By Bjorn Fehrm

August 9, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

We have covered the problem areas of (Figure 1) compression in the compressor and combustion. Now, we look at the power-generating section, the turbines.

Figure 1. The gas turbine cycle and its parts. Source: Rolls-Royce: The Jet Engine.

The Turbine

After the combustor, which produces large volumes of high-pressure gas, we have the turbines, where, for all engines but jet engines, most of the gas energy is extracted to shaft power (jet engines use a lot of the gas energy to accelerate to high speed in the exhaust nozzle, thus creating thrust).

To extract the power efficiently, the turbine is divided into two sections for most engines, the High Pressure Turbine section and the Low Pressure Turbine section, Figure 2

Figure 2. A two-shaft turbine with its parts. Source: Rolls-Royce: The Jet Engine.

The High-Pressure Turbine (HPT) drives the High-Pressure Compressor via its common high-pressure turbine shaft. The Low-Pressure Turbine drives the Low-Pressure Compressor, often called a Booster, in a two-shaft turbofan engine. It also drives the fan in such an engine (for three-shaft engines, the fan has its own turbine). That’s why the Low-Pressure Turbine has more stages and a larger diameter than the High-Pressure Turbine.

The larger diameter gives a higher blade tip speed for a low shaft RPM, which is needed as the fan needs a low shaft RPM to avoid supersonic tips at cruise RPM (and thus losing fan efficiency). The blades’ high tip speed also efficiently converts turbine pressure to shaft power.

The working method of the turbine stage is essentially the reverse of the compressor stage (Figure 3). The expansion of the combustor gases is accelerated through the High-Pressure Nozzle Guide Vanes (Figure 2) to hit the HPT blades at about Mach 1, which is 2,800km/h or 1,750mph due to the high temperatures at the HPT stage.

The principle of the turbine stage can be either the Impulse Turbine, which uses the water wheel principle to consume all speed and thus pressure of the stream, or the reaction turbine, which uses the blades as aerofoils to change the direction of the stream and thus create a rotating force.

Figure 3. Turbine principles. Gas Turbine turbines use a mix of the Impulse and Reaction turbine principles. Source: Rolls-Royce.

In both cases, the Nozzles (the name for the stator vanes in a turbine) change the speed and direct the flow to the optimal speed and angle for the following rotor stage. Engine turbines normally use a combination of the Impulse and Reaction turbine principles to tune the work extraction and pressure loss over the stages to what is suitable for that part of the engine (high-pressure, low-pressure system).

As we explained before, the higher the combustion temperature and, thus, the volume expansion of the combustion gases, the more power the turbine section of a core can extract from the stream. That’s why every degree of temperature the turbine section can handle is such an important design parameter in gas turbine development. The higher the temperature increase, the smaller the core and thus the smaller, lighter the engine.

It leads to today’s HPT Nozzle entry temperatures (the so-called T41 or TIT, Turbine Inlet Temperature) of 1,600°C for short-haul and 1,800°C for long-haul engines. The stresses on the turbine nozzles and rotating stages at these gas temperatures are extreme.

In the next Cormers, we will examine how the combustor and turbine section are designed to withstand the extreme temperatures, pressures, gas speeds, and centrifugal forces on the rotating parts.