Years ago I was involved in the design and construction of a special purpose engine generator set designed to deliver 120/208V 3-phase supply at 400Hz at a continuous output of 10kW with peaks up to 20kW. The alternator needed to run at 4000rpm which necessitated a belt drive from the engine which was running at 2500rpm. Obviously direct drive was impractical due to the high alternator speed. Calculations showed that even with multiple parallel V-belts, belt life would be short and maintenance (belt tensioning) would be frequent. The only practical solution was a toothed timing belt (about 3" or 75mm) in width similar to that used to drive superchargers on drag racing cars. The engine pulley was around 8" or 200mm in diameter and the alternator pulley around 5" or 125mm.

You not only need to choose an engine speed where the torque (and hence power) delivered is sufficient, but you also need to keep the engine speed to something acceptable for continuous operation of that engine. An engine will not be happy running continuously near idle speed nor in its upper speed range. Somewhere around the engine's torque peak is ideal. Direct drive is preferable if the speed is suitable for both the alternator and engine however mechanical issues such as alignment and coupling between the engine and alternator need to be considered. A means of keeping the engine speed constant (to keep electrical frequency constant) with varying load also needs to be considered.

You also need to have reserve power capacity. Car engines are not designed for continuous operation at or near full throttle operation. In an automotive situation power delivery (and required throttle opening) is only a fraction of the maximum available for much of the time. You will also need to consider engine cooling. The radiator and cooling fan required for a stationary application such as a generator set is very much larger than that required for a car where air velocity is usually very high under high load conditions (due to the vehicle speed).

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