Module 14 — Propulsion
14.1(a) — Turbine Engines
Introduction to Gas Turbine Engines
The gas turbine engine is the dominant powerplant for modern commercial, military, and many general aviation aircraft. It operates on the Brayton cycle (also called the gas turbine cycle or constant-pressure combustion cycle) and produces either thrust (turbojet/turbofan) or shaft power (turboprop/turboshaft) by accelerating a mass of air.
Fundamental Energy Concepts
Potential Energy: Energy stored due to position or state. In propulsion context: chemical potential energy in fuel, pressure energy in compressed air.
\[ PE = mgh \quad \text{(gravitational)} \]
Kinetic Energy: Energy of motion. This is what produces thrust — accelerating air rearward.
\[ KE = \frac{1}{2}mv^2 \]
A jet engine converts chemical energy (fuel) → heat energy (combustion) → kinetic energy (high-velocity exhaust).
The Brayton Cycle
The gas turbine cycle consists of four continuous processes (unlike the intermittent piston engine cycle):
| Process | Section | What Happens | Thermodynamic Change |
|---|---|---|---|
| 1. Intake | Air intake / diffuser | Air is drawn in and directed into the compressor. At high speed, the intake acts as a diffuser (slows air, increases pressure). | Slight pressure rise, slight temperature rise |
| 2. Compression | Compressor | Air is compressed to 10:1 – 50:1 pressure ratio (depending on engine type). Temperature rises significantly. | Large pressure rise, large temperature rise (adiabatic compression) |
| 3. Combustion | Combustion chamber | Fuel is injected and burned at constant pressure. Temperature rises dramatically (up to ~2,000°C). Only ~25% of the air is used for combustion; the rest cools the flame and dilutes the exhaust to a temperature the turbine can withstand. | Constant pressure, very large temperature rise |
| 4. Expansion | Turbine + exhaust nozzle | Hot, high-pressure gas expands through the turbine (which extracts energy to drive the compressor) and then through the exhaust nozzle (which converts remaining pressure into kinetic energy = thrust). | Pressure falls, temperature falls, velocity increases |
Newton's Laws Applied to Jet Propulsion
| Law | Application |
|---|---|
| 1st Law (Inertia) | Air at rest stays at rest until acted upon by the compressor/combustor. The aircraft in flight continues at constant velocity unless a net force (thrust or drag) acts on it. |
| 2nd Law (F = ma) | Thrust = mass flow rate × change in velocity. \( F = \dot{m}(V_j - V_0) \) where \( \dot{m} \) = air mass flow, \( V_j \) = jet velocity, \( V_0 \) = intake velocity. |
| 3rd Law (Action/Reaction) | The engine accelerates air rearward (action); the reaction force pushes the engine (and aircraft) forward. This is the fundamental principle of jet propulsion. |
Thrust equation:
\[ F = \dot{m}(V_j - V_0) + A_e(P_e - P_0) \]
Where the second term accounts for any pressure imbalance at the nozzle exit. For most cases, this simplifies to:
\[ F \approx \dot{m} \times \Delta V \]
To produce more thrust: increase mass flow (\( \dot{m} \)) and/or increase the velocity change (\( \Delta V \)).
Engine Types
Bypass Ratio
Bypass Ratio (BPR) = mass flow of bypass air ÷ mass flow through the core
\[ BPR = \frac{\dot{m}_{bypass}}{\dot{m}_{core}} \]
| Type | Typical BPR | Characteristics |
|---|---|---|
| Turbojet | 0 (no bypass) | All air through core. High jet velocity. Noisy. Efficient at very high speeds. |
| Low-bypass turbofan | 0.3 – 2:1 | Military fighters. Afterburner capable. Balance of speed and efficiency. |
| High-bypass turbofan | 5:1 – 12:1 | Modern airliners. Large fan moves enormous mass of air at moderate velocity. Very fuel-efficient, quiet. |
| Ultra-high-bypass | 12:1+ | Latest generation engines (geared turbofans). Maximum propulsive efficiency. |
Why high bypass is efficient: Propulsive efficiency is maximised when the jet exhaust velocity is only slightly faster than the aircraft speed. A high-bypass fan accelerates a large mass of air by a small amount — same thrust, less wasted kinetic energy, less noise.
Engine Constructional Arrangement
| Section | Components | Function |
|---|---|---|
| Air intake | Nacelle inlet lip, intake duct | Delivers clean, uniform airflow to the compressor. Subsonic intakes are a simple divergent duct. Supersonic intakes use variable geometry (ramps/cones). |
| Compressor | Axial (rows of rotating blades + stator vanes) or centrifugal (impeller + diffuser) | Raises air pressure 10–50×. Axial: higher efficiency, many stages. Centrifugal: simpler, more robust, used in smaller engines. |
| Combustion section | Can-type, cannular (can-annular), or annular combustors, fuel nozzles, igniters | Burns fuel at constant pressure. Must mix fuel and air, sustain a stable flame at high airflow, and cool the liner to survive extreme temperatures. |
| Turbine section | Rows of stator (nozzle guide vanes) + rotor blades. HP turbine drives HP compressor; LP turbine drives LP compressor/fan. | Extracts energy from hot gas to drive the compressor (and fan in turbofans). Made of nickel superalloys, often single-crystal. Air-cooled blades. |
| Exhaust section | Exhaust cone, jet pipe, nozzle (convergent or convergent-divergent) | Accelerates remaining gas energy into thrust. Convergent nozzle for subsonic jets; C-D nozzle for supersonic applications. |
Thrust Reversers
Redirect engine exhaust forward to decelerate the aircraft on the ground after landing. Types:
| Type | Mechanism | Typical Use |
|---|---|---|
| Cascade (cold stream) | Translating cowl slides aft, exposing cascade vanes that redirect bypass air forward | High-bypass turbofans (most common) |
| Clamshell/bucket | Two doors (buckets) swing into the exhaust stream, deflecting it forward and outward | Low-bypass engines, older designs |
| Blocker door | Doors block the fan duct and deflect air through openings in the nacelle | Some high-bypass turbofans |
Accessory Gearbox, Bearings, and Seals
- Accessory gearbox (AGB): Driven by the HP shaft via a tower shaft/bevel gears. Provides mounting and drive for: fuel pump, oil pump, hydraulic pump, electrical generator, starter, speed sensors.
- Bearings: Ball bearings (thrust loads) and roller bearings (radial loads) support the shafts. Fed by the oil system.
- Seals: Labyrinth seals (non-contact, uses close-tolerance fins) and carbon seals prevent oil from leaking into the gas path and hot gas from entering bearing chambers.
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