Engine hp required for 5000 watt generator head?

[jeepman3071]

I have a 4000 watt Coleman Powermate generator with an 8hp Briggs and Stratton engine. The date code on the engine shows its a 1988, but it appears to have hardly any use and runs great.

My boss blew the engine on his generator and said I could have it. The engine was a 10 hp Briggs OHV with a 5000 watt (6250 watt surge) generator. The generator head is still good. Even though this generator had a 10 HP engine, could I swap the power head onto my 8 HP engine, or would I need a larger engine?

We've used the 4000 watt to power a fridge, a freezer, and a few lights, so I'm not sure what the benefit would be of having 1000 more watts of power.

Any info would be great, thanks!

 

[The_Eric]

I'm sure this is too simplistic and failing to take something into account, but 1hp = about 750watts. So 5000watts/750watts(hp) = 6.67hp.

It may be that to get satisfactory performance, life and fuel consumption, they overpowered the genset somewhat. Looks like your 8hp should do the job, but may labor doing it.

 

[Rand]

usually accounting for losses you need roughly 2hp per 1000watts.

 

[punisher]

Originally Posted By: Rand
usually accounting for losses you need roughly 2hp per 1000watts.


Ditto, 2hp/KW for a little gasoline generator.

 

[wally6934]

By definition 746W = 1HP. You now only have to add in the efficiency of the generator. A quick search found this info.

http://www.engineering.com/Ask/tabid/3449/qactid/-1/qaqid/2808/Default.aspx

Most small generators have operational efficiencies of between 93% and 97%. As a result if you want to have a 5KW generator as a power station you should plan on providing that generator with at least 5.5KW of mechanical power though the input shaft.

I am providing little bid of a safety margin here.

5.5kw / 746 = 7.37 HP

 

[Colt45ws]

I would replace it with what it had on it. I think wally is right, but thats not counting surge wattage.
Say 90% efficiency.
6250 / .9 = 6944.4
Say 7000 watts.
7000 / 746 = 9.28 HP.

 

[eljefino]

You would gain very little, but if the generator head has thicker wiring it would be slightly more efficient, particularly under heavier loads.

What you'd gain though I'd worry about losing during surgical complications.

As you probably know, generator engines have a special tapered crankshaft and share a bearing with the alternator head.

You might want to find out what blew on the boss' engine and then find a parts engine from (whatever) and make a mongrel, if the crank is still good.

Also 4000 watts is overkill for what you're asking of your generator. You might only be taking 1000 or less in emergency storm use.

 

[Colt45ws]

Originally Posted By: eljefino
You would gain very little, but if the generator head has thicker wiring it would be slightly more efficient, particularly under heavier loads.

What you'd gain though I'd worry about losing during surgical complications.

As you probably know, generator engines have a special tapered crankshaft and share a bearing with the alternator head.

You might want to find out what blew on the boss' engine and then find a parts engine from (whatever) and make a mongrel, if the crank is still good.

Also 4000 watts is overkill for what you're asking of your generator. You might only be taking 1000 or less in emergency storm use.

Yeah. I'd either fix it and sell it, or just sell it as is.
Is the motor completely blown or is it repairable?

 

[BigCahuna]

If your motor would fit, and you really want to try it ,go ahead.It probably would work, but might use more gas due to the load. If your not wanting to gamble on it, check Craigs list lawn and garden section for a used 10 hp motor. I've seen them listed before cheap, and it would be closer to what was there originally.,,

 

[jeepman3071]

Ok thanks guys, I just wondered if it would be an upgrade. I think I'm just going to leave my 4000 watt alone since it runs great and is good for what we need.

Part of the reason I asked was because I had been looking around on craigslist and there doesn't seem to be very many used engines with tapered shafts, especially in the 10hp range. I could always swap crankshafts, but at that point I'd have to buy a used engine which might be just as expensive as the new parts.

What happened to the engine is that it had a cracked plastic oil fill plug on the side of the case. It had slowly been seeping oil. This engine does have a low oil shut-off, but something must have prevented it from working. The engine wiped one of the cam lobes so now one of the valves doesn't open. The cam is no longer available from Briggs and Stratton. I did find one on eBay, but it was $70 and I'm not sure what the low oil condition did to the rest of the engine.

This generator did come with a nice cart, which mine doesn't have. If I decide not to fix it, I may just switch the cart over to mine.

After work today I'm going to finish taking it apart and inspect the bearings and piston to make sure nothing else is damaged.

 

[jeepman3071]

Well I found out why the oil sensor didn't work...

The wire that goes inside the engine somehow got disconnected from the oil sensor. Everything else looks good inside the engine, no play in the connecting rod. The crosshatching is still on the cylinder walls.

I found a local used engine for cheap with a good camshaft. The part numbers for the cams on these engines is the same, so I'm going to use that one and swap it in. This engine should be savable!

I'll post updates when I get it back together.

 

[javacontour]

FWIW, my Coleman 5kw sustained generator has a 9hp B&S engine and seems to do fine. It's rated for 6250w surge, IIRC, and 5000w sustained.

I don't think I'd swap engines.

Can you take the generator head to your local HF and see if any of the engines match up with the donated genhead?

Nevermind, I see you are trying to fix the donor engine.