Honda, Yamaha and others now sell "inverter generators". These use a small 5-10 hp gasoline engine and output 110V (I'm certain they have European voltage generators also). The most unique feature is that the engine speed is varied as the load varies. This results in much lower fuel consumption compared the other portable generators that use a constant engine speed controlled by a mechanical governor. My limited knowledge of electric generation is that the output voltage varies with the shaft speed. So how do you take a varying voltage (more or less constant current) from the generator section and convert it into a constant voltage, varying current source for the load ? A circuit would be nice, by I'm more interested in a theoretical discussion.