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Old 09-12-2009, 12:08 AM   #24 (permalink)
bwilson4web
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Location: Huntsville, AL
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17 i3-REx - '14 BMW i3-REx
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Quote:
Originally Posted by some_other_dave View Post
That is still a low throttle setting to my way of thinking. I really don't think I understand what you are saying...
What happens is the 25-30 hp point is on a steep part of the power required curve. Any slight rise or additional drag requires the engine to run up that curve into higher consumption and even more inefficient power settings:


The areas of peak engine efficiency are marked by the small, red dots. You'll notice the 25-30 hp curve fall right under the last of them. Any higher power demands push the engine into inefficient, lower BSFC regions.

So it is a combination of two effects:
  • steep power slope - so any local variance in power demand will rapidly go into much higher power regions
  • upper limit of high efficiency - anything over 30 hp is going to have lower BSFC than the 25-30 hp region.
I've thought about making a cruise control device that limits the peak HP setting, just using the ICE rpm of say 2,600 rpm AND a maximum speed. The car would slow down going up a hill but it would stay in a fuel efficient mode. But it in an actual design, I would allow excursions up to 3,400 rpm to handle hills. If any more is needed, the accelerator would be used.

Bob Wilson
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2019 Tesla Model 3 Std. Range Plus - 215 mi EV
2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade
Retired engineer, Huntsville, AL
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