Newsgroups: sci.energy
From: John De Armond
Subject: Fuel economy (was Re: 100 mpg vehicle)
Message-ID: <k24sd1_@dixie.com>
Date: Fri, 19 Feb 93 07:52:16 GMT

jbb@vcd.hp.com (Jim Becker) writes:

>: Composite bodies don't necessarily weigh less. See my posting on high
>: strength steel monocoupe structures for a rebuttal.
>:
>
>I'll take your word for it on this.  But I'd like to know why GM (and I
>assume others) are working on composite bodies and 2 - stroke engines.
>Are they going through a fruitless exercise or putting up big
>investments for much smaller gains (say a 40 mpg taurus sized car)?
>Just wondering.


You are correct that reduced weight (inertia) is the most important design
parameter.  While drag dominates the equation at highway speeds, it is of
much less design concern because of - guess what - government tinkering.
Let me explain.

Did you ever wonder why EPA fuel economy numbers never relate to your
experience?  The reason is the EPA test does not actually measure economy.
It measures how much total fuel is used during their specified driving
cycles and computes mileage from the total "distance" covered on
the test dyno.  Actually they don't even measure fuel consumption.
The compute consumption from the composition of the exhaust gases.  That
is bad enough but worse, the driving cycle has little to do with the
real world.

Consider, for example, the "US-72" city cycle.  The distance is 7.5 miles,
the average speed is 19.7 mph, and the peak speed is 56.7 mph.  There
are approx 23 stops and starts.  Steady speed is held 20% of the time,
acceleration is for 34% of the time, idle for 17.4% and deceleration is
28.8%.

This isn't so bad as long as the cycle is used only for emission
testing.  It gives everyone the same target to shoot for.  It generates
numbers that are relative only to other numbers derived from the same
test.  Unfortunately the units are "MPG" and that led congress to
tinker.  Specifically Corporate Average Fuel Economy, the darling of the
econazis.  Congress took what used to be a relative system of
measurement and assigned absolute limits.  Because those limits are derived
through a test that only distantly relates to the real world,
manufacturers are forced to engineer to a thoroughly artificial target.
The penalties for non-conformance are severe enough that achieving CAFE
and the emission limits dominate all other considerations.  Consider
that as of 1986, the last year I have numbers for, if a mfr missed the
CAFE by even 0.1 mpg, a $500 tax is applied to every vehicle in the
line.  This increases to a maximum of $3850 per vehicle.  BTW, CAFE
is one reason nice cars cost so much.  The mfr prices the nicer, lower
MPG vehicle higher so that sales are biased toward cheaper, better
MPG models.  The mfr likes it because it results in greater profits.
The government likes it, well, because it is the government.  A classic
example of big business and the government teaming up to screw the consumer
while cloaking the screwing in the pale green glare of "environmental
responsibility".  Right.

With CAFE in mind, if we now go back and look at the EPA cycle, the
contributions of inertia and rolling resistance become obvious.  The
cycle average speed for the test I outlined is only about 20 mph,
a speed where rolling resistance dominates and drag is relatively
insignificant.  Next consider that for 34% of the test, the vehicle is
accelerating.  That means energy is being pumped into vehicle inertia.  Since
only 29% of the time is spent decelerating, little of the energy
is recovered in coasting.  The only way to minimize inertia is, of course,
to shave weight.  That's why you see the vendors doing such seemingly
silly things as minimizing the weight of the battery, air filling structural
plastic, using hollow fibers in carpet and even such radical things as
Ford's continuing development of the stamped sheetmetal engine.
If you pay close attention in new cars with computerized engine management
and transmission controls, you can feel the transition out of the
EPA test cycle envelope.  The engine is more responsive and the transmission
no longer tries to keep the engine speed as low as possible.

In the real world, outside a few pathological situations such as SoCal,
inertia and rolling resistance plays much less of a role.  Even on
urban streets, it is common for speeds to reach and hold 50 mph or more.
And of course, on the interstate, at a cruising speed of 70 mph or more,
inertia plays a vanishing role while drag dominates.  That's why,
for example, my nearly 5,000 lb Fury III land yacht does around 24 mpg
on the highway.

