AC to DC converter? Its usually known as a rectifier, iow, a simple diode or an arrangement of diodes. Efficiency depends on the type and the power involved, the lower the forward voltage drop across the diode(s) the less energy is wasted as heat.Try Schottky diodes for low voltage drops, I've seen them as low as 0.1V. Ordinary silicon diodes have around 0.5V drop.
Or did you want to change the voltage as well as rectifying it?
:wink:
Just use a transformer that's efficient at your alternator's frequency to change the AC, then rectify it.
Klaus