ac640barre.gif (1651 octets)

1) HOW AN ENGINE WORKS : ( very briefly ! )

A turbofan engine is made of 4 main components :

  1. a compressor which raise the pressure of the atmospheric air
  2. a combustion chamber which mixes the compressed air with fuel and burns this melange
  3. a turbine, placed in the exhaust flow, which drives the above mentionned compressor through the shaft.
  4. a fan propeller, spinning upstream the compressor, creates a cold air flow surrounding the jet engine

engineanimgear.GIF (11891 octets)

This is what we can see in a more acurate manner on this cross section of the PW6000 engine :

PW6000cut.jpg (48116 octets)
document : Pratt & Whitney Commercial Engines

A modern turbofan , the CFM56 ( see the fan propeller on the fore front ) :



On the compressor different ports are arranged on different compressor stages providing high pressure air to be tapped according to the aircraft flight phases.

For example during the descent phase when the engine is running at low power, air is bled from the high pressure port HP. During the cruise phase where the engine is providing a high power, air is bled from the intermediate pressure IP port which has a lower pressure but this is more efficient for the engine and its fuel consumption. This is carried out with two components :

arrowgreen.gif (1026 octets)    The first role of the bleed air system is to select the right bleed port.

arrowgreen.gif (1026 octets)     The second role of the bleed air system is to decrease  pressure and                 temperature to levels acceptable for the downstream pipes and the air cooling system.

This is carried out with 2 main components :

Accordingly at the precooler outlet the air temperature is lowered.


3) THE ENGINE BLEED AIR SYSTEM ARCHITECTURE    is often as follows and one can find the hot air flow cooled down by the cold air flow inside the   precooler:

bleedarchi.gif (8577 octets)


ac640barre.gif (1651 octets)


bleed air system / air cooling system / zone temperature control system / cabin pressure control system