Jet Engines

Jet Engines

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The Basics

A jet engine can be divided into several distinct sections: intake, compressor, diffuser, combustion chamber, turbine, and exhaust. These sections are much like the different cycles in a four-stroke reciprocating engine: intake, compression, power and exhaust. In a four-stroke engine a fuel/air mixture is is brought into the engine (intake), compressed (compression), and finally ignited and pushed out the exhaust (power and exhaust). In it's most basic form, a jet engine works in much the same way.

* Air comes in the front of the engine where it enters the compressor. The air is compressed by a series of small spinning blades aptly named compressor blades and leaves at a high pressure. The pressure ratio between the beginning and end of the compressor can be as much as 48:1, but almost always 12:1 or more.
* The air now enters the diffuser, which is nothing more than an area where the air can expand and lower it's velocity, thus increasing its pressure a little bit more.
* The high pressure air at the end of the diffuser now enters the combustion chamber where it is mixed with fuel, ignited and burned.
* When the fuel/air mixture burns, the temperature increases (obviously) which makes the air expand.
* This expanding gas drives a set of turbine blades located aft of the combustion chamber. At least some of these turbine blades are connected by a shaft to the compressor blades to drive them. Depending on the type of engine, there may be another set of turbine blades used to drive another shaft to do other things, such as turn a propeller or generator.
* The left over energy not extracted by the turbine blades is pushed out the back of the engine (exhaust section) and creates thrust, usually used to drive an airplane forward.


The types of jet engines include:

* Turbojet
* Turbofan
* Turboprop
* Turbo shaft


Turbojet

The turbojet is the simplest of them all, it is just as described in "The basics" section. This style was the first type of jet engine to be used in aircraft. It is a pretty primitive style used mostly in early military jet fighters such as the F-86.

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Its use was discontinued, for the most part, in favor of the more efficient turbofans. Actually, I should clarify that. Each type of engine is most efficient under certain conditions. Turbojets are most efficient at high altitudes and speeds above the speed of sound. See the diagram at the end of this page for relative efficiencies of each style engine.

Turbofan

Turbofans make up the majority of jet engines being produced and used today. A turbofan engine uses an extra set of turbine blades to drive a large fan, typically on the front of the engine. This fan differs from a propeller in that there are many small blades and they are inside of a duct. The fan sits just in front of the normal intake, some of the air driven by this fan will enter the engine, while the rest will go around the outside. The amount of air that bypasses the engine is different for each type of airplane. The different styles are called high and low bypass engines. Bypass ratio is the ratio of how much air goes through the fan, to how much goes through the engine. Typical bypass ratios would be 1:1 for a low bypass and 5:1 or more for a high bypass. Low bypass engines are more efficient at higher speeds, and are used on planes such as military aircraft, while high bypass engines are used in commercial airliners.

Turboprop

Turboprops are similar to turbofans in that they incorporate an extra set of turbine blades used to drive the propeller. Unlike the turbofan engines, nearly all the thrust produced by a turboprop is from the propellor, hardly any thrust comes from the exhaust. These engines are used mostly on smaller and slower planes such as commuter aircraft that fly to the smaller airports. As you can see from the efficiency chart below, turboprops are very efficient over a fairly wide range of speeds. They would probably be used more often on large transport aircraft, except for one problem: they have propellors. The general public does not like propellors, as they appear to be old-fashioned and unsafe. However, the military knows better and uses them on several large transport aircraft.

Turbo shaft

Turbo shaft engines are very similar to turboprop engines, but instead of driving a propellor, they are used to drive something else. Many helicopters use them to drive their rotors, and airliners and other large jets use them to generate electricity. Also, the Alaska Pipeline uses them at the pump stations to pump oil.

Overall

Overall the big difference between these engines is how they take a chunk of air and move it. Newton's third law states that Force equals mass times acceleration. Applying this to turbine engines: the turboprop takes a large chunk and accelerates it a little bit, while the turbojet takes a small chunk and accelerates the heck out of it, and the turbofan is somewhere in between these two.

These different methods of moving air also have to do with how much noise each engine makes. The turbojet makes the most noise because there is a large difference in velocities of the blast of air coming out the exhaust and the surrounding air. The air from the fan on a turbofan engine "shields" the blast in the center by having the slower moving air from the fan surround it. Then the turboprop is the quietest of all because the air it's moving is relatively slow.

A pressure - volume diagram (or a P-V diagram) is a useful tool in thermodynamics. In this case, it relates the pressure and volume of the gas moving through the engine at different stages. A P-V diagram can also be helpful in finding the work output of an engine. Work equals the integral of pressure with respect to volume. Or is simpler form, work equals the area enclosed in the diagram above. The above cycle is the Brayton cycle, or the cycle used by aircraft gas turbine engines.

