Adding Weight to improve FE
 

I have been driving this month with an extra 20 gallons of water in containers in my Metro.
My driving style is minimum idle, minimum brakes, and pulse and glide with engine off. Extra weight makes my coasting noticeably longer, with a slight increase in power to overcome the extra mass and rolling resistance. If my Metro weighed many tons, it would coast for a long time because aerodynamic drag would be a small percentage of the kinetic energy available.
Bullets are made out of lead (or depleted Uranium) so they go farther for the same amount of initial power than lighter bullets.
If you watch a soaring championship, the gliders carry about 100 Gal. (800pds) of water as ballast. This gives them a better sectional density. CDa/Mass (????)
Doing an A-B-A test with different pulse lengths and coast lengths would be very difficult to get right.
I often read about people removing things to save weight, and here I am adding weight. I feel that my FE is better with the extra weight. What I really need is an extra 600pds to see a definite change.
If extra weight is good for gliders, and bullets, why isn't it good for me and my Metro?
Has anyone done testing to see what real effect extra weight has on P&G and FE?

This thread covers it fairly well. http://ecomodder.com/forum/showthrea...oss-13556.html

If you add weight, your car will have more kinetic energy at any given speed. This means longer glides, but equally longer pulses. If that shifts you into a regime where you can P&G better, as it did for Brucey, then it's a good thing overall.

For most people, the increase in rolling resistance, and the dramatically increased amount of energy wasted to braking (when your glide goes longer than you'd like) mean that adding weight is bad for FE.

I suspect that in certain situations extra weight may be beneficial. If we ignore the rolling resistance the retardation force during the glide at a given speed is given by

force = 0.5*density*CDA*velocity*velocity

also from Newtons Second Law

force = mass * acceleration

therefore

acceleration = force / mass

acceleration = 0.5*density*CDA*velocity*velocity / mass

where velocity is m/s
mass is kG
density is kG/m^3
CDA is m^2

If you double mass the deacceleration force halves and the glide time doubles between the two speeds.

...summary:

1) it takes *energy* to get more mass (weight) up to speed, and...
2) more *energy* must be dissipated to stop an increased mass (weight)

From the other thread:

"I don't doubt it is making a difference. But in this car, in this terrain, with this driving style, it really seems to average itself out and make no noticeable difference."

I also would like to add here that it seems possible to be helping, still. In certain conditions and proper technique I think weight can be useful for mileage.

My car doesn't like city traffic and accelerations, so I removed weight to help it in those situations.

On the highway at a constant speed, the weight doesn't seem to bother it much as my best FE ever was achieved with well over 200kg of extra load.

As Euromodder said, city vs highway is the key. In the city, with lots of stop and go, weight will typically be bad. However, on open rural roads, or on the highway, where you can maintain a fairly constant speed (or P&G), the extra momentum can smooth things out and help. However, if you're doing P&G, it may or may not help, depending on the vehicle.

Adding weight is a negative overall for a long distance. It increases the rolling resistance and increase the energy to accelerate.

If your route were to allow for a decrease in altitude you might have an advantage.

Not so. There is a lot of wasted energy in the propellant, that's what makes the bang & muzzle flash. The heavier bullet absorbs more of this energy in its trip down the barrel. Now (assuming that two bullets have the same muzzle velocity) the heavier one will have the greater kinetic energy, which means it'll go further because aero drag doesn't depend on mass.

But they're getting their energy for free, from the thermals. The heavier glider will have a lower rate of climb in a given thermal, because it takes more energy to lift the glider to a given altitude. But once it's at altitude, it has greater potential energy, and so can glide further.

With a car, you're not getting energy for free (unless you live at the top of a mountain), so you lose with more weight.

I (politely) :-) don't agree with some of your comments jamesqf.
Yes the time the power is applied on a projectile will determine the final energy, that is why I said "initial power", not powder.
The same amount of "power" provided in a gun (or car) "should" give the same muzzle energy (not velocity) regardless of the mass of the bullet. The lighter bullet will be going faster at muzzle exit, but will slow down faster because of the drag to mass ratio (sectional density). I wonder whether a car will glide farther with the same amount of power (gas used) applied to accelerate the car. The velocity will be less in a heavier car, so the energy/mass ratio should be the same.

With the glider in a thermal, the amount of energy taken from the thermal to raise the glider is the same regardless of the mass of the glider, there will just be a difference in altitude gained. The overall increase in potential energy will be the same regardless of mass.

