[dcb]

curious, because most bsfc charts that I have seen show peak engine efficiency at or near WOT (though in the 2-3000 rpm range). Though if you have a torque converter that can compound losses at higher engine torque outputs. WOT can be inefficient also if you have to brake more.

To charge batteries then discharge them is a lossy proposal, I haven't seen huge gains in peak bsfc with an engine tuned to run at one rpm and load, not compared to the losses in a mechanical->electric charge->discharge->mechanical setup that I have seen.

re "sweet spot", the real sweet spot is peak bsfc, which has a load component and an rpm component. Lets say for example peak bsfc is at 2500 rpm at 80% throttle.

Most cars have far too much engine for optimal cruise efficiency, i.e. if you are making 40 hp at 2500 rpm, you would probably going to accelerate up to 100mph with enough gears if you left the throttle at %80. Even though your engine is close to peak bsfc, you are still making too much hp for a reasonable speed (say, 70mph)/ and a reasonably sized vehicle. If your power demands at 70 are, say 25 hp, then you need an engine that is ~%40 smaller to have peak bsfc at 70mph.

If you make aerodynamic or rolling resistance changes, then your constant cruise power demands go down even more.