kgtm - Track Access Charges and Electric Power Draw

[Dungrange]

I'm looking to ascertain the typical energy draw (in kilowatt hours) of an electric locomotive hauling a freight train and the Office of Road and Rail (ORR) and Network Rail (NR) publish data that looks like it could answer my question, if I could understand what the numbers mean: in particular the term "electrified kgtm", which the footnotes describe as ** “electrified kgtm" in relation to each electrified railway vehicle, means a mile travelled on the Network, by each tonne of the aggregate weight of the electrified railway vehicle in question divided by 1,000.

 

https://www.networkrail.co.uk/wp-content/uploads/2019/03/Traction-Electricity-Modelled-Consumption-Rates-List.xlsx

 

For example, Table 4 in the above spreadsheet states that a Class 92/0 locomotive consumes 40.523 kWh / kgtm (row 498).

 

A Class 92/0 locomotive is 126 tonnes, so does that mean that a Class 92 locomotive draws 40.523 * 126/1000 = 5.1 kWh per mile travelled?  However, I'm presuming that would be the average energy of a light engine movement, rather than the energy required to move a heavy freight train (which would presumably be more).  I'm also assuming that it's probably not correct to say that I just replace the locomotive weight with the train weight in that equation to calculate the energy draw of a thousand tonne freight train.

 

If anyone can point me in the direction of a 'worked example' for calculating energy draw, I'd be grateful.

[rodent279]

Kgtm- kg train miles?

I.e. weight of train in kg, X distance in miles.

B@st@rd mixed units again.

Unless it's kg train metres?

[Jeremy Cumberland]

 

9 minutes ago, rodent279 said:

Kgtm- kg train miles?

This. It says so at the bottom:

* “electrified kgtm" in relation to each electrified railway vehicle, means a mile travelled on the Network, by each tonne of the aggregate weight of the electrified railway vehicle in question divided by 1,000

[rodent279]

So it is b@st@rd units.

[Jeremy Cumberland]

1 hour ago, Dungrange said:

A Class 92/0 locomotive is 126 tonnes, so does that mean that a Class 92 locomotive draws 40.523 * 126/1000 = 5.1 kWh per mile travelled?  However, I'm presuming that would be the average energy of a light engine movement, rather than the energy required to move a heavy freight train (which would presumably be more).  I'm also assuming that it's probably not correct to say that I just replace the locomotive weight with the train weight in that equation to calculate the energy draw of a thousand tonne freight train.

 

I am as capable of making mistakes as the next person, but something looks wrong to me. If it really is 40.523 kWh per kilogram train mile, then you need to multiply by the number of kilograms and by the number of miles to find the energy usage. However,

40.523 kWh / kgtm x 126,000 kg (weight of locomotive) = 5,106,000 kWh per train mile, which seems way too high. At 60 mph, this energy would be used in one sixtieth of an hour, so we can use energy in kWh divided by time in hours to get the average power. This gives us a whopping 300 GW, or about 100 times the output of Hinkley C, when it is built.

 

Therefore I suspect that their per kg shoud actually be per thousand tonnes, and our light engine travelling at 60 mph operates at an average power of 300 kW (or 450 hp). This, at least, is in the right ballpark.

 

[Dungrange]

2 hours ago, Jeremy C said:

I am as capable of making mistakes as the next person, but something looks wrong to me. If it really is 40.523 kWh per kilogram train mile, then you need to multiply by the number of kilograms and by the number of miles to find the energy usage. However,

40.523 kWh / kgtm x 126,000 kg (weight of locomotive) = 5,106,000 kWh per train mile, which seems way too high. At 60 mph, this energy would be used in one sixtieth of an hour, so we can use energy in kWh divided by time in hours to get the average power. This gives us a whopping 300 GW, or about 100 times the output of Hinkley C, when it is built.

 

Therefore I suspect that their per kg should actually be per thousand tonnes, and our light engine travelling at 60 mph operates at an average power of 300 kW (or 450 hp). This, at least, is in the right ballpark.

 

I agree with what you have said.  The stated unit for the 40.523 is kWh / kg tm, so I agree that multiplying by the weight of the locomotive (or technically it's mass) in kg should give an answer that is in the units of kWh / train mile.  I agree that would give 5.1 GWh of energy per train mile, which means that when travelling at a mile a minute, this would use 306 GW of power.  I agree that a number of that magnitude doesn't make sense.

 

However, the footnote states that it "means a mile travelled on the Network, by each tonne of the aggregate weight of the electrified railway vehicle in question divided by 1,000".  If you take the weight of the locomotive in tonnes and divide by 1,000 then you don't get kilograms: you get a number in thousands of tonnes, kilo-tonnes or Gigagrams.  That therefore means that the stated units and footnote appear to be inconsistent and it would appear that kgtm should actually be Gg tm.  It almost appears as though someone thought that since a kg is one thousandth of a tonne, that dividing tonnes by 1,000 gives kilograms.  That is what is confusing me. 

