[The_Techie_Stirlingite]

Low-voltage hybrids (typically 48V) are all over the place in European countries, and they provide drivability improvements at low RPM, instant engine start-stop (without the risk of burning up the starter), and as much as a 15% fuel efficiency improvement in some cases.
I have a concept for a similar system as an eco-mod, and I'd appreciate it if you could help me figure some stuff out.

Preface: this system would be for automatic/CVT vehicles only. Manual transmissions would require more wiring to auto-stop effectively (you need to know if you're in neutral or not.)

I found a permanent magnet BLDC motor online, that is rated 72 volts, around 60 lb-ft peak torque, draws about 400 amps at full load. Torquiest one I could find in an alternator-like form factor and 72-volt.

The motor would be attached to the crankshaft via a belt, similar to a supercharger. My main concern would be belt slippage, especially at max assist (about 25kW). I'd guess I'd need one heck of a tensioner, or re-use an existing supercharger drive kit.

As far as control, an Arduino microcontroller board would be plugged into the OBDII port, a relay acting as an automated kill switch, the brake light switch on the pedal, and the motor controller. The control board (I'll call it the Hybrid ECU from now on) would calculate how much power assist or regenerative braking was required, based on the calculated engine load, speed, engine RPM, and brake-light switch.

Calibration question: at what load point should assist begin (to prevent an open-loop super-rich operating condition?) I did some data logging in my parents' Accord with a K24Z engine, and normal acceleration produced about 75-85% calculated load - should I be assisting at these load levels, or should I start earlier? Should I re-gen drag the engine at a low load to improve BSFC, or should I reserve re-gen drag for DFCO only?

When the driver's foot was on the brake, the ECU would wait until the vehicle was creeping (6 mph or below to avoid killing the automatic transmission) before effectively triggering an automated EOC. As with EOC, vehicle systems would continue to run, powered by the 12V battery. Could I safely cut the engine any higher than 6 mph?

When the driver lifted off the brake or the hybrid battery was low, the assist motor would kick back in, delivering its peak 25kW to spin the engine back up to idle speed quickly. Once the engine was at a workable RPM, the crankshaft position sensor would be reconnected and the combustion engine would take over.
Would about 60 lb-ft be enough for a quick restart of an average 4-cylinder engine? The system would not be tasked to handle cold starts, the 12-volt starter would do that. I saw a product called the E-Charger online, and they reportedly started a 6.0 Vortec V-8 with 150 lb-ft via a belt drive system.

Like OEM hybrids, this system would have specific conditions in which the engine would not stop when the vehicle did. For example, the engine would not stop if it was not at normal operating temperature, because cold starts would not be possible with the assist motor. The engine would also not stop if the ambient temperature was extremely low or extremely high, to ensure cabin comfort. The ECU may also be able to parse CAN data on supported vehicles to prevent an auto stop (or kick the engine back in) if the A/C needed to run.

An additional issue would be motor cooling. If the motor was in place of the alternator, its cooling fan would be sucking in hot air, limiting peak torque and possibly making it impossible to restart the engine after an auto stop. One could possibly use a hood scoop to feed cool air to the motor, so it wouldn't overheat. That would look quite interesting. A bystander would see a hood scoop, suggesting a ram-air intake or supercharger setup. When the hood was lifted, all they would see would be an alternator-looking thing with orange electrical cabling running to it.

As far as battery capacity goes, I'd be using LTO cells, and need enough capacity to supply about a minute of maximum assist. If the vehicle had to idle for any reason, it would charge the hybrid battery as much as it could unless it was full already (engine load optimization.) During catalytic-converter warm-up periods (fast idle), the motor would impart a bit more re-gen drag, to speed up the warm-up cycle (and produce electricity for more assist/engine spool-up in the future.)

Do you have any other questions? I'd be happy to answer them!