MD

Gas Turbine Engine Overview

Overview

  • Gas turbine engine (also called the jet engine) powers large and heavy aircraft to generate the necessary thrust.
  • Unlike piston engines that use a propeller, gas turbine engines move the aircraft by rapidly pushing air out the back of the engine. This rearward air action creates forward thrust due to Newton’s third law of motion: for every action there is an equal and opposite reaction.
  • Not all gas turbine engines generate thrust primarily by accelerating air rearward; turboprops are an exception and will be explored in Chapter Nine.
  • Visual reference: Fig 7.1 illustrates Newton’s third law explaining thrust generation.

Historical Background

  • Principles of gas turbine propulsion were demonstrated over 2,000 years ago by Hero of Alexandria with the Aeolipile (a sphere connected to support tubes over a fire). When steam exits the nozzles in one direction, the sphere spins in the opposite direction, illustrating action-reaction thrust.
  • Early setups were heavy and provided limited power, limiting practical use.
  • Advancements in the 1930s made gas turbine engines viable for aircraft.
  • The first jet-powered aircraft flew in 1939 (Germany).
  • Independently developed jet-powered aircraft emerged in other nations (Britain, United States) soon after.
  • After World War II, jet propulsion led to the first jet-powered airliner in 1952, the Comet, which could fly higher and faster than earlier aircraft.
  • Today, gas turbine engines power all large airliners, many business jets, and are increasingly used on smaller aircraft.

Core Concept: Turbojet and Common Core

  • This chapter provides a general overview of the gas turbine engine, focusing on the turbojet (the original form). While turbojets have been superseded in many roles by more efficient types, they share a common structural core.
  • Most turbojet engines and related variants share five basic sections in the core: intake, compressor, combustion, turbine, and exhaust.
  • Flow path:
    • Air enters through the intake (front end) and is directed into the engine’s core.
    • The air then passes into the compressor, which squeezes and increases the air’s temperature and pressure.

Core Components and Their Functions

  • Intake: Directs air into the engine’s core for efficient compression and combustion.
  • Compressor: Raises air pressure (and temperature) before it enters the combustion chamber.
  • Combustion (Combustor) Chamber: Injects fuel into the compressed air and ignites the mixture; combustion is self-sustaining.
  • Turbine: Hot, high-energy exhaust gases expand through turbine blades, turning the turbine.
  • Exhaust: Exhaust gases exit the engine rearwards, providing thrust.
  • A shaft connects the turbine to the compressor, transmitting mechanical power to drive the compressor; this shaft runs through the center of the engine.

How the Core Generates Thrust

  • After ignition, the combusted gases rapidly expand and exit the back of the engine.
  • The turbine extracts energy from these hot gases to drive the compressor via the central shaft; this energy transfer is what sustains continuous operation.
  • The exhaust gases are expelled at high speed; the mass flow rate through the engine is very large, producing a large amount of thrust.
  • Although the core shares a common structure across engines, there can be significant design variations between individual engines.

Relationship to Piston Engines

  • Several terms and functions are shared with piston engines (e.g., intake, compression, combustion, exhaust), which can make the concepts familiar.
  • However, gas turbine engines operate continuously across the core rather than in discrete cycles within a single cylinder, enabling a much larger mass of air to pass through and producing much greater thrust.
  • Both are types of internal combustion engines that use fuel to generate thrust, but their operating cycles and flow patterns differ substantially.

Chapter Progression and Scope

  • The chapter will first explore each section (intake, compressor, combustion, turbine, exhaust) in more detail.
  • It will then cover different engine types or variants (e.g., turbojet, turbofan, turboprop).
  • A range of support systems will be examined, followed by important engine management considerations—areas that can differ significantly from piston engine operation.
  • The material connects to foundational principles of thermodynamics and fluid dynamics, and highlights real-world relevance in aircraft propulsion and design.

Figures and References (Context)

  • Fig 7.1: Demonstrates Newton’s third law as the basis for thrust generation in gas turbine engines.
  • Fig 7.2: Hero’s Aeolipile, illustrating early steam-jet propulsion principles and action–reaction thrust.
  • Fig 7.3: Depicts the main sections of a gas turbine engine (intake, compressor, combustion, turbine, exhaust).

Real-World Relevance and Practical Implications

  • Gas turbine engines enable the high thrust-to-weight ratios needed for large aircraft and modern air travel.
  • The shift from turbojet to turbofan and turboprop variants reflects ongoing improvements in efficiency, noise reduction, and flexibility for different aircraft sizes and mission profiles.
  • Engine management considerations include fuel efficiency, reliability, maintenance, and safety, all of which differ from piston engine management due to the continuous high-speed operation and high air mass flow.

Foundational Connections

  • Core principles rest on Newton’s laws of motion, energy conversion, and the management of mass flow and energy through a continuous high-temperature process.
  • The engine’s design is influenced by the need to maximize air throughput while efficiently converting chemical energy of fuel into kinetic energy of exhaust and useful work to drive the compressor.

Summary of Key Concepts

  • The gas turbine engine produces thrust by accelerating air rearward, in line with Newton’s third law.
  • The turbojet remains the original form of gas turbine engine, though turbofans and turboprops are now predominant due to efficiency advantages.
  • A gas turbine core consists of five sections: intake, compressor, combustion, turbine, and exhaust.
  • Air is compressed, mixed with fuel, ignited, and expanded to drive the turbine, which in turn powers the compressor; exhaust exits to generate thrust.
  • The process is continuous, enabling very large air mass throughput and substantial thrust, with design variations across engines.
  • Historical progression shows early principles (Hero’s Aeolipile) leading to modern jet aircraft, with major milestones in 1939 (first jet aircraft) and 1952 (Comet).
  • The chapter sets the stage for deeper examination of each core section, variants, support systems, and engine management in subsequent chapters.