Weight reduction - not always good?

[landspeed]

I was just thinking about it...

Pulse and glide is the best way to increase engine efficiency (versus driving at a constant speed in gear).

If your car had a mass of 1 gram, but was the same size, and you also weighted 0 grams, then...

If you drove at 60mph in gear, you would put out 20% less energy as there would be no rolling resistance - however, the engine efficiency would decrease due to using it at less load.

If you came out of gear, the car would decelerate to 2 mph in about 2 seconds, as the aero drag would completely overwhelm the momentum of the car.

If your car weighed 100,000 tons, then your FE would be about the same at 1 or 1000 mph, but, would again be very low due to low RR.

So, in reality, weight reduction is not necessarily good - in reality, there will be a point of maximum efficiency, which will change based on average speed.

I have no idea where this point would be, but it would be interesting to find a way to work it out!. I guess, you would need the RR, CdA, engine efficiency at load, engine efficiency while cruising, engine efficiency while idling, mass of the car, and do some complex maths to work it all out!

[bbgobie]

Ummm... If your car is super heavy you need more force to accelerate it. Yes it can hold momentum better which is what I think your getting at, but the over all picture of mass is miscalculated here. The energy requried to accelerate a 1 gram car is minimal. It'd take my whole tank of gas to accelerate the 100,000 ton car to 60mph.

[ma4t]

When I had my BMW 540i, rear wheel drive, 280hp V-8, with a custom automatic racing tranny, I found out really quickly that weight reduction was a bad thing in the winter. It also had a safety feature that was really annoying. The rear wheels would stop moving if it felt the slightest slip. So I'd be sitting there at a green light in light snow, with the little warning light flashing, and the car wouldn't move an inch. That was really annoying in rush hour, for me and everybody behind me.

Finally, a mechanic friend gave me two 5 gallon buckets of misc parts. I kept one above each rear wheel so I could still drive in the snow. I also added bags of salt. That probably hurt my FE a bit. LOL.

It was a great summer car, though. Here it is in downtown Salt Lake. See TRAX in the background, the light rail. That saved me tons in gas and parking. It was free for students!
http://img.villagephotos.com/p/2005-10/1100567/bmw540i.jpg

m

[Snax]

I don't think that the extreme examples are really applicable to the real world. I can't quantify it, but I think that relative to engine displacement, there is a sweet spot in weight that is in modern car design, without fail, less than the car weighs. This is because based on the gross mass of cars and trucks, aerodynamic drag is really next to nil relative to momentum below 40mph. It would take a huge reduction in mass to find the point where returns are significantly diminished, barring a corresponding reduction in engine displacement to match.

[96hb]

I believe weight reduction would only hinder you (based on your theory) if you are coasting more than you are accelerating or cruising, which I don't believe is the case with most of us. And unless you are taking out a great deal of weight, I don't believe it helps much at all. I drove my Civic with full interior for a tank and got about 47mpg. Then I gutted everything out but the driver's seat and dash for a tank. MPG ended up about the same. So I will probably be putting the seats back in this weekend. All that stuff equals up to 100 lbs or so. Didn't make much difference in my case, based on a couple fill-up's.

[landspeed]

Ummm... If your car is super heavy you need more force to accelerate it. Yes it can hold momentum better which is what I think your getting at, but the over all picture of mass is miscalculated here. The energy requried to accelerate a 1 gram car is minimal. It'd take my whole tank of gas to accelerate the 100,000 ton car to 60mph.

(I have also read the other posts above).

What I was getting at is a 'sweet spot'. In fact, the idea is that there is a sweet spot for economy. The 1 gram car will actually be worse (aero drag stops you coasting at all), and the 100,000 ton car would be rubbish (due to massive rolling resistance).

The question is, where is this 'sweet spot' - it will be different for each car, and it is pretty unlikely that the car designers will try to achieve it.

The best idea would probably be to reduce aero drag as much as possible, and fit low RR tyres, keeping momentum, but using low RR tyres, and spending most of the time working on Aero rather than weight. Although in stop-start driving the other way round (work on RR *and* momentum) would be more appropriate.

