An Engine is a thermal device that
converts heat energy into mechanical energy. Mechanical energy is principally
derived in the form of torque on the output shaft of the engine and is utilized
for necessary driving works.
Energy input to an engine is ‘heat’. Heat so used by
the engine is derived from different sources, such as: (i) Solid fuel, (ii)
Liquid fuel, (iii) Gaseous fuel, (iv) Nuclear fuel. Energy input system of the
engine ensures efficient release of heat from the fuel. For the case a chemical
fuel (solid/liquid/gaseous), combustion is the process that is to be
carried out to liberate heat from fuel through an exothermic reaction. For
nuclear fuel, a nuclear reaction is to be carried out in a nuclear reactor so
that energy is liberated from atoms through nuclear chain reaction in which
‘nuclear binding’ energy is released as a result of re-arrangement of atomic
particles.
Depending upon where the combustion process is
carried out, (that is, outside the engine or inside the engine), the engine is
classified as External Combustion Engine (ECE) and Internal
Combustion Engine (ICE). For an ECE, combustion is carried out externally
in a furnace and heat is utilized to produce working fluid, such as, generation
of steam from water in a boiler by heating, heat being produced by burning
coal. Here, steam is the working fluid that is taken to drive a steam engine.
ECEs have applications in the field of industrial electric power generation. In
the ICEs, combustion of fuel is carried out in a space or chamber inside the
engine itself. This space/chamber is called the combustion chamber. Metered
and atomized fuel is burnt in compressed air taken in combustion chamber and
the hot gas so produced, called the flue gas with heat energy is the
‘working fluid’ that is directly used to stroke a piston or rotate a turbine
wheel. ICEs are compact, all sections being integrated into single unit and
hence, they have got wide-spread application in industries, locomotives and
aircraft. They are in the form of piston engines and gas turbine engines.
A
gas turbine engine is an ICE that uses turbines to convert heat energy of a gas
into torque; the gas is the combustion-product produced by burning fuel in
compressed air in its combustion chamber inside the engine. A turbine is a
rotary device with arrangement of series of blades around the periphery of a
wheel mounted on a shaft so that energy of the working fluid, when impinged
over blades, will rotate the wheel. In short, turbine is an energy transfer
mechanism, transferring energy from working fluid to its shaft in the form of
rotation or torque.
In aircraft application, an engine is a propulsive
device that provides propulsive force (thrust) to propel the aircraft forward overcoming atmospheric drag. Thus, as long as aircraft propulsion is
concerned, the objective of the aircraft gas turbine engine is not directly the
work output at its shaft but is the propulsive force. This is fundamental difference between the primary objective furnished by
the gas turbine engines in industrial applications and in the aircraft
applications. However, the basic aerodynamic and thermodynamic considerations
are almost the same.
