Why some jet engines smoke when starting up?

[Mr_Joe]

It’s cold out, on start.

 

[CleanSump]

Haha I spent my entire career in Naval Aviation and I've seen fireballs come out of F-8s and A-7s, F-4s smoked like a coal burning locomotive. It's incomplete combustion like an old diesel truck.

 

[Cujet]

https://en.wikipedia.org/wiki/General_Electric_CJ805

This engine for the 880 had high specific fuel consumption. Jet engine technology was still in a learning curve. Thrust, not efficiency, was the major concern at this point in time.

The very early jet engines were terrible. But it did not take long before they morphed into great performers in high speed cruise flight. Engineers addressed key issues right away. Inlet shock formation, TSFC at altitude and added low bypass fans.

The long range 707 for example could make better than 5500 nautical miles in passenger service, and it was fast too, long range cruise 0.82, normal cruise 0.85.

The Gulfstream GIII had the fantastic old Spey low bypass. The GIV with it's 'better' higher bypass Tay...

GIII 28,000 pounds fuel, 3,800 NM NBAA range (old Spey engine)
GIV 29,900 pounds fuel, 4,200 NM NBAA range (new Tay high bypass engine)

Any way you slice it, the addl fuel resulted in more range.

Both had similar cruise speeds and fuel flow in cruise. The newer Tay used less fuel on TO and initial climb. But created much, much more drag in descent. So much so we needed to fly 50 addl miles before starting descent from FL450.

This GIII was one of those we flew all over the globe: