A Turbojet has a large advantage over a reciprocating engine (piston engine) when it comes to size per horsepower, vibration, and T.B.O. (time between overhauls) however it falls short in fuel efficiency.

http://www.boeing.com/companyoffices/gallery/images/commercial/737-01.html

Turbojets burn about .52 to .7 lbs fuel/hp
Reciprocating engine burn about .4 to .52 lbs fuel/hp with the record being .29 lbs fuel/hp

In order to improve turbojet efficiency multi-stage engines were introduced.

Page 3-18 *
This arrangement split the turbine section in to fore and aft sections and the compressor section into fore and aft. sections. The inner turbine powers the inner (N2) compressor separate from outer turbine which powers the outer (N1) compressor.

The fcu (Fuel Control Unit) regulates N2 speed and allows the N1 section to automatically find its most efficient speed depending on air density and power setting.

Page 3-33*
 
 

*Whenever there is a page number underneath a diagram the picture is from
A&P Technician Powerplant Textbook published by Jeppesen Sanderson Inc., 1997