I was aware of the coal-dust, but forgot thanks for the reminder.
However I still think the answer is an Omnivorous Engine.
I am going to go out on a limb here, watch the Wankel....
A great deal of research was done to make it Omnivorous and even throtte-less and has low pumping losses. Combine that with it's small size and power density, and you have a natural fit as a prime-mover engine for a Series Hybrid.
Go here for more research on the subject, it is amazing what was done not that long ago:
You can not compress a Wankel; the most you get is about 12 to 1 if you're lucky. The design of the rotor does not allow high compression. The engineers barely got it to seal right for the gasoline use.
The Diesels need at least a 22 to 1 ratio to fire up for which the Wankel's are not up to. OTOH the Wankel"s are Rev happy!...good for racing!