Introduction of jet engine

• The image above shows how a jet engine would be situated in a modern military aircr

aft. In the basic jet engine, air enters the front intake and is compressed (we will see

how later). Then the air is forced into combustion chambers where fuel is sprayed into

it, and the mixture of air and fuel is ignited. Gases that form expand rapidly and are e

xhausted through the rear of the combustion chambers. These gases exert equal forc

e in all directions, providing forward thrust as they escape to the rear. As the gases le

ave the engine, they pass through a fan-like set of blades (turbine), which rotates a s

haft called the turbine shaft. This shaft, in turn, rotates the compressor, thereby bringi

ng in a fresh supply of air through the intake. Below is an animation of an isolated jet

engine, which illustrates the process of air inflow, compression, combustion, air outflo

w and shaft rotation just described.

• The process can be described by the following diagram adopted from the w

ebsite of Rolls Royce, a popular manufacturer of jet engines.

•

This process is the essence of how jet engines work, but how exactly does s

omething like compression (squeezing) occur? To find out more about each

of the four steps in the creation of thrust by a jet engine, see below.

• SUCK

The engine sucks in a large volume of air through the fan and compressor stages. A typical commercial jet engine takes in 1.2 tons of air

per second during takeoff—in other words, it could empty the air in a squash court in less than a second. The mechanism by which a jet

engine sucks in the air is largely a part of the compression stage. In many engines the compressor is responsible for both sucking in the

air and compressing it. Some engines have an additional fan that is not part of the compressor to draw additional air into the system. The

fan is the leftmost component of the engine illustrated above.

•

SQUEEZE

Aside from drawing air into the engine, the compressor also pressurizes the air and delivers it to the combustion chamber. The compress

or is shown in the above image just to the left of the fire in the combustion chamber and to the right of the fan. The compression fans are

driven from the turbine by a shaft (the turbine is in turn driven by the air that is leaving the engine). Compressors can achieve compressio

n ratios in excess of 40:1, which means that the pressure of the air at the end of the compressor is over 40 times that of the air that enters

the compressor. At full power the blades of a typical commercial jet compressor rotate at 1000mph (1600kph) and take in 2600lb (1200k

g) of air per second.

• Now we will discuss how the compressor actually compresses the air.

•

As can be seen in the image above, the green fans that compose the compressor gradually get smaller and smaller, as does the cavity thr

ough which the air must travel. The air must continue moving to the right, toward the combustion chambers of the engine, since the fans

are spinning and pushing the air in that direction. The result is a given amount of air moving from a larger space to a smaller one, and thu

s increasing in pressure.

•

BANG

In the combustion chamber, fuel is mixed with air to produce the bang, which is responsible for the expansion that forces the air into the tu

rbine. Inside the typical commercial jet engine, the fuel burns in the combustion chamber at up to 2000 degrees Celsius. The temperature

at which metals in this part of the engine start to melt is 1300 degrees Celsius, so advanced cooling techniques must be used.

• The combustion chamber has the difficult task of burning large quantities of fuel, supplied through fuel spray nozzles, with extensive volu

mes of air, supplied by the compressor, and releasing the resulting heat in such a manner that the air is expanded and accelerated to give

a smooth stream of uniformly heated gas. This task must be accomplished with the minimum loss in pressure and with the maximum heat

release within the limited space available.

• The amount of fuel added to the air will depend upon the temperature rise required. However, the

maximum temperature is limited to certain range dictated by the materials from which the turbine

blades and nozzles are made. The air has already been heated to between 200 and 550 °C by th

e work done in the compressor, giving a temperature rise requirement of around 650 to 1150 °C f

rom the combustion process. Since the gas temperature determines the engine thrust, the combu

stion chamber must be capable of maintaining stable and efficient combustion over a wide range

of engine operating conditions.

• The air brought in by the fan that does not go through the core of the engine and is thus not used

for combustion, which amounts to about 60 percent of the total airflow, is introduced progressivel

y into the flame tube to lower the temperature inside the combustor and cool the walls of the flam

e tube.

• BLOW

The reaction of the expanded gas—the mixture of fuel and air—being forced through the turbine,

drives the fan and compressor and blows out of the exhaust nozzle providing the thrust.

• Thus, the turbine has the task of providing power to drive the compressor and accessories. It do

es this by extracting energy from the hot gases released from the combustion system and expan

ding them to a lower pressure and temperature. The continuous flow of gas to which the turbine i

s exposed may enter the turbine at a temperature between 850 and 1700 °C, which is again far a

bove the melting point of current materials technology.

• To produce the driving torque, the turbine may consist of several stages, each employing one ro

w of moving blades and one row of stationary guide vanes to direct the air as desired onto the bla

des. The number of stages depends on the relationship between the power required from the gas

flow, the rotational speed at which it must be produced, and the diameter of turbine permitted.

• The desire to produce a high engine efficiency demands a high turbine inlet temperat

ure, but this causes problems as the turbine blades would be required to perform and

survive long operating periods at temperatures above their melting point. These blade

s, while glowing red-hot, must be strong enough to carry the centrifugal loads due to r

otation at high speed.

To operate under these conditions, cool air is forced out of many small holes in the bl

ade. This air remains close to the blade, preventing it from melting, but not detracting

significantly from the engine's overall performance. Nickel alloys are used to construc

t the turbine blades and the nozzle guide vanes because these materials demonstrate

good properties at high temperatures