Weight reduction - not always good?

[bbgobie]

The other thing to consider is more and more people on this forum seem to be using WOT or near it. Shouldn't matter if your car is light or heavy, your forcing your car to run at peak efficiency. The only difference is with heavy your spending more time in WOT to get your speed up.

The extra force required to speed up a heavy car I dont think can outweigh the extra coast. Don't forget, if 2 cars start from stop, accelerate to 60 and coast, the light one reaches 60 first as well.

The more accelerations needed, ie traffic I think the more the difference would be.

[lca13]

The "downhill" factor can change things and make it look like you get better FE with a heavy or sweet spot car... I am still not convinced you win overall.

Longer glides are a factor of increased mass, as the deceleration is the wind resistance force divided by mass (Accel = 1/2*Cd*A*p*V*V). But you can't get to speed without overcoming the same intertia.

My guess is that, since we only tend to glide on downhill runs, and heavy cars coast much longer, we "think" we are benefiting. If we were behind the car pushing by hand to get to speed, however, we wouldn't be so excited.

I agree that P&G works as you are not wasting energy during glides. But I still think that for a particular P&G scenario, you will always win by decreasing mass.

Again, I admit I might be missing something, but I need someone to show me some aspect of the physics that I am missing. The observations of "much longer glides" would need to tie back to some aspect of the force equations showing mass is at least a wash within the experiment.

Bill, anywone, what I am missing?

[Bill in Houston]

Try this on for size. I have two Honda Elements. One weighs 5000 lbs with me and my stuff in it. The other weighs 200 lbs due to the extreme lightening measures I have taken.

In the heavy Element, I drive over to Austin at 90 mph. Because it is so heavy, I can shut off my engine, or just coast in neutral, on the relatively shallow downhills, and then apply power going up hills. My engine runs about 50% of the time at about 40% efficiency.

In my featherweight Element, I take the same trip, also at 90 mph. I don't really notice the lower rolling resistance because I am going at a speed where aero losses are a huge percentage of my overall power requirement. Since my car weighs 200 lbs, it has very little kinetic or potential energy, and I never ever get to coast. So, my engine runs at 25% efficiency 100% of the time.

In the contest above, my heavy E gets better mileage than my light E.

A few assumptions are required to make this work.
-I am able to operate my engine at 40% efficiency when I want to (i.e. I know where the best point on the BSFC chart is and I can always hit it.
-I am driving at a speed where rolling resistance is small compared to aero resistance.
-I pretty much never ever hit the brakes.

I think most of us could buy that if the assumptions are followed, I really could do better in the heavy Element.

If you buy all that, LCA, then my work is done.

BUT, the real world is different from my little situation I made up. We don't go 90 mph, and we do have to touch the brakes, and we can't operate right on the max efficiency point. So the question is, on the particular trip you are making today, what is your optimum weight? For stop and go, drive the light Element. For a nice long highway trip on rolling hills, maybe drive the heavy E.

[lca13]

I get the idea, but the numbers we are throwing out are pure conjecture. It would be nice to have some experimental data to support them, or theory to predict them.... I am not saying they are wrong, just failing to understand the basis.

The more I think about this the more I think it can't be true, with two exceptions:

1. The sweet spot does exist, but only because at some low weight point all energy expenditure to waste is large WRT to energy used to travel (this is similar to the "best MPH for MPG" phenomena).... at this point decreasing mass does not help any more.

2. Downhill may play a factor that I do not understand yet, but only if the entire trip has a net downhill, in which case the heavier car may get an advantage.

Maybe item two here is what I am missing.... that you get more "gravity assist" in the heavier car versus lighter and thus end up better.... this should be demonstratable in the math though..... I suppose that is next... actually setup the equations and then differentate WRT mass.

[lca13]

ok, I am dense, dense, dense on this.... Intuition tells me you are all right and I am missing something.

So the argument is, even though it does take more energy in the greater mass case, you can generate it with such an increased efficiency that the new energy lost level is less than that for which must be added due to the increased mass ??????

Is this the argument?

[Bill in Houston]

Yep!

[slurp812]

Originally Posted by landspeed

lca13 :

In a light car, the acceleration will take less time, but the coast will be shorter too. If the car only weighed 1 gram, then the coast would be non-existent.

So, looking at extreme cases (very heavy car and very light car), it can be seen that they are bad for economy, but, in the middle somewhere will be the 'sweet spot'. The question is, where is that sweet spot?. I know that when I drive a Nissan Micra (small car), the coasts are not as good, and yet when I drove a Toyota Avensis in the same way, the coasts were brilliant and I got a much much better fuel economy (the avensis had LRR tyres, I think, and it was very heavy, and had good aero).

A really light car gets super mileage just driving normal. Coasting is a way to take advantage of all that precious fuel burnt accelerating a heavy car. If you have a super light car, no need burning all that extra fuel to get it moving

[lca13]

So in the actual car case, however, the argument won't work though, since the efficiencies at the various loads are going to be pretty much the same. The acceleration loads are always at worst efficiency.

So take the 2000 lb car with one pulse and glide. It takes some 1/2mV*V of energy to get to speed, after which you will glide some distance d. The 1000 lb car will take half of the energy to get to speed, and will glide some distance less than d, (but not necessarily half of d... or will it)???

This describes an easy test I think. If you halve the weight, you can do two pulse and glides for the price of one. Which of these cases allows you to travel farther? It only works if the heavier car goes futher in one P&G than the lighter car does in two.

[landspeed]

Originally Posted by slurp812

A really light car gets super mileage just driving normal. Coasting is a way to take advantage of all that precious fuel burnt accelerating a heavy car. If you have a super light car, no need burning all that extra fuel to get it moving

If you have a super light car, then the engine is on all the time, so, you will be wasting a lot of power to turn the engine over, with a tiny bit of fuel for actually moving the car.

If the car is heavier, you can 'charge up' the car's kinetic energy, with the engine being used more efficiently, and switched off otherwise.

Overall, a long journey might take 1 megajoule (complete guess).

At 20% efficiency, you burn 5 megajoules of petrol to get 1 MJ of energy (light car)
At 40% efficiency, you burn 2.5 megajoules of petrol to get 1MJ of energy (heavier car)

So, in this example, the heavier car gets better economy.

Eg. my car - driving style is the main mod, with minimal aero mods that have mostly come off now, and I get nearly 100% above EPA

[landspeed]

PS for electric motor cars, the lighter the better! The above discussion is only regarding internal combustion engines which work more efficiently at greater load (to a point).