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Engine Size for 35 kw Generator Endthis thread has 12 replies and has been viewed 7835 times


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#1




Engine Size for 35 kw Generator End
OK, so I have been searching the web for someplace that will give me a formula to match generator to engine size. Does anyone know how to figure what hp engine would work for a 35 kw, 1800 rpm generator end. I happen to have a continental diesel rated at 50 hp at 1800 rpm. Is that too small?
Anyone know the way to calculate this kind of stuff? 
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#2




Re: engine size for 35 kw generator end
The base rule of thumb is 2 hp per KW.
You can mate that to your 50 HP diesel, but you will run out of HP before you run to KW capacity, which is not an all that bad of a thing to have. Sort of puts an insurance plan in place.Not likely to produce more than 25  30 kw, so you won't be likely to let the magic smoke out. 
#3




Re: engine size for 35 kw generator end
A 35KW generator is 35000 Watts. 1 Horsepower is 746 watts, this means you would need 46.92 Horsepower to get the 35Kw. Not much left over. If the load will run around 20Kw it will work with the odd peak surge, say to start a motor. More horsepower is always good even if you don't intend to use it.

#4




Re: engine size for 35 kw generator end
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#5




Re: engine size for 35 kw generator end
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#6




Re: engine size for 35 kw generator end
As Ed posted, you need close to 2 HP per kw of continuous output. An engine driven WOT or full rack all day will wear fast. You need a bit of overhead for surges and mechanical wear, or there will be frequent rebuilds to maintain near rated output.

#7




Re: engine size for 35 kw generator end
I have 26KW PTO generator that I want convert to a GENSET. It has a bell housing with a normal SAE pattern to mate to ( I am hoping) a 453 Detroit Diesel (nonturbo) producing approx 170 hp at 1800rpms. Is this too much engine, will it hurt the alternator? I may draw 1012KW max in an emergency situation if my neighbours don't also plug in.
Wayne B 
#8




Re: engine size for 35 kw generator end
While a popularly repeated rule of thumb is 2 hp in per kw out, this assumes a ghastly generator efficiency, but does result in plenty of extra hp for brief periods of overload, but also results in plenty of extra fuel consumption due to the oversized engine.
Probably fine for small sets that are more likely to see intermittent overload and have somewhat less efficient generators. Lacking the specified generator efficiency, I find 11/2 hp per kW to be a vastly more realistic guess. So to get 35 kw, you'll be needing right around 53 hp, give or take, to drive the generator. The generator will be somewhat less efficient at less than full load, and at relatively light load it could be as poor as 2 hp per kW. It'll work just fine, but a 170 hp engine driving 20 or 30 hp load will be very inefficient, unless the engine speed can be lowered approximately in proportion. But the generator must (presumably) run at 1800 rpm. Unless it has a gear drive on it, as most PTO generators do. If the input to that is 1000 rpm, it might just work dandy. A 170 hp 453 sounds like turbocharged and running at least 2500 rpm. I would think that it might be more like 100 hp at the 1800 rpm required for your generator if it doesn't have the PTO gear drive input. In that case, still way too big IMHO. But, if this is all stuff you already have, and the usage will be infrequent, it may still make sense for you. 
#9




Re: engine size for 35 kw generator end
I have found
35 kW = 3.5 liter engine To be almost universally true. Turbocharged is a totally different story. You can get like 2/3 more power from a turbo engine which if this was a diesel would make it a 2 liter turbo but more likely a 2.2 or 2.4 because those sizes are more popular. Just some general patterns I have observed. 
#10




Re: engine size for 35 kw generator end
Of course being that the Detroit is a two stroke we would need to apply some conversion math.
The working portion of the stroke in a Detroit is said to be around 70% of that of a four stroke engine. Since it's obviously firing twice as often, one can reasonably expect ~1.4 times as much power from a given displacement at any given speed. So if a four stroke, naturally aspirated, diesel typically delivers 10 kw / liter (I assume at 1800 rpm) we'd expect the naturally aspirated Detroit to deliver 14 kw / liter. Lets see if this works out. A naturally aspirated 71 series Detroit will be found almost universally coupled to generators that deliver 15 kw per cylinder at 1800 rpm or 10 kw per cylinder at 1200 rpm. Detroits are American, and are therefore (appropriately) specified in Cubic Inches, a tradition that should remain embraced to this day, but sadly isn't. There are ~61 cubic inches per liter, so a 71 series Detroit should be found to be driving a generator of ~16.3 kw per cylinder, at 1800 rpm. Pretty close! It then follows that a 53 series ought to do around 12 kw per cylinder, again assuming 1800 rpm. So right close to 50 kw for a 453. I'm not sure what is typical, but I wouldn't be surprised if they were typically running at 10 kw per cylinder, for 1800 rpm sets. Are you SURE that you have a naturally aspirated 453 that makes 170 hp at 1800 rpm??? That doesn't sound right. I want to say that I recall it being considered reasonable to hop up a 453T to around 175 hp at 2500 rpm. Last edited by Vanman; 12012017 at 04:40:04 PM. 
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