RECIPROCATING ENGINES

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RECIPROCATING ENGINES

• OPERATING PRINCIPLES

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Operating cycle

• The operating cycle of a reciprocating engine
  include 5 events
• Intake
• Compression
• Ignition
• Power
• Exhaust

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Four-Stroke Cycle

• Most aircraft reciprocating engines utilize the
  four-stroke cycle
• Intake
• Compression
• Power
• Exhaust

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Four-Stroke Five-Event Cycle
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INTAKE STROKE

• The intake valve opens as the piston moves
  downward sucking the fuel/air charge into the
  combustion chamber.

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COMPRESSION STROKE

• The intake and exhaust valves are closed as the
  piston moves upward compressing the fuel/air
  charge.

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IGNITION EVENT

• The intake and exhaust valves are closed asthe
  spark plug fires igniting the compressed fuel/air
  charge.

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POWER STROKE

• The intake and exhaust valves are closed as the
  piston is forced downward by the rapid expansion
  of the burning fuel/air charge.

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EXHAUST STROKE

• The exhaust valve opens as the piston moves
  upward forcing the burned exhaust gases out of
  the cylinder.

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DEFININTIONS

• Top Dead Center (TDC) The position of the piston
  at its highest point in a upward stroke.
• Bottom Dead Center (BDC) The position of the at
  its lowest point in a downward stroke.
• Stroke The distance the piston moves from TDC to
  BDC.
• Bore The inside diameter of the cylinder.
• Piston Displacement The volume displaced by the
  piston.
• Compression Ratio A comparison of the difference
  in cylinder volume between TDC and BDC.

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Valve Timing

• Intake valves The intake valves are timed to
  open before the piston reaches TDC on the exhaust
  stroke. The speed of the exhaust gases leaving
  the combustion chamber create a low pressure
  which helps suck the fuel/air mixture into the
  chamber. By opening prior to TDC the engine
  maximizes the intake of fuel/air mixture to
  increase horsepower. If the engine is timed to
  open the intake valves too early in the exhaust
  stroke, some of the hot exhaust gases may enter
  the intake manifold igniting the fuel/air
  mixture. (backfire)

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Valve Timing

• Exhaust valves The exhaust valves are timed to
  open before the piston reaches BDC on the power
  stroke. This way exhaust gases start evacuating
  the cylinder to residual pressure in the
  cylinder. It aids in complete removal of waste
  heat once the useful energy of expansion has been
  used. Complete removal of exhaust gas before the
  intake stroke is essential to providing an
  undiluted fuel/air charge.

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ENGINE POWER AND EFFICIENCY

• Work force times distance. Work is commonly
  measured by the foot-pound. (1lb mass raised 1ft
  1ft-lb of work)
• Horsepower common unit of mechanical power.
  (HPft-lb per min/33,000)
• Indicated Horsepower (IHP) the power developed
  in the combustion chambers without reference to
  friction losses within the engine.

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ENGINE POWER AND EFFICIENCY

• Brake Horsepower (BHP) the power delivered to
  the propeller shaft takes power loss due to
  friction and engine driven accessories
  (generator, alternator, A/C, oil pumps, fuel
  pumps, etc.) into account. Brake horsepower is
  measured with a Prony brake. IHP-FHPBHP

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ENGINE POWER AND EFFICIENCY

• Shaft Horsepower (SHP) the power delivered to
  the propeller shaft takes power loss due to
  friction and engine driven accessories
  (generator, alternator, A/C, oil pumps, fuel
  pumps, etc.) into account. Shaft horsepower is
  measured with a dynamometer. IHP-FHPSHP
• NOTE BHP and SHP are commonly used as
  interchangeable terms. The only difference being
  the technicality of what type of device was used
  to measure engine output.

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ENGINE POWER AND EFFICIENCY

• Friction Horsepower (FHP) the power loss in an
  engine due to friction between moving parts,
  drawing in fuel, expelling exhaust, and engine
  driven components.

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ENGINE POWER AND EFFICIENCY

• Equivalent Shaft Horsepower (ESHP) shaft
  horsepower plus additional thrust provided by
  exhaust gases directed rearward on turboprop
  engines.
• Thrust Horsepower (THP) the result of the engine
  and propeller working together. Propeller
  efficiency varies with engine speed, altitude,
  attitude, temperature, and airspeed.
  THPBHPPropeller Efficiency.

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• Engine efficiency is measured in three ways
• Thermal efficiency ratio of the useful work done
  by the engine to the amount of heat energy
  developed by combustion.
• Mechanical efficiency ratio that shows how much
  of the power developed by the combustion process
  is delivered to the output shaft. BHP/IHP
• Volumetric efficiency comparison of the volume
  of fuel/air charge inducted into the cylinder to
  piston displacement. Varies due to atmospheric
  conditions and complete or incomplete removal of
  exhaust gases from the cylinder.