Well after a cursory glance of

http://www.allaboutcircuits.com/vol_2/chpt_13/6.html#02461.png

I am guessing that 2 phase BLDC is OK in low load start ups applications as with
computer fans. All bigger BLDCs, relevant our application, seem to be 3 phase or more.

I didn't know they (2 phase) exist....

You would still need a (bridge) rectifier if you used one as a generator.

Carl, I have a small brushless cooling fan motor that runs off a battery. If I rotate the motor, using an IC engine for example, why dont I get DC out, since that is what goes in.

Difficult to say without seeing it or putting a meter on it.

But not all motors generate or give out electricity, there maybe some cicuitry preventing it.

Also it could be that there is a controller on the input which converts the straight DC to operate the brushless motor.

If that is the case you could try putting a bridge rectifier on the connections nearest the motor and see if anything comes out.

Oh dear Carl,
what have you done, cant get the idea of a 1/6 scale live diesel out of my head. I am building 15" gauge on gauge 3 size track and trying to find drawings of Captain Howey of the RHDR, that baby will be 40" long and 8' wide and about 12" tall, was going to do battery, but you got me thinking now,loads of space in the engine compartment.
All the best,
Richard.

Can't help you with the drawings but it's an impressive locomotive and looks similar to many metric gauge diesels like Faur.

At 8" wide you could easily put a small cheap petrol engine from a strimmer or similar connected to small (motorbike) alternator.