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Old 05-11-2004, 11:16 PM   #1
brunetmj
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Default Gas mileage and weight reduction

Yes I know one cannot expect much in the way of gas mileage in a STI. However due to current gas costs I want to take some measures to economize. I currently get 20 MPG around town, which is mostly what I do. Between my tools and my sealed box woofer and my amp I carry in my trunk the weight equivalent of another person. If I removed these items would anyone think I would save anything? I intend to just use rear speakers for fill. Any thoughts on the effect of this weight reduction on gas mileage?
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Old 05-11-2004, 11:35 PM   #2
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I'm sure it might help a little but you probably wouldn't see any significant gains unless you removed a lot of weight. Try removing some weight from that right foot, that would help a lot.
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Old 05-11-2004, 11:36 PM   #3
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Quote:
Originally posted by STIenvy
Try removing some weight from that right foot, that would help a lot.
yeah. the back seat could go, too.
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Old 05-11-2004, 11:37 PM   #4
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It won't be that much. My system is only about 90 lbs., but I get the same gas mileage now that I got when I didn't have my spoiler or the system.
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Old 05-11-2004, 11:51 PM   #5
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Quote:
Try removing some weight from that right foot, that would help a lot.
hmm would it? Actually for the most part I already drive fairly conservatively. Well i will post back if I notice any gains.
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Old 05-12-2004, 12:52 AM   #6
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Quote:
Originally posted by skrib09
yeah. the back seat could go, too.
The back seat isn't very heavy.

john
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Old 05-12-2004, 02:02 AM   #7
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WOW Brunet! I'm impressed with your gas mileage as it is....20 miles around town is very good for an STi....I've heard of people getting well under 15mpg around town. I'm lucky to get 20mpg with my regular WRX on in town driving!
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Old 05-12-2004, 08:17 AM   #8
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Quote:
I'm impressed with your gas mileage as it is....20 miles around town is very good for an STi.
Well this area is really rural route driving. I think more closer to highway then city driving. I get 24mpg on pure highway. One thing I found is to make sure the air filter is very clean. I found with all my Subaru's this was critical for good mileage and I think sometimes neglected with many drivers.
I also know that my driving style effects mileage. However it is a high performance car and I intend to drive it like one. But in this thread I was curious about people's experience with pure weight reduction and still keep the car normal (not remove seats).
It sounds as if the consensus is that it does not within the weight amounts I specified.
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Old 05-12-2004, 11:21 AM   #9
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Quote:
Originally posted by brunetmj
Well this area is really rural route driving. I think more closer to highway then city driving.
I recall reading an article in MSN that reducing mass can reduce fuel consumption. However, imho, I agree with the consensus here saying otherwise. Here's an oversimplified argument.

(1) Suppose less thrust force exerted by the engine results in less fuel consumed, and vice versa.

(2) Suppose that the thrust force of the engine contributes to the acceleration of the vehicle, minus drag losses and drivetrain losses. In other words:

F=(m a) + f_{drag}(v) + f_{drive}(v,?)

Where:
F= thrust force of the engine (after transmission)
m = vehicle mass
a = vehicle acceleration
f_{drag}(v) = (aerodynamic) drag force, which is dominantly a function of velocity (v)
f_{drive}(v,?) = driveline losses, which is a function of shaft rotation, or equivalently vehicle velocity, plus perhaps something else (?) like nonlinear friction

(3) Also, suppose that the driving behavior remains unchanged; i.e. acceleration, deceleration, and steady-state speeds are maintained in the same manner.

(4) 100-200lbs of weight is removed, and the initial total weight is about 20-30 times of that removed weight. There is about 1/30 to 1/20th difference in total mass between the before and after.

Case 1:
For a mostly city (accelerate-and-decelerate) driving, and given the 4 assumptions above, drag and driveline losses should remain unchanged. Since acceleration (& deceleration) occurs frequently in city driving, then the effect of mass reduction to the first term "m a" would be more dominant than constant-speed-highway driving. So, there should be some savings.

Even so, I would not expect anything more than 1/30 to 1/20th of the current mileage-per-gallon.

