[dcb]

I should just get my own thread. But for here mech added some links to a hydraulic hybrid that was %83 efficient in series mode (not including the accumulator). If you have a right sized engine (including what you recover downstream plus some minimal safety margin) then it should be pretty close to bsfc anyway. I expect series electric to be similiar to worse (90% * 90%), but you are leaving about 18% peak efficiency on the table.

With parallel, you can still add or remove from the accumulator to tweak the engine load if it actually nets a gain when the stored power is reused, or if you just need a little more power. But it allows for a direct drive option as well.

If you are trying to make something idiot proof, firstly good luck, secondly it will always be a compromise with what is really achievable. Some things computers are good at, i.e. the timing demands and lack of drastic changes in the combustion chamber between strokes, but some things require intelligence, coordination, predictive skills, superb sensory awareness, experience, and even values, to navigate most efficiently, i.e. driving.

Series is also a way to "automate away" those higher level decisions and dilute our expectations for drivers(and engineers), at the cost of significant efficiency, when the number 1 cost effective (and probably most effective period) mod here is "Adjust the nut behind the wheel".