> I have often considered using a V-8 automotive engine with much
 > altered cam and ignition timing running at 540 RPM (PTO speed)
 > to run a generator.  By careful selection of components, it
 > would probably be made to be quite efficient at that speed.
 > Maybe even run it off piped-in natural gas for co-generation.
Craig,
 I doubt that this lashup would be "quite efficient" compared to an engine 
more closely sized to the load. There's lots of friction loss in such 
engines, and it would be quite significant at low RPM and load.
Think about engines like a Geo Metro or other mini-car. Run it at a 
reasonable speed, and belt the alternator to also run at a reasonable speed. 
I'd define "reasonable" to be the RPM that it runs when the donor auto is 
doing maybe 45 MPH. Slower would get it way off the torque curve, and faster 
just wouldn't be necessary for this application.
FWIW: I jacked up my VW Jetta Diesel and had a friend run it at 45 MPG in 
high gear while I checked the engine speed with an electronic tach. It was 
about 2200-2400 RPM, or about 50 RPM/MPH. Running the car at 67 MPH would 
turn the engine at 3600 RPM; it sounds like it's really cranking at that 
speed, but could easily operate like that for extended periods.
Running that Geo engine on natural gas and using the cooling water for heat 
is eminently practical. Putting it into practice will "keep you out of the 
bars and arms of loose women" for quite a while. (to coin a phrase)
Good luck!!
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