[RedDevil]

Originally Posted by smallscaleH2

You're looking at it from an energy-efficiency perspective.
This project idea was never about energy-efficiency.

You describe HHO as a good fuel - what makes it good, then, if not its fuel efficiency?
Emissions maybe - but you start off with electric power, and from that point on a regular EV has no emissions at all. Diesel engines typically run lean under light loads, and you need to do the same (e.g. mix in air) to have a chance to make it work. But then you'd produce nitrous oxides - unlike the EV.
With efficiency and the environment out of the room, practicality and safety never in view, what remains?

Originally Posted by smallscaleH2

Regarding the cost, I don't agree. I agree the fuel costs would be higher, but if you look at the whole thing (so including conversion costs), it would be cheaper -electric motor, battery and motor controller tend to cost a lot-

Hey, there is a battery in your setup too. You actually need a big one to get anywhere.
If we just look at the differences:
EV conversion: + motor, motor controller;
HHO-Diesel: + Diesel engine, cooling system, tranny, exhaust system, electrolyser, power regulator...
I'm not sure your setup would be cheaper. (that's an euphemism)
You could buy a nice used Leaf for the cost of a 50 kWh battery alone.

Originally Posted by smallscaleH2

If I find that I can just use hydrogen in Diesel engines, then that would even be better than if I were able to use HHO.

Yes, definitely. But you might find that a very light fuel like hydrogen works better in a gasoline engine than a diesel engine. Lower compression ratio to prevent detonation, spark ignition, throttle valve - all gasoline engine territory.
Still, if it has to be a diesel engine for whatever reason, chances to get it to work at all are higher with hydrogen than when using Brown's gas.