John

From: tgl@slee01.srl.ford.com (Tom Leone)
Newsgroups: sci.energy
Subject: Re: Fuel economy (was Re: 100 mpg vehicle)
Date: 26 Feb 1993 17:12:39 GMT
Message-ID: <1mlj28INN2ig@fmsrl7.srl.ford.com>

John De Armond (jgd@dixie.com) wrote:
:
: You are correct that reduced weight (inertia) is the most important design
: parameter.  While drag dominates the equation at highway speeds, it is of
: much less design concern ...
:
: Consider, for example, the "US-72" city cycle.  The distance is 7.5 miles,
: the average speed is 19.7 mph, and the peak speed is 56.7 mph.  There
: are approx 23 stops and starts.  Steady speed is held 20% of the time,
: acceleration is for 34% of the time, idle for 17.4% and deceleration is
: 28.8%.

Why do you ignore the highway cycle?  Government CAFE requirements
are based on "metro-highway" fuel economy, which is 55% city cycle
and 45% highway cycle.  The highway cycle is about 13 minutes of
fairly steady driving.  There are no complete stops, maximum speed is
60 mph, but mostly it is 48 to 55 mph.  Steady speed is 56% of the
time, acceleration is 28%, and deceleration is 16%.

I agree that this is not representative of most peoples' driving
habits.  Personally, I think the highway cycle should include higher
speeds, so that the benefit of higher gearing (for actual drivers)
will also be in the best interest of manufacturers.  But it is
important to have a consistent test cycle so that manufacturers can
meet the regulations.

: This isn't so bad as long as the cycle is used only for emission
: testing.  It gives everyone the same target to shoot for.  It generates
: numbers that are relative only to other numbers derived from the same
: test.  Unfortunately the units are "MPG" and that led congress to
: tinker.  Specifically Corporate Average Fuel Economy, the darling of the
: econazis.  Congress took what used to be a relative system of
: measurement and assigned absolute limits.  Because those limits are derived
: through a test that only distantly relates to the real world,
: manufacturers are forced to engineer to a thoroughly artificial target.
: ... A classic
: example of big business and the government teaming up to screw the consumer
: while cloaking the screwing in the pale green glare of "environmental
: responsibility".  Right.

Whoa, where did the consiracy come from?  Car manufacturers have
always opposed CAFE standards.  The Big Three support a higher gas
tax to give the CONSUMERS an incentive to save gas.  Statistically,
as average fuel economy of vehicles goes up, so does the average
number of miles driven per vehicle.  The CAFE standards are not
saving gas because people limit their driving based on cost, not
number of miles.

: With CAFE in mind, if we now go back and look at the EPA cycle, the
: contributions of inertia and rolling resistance become obvious.  The
: cycle average speed for the test I outlined is only about 20 mph,
: a speed where rolling resistance dominates and drag is relatively
: insignificant.

Again, this is only 55% of the fuel economy number.  The other 45% is
from the highway cycle.  Aerodynamic drag is important, too.

: In the real world, outside a few pathological situations such as SoCal,
: inertia and rolling resistance plays much less of a role.  Even on
: urban streets, it is common for speeds to reach and hold 50 mph or more.
: And of course, on the interstate, at a cruising speed of 70 mph or more,
: inertia plays a vanishing role while drag dominates.  That's why,
: for example, my nearly 5,000 lb Fury III land yacht does around 24 mpg
: on the highway.

Where's your data?  Last time I checked, most trips were less than
20 miles each.  I'll bet they weren't on the interstate.  I think
that most people drive back and forth to work or school every day,
and not on the interstate.  City fuel economy in stop-and-go traffic
is important.

: Did you ever wonder why EPA fuel economy numbers never relate to your
: experience?  The reason is the EPA test does not actually measure economy.
: It measures how much total fuel is used during their specified driving
: cycles and computes mileage from the total "distance" covered on
: the test dyno.  Actually they don't even measure fuel consumption.
: The compute consumption from the composition of the exhaust gases.

Did you ever wonder why they chose to do it that way?  It's because
emissions measurements give you the fuel consumption much more
accurately than liquid flowrate measurements.  Not to mention the
fact that most fuel systems in vehicles have fuel return lines to
the gas tank, which means you have to measure NET flow to the
engine.  The real reason that "your mileage may differ" is because
you may not drive a typical EPA cycle.  That will be true no matter
what the EPA cycle is.

Tom Leone <tgl@slee01.srl.ford.com>