I get you:
If you have a normal ICE car and you want to turn it into a plug in hybrid; efficiency aside, this is probably the easiest way:
You don't need to get the power from an electric motor to the wheels and you don't need to integrate a motor controller with the accelerator and brakes.
Some regen is possible if you have a cutoff valve/solenoid activated by the brakes or a vacuum sensor and a ...'balloon'/reservoir after that in the hydrogen feed line and gear down while slowing.
Efficiency ideas:
The RC aeroplane industry has some very efficient motors that can be modded into alternators. (95+% IIRC)
Electrolyzers are more efficient when a properly set up magnetic field within the cell/s is involved.
There is research proving this and some vids on YouTube.
Pulsed power at certain frequencies also seems to help. Possibly because the pulses set up a magnetic field..?
If you use pulsed DC; use pulsed ~2.1 Volts in a parallel cell;
An open cell is way easier to keep full vs a closed (efficient) series cell, without the efficiency loss.
The exhaust O2 sensors look like they need some fooling or they pick up on excess O2 and the injection system adds fuel..?
In fact you want to run slightly lean and let the HHO 'fix' the arising burn issues.
or
Retard ignition timing slightly, thx to a faster burn..?
Add the HHO after the airflow meter so the system does not add fuel to what is essentially the ideal O to H/fuel ratio already.
According to the science; the most efficient voltage across the HHO cell/s is 1.61 Volts IIRC.
But a voltage of just over 2.1 Volts means some of the O is turned into Ozone (O3) which is good. Both volumetrically and according to a # of studies.
That's probably why tests seem to favour 6 cell (in series) electrolyzers over 7..? (2.2 V per cell vs 1.8)
Engine coolant can be used to quickly heat the HHO cell to a more efficient temperature and then keep it there/cool it.
Using a good old radiator cap on the 'above idea cell' will slightly pressurise it, avoiding any tendency for the electrolyte to boil while at the above more efficient, higher temperature.
That's akin to using a tiny bit waste heat to help produce the HHO.