Interestingly.... If RR was the same on the 1 gram and 100,000 ton car, then, the 100,000 ton car would get the best economy if driven on a very very long straight road. You could put the energy into it, at 40% efficiency, then coast the rest of the way, while the 1 gram car would need the engine to be on all the time, so it would be maybe 22% efficient, so only get half the MPGs :D

[bbgobie]

Drag would be the same on heavy and light car.
Assuming no rolling resistance the force to keep both cars at a constant speed is the same isn't?

So the variable is acceleration, and F=ma. Maybe I'm not getting it, but I dont think there is a sweet spot, or at least not a realistic one that can be reached. If you could strip 10% off the mass of your car, I dont think it would be less efficient, except the winter driving case.

[Bill in Houston]

On the gently rolling hills north and west here, a really heavy car could P&G with the flow of traffic, since a heavy car will keep its speed up well on a slight hill (as compared to a light one).

But, your mileage would get killed in traffic...

[rvanengen]

Hmm...this brings back memories of when I used to drive a bus. We were on a charter trip, and after fueling my bus, I found that the batteries were dead. We needed to move the bus away from the fuel pump since it would be a few hours before the repair crew would arrive. One of the older drivers told me to pop it in neutral (this is a Greyhound size bus!) and laughed when I looked at him like he was crazy when he said we would just push the bus a few hundred feet away on level ground. ;)

Well...we did push it, and I had to RUN once it was up to speed to get back in the driver's seat. It was MUCH MUCH easier to push that bus than ANY car I have ever owned. Talk about a lack of rolling resistance...and this is a 28000lb bus! This tells me that it was less than a horsepower to overcome RR on this vehicle, but on the interstate, it is almost 100% air resistance (shaped like a beveled brick).

[landspeed]

Drag would be the same on heavy and light car.
Assuming no rolling resistance the force to keep both cars at a constant speed is the same isn't?

So the variable is acceleration, and F=ma. Maybe I'm not getting it, but I dont think there is a sweet spot, or at least not a realistic one that can be reached. If you could strip 10% off the mass of your car, I dont think it would be less efficient, except the winter driving case.

The three variables are:

Rolling resistance ( = weight * rolling_resistance_coefficient)
Momentum (is proportional to weight) (related to kinetic energy)
Aero drag.

So, the heavy car means you can 'charge' up the momentum, and then coast for ages, whereas the light car has no momentum to charge.

I suppose what I'm saying is that....

If you could half the RR coefficient on the tyres *OR* half the weight of the car,

then you should half the RR coefficient on the tyres - because then you will have half the RR but the same kinetic energy which you can use in a 'hybrid' fashion.

[lca13]

Doesn't conservation of energy prohibit this? You cannot accelerate any mass and decelerate it in such a manner as to gain energy or keep it a constant, you will only lose it.

So while it seems the P&G idea yields much longer glides with heavy cars, and it does, it takes more energy to get to speed.... more than you gain in return in either heavy or light cases, and since there is waste in the process, the greater mass case must waste more energy than the lighter mass case.

[Bill in Houston]

You are right about the conservation of energy. It takes a certain amount of energy to get a vehicle from point a to point b at a given rate. Sooo, if you just set the cruise, your engine will run in a band of 20% to 30% efficiency as you go up and down hills and such. BUT if you did some kind of perfect P&G, you could get all of your energy at maybe 35% efficiency.

You have to expend the same amount of energy, it's just whether you get it all at 25% efficiency or 35% efficiency.

[cfg83]

Hello -

How about a sweet spot in terms of engine efficiency? If the car is super light, it may be easy to accelerate, but maybe you are not in the sweet spot of the engine's operating efficiency. But, you would have a shorter glide.

If you added weight until you reached the "sweet spot" of engine efficiency, the longer glide may result in a higher "pay back" and thereby justify the extra weight.

Segway/Hijack : Ideal Commute Scenario
Let's say your main commute has an elevation difference. On the way down, there is always "stuff" you can put in your car to add weight and therefore give you longer glides. That "stuff" would not be in your car on the way back (delivering it?), so you would be lighter going uphill.

