Chapter 19: Introduction to Gas Turbine Engines
19.1 The Principle of Jet Propulsion
- Jet propulsion is the force generated in the opposite direction to a flow of gas or liquid
- escaping under pressure. It is a direct application of Newton's Third Law of Motion: "For
- every action, there is an equal and opposite reaction."
- The thrust generated by a gas turbine engine is a product of the mass of the air it moves and
- the acceleration it imparts to that air (Force = Mass × Acceleration). Unlike a propeller
- which moves a large mass of air slowly, a pure jet engine moves a smaller mass of air very
- quickly.
19.2 The Brayton Cycle (Constant Pressure Cycle)
- While a piston engine operates on the Otto Cycle (constant volume), the gas turbine engine
- perates on the Brayton Cycle, which is a continuous cycle of induction, compression,
- combustion, and exhaust.
- Key Difference: Combustion in a gas turbine occurs at a nearly constant pressure, not
- constant volume. This continuous process allows for a much greater power-to-weight ratio
- than a piston engine.
- Temperature Limits: The gas turbine is a heat engine; higher combustion temperatures lead
- to greater efficiency. However, the maximum temperature is limited by the metallurgical
- capabilities of the materials used for the nozzle guide vanes and turbine blades.
19.3 Fundamental Gas Laws and Duct Design
- Combined Gas Law: The relationship between pressure, volume, and temperature of the air
- as it passes through the engine is governed by the combined gas law: (Pressure × Volume) /
- Temperature = Constant.
- Duct Design: The shape of the ducts through which the air flows is critical for engine
- efficiency.
- Divergent Duct: A duct that widens. It slows down the airflow, converting kinetic energy
- (velocity) into potential energy (pressure). This is used in subsonic intakes and after the
- compressor.
- Convergent Duct: A duct that narrows. It speeds up the airflow, converting pressure energy
- into kinetic energy. This is used in the nozzle guide vanes and the final exhaust nozzle.
19.4 Types of Gas Turbine Engines
- Turbo-Jet: The simplest form. All air passes through the engine core (compressor,
- combustor, turbine). Thrust is produced entirely by the high-velocity jet exhaust.
- Turbo-Prop: The turbine section is extended to extract most of the energy from the gas
- stream. This energy is used to drive a propeller via a reduction gearbox. Thrust is primarily
- from the propeller (approx. 90%), with a small amount of residual jet thrust (approx. 10%).
- Total power is measured in Equivalent Shaft Horsepower (ESHP).
- Turbo-Shaft: Similar to a turbo-prop, but the output shaft drives something other than a
- propeller, such as a helicopter rotor or an electrical generator in an APU. Often uses a free
- power turbine, which is not mechanically connected to the engine's main compressor spool.
- By-Pass Engine (Turbo-Fan): A portion of the incoming air, accelerated by a fan at the
- front, is ducted around the engine core.
- By-Pass Ratio: The ratio of the mass of air that goes around the core (cold stream) to the
- mass of air that goes through the core (hot stream).
- Low By-Pass Ratio (e.g., 2:1): Found on older jet airliners and military fighters.
- High By-Pass Ratio (e.g., 5:1 or higher): Found on modern airliners. Thrust is mostly
- generated by the fan's cold stream. These engines are much quieter and more fuel-efficient at
- subsonic speeds.