Water 4 Gas

[GreyICE]

True enough, a gallon of gas is about 6 pounds depending on the time of day, and the old saying is not very accurate, but I think it still makes the point. A 15 gallon tank of gas weighs about 90 pounds, and if you could eliminate that weight over a length of time without any other penalty, you'd probably find a small but measurable gain in economy.

Which is mitigated by having to drive to the gas station, pull in, stop the car, fill up to exactly the amount you need, restart the car (starting the car takes a lot of energy) and drive back to where you were before.

Yeah, don't think there's much savings there.

 

[quarky]

as mentioned.

Yet, its a sane idea to consider the weight of that which we choose to shove around, from here to there. On a bicycle, gross tonnage adds up in a very noticable way.
Riding a bike is a good teacher for car driving. It becomes very obvious if the tires are low; bearings damaged. It becomes apparent that , above 40mph, almost all the force is involved in pushing air out of the way.

Too bad we're in such a hurry. Going 40mph is pretty dang efficient, and much less deadly. Life could be good at 40mph, if we could only believe.

 

[tesscaline]

Too bad we're in such a hurry. Going 40mph is pretty dang efficient, and much less deadly. Life could be good at 40mph, if we could only believe.

Um. Where I live, driving 40mph on a residential road is illegal, and the majority of the driving we do is on residential roads.

 

[bruto]

Which is mitigated by having to drive to the gas station, pull in, stop the car, fill up to exactly the amount you need, restart the car (starting the car takes a lot of energy) and drive back to where you were before.

Yeah, don't think there's much savings there.

If you pass right by the station on your regular route, and if, for example, it's a convenience store where you're accustomed to getting your morning coffee or something anyway, then it still might be worthwhile. You wouldn't have to be super-precise. If you found you needed, say, 5 gallons every three days, it would probably pay to get around that amount every three days rather than to fill up a 20 gallon tank as needed.

Starting a reasonably well tuned gasoline engine does not take very much energy, especially when it's warmed up already. I think it's pretty well established that it is more economical to stop and restart than to idle for more than a very short while.

Of course for much of my driving life I've been motivated to run with small fills for other reasons: many of the cars I drive are such beaters that I have to reckon the convenience of a full tank against the possibility that I'll have to abandon it unused.

 

[tesscaline]

If you pass right by the station on your regular route, and if, for example, it's a convenience store where you're accustomed to getting your morning coffee or something anyway, then it still might be worthwhile. You wouldn't have to be super-precise. If you found you needed, say, 5 gallons every three days, it would probably pay to get around that amount every three days rather than to fill up a 20 gallon tank as needed.

Starting a reasonably well tuned gasoline engine does not take very much energy, especially when it's warmed up already. I think it's pretty well established that it is more economical to stop and restart than to idle for more than a very short while.

Of course for much of my driving life I've been motivated to run with small fills for other reasons: many of the cars I drive are such beaters that I have to reckon the convenience of a full tank against the possibility that I'll have to abandon it unused.

I personally think that the greatest benefit of driving around on less than a full tank (i.e. only the gas you think you'll need for the day) is the psychological one. If you know you only have enough gas to get to work and back, you'll be less likely to make unnecessary side trips. And since driving less is the most obvious way to save gasoline... Well, you save.

 

[TMiguel]

I personally think that the greatest benefit of driving around on less than a full tank (i.e. only the gas you think you'll need for the day) is the psychological one. If you know you only have enough gas to get to work and back, you'll be less likely to make unnecessary side trips. And since driving less is the most obvious way to save gasoline... Well, you save.

But it isn’t driving around whit less gas means less weight, and so less gas you need to consume to make the same trip. Although this effect may not seam so visible in short term applications, it is one of the most problematic aspects for example for putting a rocket into orbit, because you need much more fuel just to carry the fuel then to carry whatever you want to put in orbit.
It makes a difference.

 

[Cuddles]

Too bad we're in such a hurry. Going 40mph is pretty dang efficient, and much less deadly. Life could be good at 40mph, if we could only believe.

Slower does not necessarily mean better, since it means you have to drive for longer. Going at around 50-55mph is generally more efficient, and with modern cars the best efficiency can be at speeds as high as 65mph.

 

[Toke]

Slower does not necessarily mean better, since it means you have to drive for longer. Going at around 50-55mph is generally more efficient, and with modern cars the best efficiency can be at speeds as high as 65mph.

My bolding: Driving is risky, the longer you stay on the road the more risk you are exposed to.
Therefore it is safer to drive as fast as possible and minimice your time on the road

Jokes aside, there are mechanical reasons for a minimum economical speed, at least for ships.

 

[TheDaver]

There’s a point at which the efficiency losses from aerodynamic drag will be greater from the gains from gearing the engine any higher. Optimum fuel economy will be had at the engine’s most efficient RPM in that gear.

 

[quarky]

Slower does not necessarily mean better, since it means you have to drive for longer. Going at around 50-55mph is generally more efficient, and with modern cars the best efficiency can be at speeds as high as 65mph.

isn't that because they are designed accordingly, as per engine rpm's and highest gear ratios? It wouldn't be the case with an all electric, would it?
The amount of energy lost to air friction increases with speed much more than the increase of rolling resistance or bearing friction.

(correct me if I'm mistaken)

 

[TMiguel]

isn't that because they are designed accordingly, as per engine rpm's and highest gear ratios? It wouldn't be the case with an all electric, would it?
The amount of energy lost to air friction increases with speed much more than the increase of rolling resistance or bearing friction.

