Aeroplane Basics
• Super Charger: Super Charger is
basically an engine driven air pump that increases manifold pressure and forces
the fuel/air mixture into the cylinders. A Super Charger is typically driven by
an engine's crank shaft through a gear train at one speed, two speed or variable
speeds.
• Turbo Charger: Turbo Charger
itself is located between the air intake and the fuel metering device.
• Energy Transformation: An aircraft engine is a form of heat engine that converts the chemical
energy of fuel into heat energy. Once converted, the heat energy causes an
increase in gas pressure within a cylinder. The increased gas pressure is then
converted into mechanical energy when the expanding gases force the piston
downward. Since the fuel used to produce heat is burned inside the engine, an aircraft
engine is referred to as Internal Combustion engine. When fuel is burned
outside an engine to produce mechanical energy, the process is called External
Combustion.
• Operating Principle: Like the piston
engine, a gas turbine engine is a form of heat engine that converts the
chemical energy of fuel into heat energy. Once converted the heat energy causes
an increase in gas pressure that is converted into kinetic energy in the form
of a high velocity stream of air. The kinetic energy is then converted into
mechanical energy when the expanding gases rotate a series of turbine wheels
that drive a compressor and accessories. In the case of Turbo-Prop or
Turbo-Shaft engines, the expanding gases may also drive a second power turbine
which drives a propeller or gearbox.
• Design & Construction: All heat engines
convert heat energy into mechanical energy by taking in a specific volume of
air and heat it through the combustion of the fuel. The heated air expands,
creating a force that is converted into mechanical energy to drive a propeller
or other device. The most common type of heat engine is the Reciprocating
Engine. Reciprocating Engines derive their name from the back-and-forth, or
reciprocating movement of their pistons. It is this reciprocating motion that produces
the mechanical energy needed to accomplish work.
• Types of Reciprocating Engines: Classification on
the basis of:- a) Cylinder Arrangement with respect to
the Crankshaft (radial, in-line, v-type, or opposed)
b) The Method of Cooling
i) Liquid
Cooled
ii)
Air Cooled
• Radial Engines: Radial Engines
consists of a row, or rows of cylinder arranged radially about a central
crankcase. The 2 basic types of radial engines includes:-
• The Rotary-Type radial engine.
• The Static Type radial engine.
• In-Line Engines
• V-Type Engines
• Opposed-Type Engines
• Four
Stroke cycle
• Intake Stroke
• Compression Stroke
• Power Stroke
• Exhaust Stroke
• Valve
Timing
• Firing
Order
• Power
Impulses
• Two Stroke
Cycle
• Horse
Power= (Force * Distance)/(33000*Time)
• Indicated
Horse Power=PLANK/33000
Where P=The Indicated Mean Effective Pressure or IMEP inside the
cylinder during a Power Stroke.
L= The Length of the Stroke in feet or fractions of a foot.
A= The Area of the Piston Head in sq. inches.
N= The Number of Power Strokes per minutes for one cylinder. On a Four
Stroke Engine, this is found by dividing the RPM by two.
K= The Number of Cylinders on the Engine.
Ques. Compute the Indicated HP for a 6-Cylinder Engine that has a bore
of 5 inches and is turning at 2750 RPM with a measured IMEP of 125psi per
cylinder.
Ans. P=125 psi; L=0.416 ft.; A=
3.14(2.5)**2;N=2750/2; K=6
IHP=PLANK/33000
IHP=(125*0.416*19.625*1375*6)/33000
=255.125
• The power
required to overcome the friction and energy losses is known as Friction
Horsepower.
• Brake
Horsepower=(2*3.14 * Torque * RPM)/33000
Given: Torque= 600 foot-pounds
RPM= 2700
Brake Horsepower=(2*3.14*600*2700)/33000 =308.30
• Piston
Displacement
• Thermal
Efficiency= (Horsepower*33000)/(F*BTU*K)
Horsepower=An Engine's Brake or Indicated Horsepower.
33000=Number of foot-pounds of work per min in One Horsepower.
F= Weight of Fuel burned per minutes.
BTU=Heat Value of the Fuel burned measured in BTU.
K=Constant representing the Number of Foot-Pounds of Work each BTU is
capable of doing in one second.
By multiplying the pounds per minutes of fuel an engine burns by
20000, you get the Total Number of BTU, or Total Heat Energy that is produced
in a given engine.
1 BTU=778 Foot-Pound of Work
Ques. Determine the Brake Thermal Efficiency of a Piston Engine that
produces 150 Brake Horsepower while burning 8 gallons of aviation gasoline per
hour.
Ans. Horsepower=150;F=8*8.345*60 lb.;
BTU=(8*8.345*60)/20000=4005.6/20000;K=(778*0.2003)*3600
Brake Thermal
Efficiency=(150*33000)/(4005.6*0.2003*561000.24)
=4950000/450102655.04=0.011
22. The Induction System is designed to supply air to the
engine so that, when fuel is added, combustion can take place. On reciprocating
engines, outside air passes through an air intake and is then routed to a carburetor
or other fuel metering device. Once fuel is added, the fuel/air mixture is
delivered into an intake manifold where it is ducted to the cylinder for
combustion. In a turbine engine Induction system, large quantities of air are
ducted through an inlet into a compressor. Once through the compressor, the
resulting high pressure air mass is diffused, mixed with fuel and ignited in a
combustion chamber to produce thrust. Due to the large quantities of air
consumed by a turbine engine, the Induction System plays a very large role in
the level of efficiency that the engine is able to attain.
23. In both a reciprocating and turbine
engine, the purpose of the exhaust system is to remove the spent gases of
combustion and safely route them overboard. For an engine to operate at its max
efficiency, these systems must function properly. In addition, because a
failure of this system could have disastrous results, such as fires or
introducing toxic gases into the cabin, it is imperative that the system be
inspected and maintained according to the manufacturer's recommendations.
24. Engine Lubrication: The primary
purpose of a lubricant is to reduce friction between moving parts and, to a
lesser degree, help in engine cooling. It is also used to seal and cushion
moving parts, clean the engine interior and protect against corrosion. Since
engines requires a lubricant which can circulate freely, liquid lubricants such
oils are the most widely used in aircraft engines.
25. Cooling Systems: Aircraft engines are
designed to convert heat energy into mechanical energy. However, in doing this,
only about 1/3rd of the heat produced is converted. The remaining 2/3rd of the
heat energy is wasted and must be removed from air engine. Therefore, the
cooling systems are designed to remove the unused heat energy produced by
combustion and allow an engine to operate at its peak efficiency.
26. Propellers: Propellers have been used
to convert aircraft engine power into thrust. Although many modern transport
category aircraft are powered by Turbojet or Turbofan engines. Most of the
aircraft in use today are propelled by one or more Propelled by one or more
propellers that are driven by either a turbine or reciprocating engine. Regardless
of the engine type, the primary purpose of a Propeller is to convert engine
power to thrust.
27. Rotational Velocity= 2*3.14*r *RPM
Eg. To determine the blade velocity at a
point 18 inches from the hub that is rotating at 1800 RPM
Use-2*3.14*18*1800=203575
At a point 18 inches from the hub the
blade travels 203575 inches per minute.
To convert this to miles per hour
Divide 203575 by 63360, (the number of
inches in one mile, and multiply the product by 60, the number of minutes in
one hour).
1 mile=63360 inches
Therefore, Velocity=(203575/63360) * 60
=192.7 miles per hour.
Also, the speed of the propeller at
station 18 is 192.7 miles per hour.
The speed of the propeller at station 48
is 514 miles per hour.
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