By Bjorn Fehrm

June 21, 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.

After covering the main thrust-generating device, which we can call a propeller, fan, or open rotor, depending on the application, we now look at the core, which provides the power to the thrust device. And there, we look at how we use the properties of the air as a gas to get it into a state that the gas turbine needs for different sections.

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

Core processes

In the last corner, we learned fundamentals about how air reacts to changes in pressure, volume, and temperature to understand what happens in a gas turbine.

The relationship is: Pressure * Volume = a constant * Temperature

The air follows the Ideal gas law for its properties.

We now examine how this relationship enables a number of necessary changes in the state of the air as it passes through the engine. To simplify the discussion, we look at cruise conditions.

Airspeed changes in the engine

As the air passes through the engine, there are three main axial air speeds that must be achieved:

• The first is when the air meets a fan or a compressor stage. The desired axial speed is around M0.5. This simplifies the aerodynamic design of the fan/compressor stage as the relative speed of the air to the fan/compressor blade can be kept below M1.5 for the tip of the blade, and large parts are not in supersonic flow.
• The axial speed of the air past the combustion flame cannot be faster than M0.2 to maintain a stable flame and, thus, a stable combustion in the combustor.
• The exit speed of the air from the engine, both for bypass and core air, must be set at the desired Overspeed of the engine to get optimal engine efficiency.

As the air that enters the engine hits the intake with the cruise speed, which we set to M0.8, there are several retardations and speed-ups of the air we need to do as we pass through the engine.

To do this, we use the Ideal gas law and the addition and extraction of work (=energy). We use the principle of a diffuser and nozzle to change air speed, Figure 2.

Figure 2. The state change of air in a diffuser and nozzle. Source: Rolls-Royce, the Jet Engine.

• A diffuser decreases the air velocity through an expanding volume that increases pressure and temperature.
• A nozzle increases the air velocity by decreasing the volume and, by it, decreasing pressure and temperature.

The use of the diffuser and nozzle

To get the intake air from M0.8 to M0.5, the nacelle intake contains a diffuser section just before the fan/first compressor stage. A benefit is that the pressure is increasing, which augments the engine’s Pressure Ratio (PR). The temperature increase of the air is an undesired consequence of the process.

The compressor then compresses the air over several stages by adding work from the turbine. As a consequence of the pressure increase, the volume of the compressor channel is gradually reduced (high-pressure air occupies less volume). This results in an increased temperature along the compressor, which forces the use of high-temperature alloys for the last stages of the compressor in high-pressure ratio engines (initial stages are made in titanium, then typically Nimonic or similar high strength, high-temperature alloys are used).

When the compressed air enters the combustor, a diffuser lowers the speed to around M0.2 around the fuel injectors to safeguard a stable combustion.

The burning fuel adds energy to the air, increasing its volume, which results in an increased velocity of the air as it’s not a volume-constrained process. The turbine nozzle adds even more velocity to the air. The turbine then extracts work from the gas, reducing pressure and temperature.

Finally, at the exhaust of the core and bypass, we use the remaining pressure in a nozzle to increase the flow speed to the correct Overspread.

In summary, the gas turbine needs a range of state changes of the air/combustion gas, where the Ideal gas law behavior of the flow is used to create changes in volume in diffusers and nozzles and, by it, velocity and pressure changes. The temperature changes as a consequence of changes in the other properties.