AIRCRAFT ENGINE TYPES

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AIRCRAFT ENGINE TYPES

• THE HEAT ENGINE

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THE HEAT ENGINE

• Modern heavier-than-air aircraft utilize thrust
  to remain in flight.
• This thrust is provided by a heat engine.
• All heat engines convert chemical energy (fuel)
  into heat energy.
• This heat energy is converted into mechanical
  energy which is harnessed to provide thrust.
• In all heat engines the working fluid (fuel/air
  mixture) is compressed to a high pressure
  relative to the atmosphere.

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ENGINE TYPES

• Reciprocating - utilizes reciprocating pistons.
• Turboprop - turbine-driven compressor.
• Turbojet - turbine-driven compressor.
• Ramjet - ram compression due to high flight
  speed.
• Pulse-jet - compression due to combustion.
• Rocket - compression due to combustion.

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GENERAL AVIATION REQUIREMENTS

• Efficiency- the engine must be able to operate
  efficiently under a wide range of atmospheric
  conditions.
• Economy- the engine must be economic to produce,
  run, and maintain.
• Reliability- the engine must be able to endure
  long periods of operation at high power settings
  without failure.

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OPERATION SPECIFIC ENGINES

• The engine selected for an aircraft depends on
  the type of flying it will do.
• Propeller driven aircraft are more fuel efficient
  at low speeds, while jet powered aircraft are
  more efficient at high speeds.
• This high speed efficiency is more economical on
  long trips.
• Turboprop aircraft combine the reliability of a
  turbine engine with the low speed (short trip)
  efficiency of a propeller driven aircraft. These
  turbine driven engines are able to operate at
  higher altitudes, giving them an operational
  advantage. (these benefits come at a cost)

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RECIPROCATING ENGINE TYPES

• Reciprocating engines are normally classified by
  cylinder arrangement.
• In-line
• V-type
• Radial
• Horizontally opposed
• Diesel

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RECIPROCATING ENGINE TYPES

• They are further categorized by the number of
  cylinders and the method of cooling.
• Engines are either air cooled or liquid cooled.
  In either case excess heat is transferred to the
  surrounding air.
• An air cooled engine transfers heat from the
  cylinders directly to the air flow routed around
  the cylinders.
• A liquid cooled engine transfers heat from the
  cylinders to a liquid coolant which in turn
  transfers the heat to the airflow through the
  radiator.
• Most aircraft engines are air cooled. (this
  method is lighter and cheaper but not as
  effective)

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In-line

• Advantages
• streamlined (less drag)
• visibility (if inverted)
• Disadvantages
• long crankshaft (limits power output)
• ground clearance
• ineffective cooling of rear cylinders

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Horizontally Opposed

• Advantages
• streamlined (less drag)
• visibility
• less vibration
• Disadvantages
• limited power (crankshaft length)
• uneven cooling

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V-type

• Advantages
• visibility
• Disadvantages
• limited power (crankshaft length)
• uneven cooling

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Radial

• Advantages
• power (rows of cylinders can be added while
  maintaining a short crankshaft)
• cooling
• Disadvantages
• drag
• hydraulicing (oil tends to pool in the low
  cylinder during extended shutdown periods) This
  problem cause major engine damage if the engine
  is started. The problem can be detected by hand
  pulling the engine and then draining the oil by
  removing the spark plug.

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Hydraulic Lock (hydraulicing)
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Multi-row Radial
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Radial Engine
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Diesel

• Advantages
• fuel consumption
• fuel cost
• less maintenance (longer duration between
  overhauls)
• fuel availability
• Disadvantages
• weight
• cost
• (both of these aspects have been improved upon as
  more research driven by high fuel costs has been
  devoted to developing diesel engines for light
  aircraft)

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Reciprocating Engine Components

• Basic components and mechanical principles are
  shared by all reciprocating engines.
• Different types of cylinder arrangement, cooling
  type, and fuel require different component
  arrangements.
• Engine components are made of materials chosen
  for their combination of strength, durability,
  weight, and heat resistance (ability to maintain
  structural integrity over repeated heating and
  cooling cycles).

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Crankcase

• The crankcase is the main structure of the engine
  which contains the bearings for the crankshaft.
• The crankcase is designed to house the
  crankshaft, camshafts, and lubricating oil.
• Externally the crankcase must accommodate the
  cylinders and peripheral or accessory components.
• Aircraft crankcases are usually made of cast or
  forged aluminum alloy because of its lightweight
  and strength. (Forged steel is used in some high
  output engines).
• The crankcase must be able to endure
  multidirectional forces, vibration and extreme
  operating temperatures.

