Adding Weight to improve FE

[kgwedi]

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?

[RobertSmalls]

This thread covers it fairly well. Weight, where is the loss?

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.

[cr45]

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.

[Old Tele man]

...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)

[brucey]

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.

[euromodder]

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.

[comptiger5000]

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.

[Varn]

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.

[jamesqf]

Originally Posted by kgwedi

Bullets are made out of lead (or depleted Uranium) so they go farther for the same amount of initial power than lighter bullets.

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.

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 (????)

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.

[kgwedi]

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. :-)