Thanks to everyone for some very good explanations. I think driving style and terrain will determine whether there is any noticeable gain in FE with the addition of extra mass.
I am still carrying extra weight to improve my FE. :-)

Sean Costin a record holder in bicycle racing told me a story about how they added 70 pounds of weight to their streamliner for battle mountain a number of years ago.

Battle mtn has a 200 foot drop in the 5 miles of run up. About the limit of what the rules allow. They were hoping that the increase in weight would improve their top speed on the flat section 1km and 200m that follows. The gist of it was that their top speed was the same. No free lunch.

The only way you could consistently improve your mpg by adding weight would be adding the weight at the top of a long downhill grade, and driving back up the hill without the weight. Of course, rivers do the same thing, for free.

It might help on the highway in a hilly area if it adds enough momentum to reduce the number of times you downshift on a hill, allowing the engine to be kept in a better BSFC range.

The problem in comparing a bullet and a car adding weight is aerodynamics. A heavier bullet will be longer, for a given diameter, than a lighter bullet. Adding weight to your car is internal and doesn't make it longer and more aerodynamic. At long range competitions they are shooting very heavy-for-caliber bullets, but it is more to reduce wind drift than anything else.

In town with the stop and go weight will hurt you. On the highway it doesn't make much difference, just a little on the rolling resistance. That is somewhat offset by increasing the momentum and allowing you to coast a little further.

A-B-A with different weights.
 

I finally decided to some A-B-A tests with different weights in my Metro.
Because I do P&G it would be very hard for me to be consistent with regards to acceleration rates and shifting gears. I opted for increasing the distance to help even out the inconsistencies.
I did my testing using LOD of 34 to accelerate up to 55 MPH then coast engine off till 40 then accelerate again. I drive this road often and have specific markers to start coasting and to start accelerating when it is more efficient. I started outside of one town and stopped before the next town so there were no restrictions from stop signs or lights. I drove totally in the emergency lane so traffic was not a factor at all.
I got some canvas bags, and filled them with sand (454 pds). With the spare tire and other thing in the car the weight I removed between tests 510 pds. My two places (E and M) to stop and turn around were 22 miles apart, and had a 200 hundred foot difference in altitude. The wind was blowing from E to M at 8-10 MPH, and remained steady throughout the test. I filled the gas tank, and put all tires at 44 PSI. Here are my weight load, average speeds and MPG for each run.

E -> M Light-- 46 MPH 81.6 MPG
M -> E Light-- 43 MPH 54.8 MPG
E -> M Heavy- 44 MPH 87.4 MPG
M -> E Heavy- 43 MPH 53.2 MPG
E -> M Light-- 45 MPH 86.5 MPG
M -> E Light-- 43 MPH 54.6 MPG

This shows almost no advantage to either being light or heavy. I tried another test that was A-B-A going on a more level road only in the one direction. This leg was 9.7 miles.

E -> A Heavy- 43 MPH 73.5 MPG
E -> A Light-- 44 MPH 75.0 MPG
E -> A Heavy- 44 MPH 76.8 MPG

I conclude that with my driving style, it doesn't matter whether I am heavy or light. Being light makes it easier to P&G because less shifting is required. When heavy, I used third gear to accelerate from 40 MPH.
Only with large altitude differences will I be concerned about weight in the future.

We use a similar concept in rocketry to determine the optimum weight to achive maximum altitude... often, adding weight will increase the final altitude. However, I think the system is different enoguh from automobile FE that it is difficult to draw any conclusions. For one, there is only one pulse and one glide on a rocket, rather than a repeating cycle.

I guess the takeaway is that there is no hard and fast rule about weight. In general, lighter will be better... however, there may be cicumstances under which you may benefit from an increase in weight. My thought is that these conditions will be highly specific, difficult to indentify, and unlikely to be extrapolated to "general use."

A contest may be an area where enough of the variables are known that you can make some improvements, but daily driving is such a crapshoot that it'd be a heroic effort. Not that that should stop us from trying. :D


(Random thoughts... would a lighter bullet undergo increased acceleration from the same amount of propellant, therefore exiting the barrel more quickly and experiencing less total force than a heavier bullet? Thus, a lighter bullet may actually have less total energy for a given propellant mass and barrel length than a heavier bullet. As can't think of a corrolary of this with respect to cars, the bullet analogy might not work.)

