Increasing weight for longer glides - Fuelly Forums

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Old 01-20-2007, 09:28 AM   #1
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Increasing weight for longer glides

I watched psyshack's Okie FAS vids on CleanMPG this morning:

http://www.cleanmpg.com/forums/fuel-...-fas-2650.html

In the second one, he comments that his old Accord would coast a lot longer (about a mile) than his Civic in the video (5/8-3/4 mile). My guess is that this is because the Accord adds more weight than it does wind drag when compared to the Civic.

Glide times where speed normally decreases on a flat or slight downhill should increase as the car's weight is increased, all other things being equal. The amount of kinetic energy the car has is proportional to weight, and the rate at which speed is lost on flat land is a function of weight, aerodynamic drag and rolling friction -- increasing weight, lowering drag and lowering friction will all increase glide times.

The benefit of an increased glide time could be used two different ways: the engine could be ignited less frequently for the same speed range (fewer big pushes of gas at startup with an automatic, or fewer clutch slips for bump-starting a standard shift), or the speed range could be lowered (less variation in speed) for the same length of glide. The former would benefit fuel economy, the latter social acceptability. Either would be a plus.

Increasing weight would also dampen acceleration as well, which should also benefit fuel efficiency in cars with a high power/weight ratio (ie. cars for the American market) because a wider throttle position could be used to obtain the same rate of acceleration, and short of WOT, a wider throttle position generally yields a better engine efficiency.

Climbing ability during a glide should not be adversely affected; the energy lost during an ascent (U=mgh) is proportional to mass, but so is the kinetic energy of the car (E=1/2mv^2), so these should cancel each other. An ascent under engine power would require more throttle, but again, this should yield a greater engine efficiency and a longer glide on the descent.

Additional weight would also lower the aerodynamic profile in cars without ride height compensation. In general, lowering the front and raising the rear has generally been found to positively benefit fuel economy, so any weight added should be as forward in the car as possible. Probably the easiest would be steel weights or water jugs in the passenger seating areas, placed as far forward as practical.

A few caveats:

All other variables being equal, increasing weight would normally increase rolling friction. This is because the tires are deflected more with the increased weight, and tires constitute the main component of rolling friction.

The easy way around this would be to inflate the tires to compensate for the additional weight. If the height of each tire at its contact patch is precisely measured prior to adding weight (e.g. ground to the lowest point of the rim), then the tire could be inflated to achieve the same height after loading of the weight to compensate. This should yield very little net loss in rolling friction.

Also, this would mainly benefit commutes with long stretches of relatively flat (not mountainous) highway and few stops. If you normally brake much during your commute for stops or downhills, all the extra kinetic energy goes up in smoke (literally) and you end up with a net loss.

This would favor using water instead of steel -- if you find yourself stuck in traffic, you could just coast to a stop and dump the water for a more agile vehicle.

In hillier terrain, the speed variation would be greater because rate of descent increases with increased mass and decreased aerodynamic drag and rolling friction. So, if the car normally accelerates on a downhill, with more weight it will have a greater velocity at the bottom of the hill than it would otherwise. Depending on the grade, only so much can be done to compensate for this efficiently.

Unfortunately, I can't exploit this possibility for my particular commute because it's very hilly with a number of stops.

I'm drawing from these concepts from my time on the bike. Heavy riders (who can generate more power for their drag) and aerodynamic bicycles (e.g. faired recumbents) are generally faster on flat country roads, but the lighter diamond-frame bikes and riders are faster in hilly terrain and city traffic (because they effectively lose less energy to downhill wind drag and braking), and on bikes speed is generally an efficiency game.

I understand that this flies in the face of the general recommendation to remove any excess weight, which certainly benefits the majority of drivers with normal driving patterns.

Any thoughts?
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Old 01-20-2007, 10:04 AM   #2
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I agree totally. Provided you never touch your brakes (or use engine braking), increase your tire pressure to compensate for the increased weight (maintain the same size tire footprint), and the grade on your downhills isn't steep enough to greatly increase your terminal velocity, increased weight doesn't hurt your mileage. With the increases in the efficiency of your pulse driving due to the increased weight, you might actually get better FE under those specific conditions of pulse driving on flat to gently rolling rural roads.
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Old 01-22-2007, 09:45 AM   #3
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This "experiment" has been done!

Quote:

Glide times where speed normally decreases on a flat or slight downhill should increase as the car's weight is increased, all other things being equal. The amount of kinetic energy the car has is proportional to weight, and the rate at which speed is lost on flat land is a function of weight, aerodynamic drag and rolling friction -- increasing weight, lowering drag and lowering friction will all increase glide times.



Increasing weight would also dampen acceleration as well, which should also benefit fuel efficiency....
An ascent under engine power would require more throttle, but again, this should yield a greater engine efficiency and a longer glide on the descent.

Probably the easiest would be steel weights or water jugs in the passenger seating areas, placed as far forward as practical.

A few caveats:

All other variables being equal, increasing weight would normally increase rolling friction. This is because the tires are deflected more with the increased weight, and tires constitute the main component of rolling friction.

So, if the car normally accelerates on a downhill, with more weight it will have a greater velocity at the bottom of the hill than it would otherwise.

I understand that this flies in the face of the general recommendation to remove any excess weight, which certainly benefits the majority of drivers with normal driving patterns.

