[alfa] RE: Fuel Economy vs. RPM

[RonHorowitz <RonHorowitz@xxxxxxxxxxxxxx>]

Howdy, Folks,

Been a while since I've posted, I think... My .02:

IIHO, if you want more fuel efficiency, either drive at a slower cruising
speed or buy a more efficient car. Having said that....

> (the e standing for Eta, a Greek letter
>having something to do with thermodynamics I believe)

Eta is often used as notation for efficiency, i.e., volumetric efficiency,
combustion efficiency, thermal efficiency, mechanical efficiency, and / or
net efficency.

> you should drive a gasoline powered car as close to wide open throttle
>as possible,

Not necessarily _at_ wide open throttle, but perhaps _near_ wide open
throttle, or at least on EFI cars. On many EFI systems (all?) at WOT, the
EFI goes "open loop" disregards the input from the oxygen, EGO, or "lambda"
sensor (mixture stoichiometry is notated with greek letter lambda) and
enriches the mixture, perhaps from somewhere in the range of 14-15 to 1
down to something closer to perhaps 12-13 to one.

Just off the top of my head, what BMW was likely refering to was the losses
incurred due to partially closed throttle plates. The volumetric efficiency
necessarily decreases as the throttle plates close (partially closing the
throttles permits modulation of the power output) and the amount of
"negative work" the engine has to do to overcome manifold vacuum
necessarily increases as the manifold vacuum goes up. Think of it this way,
each piston has so many square inches of surface area, there are so many
pounds _per_ square inch of vacuum, so the higher the manifold vacuum, the
more pounds of force there are "pulling" the pistons back towards TDC on
the intake stroke.

Anyway, long story short, high manifold vacuum doesn't work in your favor.

If you crank the throttles wide open the manifold vacuum drops, so the
efficiency goes up. You accelrate if the engine is producing more power
than you need to maintain a steady speed, but depending on where you're
cruising in terms of RPM and road speed, you only need as wide a throttle
opening as you need in order to cruise at a steady speed. At lower RPM, the
engine speed is lower, so, power being equal to torque times speed, for the
same power at a lower RPM, you need more torque, ergo, you have to open the
throttle more.

>at as low rpm as could be used to do the job (i e accelerate
>the car) then cruise at the desired speed with the throttle as open as you
>can without accelerating the car. In other words,

One of the problems with this approach with a 1300 Veloce or something like
that is that is simply doesn't work very well at low RPM. The universal
Italian cross-flow hemi-head neither breathes nor burns efficiently at low
RPM (low volumetric efficiency, low combustion efficiency, and the
"all-alloy" design even has a little bit lower thermal efficiency than a
cast iron block and head design.)

IIRC, the BMW e engines were set up to produce peak torque at relatively
low RPM. Those engines were designed to operate at large throttle angles at
low rpm. Don't try that with a Porsche RSK or whatever the hell it was with
the roller bearing crank, for instance. I guess what I'm trying to say is
that a _little_ knowledge can be a dangerous thing. Don't try to draw too
many conclusions if you haven't got a pretty good handle on what's what.

>the old jack rabbit start
>so pooh poohed by the so called experts was proved to be most efficient...

Not necessarily at wide open throttle, though.

>For cars equipped with Bosch Jetronic EFI, full open throttle triggers
>the full-throttle switch that activates what Braden describes as a
>"damn-the-torpedoes mode. (pg 48)"

Good ol' Pat. I miss the guy.

>The injectors fire twice (instead of
>once) per 4 revolutions,

On most EFI systems, whether batch fired or not, what's modulated is the
injector bandwith, or if you will the open time per revolution, not the
number of times it fires.

>and the oxygen sensor reading is ignored,
>allowing a richer than normal mixture.

Yes.

>This leads me to believe that,
>while close to wide open throttle (as described above) may be most
>efficient,

Probably.

>"floored" full throttle throws economy out the window.

Colorful, if not analytically specific prose....

RON
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