The engines on the 880/990 were GE CJ-805s, a commercial derivative of (I believe) the J-59 which powered the B-58 Hustler supersonic bomber (and may have been on the F-104, I don't know).
The 880 was a straight jet, while the 990 had a version of the CJ-805 which added a fan on the aft portion of the engine, although the core was basically the same as the 880 engine. The 990 engine was fatter because it included the fan and if you look at the 990 head-on you can see the central core surrounded by space where the air flowed back to the fan.
Even stranger was the Caravelle VII (Caravelle Horizon) which was a proposed version of the Caravelle with CJ-805s like the 990. Rather than having the big cylindrical pods, only the aft portion had a cowl around the fan, and it looked like a 707 fan engine turned around backwards. No one bought this, preferring instead the Pratt-powered version of the Super Caravelle since the Pratts had a much better reputation.