Any thoughts?
I have but one large thought: A famous man we all know about (L. DaV.) dropped two masses of different weights from the Leaning Tower of Piza (ONE "z", pizza fanatics!) and discovered...two different masses fall at the SAME RATE ! Where does this leave the heavier / lighter coasting masses argument?
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Old 01-22-2007, 09:58 AM   #4
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Quote:
Originally Posted by Ted Hart View Post
I have but one large thought: A famous man we all know about (L. DaV.) dropped two masses of different weights from the Leaning Tower of Piza (ONE "z", pizza fanatics!) and discovered...two different masses fall at the SAME RATE ! Where does this leave the heavier / lighter coasting masses argument?
They are completely different.

Two free-falling objects affected only by air friction and gravity will accelerate at the same rate. This assumes that they have the same basic shape (a bowling ball and a golf ball, etc.)

If we were talking about dropping a civic and an accord from a tower in a vaccuum, then they would accelerate at the same rate.

When it comes to real-life applications of coasting, we have to factor in the mass of the object, rolling resistance, aerodynamic drag, and other factors.
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Old 01-22-2007, 10:36 AM   #5
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Given an EXTREMELY specific set of circumstances (i.e. PERFECTLY spaced and sized hills), more weight could possibly help. But by and large extra weight will hurt.
1. cars are not 100% efficient.
2. it takes more energy to accelerate a heavier object up to speed, ignoring drag/friction.
3. a heavier car has more rolling resistance. It's not fair to crank up the tire pressure on one and not the other then draw comparisons
4. A heavier car probably has more area and thus more drag.


Sorry, but I don't think it works out for the accord IMHO.
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Old 01-22-2007, 10:53 AM   #6
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skewbe-

Quote:
Originally Posted by skewbe View Post
Given an EXTREMELY specific set of circumstances (i.e. PERFECTLY spaced and sized hills), more weight could possibly help. But by and large extra weight will hurt.
1. cars are not 100% efficient.
2. it takes more energy to accelerate a heavier object up to speed, ignoring drag/friction.
3. a heavier car has more rolling resistance. It's not fair to crank up the tire pressure on one and not the other then draw comparisons
4. A heavier car probably has more area and thus more drag.


Sorry, but I don't think it works out for the accord IMHO.
I am guessing that the glide is longer, but the overall MPG gain is not. That is to say, with all other conditions being equal, the heavier Accord will have worse MPG over the same X miles of driving than the lighter Civic.

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Old 01-22-2007, 12:15 PM   #7
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Case in point - my Geo (alas poor Geo I knew him well) and my xB on this certain streatch of road the Geo would slow down and the xB speeds up but by comparison on the Newport bridge the xB reaches a terminal speed that is about 5mph slower than the Geo and we all know the xB is heavier by about 1000 lbs.

On a side note I did a really low RPM 2nd gear takeoff yesterday and only lost 0.1mpg from a stop sign acceleration back up to 30mph on my homeward trip when I usually loose 0.5 - 1mpg on the trip average. Traveling a few gentle down hill roads on the way back last night boosted my 39-40mpg up to 45mpg during the last few miles of an 18 mile trip.
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Old 02-05-2007, 11:34 AM   #8
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Hello -

Here is a specific idea for gaining MPG. *IF* your route has a net elevation change, and *IF* there is something usefull that you can transport on the downhill portion of the round trip, then take that with you in order to increase your downhill MPG. On the uphill run your car will be lighter so that you can maximize MPG in the uphill direction.

This would be perfect for someone who always or at least frequently has to deliver something from point A (high elevation) to point B (low elevation).

Example : If your home is at the downhill point of your commute, then you could do grocery shopping at the uphill point of the commute to increase your MPG.

Maybe this would make a good experiment, eh?

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Old 02-05-2007, 11:57 AM   #9
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Case in point - my Geo (alas poor Geo I knew him well) and my xB on this certain streatch of road the Geo would slow down and the xB speeds up but by comparison on the Newport bridge the xB reaches a terminal speed that is about 5mph slower than the Geo and we all know the xB is heavier by about 1000 lbs.
The XB is an aerodynamic brick compared to a Geo. There's no mystery why it won't coast as fast.
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Old 02-07-2007, 12:26 PM   #10
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I have but one large thought: A famous man we all know about (L. DaV.) dropped two masses of different weights from the Leaning Tower of Piza (ONE "z", pizza fanatics!) and discovered...two different masses fall at the SAME RATE !
So, drop a piece of feather and a brick...

On the moon it would work, and NASA did that at one point, but on Earth the air drag buggers things up. Specifically the drag/weight ratio. I believe that Mr. DaVinchi had to use two identical boxes with different weights inside each.

Take a car, and take the same car but with four people in it. The drag force is essentially the same on each (small increase in rolling resistance, but let's do this on the highway where air drag dominates). But the second car is heavier, and as we know F=m*A. Same force, different mass -> less decelleration -> longer 'glide'.
But, you'll use more gas getting up to speed so lighter is still more efficient (unless you have to move four people, then carpooling is good)

cfg83 - your point about picking up extra mass at a higher elevation is something I think I have noticed. I go by a junkyard frequently that is at a higher elevation than home by a few thousand feet. Sometimes I will add significant weight to the car at the junkyard ( ) and it *seems* that I get better round-trip economy when I do that. Bear in mind we could be talking about 100lb of metal in the trunk here. No hard data though, but the theory is sound as long as you don't have too many rolling hills to go through on your way to a lower elevation.
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