Why do three engines have humps and one does not?

You'll notice that the outer engine in the third picture is the only engine without a "hump" that runs all the way to the cowling. And, if you remember, no other engines in the pictures I've shown you so far have had these humps. These four engines are from the Boeing 707 (first flown in 1957), the plane that ushered the world into the jet age in the late 1950s. I thought that the interesting question (to me) of why those engines are different would be a good introduction into Bleed Air Systems and their outdated counterparts, Turbocompressors.

Bleed air systems are (or soon to be "were") a key system within most commercial airliners. Bleed air is quite simply air that is taken or "bled" from the jet engines, usually from after the compressor stage and before the combustor (so there is no fuel in it). Since the air is bled from after the compressors, it is both very hot and under very high pressure. This hot and high pressure air is fed through pneumatic lines into the aircraft and can be used for a variety of functions including heating, de-icing, pressurization, and actuators. Bleed air is being (very) slowly phased out of newer aircraft designs because of its inherent inefficiencies. Bleeding air from the engines increases fuel consumption for a given output of thrust. Also, the air that leaves the engine through the air lines is far too hot to be immediately used in the cabin, so it has to be refrigerated (entailing more weight for chemicals and electrical components). Next generation planes such as the Boeing 787 (which first flew in late 2009) rely on electrical generators for heating, de-icing, etc. instead of bleed air.

So why the weird engines on the 707? The Boeing 707's comparatively archaic turbojets were not powerful enough to loose air to the bleed air system without seriously decreasing the performance of the Pratt & Whitney JT3Ds. Instead, the Boeing 707 has turbocompressors mounted above three of its four engines (thus explaining the humps). The turbocompressors are mini turbines that are mechanically driven by the main turbojets, and compress air from their own independent air inlet directly above each turbojet's cowling (so as not to draw air away from the JT3Ds). Since three turbocompressors could provide ample air for the other systems on the aircraft, there was no need to add the unnecessary weight of a fourth one on the port outer engine.

Just as a final useless factoid, the first commercial engine designed to provide bleed air was the Pratt & Whitney JT8D. The JT8D was first used on the Boeing 727 (first flown in 1963), the next airliner designed and built by Boeing after the 707.