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.

Why erosion is funest for helicopter engines

Recently, I have started my graduation internship at the Netherlands Aerospace Centre (NLR). During my internship I am trying to map the severity of erosion on a helicopter engine. In this article I will briefly explain why erosion is funest for gas turbine engines, especially for helicopters.

First let me start off by giving a definition on erosion from found literature: Finnie, Wolak and Kabil defined erosion as “The removal of material from a solid surface by the action of impinging solid or liquid particles” [Source 1]. Sundararajan and Roy defined erosion as “the removal of material from component surfaces due to impact of hard particles travelling at substantial velocities” [Source 2], which is a more applied definition to erosion.  Erosion is a broad term which also is used in the geology. The picture below is known as Árbol de Piedra, an eroded rock formation in Bolivia.

The rock is eroded due to grains of sand that have been carried away by the wind and impacted into this rock formation. One grain of sand does not make a difference, but if this happens often enough, one could see the effect: erosion.

This is also a problem for gas turbines, especially the ones operating in sandy environments. Sand that has been ingested in the engines will pass all the blades and vanes inside the gas turbine and will erode them on long term. Especially helicopter engines suffer even more from this erosive effect due to blown up sand due to the downwash of the rotors, also called a ‘brown-out’. The picture below shows a V-22 Osprey demonstrating a brown-out landing. 

Brown-out landings are mostly notorious for the hazard of the loss of situational awareness of the pilot. An example is the crash of a Sikorsky MH-60K Black Hawk of the US Army in Afghanistan, 2001 [Source 3]. The secondary effect of a brown out is excessive sand ingestion in the engine, with more erosion as a result.

What is the effect of erosion on gas turbine components? The late Professor Widen Tabakoff did extensive research into erosion and the effects on gas turbines. He described in his researches, along with other researchers that for the compressor, erosion resulted in (see also the illustration on the right):

• ·         Blunted leading edges (1);
• ·         Sharpened trailing edges (2);
• ·         Reduced blade chords (3);
• ·         Increased pressure surface roughness (4).

What is the effect of these four erosion results? Overall, the efficiency of the compressor will degrade. What is the effect of compressor efficiency degradation? Higher fuel flows. To explain this, let me start by explaining how a turboshaft engine works. The illustration below is a basic schematic of a turboshaft, a gas turbine engine type applied on helicopters. The green part is designed to deliver a high energetic gas to the purple part, the free power turbine (3). The Free Power Turbine extracts the energy from the gas and transfers it to a power shaft which is coupled by gearboxes to the rotors of the helicopter. The engine is designed in such a way that the Free Power Turbine always delivers the same amount of work on the Power Shaft.

If it happens, due to erosion, that the compressor efficiency drops, it means that the compressor (1) requires more energy which is ‘harvested’ from the compressor turbine (2). This requires more energy from the compressor turbine and, compared to a more efficient engine, would deliver less energetic gas to the Free Power Turbine. To keep delivering a constant amount of work to the Power Shaft, the Free Power Turbine requires more energetic gas and the engine will adjust this by injecting more fuel. More fuel usage means less range or bringing along more fuel which means more costs. Either way, no positive effect for the operator but no hazardous conditions.

A more hazardous effect is the potential of an engine surge, or compressor stall. A compressor is designed to do a very unnatural thing: moving air from low pressure to high pressure. To achieve this, the compressor is designed very precisely. When the compressor blades are off-design, for instance as result of erosion, disruption of air could occur within the engine and the high pressurized air will flow out of the front of the engine: an engine surge. 

This video [link] is an example of what happens during an engine surge. The GE90 engine was mounted on the first 747 during its test program. You could see (and hear) a violent bang with flames coming out of the front of the engine, which is the high energetic gas mixture from the combustion chamber. An engine surge damages the compressor section and will result in an engine shutdown. For a Boeing 747 with three other fine working engines this does not have to end catastrophically, but for a low-flying helicopter it certainly could.

To summarize: brown-outs cause excessive erosion to helicopter engines. Blunted leading edges, sharpened trailing edges, reduced blade chords and increased pressure surface roughness is the effect of erosion. This results in higher fuel usage due to compressor efficiency detoriation and could lead to the potential of engine surges which could end catastrophically to a low-flying helicopter.

Sources:

[1]       Finnie, I., Wolak, J., & Kabil, Y. (1967). Erosion of Materials by Solid Particles. Journal of Materials, Vol. 2, No. 3, 682-700.

[2]       Sundararajan, G., & Manish, R. (1997). Solid particle erosion behaviour of metallic materials at room and elevated temperatures. Tribology International Vol. 30, No. 5, 339-359.

[3]       https://aviation-safety.net/wikibase/wiki.php?id=77600

[4]       Hamed, A., & Tabakoff, W. (2006). Erosion and Deposition in Turbomachinery. Journal of Propulsion and Power, 350-360.