[serialk11r]

I've spent a lot of time thinking about how to do this.

Brushed motor isn't a great idea. The brushes don't take kindly to high voltages and there's a regen vs. drive tradeoff. It's very easy to produce a high voltage in a car because the engine has such a large operating speed range.

Say you use a 48v motor, then you'd want it to have a back emf of 48v at 7000rpm or whatever your engine's rev limit is plus a little extra. Then at 700rpm, it's only putting out 4.8v which is not a lot to work with.

I think for this application it's best to directly drive the crank or somehow the transmission input shaft with a brushless DC motor. Outrunner is nice because you can reinforce the casing to let it take higher rpms.

The way to think about the power vs. voltage vs. current vs. torque vs. current of the motor is this:
-Back emf (voltage) increases linearly with speed. If you keep the motor spinning at 1000rpm and you measure 10v, then at 2000rpm it'll put out 20v.
-Current is given by the voltage applied minus the back emf, divided by resistance of the motor's windings. So if you apply 20V at 1000rpm for the example motor, the torque output is similar to applying 10V at 0rpm, the current is the same, but the electrical power is doubled.
-Torque output is approximately linear with respect to current.
-This all works in reverse. If you can hold the motor at 1000rpm, there will be 10V with no electrical load. Think of it as a 10V battery in series with a resistor that has the same resistance as the winding resistance. If the motor windings have a resistance of say 0.1 ohm, then if you keep the motor at 1000rpm, you can draw 10A current at 9V, 20A current at 8V, etc.

I know this sounds crazy, but I've come to the conclusion that a 120mm hobby aircraft outrunner is pretty well suited for this. At 7lbs and ~20hp output they're very powerful, and should be able to put out around 20Nm of torque. The Astro 3220 has 10Nm peak torque but it's a smaller diameter inrunner with similar length as the biggest 120mm outrunners, with samariam cobalt magnets which are a little weaker than neodymium iron boron. 20Nm is probably enough to start your car engine, and some of them are rated to 7500rpm which is similar to a car engine's typical speed range.