Weight reduction - not always good?

[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]

Originally Posted by 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.

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