(correct me if I'm mistaken)

One is increasing in a non-linear way the other is decreasing in a non-linear way, you add the 2 effects together and the minimal will be something in between. It depends of the value of which.

 

[quarky]

The force of the wind is cubed by its speed.

I must insist that, with a vehicle designed for optimal fuel economy, you would be able to go farther on the same charge, or gas, at 30 mph than at 40mph, much less 65.

I have no argument that said journey would take longer, and have its own inefficiencies, but I simply can't be wrong regarding the above.

 

[balrog666]

The force of the wind is cubed by its speed.

I must insist that, with a vehicle designed for optimal fuel economy, you would be able to go farther on the same charge, or gas, at 30 mph than at 40mph, much less 65.

I have no argument that said journey would take longer, and have its own inefficiencies, but I simply can't be wrong regarding the above.

You can't be wrong? All hail the new quarky, come to save us from our petroleum-based sins!

What about 13mph, would that be even more efficient? Should all hail the new god, EFFICIENCY?

 

[TjW]

The force of the wind is cubed by its speed.

I must insist that, with a vehicle designed for optimal fuel economy, you would be able to go farther on the same charge, or gas, at 30 mph than at 40mph, much less 65.

I have no argument that said journey would take longer, and have its own inefficiencies, but I simply can't be wrong regarding the above.

I think you'll find that the force -- drag, in this case -- goes as the square of the air velocity. The power will scale as the cube.

 

[quarky]

You can't be wrong? All hail the new quarky, come to save us from our petroleum-based sins!

What about 13mph, would that be even more efficient? Should all hail the new god, EFFICIENCY?

yes, I worded it poorly, and didn't mean to be that obnoxious.

I recall a challenge in the early 70's, I think, to go as far as possible on one gallon of gas in an internal combustion engine. The winners had a honda 50cc motor on a light rig. They took it up to 14 mph, coasted down to 7mph; repeat.
If my memory is correct, they managed 1500mpg.

Obviously not real practical.

 

[balrog666]

yes, I worded it poorly, and didn't mean to be that obnoxious.

I recall a challenge in the early 70's, I think, to go as far as possible on one gallon of gas in an internal combustion engine. The winners had a honda 50cc motor on a light rig. They took it up to 14 mph, coasted down to 7mph; repeat.
If my memory is correct, they managed 1500mpg.

Obviously not real practical.

Neither did I.

I just meant that there are always other considerations than simple thermodynamic efficiency.

I remember the same contest. IIRC, in one case, an entry used a large flywheel and shut off the engine during the coast phase.

 

[The Atheist]

Crazy people.

Motors use petrol while they're going, so the key to petrol consumption reduction is to run the car for as short a time as possible.

Drive as fast as your car will go, never stop at red lights or stop signs and do not slow down for give way signs. (yield signs to you Seppos) Reduce your driving time & save gas!

Drafting is also excellent for reducing consumption, and the best things to follow are fire engines. Wait outside the fire station and call your local emergency service, ringing in a false alarm to one of your neighbours' properties. Stay within 3m (10 feet) of the rear bumper of the fire engine for maximum efficiency. Real daredevils may choose to use a tow-rope. If so, ensure you car is in neutral with the engine OFF.

Saving gas is all about attitude.

Another approach is to buy every fuel-saving device and fit them all to your car. A mechanic friend of mine has Fuelstar, Gaswiz, Ecotube, a hydrogen generator & burner, Maxsys and a Fitch calalyser on his V8 that he gets a net positive saving and he has to siphon his gas tank every few days or it overflows.

 

[Cuddles]

but I simply can't be wrong regarding the above.

You can be, and are.

Just take your argument to the logical conclusion. Going slower is more efficient. Therefore going at 30mph is better than 40mph. And 20mph is better than 30mph. And 10mph is better than 20mph. Now, what would be better than 10mph? See the problem? Clearly it can't be more efficient to go at 0mph because then you would never go anywhere, but would still be using some fuel and would be infinitely inefficient. However, this then destroys your whole argument. Since it is obvious that 0mph can't give the highest efficiency, the best efficiency must be at some finite speed. But there is nothing to distinguish between all the different finite numbers as far as the maths is concerned, there is no ideal car that all real cars strive to be as close to as possible, it all simply depends on the design of the car itself. You could certainly design a car to be more efficient at 30mph than any other speed, but you can also design one that's more efficient at 60mph.

 

[quarky]

I don't think so. the % of power that's needed to move the air out of the way increases geometricly with the speed. Below 40 mph, those losses are a small % of overall frictional loses; above 40mph, they become increasingly significant.

There's little to be gained in making an aero-dynamic horse and buggy; much to be gained in making aero-dynamic sports cars.

In the case of human powered vehicles (HPV) which are necessarily limited by the 'engine' output, this becomes more obvious.

In a vaccuum, we wouldn't be having this argument.

 

[Cuddles]

I don't think so. the % of power that's needed to move the air out of the way increases geometricly with the speed. Below 40 mph, those losses are a small % of overall frictional loses; above 40mph, they become increasingly significant.

And what exactly do you think is magic about 40mph? Why didn't you choose 50? Or 30? In any case, what you, or anyone, thinks is irrelevant. The data exists and it says you are wrong.
http://cta.ornl.gov/data/tedb27/Edition27_Chapter04.pdf
Page 23 onwards.