"Rich Grise" wrote in message
news
On Wed, 09 Sep 2009 17:33:46 -0500, cavelamb wrote:
I'll go one step farther on this topic...
You want to save fuel?
Slow down!
Isn't that somewhat of a trade-off? I certainly wouldn't expect
to save gas by putting the car in first and just idling at 2MPH
or so to the store and back!
The point being, don't IC engines have a certain range of RPM
where they're most efficient?
Yes, but the bigger factor is the percent of available torque that's being
used at any given RPM. At low RPMs or high ones, when the throttle is mostly
closed your effective compression ratio goes 'way down, thermal efficiency
goes to pot, and your mileage takes a dive. The most efficient way to run a
conventional spark-ignition engine is at some moderate level of RPMs and
full throttle -- assuming you get good fuel atomization. (That's *engine*
efficiency, not total efficiency of the car, which falls off sharply with
speed above 50 mph or so.) Fuel injection has solved the low-RPM problem;
you get good atomization at all engine speeds.
BMW ran extensive tests a decade or so ago, with the pretty good
computer-controlled port-injection system they were then using, and found
that the best fuel efficiency in acceleration was accomplished at FULL
throttle, but by shifting at low RPMs. That was the opposite of the old
idea, but that had been with carburetors.
The new stratified-charge direct-injection systems are throwing all of the
values into a cocked hat. The latest ones hardly use a throttle at all;
Ford's new one "throttles" mostly by controlling the amount of fuel
injected -- more like a diesel than a conventional IC engine. Partial-load
fuel efficiency is up to unheard of levels in these new engines, which are
taking over in the new designs from most manufacturers.
At what point does wind resistance
come into it?
It's a square-ratio effect, but typically it becomes a major factor
somewhere between 50 and 60 mph. That best-tradeoff speed may drop with the
new engines -- it probably will, in fact, based on theory.
--
Ed Huntress