To which KigAir asked:
That's "burning more fuel" IN THE OPERATING ENGINES (than each one would have been in the "all engines operating" regime). Total fuel burn per mile, in the Falcon's case, doesn't much change.
If 9000pph = 9000pph = 9000pph independent of the number of engines burning that 9000#, then the number of operating engines wouldn't much matter. HOWEVER, some engines have an "overhead" such that you get the same thrust out of 1900pph on 3 that you do out of 1500pph on 4 -- so for the same thrust you save 300 pph shutting down one engine. Gas for free? Not exactly; you aren't operating any of the pumps (hydraulics, oil), generators, or compressors (i.e. producing bleed air) on that engine. On a perfectly efficient airplane, all of that same work (producing the needed bleed air, generating the necessary electrical load, etc) would shift exactly to the other engines, but the reality is that you DO gain some efficiencies by shutting down an engine.
That is why, to answer ClassG's question, setting the engine to zero-thrust instead of shutting it down will NOT be a good deal for you -- all of the fuel needed just to "keep the engine running" will still be burned, but the remaining engines will have the fuel consumption that they would have had if you were just running on (N-1) engines.
Different engines (and airframes) work differently; your milage may vary. But in a P-3 or C-130 (similar airframe, same engines) it IS more fuel efficient to shut down an engine or two (higher temperature in the engine = more energy obtained per pound of fuel) (all other things -- altitude, winds, etc being equal).