Explanation of the above cycle:

* Air enters the inlet at point 1 at atmospheric pressure.
* As this air passes through the compressor (from point 1 to 2), the pressure rises adiabatically (no heat enters or leaves the system).
* Now the air enters the combustion chamber (from point 2 to 3), is mixed with fuel, and burned at a constant pressure.
* Finally, the air goes through the turbine and out the exhaust (point 3 to 4) where the gases expand and do work. Thus, the pressure drops and the volume increases.


The Compressor

There are two main styles for turbine compressors: the axial and the centrifugal.

The Axial Compressor

* The axial type compressor is made up of many small blades, called rotor vanes, arranged in rows on a cylinder whose radius gets larger towards the back (as can be seen from the above picture). These blades act much like small propellors.
* In between these rotor vanes are stator vanes which stay in a fixed spot and straighten the air coming out of the previous stage of rotor vanes before it enters the next stage.
* On some newer engines, the angle of these stator vanes can be adjusted for optimum efficiency.
* Each stage (1 row of rotor and stator vanes) generally provides for a pressure rise of about 1.3:1 (so after the first stage, the pressure would be 1.3 above atmospheric, after the second it would be 1.69, 2.2, etc...).

The Centrifugal Compressor

* Air enters the centrifugal compressor at the front and center. The blades then sling the air radially outwards where it is once again collected (at a higher pressure) before it enters the diffuser.
* Pressure rise per stage is usually about 4 to 8:1 (higher than axial). These can be sombined in series (that is the exit of the first leads to the entrance of the next) to produce a greater pressure rise. But more than two stages is not practical.

Misc. Compressor Stuff

* The axial and centrifugal compressor described above can be combined together with the axial compressor in front.
* The horsepower required to drive a compressor is very large. The formula for figuring this out is:

HP = Cp x (T2-T1) x Wa x 1.414
Where Cp is the specific heat of air, (T2-T1) is the change in temperature across the compressor, and Wa is the airflow through the compressor in pounds /sec.
A typical example is the JT-9 used on 747's.
HP = .24 x 670(deg F) x 247(lbs/sec) x 1.414
HP = 56160 horsepower, assuming 100 percent efficiency

* The compressor can be tapped near the end to provide pressurized heated air to other systems of the airplane. Most planes use this "bleed air" for pressurizing and heating the cabin. Also, many pump it through the leading edge of the wings to melt ice during icing conditions. This reduces the efficiency of the compressor some, but is usually not much of a factor.
* An engine running at take off power can consume more than 2000 pounds of air per second. That's about 27000 cubic feet of air/second (at atmospheric pressure and 70 deg F). PV = mRT => V = mRT / P = (2000lbm x 0.3704(psia x ft3/lbm x R) x (70 + 460)deg R ) / (14.7 psia) = 26709 ft3.

Miscellaneous

- Jet engines are rated in "pounds of thrust," while turboprops and turboshaft engines are rated in "shaft horsepower" (SHP). This is because it is difficult to hook up a dynamometer (power measuring device) to the column of air coming out of a jet engine, while it is easy to hook one to the shaft of a turboprop.

- An equivalent measure to horsepower is thrust horsepower (THP). THP = (Thrust x MPH) / 375. or THP = SHP x 80% in the case of turboprop engines (the 80% is because the propeller "slips" a little in flight).

- Exhaust gases exit the exhaust at upwards of 1000 mph or more and can use 1000 gallons of fuel/hour or more.

- Turbine engines run lean. Unlike gasoline engines, turbines take in more air than they need for combustion.

- Fuel can be injected into the exhaust section to burn with this unused air for extra thrust. This is called an afterburner.

- A water/methanol mixture can be injected into the intake to increase the air density, and thus increase thrust.

- Turbine engines can be built on a small scale as well. The turbine pictured below has a diameter of 4mm and runs at 500,000 rpm. It was built by at MIT for purposes of powering an aircraft with a wing span of about 5 inches that was projected to fly about 35 - 70 mph with a range of about 40 - 70 miles.
micro turbine


- The ignition system on turbine engines is only necessary for starting, afterwards it is self sustaining. In jets, the ignition system is also turned on for added saftey in "critical" stages of flight, such as takeoff and landing.

- A device similar to a spark plug is used for the ignition process, but it has a larger gap. The spark is about 4 to 20 Joules (watts/second) at about 25000 volts and occurs between 1 and 2 times per second.

- Turbine engines will run on just about anything, they prefer Jet-A (AKA diesel, kerosene, or home heating oil), but can burn unleaded, burbon, or even very finely powdered coal!


- The above snowmachine uses an Allison turbine engine, a very common engine in helicopters (such as the Bell 206 Jet Ranger shown below). A lot of horsepower can be put into a small package! Note the intake and compressor are at the front of the engine, then the two side tubes take the compressed air and bring it around back to the combustion chamber and turbine and the exhaust exits out the middle. There are many engines out there with strange configurations like this.
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