Thanks for taking the time to do the test. You said that you used third gear when heavy, does that mean you didn't use third gear when you were light?

500+ lbs more or less is a serious difference in weight for a car the size & weight of a Metro.
I'm a bit surprised that it doesn't show more clearly than just 1.5 mpg going from M to E.

It does however show the effect of wind and - more importantly - the change in altitude.
A whopping 30 mpg :eek:


It also shows that variation in this kind of real-world testing can be quite high - 5 mpg for the same situation.

IMO this kind of real-world testing simply cannot bring out the minor effect of small mods, as the testing environment isn't stable enough.

Not necessarily. If you have three bullets, one lead, one depleted uranium, and one lead free, all the same size & shape, they're going to have different weights.

You are absolutely right. But considering that 90+% of the bullets used in the world are basically lead (DU is real hard to come by for some reason!:)), with some small amount of copper on jacketing them, they are basically considered lead.

Changing density but not changing shape is what kgwedi did by adding weigh inside his car but not effecting it's aerodynamics. And I applaud his diligence in his testing, no small effort that. And predictably, on a highway run, the weight mattered little. In town I am sure he would have seen a much greater effect, and the weight would show a big disadvantage in most situations. The only place I see weight coming in as an advantage is if you encounter intermittent resistance such as small hills, or snow and slush, that the weight helps you plow through without having to power through, but rather you use the kinetic energy you have already input into you car to get you where you need to be.

I usually use the highest gear possible without lugging the engine. I consider 1600 rpm to be the minimum. I used to use BSFC graphs, but found that in practice a slower turning engine and higher manifold pressure to be better. That is based on experience, and not science. :-)
For day to day driving I use 1st, 3rd, and 5th only.
For my A-B-A test I used 4th also.

Seems like you would be adding wear to the main bearings of your engine by not using all of your gears when you accelerate to speed.

Thanks for the write up.
Theoretically, if your on level ground, with predictable losses, starting from 55mph, with a 100%efficient engine and drive train; you should see no difference. The question then becomes: what is the engine doing. This is what could favor one weight over the other.
First off; the frequency of starting and killing the engine will slightly favor the lower frequency of the car with greater mass.
Second, the need for greater power to accelerate the higher mass car may effect eff of engine. Though, by expecting slower response and doing everything the same (gear/throttle position) this should be almost identical for both.
Third; aerodynamics, The higher mass car will sit lower to the ground. This may or may not be a good thing, but I would give the high mass car better odds on this one.
But in life; you must start from a stop, and there are hills, and people driving behind you. Acceleration is clearly bad with the high mass car, which will balance out gains that may have been possible.


As for the bullet comparison: I think people are missing some rather important points on this. If you have a identical lead bullet and a solid copper bullet with the same power behind them (in a theoretical "perfect gun") then they will both have the same power at the muzzle. The copper bullet will be going much faster. Does anyone here know anything about aerodynamics and speed? Really? So the faster bullet will have far more drag on it, taking away more energy, until it reaches the same speed as the lead one. But at that point it will have less energy because it has less mass, so it will continue to slow at a greater rate.
With the metro you have the same speeds, and different masses. So less powder is needed to get the copper bullet up to the same speed as the lead one. But the lead one will go further, as it has more energy. In a perfect world: It would take exactly the same amount of powder to send the lead or copper bullet one yard in this experiment.

Not true. In theory (and in practice) rolling resistance is proportional to vehicle weight. The question is whether the improvement in engine efficiency from easier P&G outweighs the increased rolling resistance. For most, the answer is no.

oh yeah, forgot that one. The tire patch will be proportionally larger on the higher mass car. And there is also mechanical rolling resistance, and this is one that could go either way. It is far more likely to favor the low mass car (less weight on the bearings) but depending on the suspension more mass could make the universal connector have to bend less and reduce resistance there (or make it worse).
There are really too many variables to accurately predict any outcome of this, but as always the key variable is the driver. It take time to get good at P&G with a new car, and that's basically what the high mass mod is. On this point, if you can get the same mpg right from the start, would a few months of practice give you better mileage? there is really only one way to find out.
If we look at it in the real world I think the differences between the high mass and low mass car will be minute, with the exception of getting up to speed. A little city driving could really kill a tank's number.

Varn - As long as you're not revving overly high or lugging the engine too low, it has no effect on the engine. In some vehicle, the tranny ratios are spaced closer than they need to be, to optimize for heavy loads, or fast acceleration. When neither is needed, some gears can be skipped without any issue.

