12 v dc dynamo

[the highwayman]

Worth remembering that the output voltage (without regulator) will depend on engine rpm, if you are looking at a steady load then simply setting the regulator and keeping steam constant will probably suffice, if your not to worried about lamps dimming when steam and revs drop a bit, if the load is variable, then you'll be relying on engine governors, which in 4" are probably not too reliable, so could lead to blown bulbs etc.