rpm vs. hp
 

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.

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).

I wonder if using the engine AND transmission to power the generator would work?

1800 rpm generator means 1.5:1 gearing (don't know which gear) to give 2400 rpm engine speed, right in the fat part of the torque curve.

Then, use the speed control to keep driveshaft rpm constant at varying load, and you have a crude, 3rd world-worthy generator. With the right size radiator and fan, it might work better than imagined...

What say ye all?

Best Regards,
Jim

Thanks to you all
 

Thanks to all who have responded to my query. I would like to learn more about the torque curve as referred to in one of the responses. If I am to consider this as a viable approach, I need to know more about the torque curve of the engine I have. If anyone can direct me to a site that has these kinds of specs, I would very much appreciate it. I have considered a lovejoy direct coupling but have read a lot about using slower speeds with large and heavy flywheels for greater torque and momentum. That is why I was looking for information relative to my engine at idle speeds. I wonder where the point of diminishing returns would be operating the engine at 1800 rpm with a direct drive coupling to the generator off the pto end and a heavy flywheel on the other. Could this be done without causing too great a load on hp while gaining momentum for demanding loads on the generator? (starting large induction motors and the like) The primary use would be for power backup during power losses from hurricanes or other long duration events. As hurricanes are in the heat of the summer, I would want to run my air conditioner as well as water pump and hot water heater.

Thanks, Roger

It may appear tempting to retain the auto transmission however the numerous ratios are not required to run an alternator at constant speed (versus great variations in road speed). The transmission would also add unecessary losses and would also require the speed feedback signal be derived from the alternator. Assuming the auto transmission was retained and the speed (frequency) signal was derived from the alternator, the only possible advantage I could see with this arrangement would be that it may allow the engine speed to vary (by way of torque converter) as varying loads demanded differing amounts of torque from the engine. The speed controller would be the complex part of such a system.


Being a diesel, the torque curve of that engine would be fairly flat. However, unlike diesels intended for use in generator sets, the governor in the injection pump will not attempt to maintain constant engine speed (other than providing overspeed protection). Being intended for an automotive application the governor will attempt to maintain constant torque for a given "throttle" setting. This is not ideal for use with a generator set where the result would be varying speed (and hence frequency) with varying load. You would need to obtain a different (or possibly modified) injection pump with a constant speed governor, or obtain an electronic speed control that could control your existing injection pump. With regard to the use of a flywheel, even very large ones will only compensate for very brief transients in electrical loading. The control of speed (frequency) in a generator set with greatly varying electrical loads is a difficult task. The generator set I mentioned in my previous post in which I was involved with the design (actually 15kW with peak loads to 30kW now I think of it) used the frequency of the alternator (400Hz) as a feedback signal for the electronic speed controller (governor). This was a complex PID (proportional, integral, differential) design carefully tuned to the application which had to deal with transient and regularly cycling loads varying from no load to full load. The electronic controller drove a rotary solenoid which operated the engine throttle on the petrol (gasoline) engine required for this application. Watching the very rapid response of the throttle to the varying electrical loading was amazing.

my friend here in hurricane alley built a gen. from a vw diesel, he tried the trans and its output shaft as well and it didnt work, he finally ended up using the motor with a shaft that he built that ran off the front of the motor like a "live drive" setup used on many commercial trucks. it worked fine and still does. the vw diesel is much smaller and lighter and has only 50hp and 50ft lbs of torque, it works though. keep in mind you will have to have any external shaft balanced.

torque
 

Does anyone know the torque developed in the 5 cyl diesel from a 76 300D?