Diesel Engine Ratings.

[Gbawn]

Hi All,

I was wondering if somebody could help clarify the power ratings for a diesel generator for me. A vendor is trying to sell me this generator (I think it is all he has in stock...) and I am convinced that it is larger than what I require. I require a 80 kW prime generator to service a variable load that typically varies between 35 kW minimum and 76 kW maximum. The vendor provided me with the attached datasheet and tells me that this is a 90 kW prime generator and would be suitable for me. As far as I can tell this is a 114 kW prime generator with a parasitic load of 2.8 kW giving a net prime power of 111.2 kW. Am I missing something? Is there a reason why the vendor is telling me that this is a 90 kW generator? I'm concerned he is trying to sell me a generator that is too large simply because it is all he has in stock.

I am confused as to why the datasheet includes the following:

Generator available power @ Prime power kW 80 100
Generator available power @ Stand by kW 89 110

I don't understand the difference between the "Generator Available Power @ Prime Power" and the "Prime power (net)". Are they not the same thing?

I appreciate any advice.

Gino

 

[TugboatEng]

Most 4 cylinder 4.5ish liter diesel engines are rated at 125 kW. 99kW prime generating heads are common on these engines. If you need this engine to run regularly I strongly suggest not going to a smaller engine. They are not durable. We run our 99kW sets at 12-15% load and get an easy 35k hours out of a set. They're replaced on a time out basis and not wear out.

Also,if you're in an emissions controlled region, it's extremely expensive to certify an engine so manufacturers are having to cover a broader range of applications with a smaller catalog of models.

 

[GregLocock]

I don't understand the difference between the "Generator Available Power @ Prime Power" and the "Prime power (net)". Are they not the same thing?

Generators are not 100% efficient, so the difference between 80 kW and 89.4 are the losses in the genny, 10%, not bad. You can get better if you spend more money no doubt.

I doubt you'll save any real amount of money with a smaller diesel, after you've poked 4 holes in a lump of cast iron the size of the hole doesn't make much odds.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Gbawn]

Thanks for the responses. I believe the genset has a 110 kW, prime rated, Stamford End as per the following:

Surely, given the variable nature of the load and the fact that on average the load is between 35 kW - 40 kW with brief spikes up to 76 kW it would be more fuel efficient to have a smaller engine running at a higher load factor?

 

[TugboatEng]

No. The BSFC of larger engines tends to be lower than smaller engines at any load above 25%. Engine manufacturers don't typically publish BSFC for loads under 25%.

 

[LittleInch]

What you also need to factor in her is the starting or inrush current to whatever your device is.

If it's a large motor this can be very significant and the generator may trip or struggle to load up before tripping or browning out.

Have you actually run your load using a generator?

The two columns are for different frequency - What's your frequency?

Prime power looks to me like shaft output power from the engine - you need generator power which is typically 10-15% below engine shaft power. Use of the word "Prime" can be confusing.

Most generators like to run at about 60-80# power for best efficiency, but normally as long as they are >25% your efficiency is pretty good.

But you can't neglect starting current/power

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