Turboshaft Engine – Definition, Function, and Types

What is a Turboshaft Engine?

A turboshaft engine is a gas turbine designed to produce shaft power rather than thrust. It’s essentially a jet engine variant optimized to generate rotational force (horsepower) for driving machinery instead of pushing an aircraft forward.

The engine’s key advantage is its exceptional power-to-weight ratio, making it essential for weight-sensitive applications. Key applications include:

• Helicopters: As the primary power plant, driving the main and tail rotors.

• Auxiliary Power Units (Apus): On large aircraft, driving generators to supply electrical and pneumatic power while on the ground.

• Ground and Marine Vehicles: Powering specialized vehicles like the M1 Abrams tank, high-speed boats, and hovercraft that require high performance from a compact engine.

How Turboshaft Engines Work

A turboshaft engine operates on the same principles as a turbojet but is designed to harness mechanical energy rather than produce thrust. The process involves two primary sections: the gas generator, which creates hot, high-pressure gas, and the power turbine, which converts that gas energy into rotational force.

The cycle begins as air is drawn into the engine’s inlet and fed into a compressor. This component, consisting of multiple stages of rotating blades, rapidly squeezes the air, significantly increasing its pressure and temperature.

The high-energy gas first passes through a turbine connected to the compressor, extracting enough power to keep the gas generator running. The remaining gas then flows to a second, mechanically independent turbine—the ‘free power turbine‘—which captures the remaining energy to spin the output shaft. This turbine spins at incredibly high speeds—often exceeding 30,000 RPM—which is far too fast for most applications.

Modular Engine Designs

Many modern turboshaft engines also feature a modular design, constructed from distinct, easily replaceable sections like the compressor, turbine, and gearbox.

This combination of high power output from a compact, lightweight package makes the turboshaft engine ideal for demanding applications, revolutionizing industries from aviation to ground transport.

Helicopters

The most prominent application of turboshaft engines is in helicopters, where they are perfectly suited to drive the main and tail rotors.

Ground and Marine Vehicles

Beyond aviation, these powerful engines drive specialized ground and marine vehicles.

Auxiliary Power Units (Apus)

Turboshaft engines also serve as Auxiliary Power Units (Apus) on large aircraft and some ground vehicles. In this role, the turboshaft engine doesn’t provide propulsion but instead drives a generator to supply electrical, pneumatic, and hydraulic power. This allows the vehicle’s systems to operate on the ground without running the main engines, saving fuel and reducing wear.

Performance Metrics of Turboshaft Engines

Key performance metrics are used to evaluate a turboshaft engine’s efficiency, output, and operational health.

• Power Output: Measured in shaft horsepower (SHP), this quantifies the mechanical energy the engine delivers.

• Specific Fuel Consumption (SFC): Measures how much fuel is consumed to produce a unit of power. A lower SFC indicates greater fuel efficiency.

• Engine Efficiency: Reflects how effectively the engine converts fuel’s chemical energy into useful mechanical work.

• Time Between Overhauls (TBO): The manufacturer’s recommended operational hours before an engine requires a comprehensive inspection, impacting maintenance and costs.

• Real-time Health Indicators: Monitored parameters like turbine inlet temperature and torque provide a continuous assessment of the engine’s health.

A key feature of modern turboshaft engine design is the free power turbine. Unlike single-shaft designs where the compressor and power output are linked, this configuration mechanically separates the turbine that drives the output shaft from the gas generator turbine. This separation provides several key operational advantages.

This separation allows rotational speeds to be optimized independently and enhances flexibility, letting the engine match its output to fluctuating load demands.