Case 2:
For mostly highway driving, the amount of savings should be even less, because changes in acceleration does not occur as frequently.

my 2 cents
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Old 05-12-2004, 12:38 PM   #10
DrDRum
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Quote:
Originally posted by satrya
Here's an oversimplified argument.
Quote:
Originally posted by satrya
(1) Suppose less thrust force exerted by the engine results in less fuel consumed, and vice versa.

(2) Suppose that the thrust force of the engine contributes to the acceleration of the vehicle, minus drag losses and drivetrain losses. In other words:

F=(m a) + f_{drag}(v) + f_{drive}(v,?)

Where:
F= thrust force of the engine (after transmission)
m = vehicle mass
a = vehicle acceleration
f_{drag}(v) = (aerodynamic) drag force, which is dominantly a function of velocity (v)
f_{drive}(v,?) = driveline losses, which is a function of shaft rotation, or equivalently vehicle velocity, plus perhaps something else (?) like nonlinear friction

(3) Also, suppose that the driving behavior remains unchanged; i.e. acceleration, deceleration, and steady-state speeds are maintained in the same manner.

(4) 100-200lbs of weight is removed, and the initial total weight is about 20-30 times of that removed weight. There is about 1/30 to 1/20th difference in total mass between the before and after.

Case 1:
For a mostly city (accelerate-and-decelerate) driving, and given the 4 assumptions above, drag and driveline losses should remain unchanged. Since acceleration (& deceleration) occurs frequently in city driving, then the effect of mass reduction to the first term "m a" would be more dominant than constant-speed-highway driving. So, there should be some savings.

Even so, I would not expect anything more than 1/30 to 1/20th of the current mileage-per-gallon.

Case 2:
For mostly highway driving, the amount of savings should be even less, because changes in acceleration does not occur as frequently.

my 2 cents
That was oversimplified...
It looks like one of my "oversimlified posts" . I agree tho', that looks quite right.

DRum
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Old 05-12-2004, 01:08 PM   #11
satrya
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Quote:
Originally posted by DrDRum
That was oversimplified...
Oversimplified in the sense that I didn't factor in the fact that engine efficiency varies as a function of steady-state rpm & other more complex conditions.
Oversimplified in the sense that the thrust force only goes to 3 elements: acceleration of mass, viscous air drag, and driveshaft loss, using a mostly linear representation.
Etc.

I didn't mean to use the word oversimplified in a sarcastic way. Imho, it was oversimplified, but not necessarily concise.
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Old 05-12-2004, 02:20 PM   #12
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The model is good enough to get a SWAG. In that sense it is at about the right fidelity. As a model for stop and go it should give you a number that is within a factor of 2 or 3--if you drop total weight by 10% you should see somewhere between 3-10% increase in fuel economy for a given power application.
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Old 05-12-2004, 04:01 PM   #13
brunetmj
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Well that formula is a little over my head. However it seems
to imply that the mass is equally distributed. That is , all mass responds equally to the force applied . My logic, which is definitely simplistic, goes something like: The mass to be removed is all concentrated directly above the main power wheels in a 65-35 back to front split. If you decrease the mass in that location the power to move those wheels requires less energy. In the end however, even if there was any MPG to be gained, I never anticipated it would much.
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Old 05-12-2004, 05:10 PM   #14
satrya
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Quote:
Originally posted by brunetmj
However it seems to imply that the mass is equally distributed. That is , all mass responds equally to the force applied .
I hope I don't misinterpret your statements above too much.

Lumping all of the mass, imho, is the simplest assumption we can make when it comes to considering how much force is required to push the vehicle. There is only one spatial degree of freedom: horizontal, along the forward direction of the car.

Since the topic was improving fuel efficiency, I assumed that the transient weight transfers during acceleration and deceleration do not play a significant role in my arguments. Hence, there is no need to involve an additional degree of freedom; which is pitch in this case.