Example : Offer to drive someone to work, but drop them off "at the train" for the ride home, or visa versa.

Crazier example : Drop someone off at the train on the "up" run. Pick them up at the top. Drive them home without the train on the "down" run.

CarloSW2

[lca13]

Interesting thought experiment i have to admit. Something doesn't click in my head though because still, basically, your deceleration rate (linear with respect to your mass) is the inverse of your acceleration rate (also linear WRT mass).... or acutally, the energy of 1 and 2.... so I guess I am stuck on, "You may have longer glides, but only because you spent the $$ of front to get to speed"... you still lose .

But, the efficiency thing is interesting. Let's say you have 100% efficiency on accelertion and 0% consumption on deceleration. Now, how does varying the mass change things? Greater mass = longer glides but you still lose because of the acceleration penalty and thus always win with less mass.

Forgetting all that for a minute though.... acceleration in the car is always the worst efficiency... A/F ratios are dropped so that puts a damper one aspect of the idea.

Nevertheless, it sure "seems" like the idea has merit. But I can't get past this annoying, "you can never save energy later by burning more earlier" problem.

[lca13]

Here's another thought.... the railroad industry certainly would know the answer to this question, is a train is the ideal experiment vehicle. I would be very surprised if they said that adding train cars acually helps mileage....

but hey, maybe that is the case and they do optimize their load with the engines becuase there is a sweet spot or window.

[basjoos]

Here's another thought.... the railroad industry certainly would know the answer to this question, is a train is the ideal experiment vehicle. I would be very surprised if they said that adding train cars acually helps mileage....

but hey, maybe that is the case and they do optimize their load with the engines becuase there is a sweet spot or window.

Its not the same thing. Adding more railcars adds more than just weight,it also means adding more axles, thus more rolling resistance (and also more aero drag from the additional cars). A diesel-electric locomotive has an entirely different concept of "sweet spot" compared to a direct-geared, gasoline ICE powered car.

[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).

[JanGeo]

Yeah you need enough weight to maintain a coast downhill without stopping thus NOT using the engine at all and that alone give the greatest efficency since you don't run the engine. Getting up the hill doesn't matter as much so it comes down to the hills that you have to deal with. Now if you have steep hills then less weight would be better since you have sufficent mass to maintain downhill speed. You also have to factor in the aero drag with the weight so you don't have to run the engine to get down a hill.

[88HF]

Its still only efficient to accelerate an object just large enough for a given situation. If you accelerate a Cadillac to highway speed rolling resistance and wind are going to take energy out of the vehicle, if you lose speed in a smaller car due to wind resistance you still put less energy in to accelerate and there is less to take away. Unless you are fighting constant hurricane force winds I don't think there is an argument for heavier car = more FE. More dense vehicle maybe. If you are carrying a huge light air bubble its not going to be as efficient as same mass vehicle in smaller package.

If you are talking about what I think you really are then the larger the vehicle and engine size the more efficient it will be to carry a larger optimum load. basically a school bus is more efficient at transporting kids than 5 cars, much less 20 or 30. Maybe its peak efficiency would be carrying 500 kids, or sawing off the back and hauling bags of cement. The point is small cars are more efficient than wasting extra energy on moving more mass, but public transport is more efficient because its less energy per unit(person).

[Snax]

I think that makes more sense than what I said. :P

[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]

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]

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).

[landspeed]

Back to my original argument.

(1) Start by realising that pulse-and-glide is the most efficient driving technique on flat ground.

Get three identical cars (e.g. Honda Accords).

One of them has been modified, and weighs 1 gram.
One of them is 'normal' in weight
One of them has been modified, and weighs 10 tons.

The 1 gram one is not that efficient. You have to stay in gear all the time, so get a lot of engine drag. If you go into neutral, aero drag stops the car in about 0.1ms :)
The 'normal' one lets you pulse and glide -> the most efficient one
The 10 ton one is not that efficient, because the rolling resistance is too high.

So, somewhere between 1 gram and 10 tons, there is a 'sweet spot'. It is probably not at the weight of the 'normal' one, although that one is closer.

