I am looking for a correlation between hp and rpm. I have a 76 300d 5 cyl inline diesel rated at 72 hp that I would like to use to power a generator requiring 1800 rpm. If the engine is rated at 72 hp at 3600 rpm will it have 36 hp at 1800 rpm? Could I possibly run it at idle and use a pulley to get the speed for the generator? If it idles at 650 rpm would the hp be 12 hp? I think I need about 23 hp to run the generator. I would appreciate any comments or additonal ideas regarding this use.

Thanks, Roger

HP = (RPM x Torque)/5252

TO resolve the horsepower an engine is producing, torque has to measured at a known rpm and then hp is found.

When peak hp is rated on an engine it is ONLY for full throttle at the specified rpm. There is no way to find out the hp at other rpms unless you have a graph of a dyno run of the engine.

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No argument with the above analysis, but with a diesel engine you can assume that the torque curve is fairly flat and that at least 80 percent of peak torque is available from about 1200 to the power peak, in which case the engine should be able to provide the required 23 HP (17.2kW) at 1800.

Look at the peak torque rating, assume 80 percent of this at 1800 (which is conservative), compute the HP and convert to kW (1HP = 0.746 kW). That's how big an alternator it will drive.

I would recommend direct driving the alternator at 1800 rather than trying to drive it from a belt. That's a lot of power to transfer via belt.

Duke

A three or four sheave pulley set up will deliver enough power.
Direct drive will mean more engineering.

Plus....a pulley set up will allow gearing to increase engine rpm to generator rpm.


.

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the RPM that you run the generator at will determine the output frequency, therefore you need to maintain a constant regulated RPM. The trick is that the torque requirements for the generator will change as the electrical load on the generator changes. This requires you to regulate the throttle of the engine to maintain a constant output frequency (i.e generator shaft RPM).

As a rough estimate:

shaft input power(hp) = electrical load(watts) / 768 X 1/ generator efficiency

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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