Glossary of engine terms
Clean Air Engine (Claire) is a technology program jointly developed by MTU and Bauhaus Luftfahrt. It aims to drastically reduce the fuel consumption and carbon dioxide output of aircraft engines. The goal is to achieve a 30-percent reduction by 2035. At the same time, it aims to substantially cut noise levels. All key components of the Claire program have already been tested or demonstrated proof of principle, and fulfill all expectations concerning energy efficiency and economic viability.
A combustor or combustion chamber consists of an outer casing and a flame tube or ‘can’ in which the actual combustion takes place. Inside, the compressed air flowing into the chamber is mixed with fuel, which is then ignited and burns at a temperature of over 2,000 degrees Celsius. Due to the high temperatures involved, combustors require special thermal barrier coatings.
The task of the compressor is to ingest air and compress it before it is fed into the combustor. Compressors consist of bladed disks (rotors) that rotate at very high speed between stationary guide vanes (stators). In order to achieve a compression ratio of over 40:1, which is standard in all modern two-shaft engines, it is necessary to use multi-stage low-pressure and high-pressure compressors rotating at different speeds on dual concentric shafts. These are driven by the corresponding turbines.
The first rotor of the low-pressure compressor is called the fan. It accelerates the bypass stream flowing aftward and provides the engine’s main thrust. It is driven by the low-pressure turbine via the low-pressure shaft.
Geared turbofan engines consume far less fuel and generate significantly less noise than today’s engine types. They therefore have every chance of becoming the standard type for use in future aircraft. Normally, an engine’s fan, low-pressure compressor and low-pressure turbine are all rigidly connected to one shaft. In contrast, the geared fan is ‘decoupled’ from the low-pressure section by means of a reduction gear unit. This enables the low-pressure turbine and the low-pressure compressor to run at their optimum high speeds, while the fan rotates at a much lower speed (in a ratio of approx. 3:1). This results in significantly improved overall engine efficiency and greatly reduced noise levels.
INDUSTRIAL GAS TURBINES
The operating principle of an industrial gas turbine is essentially the same as that of an aero engine. However, instead of the customary low-pressure turbine used in aircraft, industrial gas turbines have a power turbine. This turbine delivers the power, either directly or via a gear unit, to an additional attached power unit such as a pump or generator. Nearly all industrial gas turbines of the lower and intermediate power classes are aero-engine derivatives.
MRO stands for maintenance, repair and overhaul. At MTU, the term ‘MRO business’ is also used more specifically to designate one of the company’s two business segments, where it refers to maintenance services for commercial engines, or commercial MRO.
At MTU, the original equipment manufacuring – or OEM - business segment refers to the development, manufacture and assembly of (new) commercial and military engines. Spare parts for (in-service) commercial and military engines and maintenance services for military engines are also included in this business segment.
RISK- AND REVENUE-SHARING PARTNERSHIP
In a risk- and revenue-sharing partnership, each partner contributes a certain share of the resources needed for a specific engine program (work capacity and funding), thus carrying part of the risk. In return, each partner is entitled to a corresponding percentage of the overall sales revenue from that program.
A complete aircraft engine is made up of a number of subsystems. These include the high-pressure and low-pressure compressors, the combustor, the high-pressure and low-pressure turbines and the engine control system.
Jet engines are generally grouped into three thrust classes: engines with a thrust of between 2,500 and around 20,000 pounds (around 10 – around 90 kN), engines with a thrust of between 20,000 and approximately 50,000 pounds (around 90 – around 225 kN), and engines with a thrust ranging from 50,000 to more than 100,000 pounds (around 225 – around 450 kN).
In a turbine, the energy contained in the gases emerging at high pressure and velocity from the combustor is converted into mechanical energy. Like the compressor, the turbine is subdivided into a high-pressure and a low-pressure section, each of which is directly connected to the corresponding compressor via the respective shaft. The turbine has to withstand much higher stresses than the compressor, as it has to deal not only with the high gas temperatures but also with extreme centrifugal forces of several tons acting on the outer rim of its disks.
TURBINE CENTER FRAME
The turbine center frame connects the high-pressure to the low-pressure turbine. It has to be able to withstand the high mechanical and thermal loads. The center frame includes struts to support the shaft bearings, clad with an aerodynamic fairing, and the air and oil supply lines.
The turbofan is an advancement of the turbojet principle, the main difference being its enlarged first compressor stage, the fan. While in turbojet engines, all of the ingested air flows consecutively through the compressor, the combustor and the turbine, turbofans separate the air stream behind the fan. A fraction of the air reaches the combustor via a number of further compressor stages and is burned. The rest, however – which constitutes a much larger fraction – is channeled around the inner components. The ratio between these two airflows is known as the bypass ratio. In modern commercial engines, this ratio can be higher than 10:1. The greater the bypass ratio, the more economical, environmentally compatible and silent the engine. Turbofans are far more fuel-efficient than turbojets.
The most noticeable external feature of a turboprop is its propeller. Inside, however, the engine differs only slightly from the turbojet and the turbofan. The turbine is larger, and drives not only the compressor but also the propeller, the latter via a gear unit. Conse-quently, more energy has to be drawn from the exhaust gas stream in the turbine of a turboprop than in that of other engine types. Over 90 percent of the energy is required for the compressor and the propeller. Turboprop airplanes can only achieve flight speeds of up to 800 km/h. They are thus slower than turbojets or turbofans, but they do have the advantage of consuming far less fuel. This predestines them for use in roles where speed is less important, such as on shorthaul routes or for air freight.
Turboshaft engines are used in helicopters and are similar to turboprops.