Yeah you are right, it will work for a while but why not use the entire range. Shifts can be made at even lower rpm by not skipping. I bought one of the first new Toyotas with a 5 speed (73). To turn it in to a 3 speed just doesn't make sense.

It seems to me that, all other things being equal (aerodynamics, etc.) except for weight, the simple fact is it takes more work to move the extra weight. It takes more gas to do more work. All this stuff about extra glide distances doesn't come for free. It takes the extra work to get the extra glide. And since the real world isn't perfect, there's plenty of inefficiency in the engine working and the car gliding. Under some conditions, the glide side may show an advantage, but overall, day in and day out, it takes more work to move more weight, and the lighter car should get the better mpg.

This is for the most part quite true, but remember; with the exception of rolling resistance, it takes the same energy to keep a identical car of any mass moving at a constant speed. If you where to load your car with 800lbs of junk and inflated the tires proportionally (assuming they were not all ready at max pressure) you would get nearly the same mileage on a long highway trip. But if you had to stop and start many times in the trip you would suffer greatly.
With proper technique, for some people, more weight could work as an advantage for long trips at a relatively constant speed. This it not true for most people or situations, but may be for some. Notable elevation changes would add another consideration to this. I commuted down about 1000 feet over 12 miles every day. I was planning some water tanks, but I came in to a better solution; I moved in to the valley.

Following is a terribly uncontrolled comparison, but I'll toss it up for illustration and discussion. I drove four of us to dinner 75 miles away, leaving from Bryce Canyon National Park at 8000' elevation and going to dinner in Boulder, UT. The extra 450 lbs of passengers in my 2200 lb. xB is a 20% weight increase. The drive has a final 1300' elevation drop to Boulder, but that doesn't tell the real story. There are 12 and 14% grades on the route that necessitated shifting to second gear to climb them. By the time I reached Boulder, I had a 50 mpg trip average per the Ultragauge. At the end of the roundtrip, back in the park, I'd averaged 42.7 mpg. I had to run the AC from time to time, and the lights on the entire trip back. I tried my usual P&G (the passengers were all interested in seeing how I got 50 mpg on the trip out).

Three days later, I repeated the trip, without the passengers, and didn't use the AC or lights, so it wasn't a very good comparison. Nonetheless, the results were much better in both directions without the load. I averaged 55 mpg to Boulder, and 47.5 mpg for the round-trip, about 5 mpg better in both directions. I averaged 140 mpg the first 7 miles to Tropic, UT, and an amazing 71.9 mpg for 48 mpg to Escalante, UT.

I probably could have improved my loaded mpg if I'd been able to concentrate better, didn't run the AC or lights, and learned to compensate for pulsing with the additional load, but I doubt I could have improved it 5 mpg in either direction. The extra load simply overwhelmed my 108 hp. The heavier load meant I simply couldn't accelerate at my usual 83 load. 83 on the Ultragauge had me barely maintaining speed, and I had to mash it into the 90s load range to accelerate on far too many of my pulses.

Interestingly, I still had to drop to second gear to climb the steepest 12% and 14% grades, but there were a lot of hills I climbed in 4th and 5th gear solo that had me a gear or two lower with the extra 450 lbs of passengers (one 230 lb male, one 90 lb female, one 120 lb. female, clothing).

So the conclusion is that extra weight hurts MPG on acceleration/hills, and doesn't matter at cruise/level, right?

Seems pretty obvious, but it's good to have the testing to back it up.

I think with proper technique weight on hills is also negligent. The only time it seems to affect me is stop and go city driving.

As already stated unless you can drop off the extra weight at the bottom of the Hill itadding weight will always be a loss ... In my opinion

there might be more to the story if you were able to Balance the car and see improvement a 50/50 ratio of weight distributed consistent throughout front to back left to right will result in better performance so that should translate to effiiencey.

quick example is pushing weight to pulling it relocating your car battery from the front of your car to the trunk will result in performance gains that is why it is an old racers must do list when hot rodding a car. perfect balance and weight transfer in acceleration will result in gains everytime compared to a stock setup on the racetrack.

Sorry I am not a numbers/ Math guy or even a scientific theory person I understand what I see.

So ask the guy in the 5500 lbs truck trying to climb the hill if fuel mileage has increased with weight :)