 

If I assume that the energy consumption is 5.1 kWh / train mile, then if travelling at 60 mph, the power would be 306 kW.  The Class 92 Wikipedia article states that the "maximum power is 5 MW (6,700 hp) on 25 kV", so that is well within the maximum.

 

If I try to work back from that then a Class 92 travelling a 60 mph and consuming 5 MW of power, would be using 300 MWh / train mile.  If I then divide that by 40.523 kWh per kilo-tonne train mile, then I think that implies a total weight of 7,403 tonnes, which if the locomotive itself weights just 126 tonnes, then that implies a Class 92 could haul a trailing load of 7,277 tonnes, which seems rather high.  Admittedly, the issue is with starting a train from rest, so perhaps if a 7,000 tonne train was already in motion at 60 mph, then a Class 92 could continue to keep it moving at that speed.  I'm not sure that it answers my question, but at least your thinking is consistent with mine.

[Jeremy Cumberland]

10 hours ago, Dungrange said:

technically it's mass

I find that multiplying by the weight is a lot easier. Given a large enough set of scales, I can measure the weight. It would be exceedingly difficult to calculate the mass.

 

Remember that this is not like Newton's laws of motion, and we do not have a precise mathematical relationship. Energy usage depends on a number of things. One of the most important is rolling resistance, and this is approximately proportional to weight, but it is only a rough approximation and there are lots of other factors that are only indirectly related to weight, if at all. The numbers in the table are some sort of average, and probably don't really warrant being expressed to five significant figures

 

You cannot use these figures (together with the locomotive's maximum power) to work out the maximum speed a locomotive can haul a particular weight of train or the heaviest train a locomotive can haul at a given speed; this is not their purpose.

 

[The Stationmaster]

16 hours ago, rodent279 said:

So it is b@st@rd units.

Which is inevitable when different systems of measurement are used for different things so as railways are measured in miles and train etc loads are measured in metric tonnes to will get the mix.  Not really a problem if you're used to working with the two systems which many railwaymen have been for several decades.

[Nearholmer]

This whole subject is a nightmare of mixed units, freight even more so than passenger, and that is terrible!

 

In my full-time working life, I used to deal with comparisons of this kind for urban passenger transport (taxi, bus, tram, metro, and heavy rail), which is a lot less befogged by problems with units than freight is, and even that took some thrashing to get a common system applied across all the bodies in the international benchmarking body in question.

 

Good units for comparitive purposes are:

 

- J/Tonne.km for freight; and,

 

- J/Passenger.km for passenger.

 

It is worth the bind of converting whatever is presented to these units for comparison, if possible. It isnt always possible, because figures get provided by "train" (what is one of those?) or by "freight car" (ditto), and the load factor is often seriously hard to unearth for both passenger and freight. 

 

The EU publishes good, solid, well-based figures expressed in sensible units, ditto UITP, UIC, and Comet (the international benchmarking body for Metros). The EU stuff is particularly good, because it allows very easy comparison between modes, and for personal cars theirs are based on real field data about vehicle occupancy, country by country. I will try to remember where in the EU Babel Tower Library it all resides, 'cos like all gems it is hard to find!

 

Thats all good for benchmarking, intermodal comparison, specific CO2 emmissions etc, but it is different from the engineering calculations used to select ratings for motive power, traction power supply infrastructure, fuel consumption by diesel locos etc., which are much more about basic equations of motion, and knowledge of system losses etc. At least the engineering calculations all follow internationally accepted units, except where the USA gets involved!

 

Having jabbed at the US, this is a particularly good introduction to what is being discussed here, even if it is all expressed in US units. Birmingham University teaches the equivalent module in the UK, and although I have their material, I don't think it is in the public domain, so I can't link to it.

 

https://www.slideserve.com/adele/module-5-tractive-effort

 

 

 

 

Edited November 20, 2021 by Nearholmer

[rodent279]

It's just something that grates with me, after having had to be fastidious with units whilst studying. I'm actually pretty fluid in both, I still use miles & chains when referring to railway distances, but it is in fact remarkably easy to convert, 1 chain being near as dammit 20m.

I can still hear the Engineering Science lecturer saying "Units Mr Rodent! You look after your units, they look after you".

That, and "Only a thousand out".

 

But we are wandering into full on, un-matriculated, dimensionless thread drift.

 

(Yeah, I know, I started it).

Edited November 20, 2021 by rodent279