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Crankcase
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Crankshaft

• The crankshaft transforms the reciprocating
  (linear up and down) motion of the pistons into
  rotary force for the propeller.
• The crankshaft is exposed to most of the forces
  developed by the engine.
• The length of the crankshaft then becomes one of
  the main limiting factors when designing an
  engine.

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Crankshaft
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Crankshaft Components

• Main journal rotates within the main bearing.
• Rod journal rotates within the connecting rod
  bearing.
• Counterweight used to balance the crankshaft and
  reduce vibration.

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Connecting Rod

• The connecting rod connects the piston to the
  crankshaft.
• It transmits forces between the piston and
  crankshaft.

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Piston

• The piston moves up and down within the cylinder.
• It draws the fuel/air mixture into the cylinder
  and drives the crankshaft on the downward stroke.
• It compresses the fuel/air mixture on the upward
  stroke.

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Piston Rings

• Piston rings fit into grooves on the side of the
  piston and form the seal between the piston and
  cylinder wall.
• The rings are designed with a gap which is forced
  closed when the piston is inserted into the
  cylinder to form a spring loaded seal.

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Functions

• Compression gas sealing Piston rings maintain
  gas compression between the piston and cylinder
  wall. They prevent combustion gas from escaping.
  A leak would cause a decrease in power.

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Functions

• Lubricating oil film control The cylinder walls
  must be coated with a thin film of lubricating
  oil, to reduce friction, and prevent damage to
  the cylinder and piston. The oil ring controls
  this thin film of oil.

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Functions

• Heat transfer Piston rings transfer heat from
  the piston to the cylinder. The heat is then
  removed form the cylinder by an air or liquid
  cooling system.

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Functions

• Piston support piston rings help keep the piston
  tracking properly within the cylinder. If the
  piston were to incline within the cylinder and
  touch the cylinder walls it would cause the
  engine to fail.

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Cylinders

• The cylinder is the portion of the engine where
  the power is developed.
• The cylinder forms the combustion chamber where
  the fuel/air mixture is ignited and burned.
• Factors affecting cylinder design
• strong enough to withstand internal pressures.
• lightweight construction.
• heat-conducting properties for efficient cooling.
• easy and inexpensive to manufacture and maintain.

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Cylinders

• The cylinder heads of an air-cooled engine house
  the intake and exhaust valves.
• The cylinder barrels house the piston and
  connecting rod assembly.
• The cylinder head of an air-cooled engine is
  usually made of aluminum alloy due to its heat
  conductivity properties and light weight.

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Firing Order

• The cylinders of a reciprocating engine are
  always assigned numbers.
• The numbering theme varies depending on the type
  of engine and the engine manufacturer.
• The firing order is the sequence the firing of
  the cylinders occurs in.
• The firing order of an engine is designated in
  such a way as to reduce vibration.

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Valves

• The fuel/air mixture or charge enters the
  combustion chamber through the intake valve while
  the burned gases are expelled through the exhaust
  valve.
• The valves are housed within the cylinder head.
• Valves are subjected to extreme operating
  conditions within the combustion chamber.

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Valve Operation

• The valves are held closed by springs and
  actuated to the open position by mechanical
  linkage made up of a tappet (lifter), pushrod and
  rocker arm.
• The lobes on the camshaft push the tappet,
  pushrod and rocker arm assembly upwards which in
  turn opens the corresponding valve.
• The opening and closing of each valve must be
  synchronized with the with the movement of the
  piston.

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Camshaft

• Valve lift the distance the valve is lifted off
  its seat.
• Valve duration the length of time the valve is
  held open.
• The camshaft is responsible for actuating the
  tappet, pushrod, and rocker arm assembly.
• The shape of the cam lobes determine the valve
  duration and lift.

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Camshaft

• The camshaft is driven by a gear which is
  directly connected to a gear attached to the
  crankshaft.
• The camshaft always rotates at half the speed of
  the crankshaft.
• This timing allows each piston to complete its
  four-stroke cycle. (the valves will remain closed
  for two of the four piston strokes)

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Bearings

• Bearing any surface which supports, or is
  supported by another surface.
• Bearings are used within engines to reduce
  friction between rotating components.
• There are three distinct types of bearings
• Plain
• Ball
• Roller

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Plain designed to handle radial loads. Used for
the crankshaft, cam shaft, connecting rods.
Lubricated through oil channels, or made of self
lubricating materials (bushings). Roller Can be
designed to withstand both radial and thrust
loads. Used for crankshafts is high performance
engines. Ball Used for superchargers impeller
shaft bearings, and some rocker arm applications.
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REDUCTION GEARING

• Propellers are efficient through a limited range
  of rpm.
• Depending on engine output and propeller design
  reduction gearing may be necessary.
• Three common forms are
• Spur and pinion
• Spur planetary
• Bevel planetary

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• Pictures\Gear Reduction.mpg