From: stephen@sprunk.org   
      
   On 04-Sep-15 12:47, danny burstein wrote:   
   > Stephen Sprunk  writes:   
   >> I'm not sure about unburned fuel, but an ICE has uneven power   
   >> output; higher RPMs mean lower fuel economy.  That doesn't apply to   
   >> electric traction, however; output is the same regardless of motor   
   >> speed.   
   >   
   > It's not necessairly "higher RPMs". It's more that the ICE's are   
   > optimized for a "sweet spot" where they get the best efficiency.   
      
   They have a "power band", i.e. a range of RPMs where they produce   
   roughly the same amount of torque.  Power is torque times speed, so   
   higher RPMs within this band mean more power, but it also means more   
   fuel consumption.  If you go off the top end of the power band, though,   
   the torque falls--and if you keep going, the engine rips itself apart.   
   Obviously, that's not a good thing.   
      
   Transmissions are designed so that shifting at the top end of the power   
   band in one gear puts you at (at minimum) the bottom of the power band   
   in the next gear, or vice versa.  More gears simply allow you to stay in   
   a narrower range (e.g. always near the top or the bottom) within that   
   power band, which provides either better mileage or more power for a   
   given engine.   
      
   > Lower RPMs can be bad, too...   
      
   If you fall off the lower end of the power band, it stalls; electric   
   motors don't have that problem.   
      
   > The related problem is that during the speedup/slowdown, there's even   
   > more ugliness until the engine stabilizes at the new speed.   
      
   I'm not sure what you're referring to here.   
      
   > - This is a BIG advantage of (properly designed) hybrid electric-gas   
   > cars. The ICE output remains a lot steadier, and speed changes are   
   > slower. The electric motor does the tweaking...   
      
   Are you thinking only of series hybrids?  In that case, the ICE can run   
   at a constant speed to maximize mileage, and it also removes the need   
   for a complicated (and heavy) transmission.   
      
   Parallel hybrids aren't that different from regular cars; the ICE has to   
   work over just as wide a range, just with the electric motor adding or   
   removing power that is stored in the batteries.   
      
   S   
      
   --   
   Stephen Sprunk         "God does not play dice."  --Albert Einstein   
   CCIE #3723         "God is an inveterate gambler, and He throws the   
   K5SSS        dice at every possible opportunity." --Stephen Hawking   
      
   --- SoupGate-Win32 v1.05   
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