A very brief explanation of this new 'hype' of bleedless engines.
An engine basically has 2 tasks:
The first is the most obvious one; i.e. to push the plane forward by converting fuel into thrust,
Secondly, an engine also supplies many vital aircraft systems with hydraulic power, air pressure, air conditioning etc...
All these secondary consumers of power are called engine bleed, since they require fuel to be burned without any propulsion resulting from that, which might be seen as a waste (although you need the systems which are dependent on this 'waste')
The idea behind this bleedless concept (which is already as old as the jet engine itself BTW) is to find alternative ways of supplying these aircraft systems so as to reduce the fuel flow of the engine. The alternative seems to be found by having these systems supplied by the electrical system rather than by engine bleed. Electricity, which also has to be generated by the engine, thus also constitutes a kind of indirect bleed BTW.
Therefore, bleedless in a wrong name, as the aircraft becomes more dependent on electrical power, which is also made by the engines (also a kind of bleed) and besides the engines will still need to supply bleed air, for instance for the air conditioning system.
Over the past decades many theoretical experiments have been done to see if a shift from direct bleed as it is most commonly used now to a less bleed dependent engine would make any sense and the conclusions then were that it is technically possible and is will indeed reduce the FF
of the engine by a few percent, but that it makes the plane more complicated to maintain, less redundant in operations and requires more maintenance. Back then, it was deemed not a really good idea.
However, it seems people in aviation research and design are convinced that with today's technology they can make this concept work cheaper than the current bleed engines, but that remains to be seen.
Anyway, what we will most probably see is that engine manufacturers will (re)design their engines in such a way they can be installed both on airplanes requiring bleed as well as on future bleedless planes.... The later will benefit from all improvements, the first one will also, except from the gains made through the bleedless concept which are only part of the overall improvements expected on new engines.
[Edited 2004-04-29 12:57:45]