictionless power.
• Friction Mean Effective Pressure
• The pressure used to overcome internal friction.
• Brake Mean Effective Pressure
• The pressure used to produce useful work.

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Reciprocating Engine Theory Of Operation

• Thrust Horsepower
• The result of the engine and propeller working
  together.
• The ratio of thrust horsepower and brake
  horsepower delivered to the propeller shaft will
  never be equal.
• Determines the performance of the
  engine-propeller combination.

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Reciprocating Engine Theory Of Operation

• Efficiencies
• Thermal Efficiency
• The ratio of useful work done by an engine to the
  heat energy of the fuel it uses, expressed in
  work or heat units.
• Mechanical Efficiency
• The ratio that shows how much of the power
  developed by the expanding gases in the cylinder
  is actually delivered to the output shaft.

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Reciprocating Engine Theory Of Operation

• Efficiencies
• Volumetric Efficiency
• A comparison of the volume of fuel/air charge
  inducted into the cylinders to the total piston
  displacement of the engine.