Weight reduction - not always good?

[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]

Originally Posted by 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.

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.
If you can operate at 237 vs 300 or 400 or 500, then you can get your kinetic energy by burning less gas.

[brucepick]

Originally Posted by landspeed

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]

Originally Posted by 88HF

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.