Quote:
The mass to be removed is all concentrated directly above the main power wheels in a 65-35 back to front split. If you decrease the mass in that location the power to move those wheels requires less energy. In the end however, even if there was any MPG to be gained, I never anticipated it would much.
Let's for the sake of argument we assume that the vehicle is an FR (front engine rear drive). As long as the rear tires maintain traction, I don't see how reducing 200lbs in the rear will result in a different fuel efficiency from reducing 200lbs on the front, at least in a non-agressive, fuel-saving style of driving. The power required to propel the vehicle should remain the same imho.
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Old 05-12-2004, 10:16 PM   #15
brunetmj
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Erwin, as always , thank you for your intelligent and well thought out posts. I will move this from the theoretical to the applied in a few weeks and post back in the event anyone is interested..
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Old 05-12-2004, 11:26 PM   #16
satrya
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Default Re: theory to applied...

You're welcome.

I'm sure there will be people interested. However, I'm a bit concerned for 2 reasons: (1) the amount of weight removed may not result in a significantly measurable difference in fuel efficiency (since it is very difficult to assure identical conditions all the time), & (2) the lack of the double-blind effect (since the tester knows beforehand what the hypothesis is) might sway the result.

Imho, the aberration could be minimized by doing a drastic car weight reduction just for the experiment, and scale down to something more reasonable after the experiment. Even then, I'm not sure if all that effort is worth your while.

One other thing I was going to bring up related to your particular driving conditions, is the use of a flexible front air dam to reduce the amount of underbody airflow. The flexible material shouldn't present a problem, and it can be installed inconspicuously. You might be able to make the dam go as low as 1 inch above ground level.

just a thought.
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Old 05-12-2004, 11:47 PM   #17
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To really simplify what Satrya (either an ME or a Physicist) is saying is light weight assists fuel economy most when there is frequent acceleration/deceleration.

A corolary to that is that aerodynamic drag which is what forms most of the drag at highway speeds is most responsible for mileage at highway speeds.

ex - a 2005 corvette with 400 hp gets 29 mpg highway while a Ford explorer with a 200 hp gets 20 mpg. At highway speed, the engine in the vette is only required to make 20 hp to maintain that speed. The engine in the explorer is required to make some amount more because of the explorer's poorer aerodynamics (Cd x frontal area).

On other item is the drag associated with an AWD system. I can feel that there is more drag in my STI than there used to be in my fwd VW Passat. There were hills that I used to coast down in the Passat where I would actually accelerate while coasting. In the STI, I decelerate coasting down those same hills. One thought, aerodynamic drag is a square function, in other words if you double the speed, the drag goes up by a factor of 4. Drivetrain drag goes up linearly. So regardless of the drivetrain losses, at higher speeds aerodynamics rule.

Don

EDIT - I guess I didn't simplify anything, well at least there were no equations. Next time . .
Xf = Xi +VT +1/2 AT(2) << - a train leaves chicago traveling east at . . . .
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Old 05-13-2004, 08:17 AM   #18
brunetmj
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Quote:
Even then, I'm not sure if all that effort is worth your while.
Well I have more primary motives for this removal. Basically my center console Is so crammed with wires and electronics I can hardly get the trim piece on anymore.

My intention is to make room for additional components for my recently installed security system which is located in the center console. To make more room I will relocate my aftermarket gauge wires and it's associated electronics in addition to revamping my audio system. I will be going to 4 speakers instead of 2 and will use what is equivalent of a stock sub woofer. I will loose my amp and sealed box both of which are in the trunk. It is an Alpine Head unit and has excellent output even without an AMP.
After I conceived this plan I began to wonder if this removal would also help with gas mileage. One of those “this will make it easier to sell the justification of more expenditure and garage time to my wife concepts”
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Old 05-13-2004, 10:37 AM   #19
satrya
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Quote:
Originally posted by brunetmj
Well I have more primary motives for this removal.
That changes everything I suppose.

Regarding the fuel mileage experiment, how about intentionally adding significant weight, say, 400lbs in the trunk or rear passenger area? 400lbs wouild give you twice the difference from 200lbs, in terms of predicted overall change in fuel economy (per discussions in the previous posts). Since removing 400lbs is probably not easy, imho you can test the hypothesis just the same by adding 400lbs. I'm not talking about permanently adding that weight of course. The numbers you get will be more reliable that way.
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Old 05-13-2004, 12:26 PM   #20
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It was pointed out to me that I should not broadcast the location of my security system and I totally agree that this is not a smart thing to do. So ...for new people..don't do that!

I think for now I will just stick with the weight reduction for now and see what happens. Thanks everyone for your input.
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