Too good to be true?

[Spacecadet]

In principle it costs nothing to move from A to B, where A and B are at an equal potential (e.g height). You just have to try to minimise heating up the car and environment when you move, which isn't possible in most cases. So in practise, and especially for that particular car, flip knows.

in principle thats rubbish - you need to accelerate and deaccelerate a mass which takes energy

[Spacecadet]

Why is it that a high performance car going slow uses more fuel than a low performance car at the same speed? Is a larger engine just that much more innefficient? How come my 2.0 N/A petrol S2000 isn't as good on fuel economy whilst motorway cruising as my 2.0 N/A petrol Primera despite the latter being heavier and less aerodynamic?

Did you not see the top gear episode recently, BMW M3 vs a Toyota Prious. All the BMW had to do was follow the Prious round a track. The BMW burnt less fuel.

I'm seeing more and more of these Prious hybrid vehicles on the motorways, lots of them being driven at 80mph+
At those speeds over a long distance a diesel would probably return a better MPG

Infact I've heard it mentioned a few times that a Prious would probably be more efficient if the (heavy) electric motors and (heavy) batteries were ripped out and it just relied on the petrol engine.

[Fred Bloggs]

Environmentally, IMO, the Prius is a carbon footprint disaster. Doubtful if any car ever made has a higher cradle to grave energy footprint. As usual, it's all smoke and mirrors.

[DimPrawn]

Environmentally, IMO, the Prius is a carbon footprint disaster. Doubtful if any car ever made has a higher cradle to grave energy footprint. As usual, it's all smoke and mirrors.

WHS++

[TimberWolf]

in principle thats rubbish - you need to accelerate and deaccelerate a mass which takes energy

The work done from going from A to B (at equal potential) is zero, even if it is hard to achieve 100% efficiency in practise. Regenerative braking recovers some acceleration costs in some vehicles.

With a frictionless air-free tunnel you could in principle travel from any point on the Earth to another without energy cost (and get anywhere in 42 minutes IIRC). In practise some friction would be present. A siphon operates in similar way, carrying water over a hill (where it had to be accelerated up hill beforehand) and down the other side.
e.g. http://en.wikipedia.org/wiki/Gravity_train

[VectraMan]

Did you not see the top gear episode recently, BMW M3 vs a Toyota Prious. All the BMW had to do was follow the Prious round a track. The BMW burnt less fuel.

IIRC the M3 still only managed something like 19mpg, and the Prius being driven like on a track day managed about 17mpg. If both had done 30mph down to the shops, the M3 probably still would have done 19mpg, and the Prius an awful lot more.

Interesting test though.

[Spacecadet]

The work done from going from A to B (at equal potential) is zero, even if it is hard to achieve 100% efficiency in practise. Regenerative braking recovers some acceleration costs in some vehicles.

Its not just hard, its simply impossible. Any sort of mechanical device is going to incurr frictional losses.
The most efficient travel for a single a->b stationary to stationary journey will involve acceleration to a limit and then using the natural friction within the system for decceleration - i.e coasting to a halt.
It is by the laws of physics impossible to recover 100% of the energy expended.

With a frictionless air-free tunnel you could in principle travel from any point on the Earth to another without energy cost (and get anywhere in 42 minutes IIRC). In practise some friction would be present. A siphon operates in similar way, carrying water over a hill (where it had to be accelerated up hill beforehand) and down the other side.
e.g. http://en.wikipedia.org/wiki/Gravity_train

Siphons still need energy to get them started and you need a height difference between the start of the hose and the end.

[TimberWolf]

Its not just hard, its simply impossible. Any sort of mechanical device is going to incurr frictional losses.

Let's bring you up to date with Newton: an object will continue in uniform motion unless acted on by an outside force. The Voyager spacecraft will continue on their merry way for instance even if there were a piston on board, but to address your strawman I never said doing mechanical work was lossless, I said no work is done in going from A to B.

The most efficient travel for a single a->b stationary to stationary journey will involve acceleration to a limit and then using the natural friction within the system for decceleration - i.e coasting to a halt. It is by the laws of physics impossible to recover 100% of the energy expended.

No, high speeds are bad news were air is involved since air resistance goes as the square of the speed, beyond quite modest speeds. Slow is better. The acceleration and coast technique works for the MPG record setters with the constraints they have, but I never said a car running on petrol was efficient. Quite the reverse, I said cars and petrol engines were hopelessly inefficient. You can see how efficient even these record breaking cars are though compared to conventional ones from the MPG figure I gave. They go slowly.

Siphons still need energy to get them started and you need a height difference between the start of the hose and the end.

Sure, to get them started. Losses and height differences actually due to friction, without which a difference in height would not be needed and no input would be needed. Experiments of this sort have been done with liquid Helium.

[Spacecadet]

Let's bring you up to date with Newton: an object will continue in uniform motion unless acted on by an outside force. The Voyager spacecraft will continue on their merry way for instance even if there were a piston on board, but to address your strawman I never said doing mechanical work was lossless, I said no work is done in going from A to B.

Any object drifting through space is using no energy, but thats completely useless, its not travelling through my point A and point B.
Secondly voyager has travelled from point A (earth) and is going to point B (whatever it crashes into or starts orbiting), from my recollection large amounts of rocket propellant were needed to get it going on its merry way.
For me to get into a vehicle and travel from point A to a point B to get off again, no matter where those points are, unless there is a potential difference in energy to be taken advantage off (e.g. going downhill) then there will have to be energy expenditure.

No, high speeds are bad news were air is involved since air resistance goes as the square of the speed, beyond quite modest speeds. Slow is better. The acceleration and coast technique works for the MPG record setters with the constraints they have, but I never said a car running on petrol was efficient. Quite the reverse, I said cars and petrol engines were hopelessly inefficient. You can see how efficient even these record breaking cars are though compared to conventional ones from the MPG figure I gave. They go slowly.

Are you agreeing or diasgreeing, its impossible to work out

Sure, to get them started. Losses and height differences actually due to friction, without which a difference in height would not be needed and no input would be needed. Experiments of this sort have been done with liquid Helium.

So to get the fluid moving you need energy, glad you agree with that.
And to keep it moving with no potential difference between start and finish requires a super fluid and in this case will also need a tube which is cold enough to allow the super fluid to remain a super fluid.
Its not called a super fluid for nothing

[DimPrawn]

You canna change the laws of physics Captain.

Except on CUK.