Pratt & Whitney PW4000 Commercial Jet Engine
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Pratt & Whitney PW4000 Commercial Jet Engine
F119-PW-100 Military Jet Engine
The jet engine takes in a large volume of air, compresses it, adds fuel, ignites the mixture and expels the hot gases to produce thrust. Each of these steps can be seen in the cutaway for the Pratt & Whitney (P&W) PW4000 engine.
The fan section is responsible for the intake of the air. It acts just like you fan at home. It is also similar to the first stroke of a four stroke engine such as the one used in your car where air and fuel are drawn into the cylinder. In commercial aircraft, the fan also can be used to supplement the flow of hot gases from the engine with extra air propelled by the fan. At subsonic speeds, it is actually more economical to use an engine that provides much of its thrust in this manner. For military aircraft, the need for high performance and quick response makes it desirable to use an engine with a smaller fan.
The fan blades can be very large and rotate at several thousand revolutions per minute. Some can be up to 50 inches or more long. They are slightly warmed during operation. Typically the temperature is less than 300 F for commercial aircraft. They work reliably for hundreds or thousands of hours between overhauls and replacements. The fan blades must also be capable of surviving an impact with a bird if one is ingested into the engine.
Typically titanium, aluminum and stainless steel have been used in the fan blades. Titanium is currently favored because of its light weight, corrosion resistance and good high cycle fatigue properties. Future planes my use fans with polymer (plastic) matrix or ceramic matrix composite blades.
Things begin to heat up literally and figuratively in the compressor section. The air coming into the engine is compressed by rotating blades pushing the air against stationary vanes. This step is similar to the compression stroke in your automobile's engine. Aircraft engines can compress that air 25X or more. As the air is compressed, it heats up. Depending on how many compressor stages the engine has and what the total compression is, the compressors can see temperatures over 1000 F.
A variety of materials have been used in engine compressor sections. Initially stainless steels were typical. Today's engines normally use an iron or nickel based superalloy. These are typically iron or nickel alloyed with aluminum and titanium for strength and chromium and rare earth elements such as yttrium for corrosion resistance. Future engines may well use ceramic matrix composites. Regardless of what is used, the material must have high strength at elevated temperatures, be able to resist creep (slow deformation by an applied stress at elevated temperatures), be able to resist the growth of cracks, not oxidize severely at these temperatures and, in the case of the vanes in particular, be able to resist erosion.
In addition to the blades that compress the air, there are disks that hold the blades and connect them to the central shaft. These disks can be very large and heavy. Some are over three feet in diameter and weight more than 200 pounds. They also operate at high temperatures like the blades on their edge. Typically the disks are made out of similar materials as the blades and need to have the same types of properties, i.e., good elevated temperature strength. The balance of properties, tensile strength versus creep resistance versus crack growth rate versus high cycle fatigue life, is different. Hence, while the same elements are typically in the two alloys used, the compositions are varied to optimize each part.
The combustion chamber is where the fuel comes together with the compressed air and is ignited or combusted. It is equivalent to the compressed fuel and gas mixture in your car engine being ignited by the spark plug. The burning of the fuel heats the air and causes it to expand. The hot gases flow out the back of the engine and provide the power to propel the aircraft through the sky.
This section of the aircraft engine is made almost exclusively from high temperature nickel- and cobalt-based superalloys. They have large amounts of aluminum and titanium for strength. Chromium is also added for corrosion resistance. Refractory elements such as molybdenum, tungsten and niobium are also common. Rhenium is also being added to the newest generation of blades. The alloys have to be able to withstand extreme heat with temperatures of 1800 F or more being common. The rotating blades must also be able to resist creeping, not oxidize or otherwise corrode, resist crack growth and have high strength at elevated temperatures. Not only are high temperature alloys required, exotic processing of the alloys is often used.
If you look at an old brass doorknob, you can see large flecks on the surface. These are actually grains within the metal. Where these grains come together are called grain boundaries. Grain boundaries are normally much weaker than the grains at high temperatures. Today's engines often use directionally solidified and single crystal turbine blades. Directionally solidified blades have all the grain boundaries aligned along the length of the turbine blade. With no grain boundaries being pulled apart by the forces generated by the blade revolving at several thousand revolutions per minute, the blade lasts much longer. The ultimate blade today is a single crystal blade. These blades literally have only a single grain for the entire blade. With no grain boundaries they are the strongest of all blades.
Today's jet engines also make extensive use of cooling. Just as your car's radiator transfers heat from the engine to the surrounding air, jet engines use air flowing through the core of blades to cool them. The design and manufacture of blades with the very complex cooling channels is one of the largest challenges to further advancements in jet engine performance. Blade casters such as PCC Airfoils and Howmet are working with engine manufactures on this problem.
Finally, the place where the combustors themselves are often made from exotic materials. In the two engines, they are where you can first see the flame moving from left to right. They are made from high temperature nickel- and cobalt-based alloys that are very resistant to corrosion and extreme temperatures. Their strength is not as critical since the part is not moving. Newer deigns may use ceramics or ceramic matrix composites for the combustors.
The jet engine uses the turbine section to provide the power to the engine to rotate all of the various moving parts. Hot gases flow over the turbine blades. The blades act much like a windmill in a strong breeze and rotate. The rotation of the blades is transmitted to the shaft running the length of the engine and on to the fan, compressor and combustor sections.
Like the other hot sections, the turbine is generally made from nickel-based superalloys in the high pressure section. The low pressure section can have nickel- or iron-based superalloys or even stainless steels if the temperature is low enough. The material requirements are similar to the combustor section, but the temperature begins to drop in the turbine section as the gases are allowed to expand.
Around the entire engine are a series of cases. Those in the hot section (compressor, combustion chamber and turbine) must be made of high temperature materials such as nickel-based superalloys. Because they are not moving, they do not need to be as strong or creep resistant. However, they must have excellent impact resistance so they can contain any part that breaks. If the cases cannot contain the flying blades, the debris can penetrate the aircraft cabin and injure or kill someone.
For the lower temperature sections, the casings are typically made from aluminum or polymer matrix composites. These tend to be lightweight and impact resistance.
At the center of the engine is a large shaft that runs almost the entire length of the engine. It is analogous to the drive shaft of your car. It transmits the energy from the turbine section to the rest of the engine to drive the various rotating blades in the fan, compressor and combustor sections. Today's jet engines have hollow shafts that carry cooling air to the blades and vanes within the engine.
The shaft is made from an alloy that is very strong and resistant to high cycle fatigue. Depending of the size of the engine, the shaft can be made from stainless steel, iron-based superalloys or nickel-based superalloys.
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