Atmospheric railway versus conventional railway

[tjej]

There is a new automated people mover (APM) opening in Porto Alegre, Brazil called Aeromovel; it is built using atmospheric railway principles (see http://www.copa2014.gov.br/en/noticia/100-national-technology-aeromovel-reaches-its-final-construction-stages-porto-alegre)

The vehicles runs with steel wheels on steel rails but are propelled by a pressure plate running in a duct below the track. A pressure differential is applied to the plate by stationary electric motors along the track either blowing air or extracting air.

Someone asked me why they don't just use electric motors mounted on the vehicles. The argument for Aeromovel is that because of the 50% to 60% lower dead-weight of the vehicle less energy is required and this compensates for other losses.

Assuming the Aeromovel vehicle is 50% of the weight of a comparable APM with electric motors, obviously KE= 1/2mv^2 means the electrical energy translated into motion is also 50% for the same top speed. Thus the losses for Aeromovel in the converting electrical energy into air pressure and then conveying that pressure along the duct would have to be less than 50%. Assuming the other APM uses regenerative braking to recoup 20% this would fall to 30%.

I started to analyse each loss in more detail such as
1. the loss along the duct using
ploss*= λ (l / dh) (ρ v2*/ 2)
where ploss*= pressure loss (Pa, N/m2), λ*= friction coefficient, l*= length of duct or pipe (m), dh*=*hydraulic diameter*(m)
2. taking into account the greater efficiency (~90%?) of large electric motors compared to (~85%) of bogie mounted motors
3. the higher frictional force on the conventional APM
etc.

My physics is a little rusty so I'm not sure if I'm making this more complicated than it should be.
Also I'm a bit stuck as to the size of motors the APM would need; assume it accelerates at a=3.33 m/s2 to v=10m/s and its weight is m=5,000kg. The time is 3 sec (10/3.33) and the distance traveled is 15m (1/2at^2) but what power would be required?

 

[sophiecentaur]

IK Brunel had an atmospheric railway on the line between Exeter and Dawlish, at Starcross (in Devon) during the 10th Century. The only thing that seriously stopped it working properly was the rats ate the leather seals!
My friend keeps his boat in what was the old pump house, I believe (or next door to it).
Nothing new under the Sun.
N.B In his day, they didn't have the option of Electric Propulsion, which involves far fewer losses!

 

[tjej]

Thanks. I was aware of the precedent. You mean the 19th century. (It would have been really impressive if it was the 10th.)

Rubber seals have replaced leather ones and electric motors have replaced steam engines.

I'm really interested to see if the efficiency of atmospheric railways can be shown on the basic physics.

 

[A.T.]

the rats ate the leather seals!

Rubber seals have replaced leather

Rats and martens love to chew on rubber.

 

[sophiecentaur]

Thanks. I was aware of the precedent. You mean the 19th century. (It would have been really impressive if it was the 10th.)

Rubber seals have replaced leather ones and electric motors have replaced steam engines.

I'm really interested to see if the efficiency of atmospheric railways can be shown on the basic physics.

A nice typo, there. William the Conqueror could have got to Hastings by train, perhaps?

For fun, perhaps but so many unknown unknowns, I think.

 

[tjej]

the rats ate the leather seals!

Rubber seals have replaced leather

Rats and martens love to chew on rubber.

There has been a system running in Jakarta for 24 years and there are no reports of rats being a problem. (martens aren't native to either Brazil or Indonesia so we won't know about that.)

Any comments on the underlying physics?