That’s a bit of an obfuscation, truthfully. There IS no “imagination engine” or “imagination machine”. But machines have motive power, and so does the imagination: it’s just a matter of discovering what “engine” runs yours.
My “imagination engine” is fueled by “what ifs” mostly. A single thought - even a partial “phrase” of a thought - will set me off on the “what if THAT happened instead” path. I even do this in dreams: don’t like the way this one’s headed? Fine, what if we go this direction instead? I’ve been known to direct dreams for long periods of time - and while I recall them imperfectly, some of the parts eventually make their ways into my writings.
The oddest things can start a train of thought - a face that seems familiar though you can’t give the person wearing it a name; the horses got out and were NOT hit by a car; a cloud-shape that seems to hang in the atmosphere forever; a strange light where there shouldn’t be one on the mountain. On and on - imagining is play much of the time, with only a nod to “using its output”.
Once you find out what sort of fuel yours uses, it’s a matter of practicing “starting the engine”. Let ‘er roll!