One for the experts

[notadriver]

Can anyone tell me which uses less energy - a slower lower powered climb uphill or a full power climb to the summit which takes less time ?

 

[O L Leigh]

According to the bods who set up the simulators for NXEA at Stratford, a lower power notch would use less juice.

When the sims were launched we were all sent on a training and familiarisation session which included an energy-efficiency challenge. We all drove the same route and our driving and power usage analysed. I had been lead to believe that the best option was to give it the beans up to linespeed and then coast as far as possible, so this is what I did. My power usage curve was appalling and not far from what they called "The Stig Curve". I had erroneously thought that using "weak field" (notch 4) used no more power than "full power" (notch 3), but that turned out to be wrong.

Of course I think it does depend on the traction because the characteristics of the traction equipment may have different effects on power usage, so the lesson I learned on the Cl315 sim may not apply to, say, Networkers or Desiros.

Diesels should be more straightforward because the higher notch you use the higher the fuel consumption. Whether a lower power notch for longer would use less fuel to get up, say, Bromsgrove Bank than just opening the taps wide and getting up marginally faster I simply could not say. It's not a question of miles per gallon but litres used per minute in each notch and, frankly, that's not something I can answer.

O L Leigh

 

[DownSouth]

Generally, the efficiency drops as the power being delivered increases. It therefore follows that the 'ideal' way to miminise fuel consumption while ascending a big hill would be to work up a good amount of speed on the flatter areas before the ascent starts (which can be done on the flat without using the top power notch), and then selecting a notch that will provide enough power to get the train up the hill but not enough to maintain the starting speed, so the speed will gradually wash off as you trade speed for height and you'll reach the summit going fairly slowly.

However, what that doesn't take care of is the fact that there are a number of valid reasons for going faster (people might not like using trains if they are slow, staffing costs, the infrastructure operator wanting to sell someone else a path behind you) and also valid reasons for going faster but not too much faster (e.g. sustained periods at full power burning out DC motors, required infrastructure/signal upgrades, pricing operators off the rails). What that leaves you with is an equation of many variables with no simple answer you can apply to all situations.

Taking the approach which absolutely minimises fuel consumption will almost never be the correct approach to completely minimise the cost of running the train - unless you build your own private tracks to avoid being dictated to by timetables, go to full driverless operation to avoid staffing costs, and carry only freight that's not time-sensitive. That sort of railway already exists, the track gauge is commonly around 16.5 millimetres and you can find them set up in spare bedrooms and garages.


It's not an issue unique to going up a steep hill though. On the Trans-Australia Railway (the flattest and straightest railway in the world) drivers will sometimes have to go at below the line speed if they are hauling a double-stack intermodal train with DC locos and they get a strong head wind - to avoid going through a set of traction motor brushes in every trip.

 

[O L Leigh]

Generally, the efficiency drops as the power being delivered increases. It therefore follows that the 'ideal' way to miminise fuel consumption while ascending a big hill would be to work up a good amount of speed on the flatter areas before the ascent starts (which can be done on the flat without using the top power notch), and then selecting a notch that will provide enough power to get the train up the hill but not enough to maintain the starting speed, so the speed will gradually wash off as you trade speed for height and you'll reach the summit going fairly slowly.

Yes, that answers the theoretical question nicely. Get a good run at the foot of the hill and then trade off all that kinetic energy as you climb the gradient so that you come to a stand just as you crest the summit.

However, the practical application of that technique is very limited because your destination is rarely at the top of the hill, so you have to consider that saving energy on the hill will be traded off by using more once you're at the top.

As an example, a Cl170 hitting the bottom of Bromsgrove Bank at 80mph on full power will crest the hill at 45-50mph depending on conditions. Yes you could select a lower power notch and crest the hill at a lower speed to save fuel, but then you have to accelerate again up to linespeed from a lower speed which will burn more fuel if starting from a lower speed.

I would have to say that judging a hill will depend on where it is relative to other features such as station stops. For XC services coming up Bromsgrove Bank I would say that the most efficient way of getting up is to burn the fuel on the hill to maintain as much of the momentum and speed as possible as a trade-off against having to accelerate on the flatter sections later. It's not a question of the most efficient way of tackling a gradient but how that gradient fits into the section of route as the gradient itself cannot really be considered in isolation.

O L Leigh

 

[142094]

That is what I've wondered as well, is it better on a relatively straight and flat section of track to whack it up into Weak Field and coast for a long period of time or notch up at a slower rate but keep it in Full Field?

