The Jet Engine 579
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 this
article are drawn from the mightiest family of engines ever to fly, the Pratt & Whitney 4000s.An Engineering Marvel: The Jet Engine
Fa n
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 com-
pressors 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 mass of the air combined with its
acceleration drives the engine, and the airplane attached to it, forward.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