Hey all, got a question about horsepower. I know how much power an engine makes at higher rpms, but how about at idle ? Anybody ever read any ratings ? Maybe there is a nice rule of thumb based on cubic inches ? The reason I'm wondering is this- I want to buy a generator for the frequent power outages my area has, and to be able to take the same generator with me on vacation to a friends house. He lives waaaaay out in the woods without grid hookup. I have found 10K generators for sale for 600$. With an engine they cost about 3600$. The ones I've looked at have a 2 cylinder engine that runs full throttle at 3600 to make 60 cycles per second. They also make a heckufa racket doing it. What to you think of the idea of using a V8 to power the generator ? If geared at 4 to 1 the engine would only have be going at a fast idle (loafing). If geared at 6 to 1 you could idle at 600 rpm, nice and quiet like. But I have no idea how much power a V8 makes at 600 or 900 rpm. Do any of you have any ideas ? DF

 

The problem with what you have planned is hooking up some type of governor system to the engine. The more power you use from the generator, the more load the engine will see. The more the load on the engine, the lower your RPM will drop without an increase in throttle. You must keep a constant RPM to keep your 60Htz at a constant so that you don't damage sensitive equipment. It may be possible to rig a cruise control system to do this for you.

 

Here's a little "Power Conversion Math' that might help.

1 HP = 743 Watts or 0.743 kWatts

therefore you need 13.46 HP (neglecting frictional loses) to make
10 kW.

A typical stock 302 with a stock 2 barrel carb and stock exhuast would be plenty powerful enough to run a 10 kW generator at idle.

RCC

 

Hey DF just thinking out-loud here. Looking at some stock HP curves, they all start around 2000 RPM. If you extrapolate the curve backwards (can you do this?) it looks like it would be around 40 HP at 1000 RPM.

The biggest hurdle I think would be regulating the frequency with the loads switching in and out. Electronic equipment is very sensitive to frequency variations of only a few hertz or so. If you ran a 4:1 gear ratio to spin the generator at 3600 RPM for 60 Hz, the engine would be turning 900 RPM. Now say we go from no-load to full-load while keeping the frequency within a range of +/- 3Hz (this is pretty wide), the engine RPM can only be allowed to vary between 855 and 945 RPM. I'm guessing it would be hard to regulate these low RPMs under switching loads even with a governor. A speed control would probably have too large a time constant to recover fast enough before the equipment blew. It would help if you had a continuous load always on the generator to stabilize it while the other loads are switching in and out. Maybe a heavy flywheel so inertia can work for you too.

Now if the loads are only lights, appliances and such, then I think it could work. Like I said, just thinking out-loud here.

Barry

 

Hmmm. these guys have some really good input on this. But I'm wondering about how the gas engine would stand up to a gen stinger if it has to run constantly. Like how long of periods will it have to run steady? I'm not positive, but I was under the impression this is the reason why diesel engines are used on bigger gen packs. Because they dont complain much when ran for thousands of hours straight.
What do you guys think?

 

The reason for diesel generators is that they last longer, get more hours of operation per gallon of fuel, and can legally burn the fuel that had the red dye which is exempt from road taxes, thus cheaper per gallon. Gas engines weigh less than a diesel of equal horsepower, but when they are big enough that they can't be transported without a trailer or forklift, who cares about the weight. Therefore, diesel is the best choice for large generators.

 

Ok. That dose make some sence to me. i was'nt sure. And did'nt want to post wrong information about them.
But would it hurt to use a gas engine if it wasnt used for extended periods of time? Like a month straight without shutdown or something like that?

 

I'm not positive about generators but I would think that they would have a frequency regulator to keep things at 60Hz no matter what. I would think if you built a 302 with the smallest cam you could find and used the most restricting heads out there it would run great at 500 - 1000 rpm and generate a good amount of torque. The 300 I6 may be a better option. I think it is a little smoother due to the longer stroke and definitely would be the best low rpm performer. We won't even mention the obvious longevity factor.

 

Ratsmoker is right about the need for frequency regulation. The frequency is regulated by the speed of the engine, which is why you need a govenor.

The reason most generators run at 3600 rpm is because 3600 rpm equals 60 cycles per second (60Hz). Math: 60 x 60 = 3600

If I remember correctly from my training days... You could run a direct coupled engine and generator at half speed if you used a generator with double the number of poles. So say if a 3600 rpm generator had 4 poles, a 1800 rpm generator would need 8 poles. Poles meaning north/south polarity that makes the alternating current cycle. A generator for a farm tractor PTO might be built to make 60Hz at 1800 rpm.