MD

Chapter 9: Types of Gas Turbine Engines

Turbofan

  • The turbofan has become the main type of engine on airliners and business jets. It differs from a turbojet in two key ways:
    • It has a large ducted (shrouded) fan located in the air intake. The fan blades are similar to those in an axial flow compressor but much larger, which enables accelerating a large volume of air rearward.
    • Not all air entering the engine goes into the core. Only some of it is directed into the engine core (primary airflow); the rest bypasses the core within the engine cowling (bypass air or secondary airflow).
  • Bypass ratio (the ratio of bypass air to core air) varies by design:
    • Early turbofan engines had a bypass ratio of about 1:1, meaning the same amount of air bypasses the core as passes through it.
    • Modern turbofan engines have a bypass ratio of at least 3:1 (for every 1 unit of air through the core, at least 3 units bypass the core). In some engines, bypass air provides around 80\% of the engine’s thrust due to the large volume of air accelerated rearward.
  • Bypass air serves several functions:
    • Cooling of the engine core
    • Mixing with exhaust gases to reduce noise levels
    • Providing a significant portion of thrust (often the majority, up to about 80\%\$\text{of thrust} in some designs)
  • Overall, turbofans are much more fuel-efficient than turbojets.
  • Configurations and spooling:
    • A common layout is for the fan to be part of the first compressor stage, so the fan and the low-pressure (LP) compressor are driven by the same turbine, forming the low-pressure spool.
    • In other engines, the fan may be driven by a dedicated turbine, creating a multi-spool arrangement. A typical three-spool design includes:
    • A fan + LP spool
    • An intermediate spool (IP)
    • A high-pressure spool (HP)
  • Practical implications:
    • Higher bypass ratios generally improve efficiency and reduce noise at the cost of a larger engine and heavier weight.
    • The core and bypass flow paths allow flexibility to tailor thrust, fuel consumption, and noise for different aircraft roles.

Turboprop

  • A turboprop uses a similar core (intake, compressor, combustion, turbine) to other gas turbine engines, but the key difference is that one turbine section drives a propeller to generate the majority of thrust.
  • Turboprops are highly efficient at low to medium speeds, which makes them ideal for short regional flights.
  • They can be compact enough for single-engine aircraft.
  • A key component is the reduction gearbox:
    • The turbine runs very fast, while the propeller must rotate much more slowly to avoid tip speeds approaching or exceeding the speed of sound, which would reduce thrust efficiency.
    • The reduction gearbox slows the turbine output to a suitable propeller speed, allowing both components to operate efficiently.
  • Direct Drive (Geared) Turboprop:
    • In early turboprops, the propeller was driven by the same shaft that connected the compressor (turbine, compressor, and propeller on one shaft).
  • Compound Turboprop:
    • A two-spool arrangement.
    • The propeller is connected to the LP spool (propeller, LP compressor, LP turbine on one shaft), while a separate HP spool (HP compressor and HP turbine) provides high-pressure energy.
  • Free Turbine Turboprop:
    • The more common modern design uses an independent turbine to drive the propeller (power turbine).
    • The engine has a compressor/turbine spool and a separate turbine with its own shaft for the propeller.
    • This layout enables interesting configurations, including reverse flow, shown in Figure 9.4, and tends to be compact.
  • Reverse Flow Turboprop (via Free Turbine):
    • The intake is often large and can sit under or beside the engine.
    • Air travels through a duct to the back of the engine, makes a 180-degree turn to head forward again, and passes through the standard compressor, combustion, and turbine.
    • The propeller is driven by a separate power turbine, with exhaust gases diverted away from the engine.

Free Turbine Turboprop (with reverse flow capability)

  • The free turbine arrangement allows a separate turbine to drive the propeller, offering flexibility in packaging and operation.
  • Reverse flow configurations aid packaging and aerodynamics, particularly on compact or smaller aircraft.

Turboshaft

  • The turboshaft engine is very similar in core to other gas turbine variants (intake, compressor, combustion, turbine).
  • The defining feature is a free turbine that drives a power shaft (shaft output) used for applications such as helicopter rotor blades.
  • The emphasis is on turning exhaust energy into shaft power for external work rather than primarily producing thrust.

Auxiliary Power Unit (APU)

  • Many large aircraft include a small gas turbine in the tail called an APU.
  • Uses and functions:
    • Generates electricity on the ground and can support electrical systems when main engines are off or during ground operations.
    • Designed to produce more air than is needed to drive the turbines, which enables starting main engines and providing aircraft systems with necessary bleed air or electrical power.
  • Real-world relevance:
    • APUs enable independent power for starting engines, cabin conditioning, and other systems during ground operations.

Connections to core concepts and practical implications

  • Across engine types, the basic core remains: intake → compressor → combustion → turbine. Variants add/fine-tune components to meet performance, efficiency, noise, and size requirements for different aircraft roles.
  • Efficiency and noise considerations drive design choices (e.g., high bypass ratio in turbofans reduces fuel burn and noise but increases size and weight).
  • Propulsion and drivetrain choices (direct drive vs geared, LP/IP/HP spools) impact engine performance, maintenance, and application suitability.
  • Practical implications include trade-offs between thrust, efficiency, noise, packaging, and weight for specific aircraft categories (airliners, business jets, regional turboprops, helicopters, etc.).

Key terms and concepts to remember

  • Turbofan: fan-driven bypass flow; higher bypass ratios enhance efficiency and reduce noise.
  • Bypass ratio: ext{bypass ratio} = rac{ ext{bypass air}}{ ext{core air}}; ext{early}=1:1, ext{modern}
    ightarrow ext{at least } 3:1.
  • Bypass air functions: cooling, noise reduction, and most of the thrust (up to ~80\% in some designs).
  • Spool: one or more shafts connecting turbines to compressors; common layouts include two-spool and three-spool turbofans.
  • Turboprop: uses turbine power to drive a propeller; includes reduction gearbox and various propeller-drive configurations (direct, compound, free turbine).
  • Reverse flow: air path that reverses direction within the engine to improve packaging/ground clearance in certain turboprop designs.
  • Turboshaft: turboshaft core with power shaft for external work (e.g., helicopters).
  • APU: small gas turbine for ground operations and engine starting, providing electrical power and bleed air as needed.