Re: fifth-gear and miles per gallon

[16V Jason <jason@scirocco.org>]

At 04:33 PM 10/31/2001, Fry, Larry wrote:
>Hypothetical situation: you have fifth-gear ratio that gives 3800rpm at
>80mph and it takes 1/4 throttle to maintain that steady speed.
>second hypothetical situation: you have, on the same car, on the same
>stretch of road, a fifth-gear ratio that gives 3000rpm at 80mph but it takes
>1/2 throttle to maintain that steady speed.
>
>question: which situation yields the best mileage?

It's tough to tell because you don't know the characteristics of that 
engine.  Let's put it another way:  You have a car that revs at 3800 rpm at 
80.  (We don't know throttle position).  Then, we replace the 5th gear so 
that it turns 3000rpm.  We again don't know the throttle position.  But we 
do know is that the torque peak is somewhere around 4500rpm.  The longer 
5th gear (=3000rpm) will get better gas mileage.

The reason:  One of the biggest killers of fuel economy in gasoline engines 
is losses that come from throttling.  Your engine is constantly working to 
suck as much air as it possibly can past the throttle.  (This is the main 
reason that diesels get so much better gas mileage than gas engines do - no 
throttling losses).

So, as long as we know that we haven't reached the torque peak yet, we know 
that the engine is able to put significantly less power to the ground in 
the 3000-rpm gear, for 2 reasons:  1, the engine is making less torque, and 
2, the torque multiplication isn't as much.  Therefore, we know that the 
throttle opening will be larger, and therefore throttling losses will be 
reduced.  Ergo, most likely* the 3000-rpm cruiser will get better fuel 
economy, as long as the throttle doesn't have to be floored.  The reason 
being that mixture stays the same until full-throttle, when it goes full 
rich.

*I say most likely here because every engine has different characteristics 
of specific torque -- that is, the amount of torque produced per <measure> 
of gasoline.  If you think about it from a physics perspective, your engine 
will most likely be operating at peak efficiency at or near
the torque peak -- it only makes sense, since if your engine was operating 
under ideal conditions all the time, the torque peak would be horizontal 
and flat.  Therefore, it might be that this engine is so much more 
efficient at 3800rpm than it is at 3000 that it actually gets worse fuel 
economy.  It's doubtful, but it could happen -- and that's the reason I 
made the torque peak at 4500rpm -- enough revs higher that it wouldn't 
interfere.

>In other words, which influences miles-per-gallon more, rpm or throttle
>position?  Or is it pretty much a wash?

Both.
The higher the RPM, the higher the internal resistance needed to keep the 
engine turning at that speed.  Think of the force it would take for you to 
(by hand) turn your engine at 7000rpm.  Your engine has to overcome those 
same forces... the maximum engine speed (without limiters) is where the 
internal forces get so high that they're the same as the amount of power 
the engine can produce.  And resistance and reciprocal forces inside the 
engine increase at the square of engine speed... so 3000 to 3800rpm is a 
big difference.

Then, until the full-throttle enrichment kicks in (and almost every car 
made has this), the wider the throttle opening, the more efficient the 
engine is.

So, your best bet for fuel economy is low-rpm, wide throttle.

BMW's new drive-by-wire double VANOS system does this automatically, it's 
cool as shit.  Under steady state cruising at lower RPMS, it automatically 
(and totally unknown to the driver) opens the throttle all the way, leans 
out the mixture, and changes the cam timing to permit what 's called "lean 
burn" mode.  This is the best fuel efficiency, since the motor is running 
exactly like a diesel.  It's the reason that the BMW 330i (with a 3.0l L6) 
uses 10% less gas than the older 2.8L from the 328i, and at the same time 
gives 20 more hp!

I love technology!
Jason




>Inquiring minds......
>
>Larry  sandiego16V
>
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