If you use an electric motor in your car, then the ligher the better, because the efficiency is (pretty much) constant, and so you just want to reduce RR and aero drag as much as possible to get more efficiency.

[Bill in Houston]

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.
I think that this is the problem. Efficiencies vary quite a bit. A lot more than I would have expected. And in ways I would not have expected. Have a look at this chart.
http://img.photobucket.com/albums/1003/thaticktockman/3VZbsfcwtrans.gifIf you can operate at 237 vs 300 or 400 or 500, then you can get your kinetic energy by burning less gas.

[brucepick]

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....

Yes however with a super light car (or a lighter one) you would give it a smaller engine because that would be sufficient for real world needs. Huge engine in small car (as in sports car) would be very fast but if aiming for FE, you'd be at very low throttle most of the time so you'd be far from the sweet spot.

[88HF]

You may be able to "never hit the brakes" in theory but when a gust of wind sets you back all the same, you are having to regain 4800lbs worth of momentum in the heavier element. Same gust of wind and you only have to accelerate 200lbs in the lighter. You can what if the road was 0 friction all day and still the lighter car is going to win in theory. Someone is going to read this thread and start telling people that they read on gassavers that you can "in theory" get better gas mileage in a heavier car in the mountains.

[lca13]

Yeah, I know, this is desperately starting to sound like a perpetual motion argument. Sure a 100 HP engine in a 50 lb car probably won't do any better than in a 100 lb car, because the waste energy is large WRT to energy expended for motion, and the 100 car might do better using P&G for that reason.

But at some point.... some point lower than I believe most here are thinking, the return on investment starts following the linear degradation curve.

Does anyone really think that shaving 500 lbs off their car will hurt mileage?

Bill, I agree with your chart, but that is not the same as taking the same engine, chassis, transmission, final drive, etc and changing the mass around it.

Gads, I am sure this can be show mathematically, its just that it has been a while and when one is away from that stuff, one usually gets it wrong on the attempts rather than right :-(

[lca13]

Another real world consideration is that real driving can never be pure pulse and glide... there will be some amount of steady state, engine load cruising, and don't think for a minute that you are maintaining a stead speed. Instead you are constantly accelerating and decelerating around your target speed, and because you are not 100% efficient in gasoline PE to KE conversion, you continually lose in direct proportion to your mass.

[Bill in Houston]

You may be able to "never hit the brakes" in theory but when a gust of wind sets you back all the same, A gust of wind DOESN'T set you back all the same. The heavy one will not be slowed as much as a light one. That's part of the point.

[97tercel]

Has anyone ever racer a derby car in the boy scouts? the cars are all the same weight and depending on the design of the car and where the weight is placed each car will travel at different speeds down an incline, some much faster than others but all weighing the same.

[baddog671]

You have to also take into account the terrain the vehicle travels, since not all of us live on flat land. For me, the two biggest fuel consumers are speeding and big hills. If you are going to the extremes of many tons, a car like that on a hill would consume so much gas that the more removed would lessen the strain on the engine. Personally, I have removed nearly 100 pounds off my car and it is now stronger and better on mpg.

Going up the hill to my gf's house I used to have to put it in second, half way shift into third, and then back into second shortly after the rest of the way. After 50 pounds I did second, shifted into third about 10 feet sooner, and didnt have to shift back into 2nd till nearly the top. Now at 95ish pounds, I can make it clear to the top on 3rd.

Its faster on hills with the same rpm= a big improvement even if not FE based.

[baddog671]

Edit^^

Course my car is a Metro with a 1.0L. Stock at 1902 pounds with no options, it could use about a 400 pound diet.

[97tercel]

So for cars weighing the same positioning the weight just right will yeild longer glides

[Bill in Houston]

Ya, baddog, the kind of hills where a little more weight might theoretically help would be hills where you would never hit the brakes on the way down. More weight is a sure loser if you are having to press the brakes going down hills.

