Discovery Center -- How it Works: Jet Engine

Over the course of the next 40 thousandths of a second you, Mr. or Ms. Molecule, will be beaten through 18 stages of compression, singed in a furnace heated to nearly 3,000 degrees Fahrenheit, expanded through a turbine and pushed out the back with a wicked headache and the bitter knowledge as the aircraft screams on that your ugly ordeal has been merely for the sake of getting Aunt Mildred to Phoenix for a much needed rest.

That’s basically what happens every day in the skies above as thousands of Pratt engines power aircraft from place to place while the passengers and crew inside those aircraft experience only the steady and reassuring thrum that results from Pratt’s precision manufacturing.

The principle of jet propulsion has been demonstrated by anyone who has blown up a balloon for a child and accidentally let go before tying it closed. The air stored up inside the balloon accelerates as it rushes to escape through the narrowest part of the balloon’s neck. This acceleration, or change in the speed of air, combines with the weight of the air itself to produce thrust. It is the thrust that sends the renegade balloon zipping madly about the room.

The original turbojet engine, which debuted in scheduled commercial service in 1952, accomplished this same trick on a breathtaking scale and with pinpoint control. The progress of jet engine technology in the past four decades has been to increase the amount of air going into the engine, and change the speed of that air with ever greater efficiency. All the examples in the article are drawn from the mightiest family of engines ever to fly, the Pratt & Whitney 4000s.


1. Fan	The propulsion process begins with the huge, 9-foot-diameter fan at the front of the engine, spinning 2,800 times a minute at takeoff speed. That fan sucks in air at the rate of 2,600 pounds per second, or enough to vacuum out the air from a 4-bedroom house in less than half a second.
2. Compression	As the air leaves the fan it is now separated into two streams. The smaller stream, about 15 percent of the total volume of air, is called primary or core air and enters the first of two compressors that are spinning in the same direction as the fan itself. As the primary air passes through each stage of the two compressors, both its temperature and pressure rise.
3. Combustion	When compression is complete, the air, now 30 times higher in pressure and 1,100 degrees hotter, is forced through a furnace or combustor. In the combustion chamber, fuel is added and burnt. The air’s temperature soars even higher, and the air is finally ready to do the two jobs for which it has been so hastily prepared.
4. Turbine	The first job is to blast through the blades of two turbines, sending them whirling just like the wind spinning the arms of a windmill. The whirling turbines turn the shafts that drive both compressors and the fan at the front of the engine. This process, in which the engine extracts energy from the air it has just captured, is what allows modern jets to operate with such high fuel efficiency.
5. Exhaust	The second job is to push the airplane. After passing through the turbines, the hot air is forced through the exhaust opening at the back of the engine. The narrowing walls of the exhaust force the air to accelerate and, just as with the balloon, the weight of the air combined with its acceleration drives the engine, and the airplane attached to it, forward.
6. Fan Air or Bypass Air	The larger air stream exiting the fan, representing 85 percent of the total, is called fan air or bypass air, because it bypasses this entire process. The engine itself is shrouded in a metal casing called the nacelle, shaped roughly like a sideways ice cream cone with the bottom cut off. Bypass air is forced through the ever narrower space between the nacelle wall and the engine, picking up speed along the way. Because of its huge volume, bypass air needs only to accelerate a small amount to produce an enormous kick of thrust. In the PW4084 engine, bypass air accounts for 90 percent of the thrust, and has the added benefits of keeping the engine cooler, quieter and more fuel efficient.

Fan

The propulsion process begins with the huge, 9-foot-diameter fan at the front of the engine, spinning 2,800 times a minute at takeoff speed. That fan sucks in air at the rate of 2,600 pounds per second, or enough to vacuum out the air from a 4-bedroom house in less than half a second.

Compression

As the air leaves the fan it is now separated into two streams. The smaller stream, about 15 percent of the total volume of air, is called primary or core air and enters the first of two compressors that are spinning in the same direction as the fan itself. As the primary air passes through each stage of the two compressors, both its temperature and pressure rise.

Combustion

When compression is complete, the air, now 30 times higher in pressure and 1,100 degrees hotter, is forced through a furnace or combustor. In the combustion chamber, fuel is added and burnt. The air’s temperature soars even higher, and the air is finally ready to do the two jobs for which it has been so hastily prepared.

Turbine

The first job is to blast through the blades of two turbines, sending them whirling just like the wind spinning the arms of a windmill. The whirling turbines turn the shafts that drive both compressors and the fan at the front of the engine. This process, in which the engine extracts energy from the air it has just captured, is what allows modern jets to operate with such high fuel efficiency.

Exhaust

The second job is to push the airplane. After passing through the turbines, the hot air is forced through the exhaust opening at the back of the engine. The narrowing walls of the exhaust force the air to accelerate and, just as with the balloon, the weight of the air combined with its acceleration drives the engine, and the airplane attached to it, forward.

Fan Air or Bypass Air

The larger air stream exiting the fan, representing 85 percent of the total, is called fan air or bypass air, because it bypasses this entire process.
The engine itself is shrouded in a metal casing called the nacelle, shaped roughly like a sideways ice cream cone with the bottom cut off. Bypass air is forced through the ever narrower space between the nacelle wall and the engine, picking up speed along the way.
Because of its huge volume, bypass air needs only to accelerate a small amount to produce an enormous kick of thrust. In the PW4084 engine, bypass air accounts for 90 percent of the thrust, and has the added benefits of keeping the engine cooler, quieter and more fuel efficient.