Miata Mailing List: December 1993, Message #224
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   From: meek@me.mech.utah.edu
Subject: Re: gasoleeeeen
   Date: Thu, 23 Dec 1993 12:23:41 -0500

>Fellow fiends:
>
>Joining the "I'm no engineer" chorus, I always thought that one of
>the principal reasons for lower mileage in the winter is that cold
>air mixed with cold fuel just doesn't burn as efficiently as the
>warm stuff does.  This goes double for the cold start (i.e. cold
>head, cold induction, cold pistons, cold everything!).  So, to get
>a certain level of power more fuel has to be shoved in because a
>lower % of that fuel actually burns (hence nasty emissions must be
>much higher too, and not just because the catalytic converter isn't
>hot yet).  As the engine warms up the mixture is leaned down via the
>automatic choke.  In warm weather this leaning down occurs many
>miles (minutes) sooner.
>
Well I am an engineer (mechanical engineering professor), but maybe I
shouldn't say that in case I say something really dumb and embarrass
myself.

The primary reason for worse milage in winter is different gas.  Gas mixed
for winter use has more pentane in it.  Pentane is less dense than octane
and so it gets metered differently by the engine.  (Pentane has a five
carbon chain and octane has an eight carbon chain.)  This is so the engine
starts easier in the cold.  The milage is a worse problem with carburated
cars than it is with fuel injected cars.


>On the other hand, I am not sure why people are reporting lower mileage
>in the summer, unless it is due to increased drag with the top down.
>I don't see why power/gallon should be down, but with the top down I
>can see why power/mph must be up (so gallon/mph would be up) (or is my
>8th grade algebra teacher spinning in his grave?).
>
>For the record, I have a 13 mile commute and seem to wander only +/-
>1 gallon from 30mpg all year.  I hazard to confess that during my
>commute (a true race to the death on a Detroit freeway with, count them,
>26-wheelers (I s--t thee not)), there is just too much noise for top
>down motoring.  By the exit ramp though, that baby is down.  During long
>highway cruising I get up to 34-35 mpg (during the summer, mostly)
>
>Doesn't driving style have a lot to do with this?  I mean, shift points.
>I don't know what the power/rmp curve looks like on the Miata, but I
>have a suspicion that peak horsepower occurs well before 6700 rpm.

The peak power of a MIata is at around 6500 rpm.  The peak torque is at
around 5000 rpm.  You don't necessarily want to shift at the peaks.  What
you want to do (for maximum acceleration) is to have the maximum torque
that you can have form the engine always applied to the wheels.  If you
shift at the maximum torque point, you will put the engine at a lower rpm
and at a sub optimal torque region.  You want to shift when you're in the
decreasing torque region of the engine's torque/rpm curve at the high rpm
and shift to an engine rpm at the same torque level on the increasing
torgue region of the engine's torue/rpm curve.  Does this make any sense. 
The bottom line is that there is an optimal shift point, but it's not as
simple as shifting at the peaks.


>Does anyone know the facts?  If peak is at 5000 rpm, is there any point
>in going beyond?  Shouldn't peak horsepower rpm be the ideal shift point?
>This gets complicated because break horse power also has something to do
>with the car's drag?  Stop me before I ask another stupid question......
>...
>

Brake horse power has nothing to do with the car's drag.  Brake horse power
refers to the measurement technique.  Power is defined as the speed times
the torque.  The engine is run a a specific throttle opening and a brake is
applied to the wheels.  The torque that the brake appies is measured and
the speed is recorded.  Multiple the two together and you've got power. 
You simple get the entire torque/power vs speed curve by applying different
brake torques and run the engine at different throttle settings.


Happy Holidays

Sandy

Sanford Meek
Dept. of Mechanical Engineering
University of Utah
Amateur radio:  KB7YSQ
meek@me.mech.utah.edu