[baddog671]

Bill, I live in Western Maryland and the "hills" I'm talking about, I drive differently than "declines". Coming down a decline I generally put it in neutral and just coast until I either hit a light,traffic, or speed up becuase Im going to slow.

A hill (a mountain if you will, but these are the Appalachins) I may ride the brakes nearly the entire way down or "pulse" the brakes. Otherwise I would gain way too much speed and loose control of the vehicle. But coming up these hills suck soo much gas since they are so steep.

[baddog671]

Edit* I thought you were disagreeing with me so I was defending myself.

[lca13]

The derby analogy only applies if you are fortunate enough to drive downhill everywhere you go.

I had a girlfriend once that use to jog a circular route around her neighborhood and claimed it was a great workout because it was uphill the entire way :-)

[Snax]

I know that neighborhood. Here's the house of a friend that lives there:

http://www.gassavers.org/attachment.php?attachmentid=622&d=1182609881

[landspeed]

Yes however with a super light car (or a lighter one) you would give it a smaller engine because that would be sufficient for real world needs. Huge engine in small car (as in sports car) would be very fast but if aiming for FE, you'd be at very low throttle most of the time so you'd be far from the sweet spot.

I was talking about weight reduction without changing the engine - most people on here don't change engines.

However, if you did massive weight reduction, and put in a 2 cylinder tiny engine that worked at the best efficiency, it would be better.

[Bill in Houston]

I had a girlfriend once that use to jog a circular route around her neighborhood and claimed it was a great workout because it was uphill the entire way :-)She should have just run the other way... :-)

[baddog671]

I was talking about weight reduction without changing the engine - most people on here don't change engines.

However, if you did massive weight reduction, and put in a 2 cylinder tiny engine that worked at the best efficiency, it would be better.

Yugo's had a 2 cyclinder, 1.1 Litre if I remember correctly. They weren't very well designed though and were less powerful than my 1.0 L. Wonder how the FE was though...

[landspeed]

Yugo's had a 2 cyclinder, 1.1 Litre if I remember correctly. They weren't very well designed though and were less powerful than my 1.0 L. Wonder how the FE was though...

They actually had a 45-horsepower 4-cylinder (Yugo 45A), and they were the butt of many jokes in Europe!. They were basically old-design Fiats (can't remember which one), so were very out-of-date. The economy probably wasn't too bad, but they were carburettor. Other 'recycled' Fiat car companies include Lada, and 'FSO' (a polish car company making the 'Polonez Prima'. This was so rubbish that, after 6 years, the body panels would actually rust through and fall off :eek: I remember, 15 years ago, I saw a 1988 one on a paper-round I used to do, with duck-tape along the seams of the wings etc, because they had completely rusted through and were about to fall off.

The Fiat 126 BIS had a 2-cylinder engine, and it was light, and very small (so a low frontal area). Cd wouldn't have been that good but CdA probably was. With some aero work it would probably be unbeatable, especially as an electric car.

http://en.wikipedia.org/wiki/Fiat_126

[baddog671]

Hmm, there must be a differnt style here in the states. I looked it up in a book at work yesterday and it said 1.1L 2 cylinder..

I think I could live with 45hp lol. My metro is rated 55hp at the crank but thank god its manual. I've driven the larger 1.3L but automatic and they are complete dogs.

[landspeed]

Probably different to the ones I used to see in the UK :)

You don't get them now because they have all rusted away!

[baddog671]

Yea, I think they were discontinued in the late 80's anyways. Ive never seen one in person..

[samandw]

Sorry to resurrect an old thread, but I'd like to make a point I didn't see made in the preceding replies.

Given two cars identical accept for the weight, the heavier car will have a greater ratio of weight to aerodynamic drag. When coasting down hills, the propelling force is a proportion of the weight of the vehicle. Where aerodynamic drag is the predominate slowing force, a heavier car could maintain a faster speed down a given slope. Why is this important? Well, because if one car maintains 52 mph EOC'ing down a given slope of the freeway, and the heavier car maintains 60 mph down the same slope, if the driver wants to maintain the minimum 60 mph in order to avoid causing a traffic disruption, guess which car burns less fuel? You guessed it, the heavier car, because the light car will require the engine on and at light throttle to maintain minimum speed (inefficient area of the poweband), while the heavy car will have the engine off.