 

[DownSouth]

As an example, a Cl170 hitting the bottom of Bromsgrove Bank at 80mph on full power will crest the hill at 45-50mph depending on conditions. Yes you could select a lower power notch and crest the hill at a lower speed to save fuel, but then you have to accelerate again up to linespeed from a lower speed which will burn more fuel if starting from a lower speed.

I would have to say that judging a hill will depend on where it is relative to other features such as station stops. For XC services coming up Bromsgrove Bank I would say that the most efficient way of getting up is to burn the fuel on the hill to maintain as much of the momentum and speed as possible as a trade-off against having to accelerate on the flatter sections later. It's not a question of the most efficient way of tackling a gradient but how that gradient fits into the section of route as the gradient itself cannot really be considered in isolation.

O L Leigh

Thanks - that's a perfect example of what I was talking about with real world train driving practices involving an equation of many variables with the 'best' answer always involving most areas (e.g. speed, fuel, wear and tear) being compromised to some degree.

I would be interested to chat to some drivers of DC locos who drive up the hill from Adelaide to Mt Lofty on a regular basis to see how they handle it. This line has a 13km incline with two short segments of level track in the middle with the rest being between 1/50 and 1/45. 13km of continuous climbing (the trains are longer than either of the flat segments) is easy for the train drivers I know (who drive only AC locos at that operator) but it would probably take a fair bit more skill to get the most out of a DC loco going up there.

 

[Legzr1]

Someone answered this question during the 1970s oil crisis.


Save fuel by simply making all you journeys downhill.

Perfect.

 

[Jamesb1974]

I suppose it all comes down to train type and load.

For example, a 12 car set of 325 postal units will breeze along the section of line between Nuneaton and Stafford at line speed in notch 2, where as a 66 with a 2300 ton sand train hanging off the back gets thrashed in notch 8, just to get up to (and maintain) 60mph.

Whilst that section of line is not flat, it doesn't have overly aggressive gradients.

It's all relative.

 

[cossie4i]

When I passed in 91 you was always told to accelerate as hard as possible to line speed.

 

[rebmcr]

Lets say there's a hypothetical scenario where there's a station start at the bottom of a hill, and you want to be going linespeed after the summit. There's probably a 'sweet spot' where the curves of "time spent taking power" and "energy consumption per second" intersect on the graph.

Whether that 'sweet spot' is further down the notches, or high up at the top, or somewhere in the middle, will entirely depend on the traction in use. I would imagine with a diesel-mechanical, that spot is somewhere between the middle and highest notches, whereas electric traction wouldn't surprise me to be right up in 'full power'.

Once you factor-in the impact of timetabling, that probably raises the desired notch quite a bit on top of the pure energy-efficiency analysis.

 

[Domh245]

I had erroneously thought that using "weak field" (notch 4) used no more power than "full power" (notch 3), but that turned out to be wrong.

Is there a similar acceleration between those notches? I vaguely remember something about DC electrics where the two notches have much the same effect up to about 40mph at which point notch 3 would equalise, and notch 4 would allow to continue up to max speed. Would it be different with AC electrics? It certainly seems different with modern "drive by wire" traction, as drivers will often slam the controller into max until they reach line speed.


I remember watching something on youtube about driving efficiently on the railway, I think it was this-
[youtube]watch?v=ca2lGLI_ICs[/youtube]

 

[edwin_m]

Weak field on a DC motor will increase maximum speed but reduce torque, so the acceleration will also be less.

 

[Domh245]

So going straight into weak field will in fact reduce the acceleration? Does this make it harder for drivers, you have to resist the temptation to whack it straight up but instead you have to time it just right.

 

[chuffchuff]

So going straight into weak field will in fact reduce the acceleration? Does this make it harder for drivers, you have to resist the temptation to whack it straight up but instead you have to time it just right.

Putting the controller into Weak Field allowed twice as much amps to the motors, as they went through series and parallel and finally into WF
On DC EMU's anyway

 

[edwin_m]

DC EMUs notch up automatically under the control of a current relay to keep the current through the motors within limits. So it was possible to put a SR EMU into weak field when standing still and it would run through all the notches and only actually get to weak field when running close to maximum speed.

 

[cossie4i]

DC EMUs notch up automatically under the control of a current relay to keep the current through the motors within limits. So it was possible to put a SR EMU into weak field when standing still and it would run through all the notches and only actually get to weak field when running close to maximum speed.

Weak field comes in around 40mph.

You can notch a 400 series unit up and hold a set power level (series and parallel), you used to drive them unlike all these modern units.