re. Optimal rpm for fuel economy???

[Huw Powell]

> >>Rob Beatty wrote:
> 
> >>Let the arguing commence :)

one way of looking at it is that the lowest consumption of fuel per rpm
should be at idle - and the furthest you will travel per rpm is in the
highest gear.  Clumsy, but borne out by driving experience - in my 90Q,
with a light foot, trapped in the 45-50 mph range, I will see up to 30
mpg.

as speed goes up, more power is required to overcome air resistance. 
unless there is some rpm that is more efficient than a lower rpm by a
factor *greater than their ratio squared* the slowest speed in the
highest gear will be the most efficient.

in other words, if 1200 rpm was more than 1.4 times as efficient in
turning fuel into horsepower, it would be cheaper than driving at 1000
rpm in the same gear.

...at these low rpms, our cars have no trouble making enough power to
overcome rolling and wind resistance.  

...in closed loop (ie - warm engine, no WOT) all rpms are equally
efficient, the air being pumped being matched by a constant ratio of
fuel delivery.  

...this could be complicated by small effects of intake tract/pump
operation variations by rpm - for instance, say 2000 rpm does not
actually draw twice as much air as 1000 rpm.  then it might be more
efficient.  and so long as it is *four* times as efficient (ratio
squared) it will be a cheaper to drive at that rpm in the same gear.

whew...

-- 
Huw Powell

http://www.humanspeakers.com/audi/

http://www.humanthoughts.org/