Just some food for thought.

[white90crxhf]

Hello -

How about a sweet spot in terms of engine efficiency? If the car is super light, it may be easy to accelerate, but maybe you are not in the sweet spot of the engine's operating efficiency. But, you would have a shorter glide.

If you added weight until you reached the "sweet spot" of engine efficiency, the longer glide may result in a higher "pay back" and thereby justify the extra weight.

Segway/Hijack : Ideal Commute Scenario
Let's say your main commute has an elevation difference. On the way down, there is always "stuff" you can put in your car to add weight and therefore give you longer glides. That "stuff" would not be in your car on the way back (delivering it?), so you would be lighter going uphill.

Example : Offer to drive someone to work, but drop them off "at the train" for the ride home, or visa versa.

Crazier example : Drop someone off at the train on the "up" run. Pick them up at the top. Drive them home without the train on the "down" run.

CarloSW2


Just put a huge water tank in your car at work and fill it up there, then empty it when you get home.

if the drag & the engine are the same i don't see how making the car heavier will work unless it's a 1 way trip downhill.

The heavier car will gain more energy going down the hill, and will lose more going up the hill. The lighter car will gain less energy going down the hill, and lose less energy going up the hill.

Also wouldn't more weight equal more friction with the road?

[Snax]

> weight = > rolling resistance

[usedgeo]

I have tried to decide whether I do better with my wife along or without her. I am not sure. Why not take several friends with you and see if it helps.

It has been said that P&G is a poor man's hybrid. The heavier your car the bigger your pseudo battery. ;)

I decided a long time ago that the extra battery in my trunk is not a penalty.

[RoadWarrior]

Ya, baddog, the kind of hills where a little more weight might theoretically help would be hills where you would never hit the brakes on the way down. More weight is a sure loser if you are having to press the brakes going down hills.

In my experience the kind of hills where weight would help and you can pick up some good momentum to make it up the other side... are the ones cops are waiting at the bottom of.... I figure on just engine braking enough to keep it in injector cutoff.

These situations argue well for a compressed air or regen braking electric hybrid system though, tempting to rig a lawnmower motor as a compressor/motor to each rear wheel with a tank or two under the car... have an extra push/pull hand throttle, for moderate braking or acceleration when you need it.

edit: another thought. I wonder if a full size spare tire spun on a good bearing up to 2000 rpm (I figure it would hold up to that) weighing about 30lb, would hold enough momentum to shove a 3000lb car up to 10mph and let you pop the clutch in 2nd to bump start, for a 5 minute hold at a red light. Might be doable with an AWD back end transmitting power on braking to the spare on a bearing in the wheel well... needs clutches...

[bzipitidoo]

I'm thinking of weight as a battery. Instead of storing energy in a chemical battery, you're storing it in a "kinetic battery", like a flywheel. In so far as that lets you optimize your engine usage, that helps fuel economy.

But that's all that more weight does for you. Everything else is negative. Rolling resistance is higher, both from the bearings and tires. It takes even more energy to climb hills and accelerate. If your engine actually operates closer to its optimum thanks to that, then the solution is not to add weight but to tune the power train. Put in a smaller engine, or taller gearing, or both then you can better reap the benefits of less weight.

This thought experiment isn't all that good. Very good from the scientific point of view of holding all other variables steady while changing just one. But you can't just toss 500 pounds and expect to get the full benefit of that weight loss if your engine is still turning at the same rpms it was before the weight reduction, and still sucking down the same displacement as before. You'll get some benefit, but not as much as you should. This fact will skew where that sweet spot is.

[8307c4]

This thread being about FE as far as weight is concerned, I do believe lighter is better.
There are some issues concerning handling, specifically the sprung vs. unsprung weight, as it is possible to negatively affect your vehicle's handling, possible I said.

When I had my BMW 540i, rear wheel drive, 280hp V-8, with a custom automatic racing tranny

I would think so long necessity is not the reason for engine size, fuel economy is no concern.