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Onan Generators

King of Overkill?


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  #1  
Old 11-02-2017, 10:51:12 AM
Utah Onan Utah Onan is offline
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Default King of Overkill?

For a while now I've thought Onans were the epitome of overkill. 1800 RPM sets (except 10 CCK's and probably some others that I don't know of), conservatively rated to take on what I'll call load surges, and engines more than adequate for what is required. From what I have seen excellent design and construction!

However ... I think the King of Overkill goes to Caterpillar (I know, I know, it's not green) back in their earlier days. I.E. Caterpillar made a D397 Diesel Electric Set (their older vernacular for generators), 261kW Continuous & 296kW 12 hour rating with a radiator fan & without said fan 279kW Continuous & 314kW 12 Hour Rating - both ratings @ 1200 RPM, that was powered by Supercharged v12 4-stroke diesel. Only this diesel was a 2,493 cubic inch BEHEMOTH

Onan's 250 DWF was rated at 250kW @ 1800 RPM powered from a 1197 Cubic Inch Turbocharged Waukesha Straight Six Diesel.

My first question, among others, is why could Onan use a much smaller engine to turn a generator that was rated for ~50kW less (Cat's 12-hour Rating with Radiator fan compared to Onan's 250 DWF's 250kW Standby rating). In my uneducated opinion, advantage to Onan.

Also, why would you then want the Caterpillar?
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  #2  
Old 11-02-2017, 11:37:16 AM
DMeed DMeed is offline
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Default Re: King of Overkill?

Quick back of the envelope math on engine displacement:

250kw vs 300 kw = 6/5 power
1800 rpm vs 1200 rpm = 3/2 the speed (and 3/2 the hp since hp is directly related to rpm)

1197 cu in * 6/5 *3/2= ~2155 cu in. That is 85% of the way to 2493.

Supercharger vs turbocharger (more efficient) may make up another few % of the difference.

Be very interesting to compare real fuel consumption numbers!

Now I'll sit back and watch someone who really knows what they are talking about correct my guessing...
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Old 11-02-2017, 11:41:24 AM
brianh brianh is offline
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Default Re: King of Overkill?

The Cat, being a 1200 RPM set, will accumulate wear only 2/3 as fast as the 1800 RPM set, all other things being equal. Naturally aspirated vs. turbocharged means less BMEP and engine stress, although less efficient.

That Cat, if taken care of, will still be running when my grandkids are old.
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Old 11-02-2017, 11:59:08 AM
Steve Dawkins Steve Dawkins is offline
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Default Re: King of Overkill?

Love that old iron, both green and yellow. My guess in engine size difference would be
the power curves on the engines, to provide the rated kW output at a given speed.

I've never seen a generator data plate that gave a continuous rating and a higher 12 hour rating. I think the definition of continuous has changed over time. A Cummins/Onan salesman told me in the 1990s their "continuous" rating was full rated load for any 2 hours out of 24. I don't know if he was serious. Generator ratings have changed over the years, and I heard that some manufacturer's sets couldn't meet their published ratings. Back in the good ol' days, things were designed and built to be more durable than they are now. They sure don't make them like they used to, as evidenced by your photos. Thanks for posting them.
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Old 11-02-2017, 01:22:22 PM
AngrySailor AngrySailor is offline
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Default Re: King of Overkill?

I've been on ships with D series CAT generators. 397 (8cyl) 398 (12cyl) and 399 (16cyl). They work good, leak oil like mad, HARD on fuel and they like to get hot under load even with water to water cheat exchangers. They are a two valve engine so they're choked up pretty bad. I saw the aftermath of a D399 runaway, there was a connecting rod shaped dent in the shipside (3/4" plate). It chucked all 16 rods out the side and broke the block pan rail to deck surface both side The "engineer" had changed the fuel pump and it was jammed full rack. Lucky no one was killed.
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Old 11-02-2017, 02:30:13 PM
Utah Onan Utah Onan is offline
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Default Re: King of Overkill?

Brian, you're spot on ... if that D397 is taken care of well, it'll run well beyond my lifetime. Machines then were simple and built to outlast those who built 'em - maybe that's why us here seek after them.

My Spec B DJC also has listed Continuous & Standby ratings on it (10kW Continuous & 12kW Standby)

I wish I could say I personally saw both the D397 & 250 DWF and took these pictures but I just found them online and saved them.

If I ever had a chance to get a D397, I'd jump on it!
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Old 11-02-2017, 02:32:37 PM
Utah Onan Utah Onan is offline
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Default Re: King of Overkill?

you'll note also that the D397 has DUAL electric starters
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Old 11-02-2017, 02:50:31 PM
AngrySailor AngrySailor is offline
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Default Re: King of Overkill?

Not using air starters...

Jk
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Old 11-02-2017, 03:43:55 PM
Wayne 440 Wayne 440 is offline
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Default Re: King of Overkill?

To me (with apologies to D.W. Onan) the old Caterpillar sets that were 1200 or even 900 RPM are the definition of "lasts forever". I would sure like to have a D330 or similar.

https://www.youtube.com/watch?v=43BvGYTWbRE
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