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3600RPM for 60HzReasonable. 2-pole. But you may have noted that motors above 5HP are often 1800RPM, 4-pole. You can scale this as far as you want. I have seen an 18-pole alternator (200RPM) but it was too big to fit in the museum.
What do we have here? 25KW is about 34 horsepower. Figure alternator efficiency as 80%, we need 42 horses at the shaft.
Flathead specs:
1934-1941, 221CID
CID HP@RPM
221 85@3800
CurvesWe have 50HP at 1800 RPM, need 42, so 1800 is a possibility.
I have power vs. fuel plots for a flattie somewhere, but not at hand. They show the usual trends: power peaks at some high RPM, perhaps 3600, but both torque and economy peak around half that RPM, say 1800. Economy is much better at 1800 than 3600.
Running 3600, the engine can make 80HP, twice what we need, so it would have to be throttled way back. On a spark engine, that's bad for economy. Expansion ratio is lower, mechanical friction is high.
At 1800, the 42HP load is just shy of wide-open, economy can be good. Mechanical friction is lower, wear is less, and inertia stresses are 1/4 what they are at 3600.
While the flattie can turn 3600, maybe past 4300 stock, how happy is it? And how long will it run?
Flattie featured just 3 main bearings. Poured babbit bearings into 1936. My Four has five mains, all thinwall type (much better), which is why it is rated to turn 5800 RPM on peaks, though the noise discourages use above 4000 except to merge onto a highway.
Comments from modern users:
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A Flatmotor MAY turn 3300RPM but it won't be happy for long
> It CAN cruise all day at 2500 RPM
> It will be VERY happy between 1800-2100
> ...doing about 2800 rpm ...it was screaming like a banshee... There are various uses for "emergency" engines. Stationary fire-pump engines may be rated high power for only 100 hours total life. I guess if your building suffers ten 10-hour fires, you have worse problems than your pump. For electric, there is the "get me through the night!" application, where line crews are coming "soon", and "power will be out a while", such as a remote site where power goes out in the first ice-storm and can't be restored until the snow melts 3 months later.
And my dad says the flattie needed major attention in 50,000 miles, which is perhaps 1,000 hours of cruising (not screaming).
My mechanical gut says that asking a flattie to run 3600 RPM for 100 hours is risky, whereas it may turn 1800 RPM for many-many hundreds of hours.
I vote 4-pole 1800 RPM.
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