By Bjorn Fehrm

June 14, 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 main thrust-generating device, which we can call a propeller, fan, or open rotor, depending on the application. To drive the main thrust device, we need a lot of shaft power, which is provided by the core. We start with how the core, which is a gas turbine, generates power.

Figure 1. The core cycle compared to a piston engine cycle. Source: Rolls-Royce, The Jet Engine.

How air pressure, volume, and temperature are connected

To understand what happens in a gas turbine, we need to understand some fundamentals about air. Air is made of a cocktail of gases that reacts to changes in one property, like volume, with changes in the other properties, pressure and temperature.

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

We only need to know that if we increase the pressure at a constant volume, the temperature will increase. Or if we increase the temperature at constant pressure, the volume will increase. And so on.

The core is a gas turbine

Figure 1 shows a jet engine cycle compared with a piston engine cycle. Jet engines were the first, simplest aircraft gas turbine engines. A jet engine and a turbofan differ in that the first compressor stage is larger for a turbofan, with a larger diameter, Figure 2. The first stage, now called a fan, pushes accelerated air both into the bypass channel and the core.

Figure 2. A two shaft turbofan engine. Source: Rolls-Royce, The Jet Engine.

In figure 1 we have a single shaft gas turbine core, in Figure 2, a two shaft core. There are also three shaft engines from Rolls-Royce and Ivchenko. We will come to why engine makers use several shafts in their engines.

The gas turbine cycle

We use Figure 1 as the example of a gas turbine as Rolls-Royce has made a nice complementing graphic (Figure 3), which we can use to explain what happens in a gas turbine.

Figure 3. The gas turbine cycle. Source: Rolls-Royce, The Jet Engine.

A car piston engine and a gas turbine engine are both heat engines. It means they generate power by heating air to increase its energy and then extract work from the heated air.

They both have;

• An air intake. In the classical piston engine, the air is mixed with the fuel in the air intake. Advanced piston engines, like the gas turbine, have fuel injection at the start of the combustion phase (direct injection engines).
• The air is compressed to make the subsequent combustion more efficient. Figure 3 shows the compression in a multistage axial compressor, with the pressure in PSI in blue. The ambient air at the intake is at 15 PSI (1013 hPa), with the final pressure at the end of the compressor 10 times higher. The compressor has a Pressure Ratio (PR) of 10. The higher the compression, the higher the combustion efficiency. A PR of 10 is low, typical of engines from the 1960s. Today’s engines have PR 40 to 60. Compression increases the temperature of the air, starting at 15° C and ending at around 400°C.
• The combustion in a piston engine is in a constrained volume, thus the pressure increases, which pushes the piston down. The combustion in a gas turbine is not constrained in volume, the combustion gas can expand towards the turbine (the compressor, which has ~150 PSI at the end, is a less attrative way for the combustion gases). The combustion under constant pressure (it declines slightly due to flow losses in the combustor) makes the gas turbine a Brayton heat engine, where the combustion is done under constant pressure and expanding volume. The 400°C at the end of the compressor has now increased to close to 1,000°C. This is a low value today, where the latest, most advanced engines almost double this value.
• The turbine stages take the expanding gas and give it a high velocity in the first static stage of the turbine, called the turbine nozzle. The high-temperature, high-velocity combustion gas now generates shaft power in the turbine stages. Finally, the jet exhaust nozzle of the gas turbine uses the remaining energy of the gases to accelerate the gas mass flow to a suitable exhaust speed, generating jet thrust. The jet thrust is 100% of the thrust for a jet engine, about 10-20% for a turbofan, and less than 10% for a turboprop or open fan engine.

We will go through the different stages of the gas turbine and analyze why it’s such a challenge for engine OEMs to make the engine and its core both reliable (running 99.9% of the time when properly cared for) and durable (staying on the wing for a lot of flights).