Suck, Squeeze, Bang, Blow - Aircraft engines, how do they work?

I started blogging with the aim to write technical articles in the field of Aviation. Next to opinion articles focused on readers with a certain background, my goal is to write educational articles. These articles will be focused on Aviation professionals, but with limited knowledge in other fields. I want to kick these articles off with one of my favourite subjects within Aviation Engineering: gas turbines, the power behind your flight.

Figure 1: A JT9D high-bypass turbofan on the 'City of Everett', the first produced Boeing 747 prototype

How does an gas turbine work: the essentials

As the title suggests, a simple way to explain the way a gas turbine works is ‘Suck, Squeeze, Bang, Blow’. Air is sucked in, squeezed, ‘blown-up’ and blown away. But how is this possible? Every gas turbine has three basis components: the compressor, the combustor and the turbine. Figure 2 shows the interaction between these three components. Air flows from left to right.

Figure 2: Schematic view of a basic gas turbine

Doing the unnatural: the compressor

The compressor works with the same principle as a bicycle pump: air is rapidly compressed. A bicycle tire is inflated up to pressures of around 6 bars, which is six times the pressure around you. Small gas turbines have similar compression ratios, but there is one difference: the compression of your tire is reached with a lot of (exhausting) pumps, which could take a while. The compression ratio of a gas turbine is reached within the blink of an eye and with a lot more air to compress.  Imagine pumping up your tire in a single stroke, and that is for a small gas turbines. One of the biggest engines currently flying, the GE90 that powers the Boeing 777, has an airflow of about 1400 kg/s. That is sucking in the air of a 100 square meters big hangar every second! The newest developed gas turbines have compression ratios reaching 60! To compare it with water pressure, this is the water pressure at about 600 meters depth. 

To achieve these high pressures, small aerofoils ‘cooperate’ to create a high pressure. Figure 3 shows how an (axial) compressor works: rows of alternating spinning and fixed aerofoils (rotors and stators) push the air through the narrowing compressor. The overall effect is high pressure air. Because air naturally moves from high pressures to low pressures (think about the daily weather forecast), the process within a compressor is very unnatural. For this fact, compressor design is one of the toughest parts within gas turbine design. 

Figure 3: An (axial) compressor

The compressor driver: the turbine

The compressor needs a lot of power to achieve the high compression ratio. This power is ‘harvested’ from the turbine. The principle of a turbine is exactly the opposite of the compressor: air moves from high pressure to a lower pressure and energy is used to drive the shaft which powers the compressor. The design of a turbine also consist of moving and static blades, but the blade design itself differ a bit (see Figure 4). The difference is because of the difference in temperature: while the compressor operates in temperature ranges of ambient temperature to 400/500 degrees Celsius, a turbine operates at temperatures around 1200/1300 degrees Celsius (or higher!). To protect the metal blades from melting away, the blades are forged from exotic metals, cooled internally and externally and coated with a ceramic coating.

Figure 4: A compressor blade (left) versus a turbine blade (right).

Where the magic happens: the combustor

A combination of a turbine and compressor could not operate on itself. Because of inefficiencies, extra input of energy is required to keep the compressor and turbine spinning. This is done by the combustion of fuel. This combustion is done with a very high efficiency with the aim to minimize the NO_x, SO_x and carbon monoxide (CO) emissions. The combustors of older gas turbines are less efficient which results in incomplete combustion (with by-products like soot and carbon monoxide), which clearly could be seen in Figure 6. 

Figure 5: A Boeing 707 in take-off

Future goals are set to repulse emissions by gas turbines and could be seen in Figure 5. The figure means that engines that are been developed by 2020 should emit 50% less carbon dioxide compared to engines developed in 2000. 

Figure 6: ACARE flight path for Europe. Image copyright of Arvind Gangoli Rao and Feijia Yin

Turbofan, Turbojet, Turbowhat?

Within (commercial) aviation, four ‘variants’ of gas turbine exist: turbojets, turbofans, turboprops and turboshafts. These could be divided in two categories: ‘using the power for thrust’ (turbojet & turbofan) and ‘using the power for torque’ (turboprop and turboshaft). Next to delivering power for thrust or lift, gas turbines drive the electric, hydraulic and pneumatic systems of aircraft.

In thrust we trust: the turbofan and turbojet

The turbofan (Figure 8) and turbojet (Figure 7) both rely on the same principle: generating ‘excess’ pressure in the exhaust to generate thrust. You could say that a turbofan is a turbojet with a huge fan mounted on front. The fan, because of the bypass, is more efficient than the turbojet and therefore is applied widely in the civil industry. Modern day turbofans are designed with bigger bypasses to enhance the efficiency.    

Figure 7: a turbojet schematic and the General Electric J85-GE-17A

For military application and in particular for fighter jets, turbofans with high bypass ratios are not useful. For most cases, a turbojet or a turbofan with a low bypass ratio is used. To enhance performance in some scenarios, an afterburner is used. An afterburner is basically a huge combustor mounted within the exhaust, so extra energy is added. With the right use of the exhaust, velocities above the speed of sound could be reached.

Figure 8: A turbofan schematic (left) and the CFM56-2

Small, but powerful: the turboshaft and turboprop

The principle of the turboprop (Figure 9) and turboshaft (Figure 10) is not to generate thrust, but to convert all the power that is generated into torque. The gas turbine is used as a ‘hot gas generator’. The energy from that gas is then extracted mostly by a power turbine: an extra turbine which is not coupled to a compressor but to a gearbox which powers a propeller or (helicopter) rotor. 

Figure 9: Turboprop schematic and the T56-15-LFE

The difference between a turboprop and turboshaft is that mostly one turboprop powers one propeller, but a helicopter rotor could be powered by more than one turboshaft. For instance, the AH-64 ‘Apache’ helicopter has two GE T700 turboshafts, which are both able to power the helicopter by itself. The extra gas turbine is installed to provide (extra) power for certain manoeuvres and for redundancy.

Figure 10: Turboshaft schematic and the T55-GA-714A

What’s else?

Gas Turbines are used for more purposes than described before. The same principle as turboprops and turboshaft could be used to drive anything other than a propeller or rotor. An Auxiliary Power Unit (APU) is a ‘mini’ gas turbine installed in the tail of most (civil) aircraft. The APU provides power for hydraulic, pneumatic and electric systems if the engines are not started yet. The high-pitched sound that could be heard at airports is coming from the APU. It could also be used if both engines fail in mid-flight to power the essential systems to keep the aircraft in control. A deviation from the turboprop is applied in the maritime field. For instance, Areal Commando Frigates use gas turbines for high speeds. Gas turbines are also used to generate high power current. Special gas turbines are designed for energy generation purposes and are enhanced with intercoolers, heat exchangers and are not limited in size. Most of the times a combined cycle is used: steam that is generated with exhaust gasses powers one or more steam turbines. This enhances the total efficiency of the power plant.

Figure 11: Three other major applications of gas turbines: the APU, maritime propulsion and energy generation

What’s next?

Because of environmental boundaries and the depletion of the oil reserves, within this century the aviation industry has to find other means of propulsion. This could be done by using other fuels, going hybrid or even fully electric. I will come back on this subject next month in a separate article on the short- and long term alternatives for gas turbines.

Figure 12: What's next? Image copyright of NASA

I hope I taught you a thing or two about the way gas turbines work. If any questions arise do not hesitate to comment below! Next two blogs I will focus on the ILA Berlin Air Show which I will visit on April 27^th. In two separate articles I will preview and review the air show, the aircraft and technologies displayed at the trade visitors’ event.