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DMU Fuel Economy - Sense Check

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Rhydgaled

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From the topic how far on a tank of fuel I have a report that a 2 car 156 will do 3mpg per set, or roughly 6mpg per vehicle.

Now, I have some figures for a bunch of Northern DMUs:
  • Class 195: 0.81 litres per mile
  • Class 170: 0.97 litres per mile
  • Class 158: 0.72 litres per mile
  • Class 156: 0.68 litres per mile
However, 0.68 litres per mile for the class 156 does not appear to be anywhere near 3mpg. Have I got the wrong conversion formula or are the above figures really in miles per litre?

I've also tried to calculate the fuel economy per vehicle. To do this I calculated the average unit length:
  • Northern Class 195: 2.56896551724138 coaches
  • Northern Class 170: 3 coaches
  • Northern Class 158: 2.15094339622641 coaches
  • Northern Class 156: 2 coaches
I then multiplied the average unit length by the fuel consumption figure above and divided the result by 0.219969157 (to convert to miles per gallon). However, the results are not what I expected. I thought the class 170s would be very poor, but the outcome implies they are actually really economical. Do these figures seem about right?
  • Class 195: 9.46 miles per gallon per vehicle
  • Class 170: 13.23 miles per gallon per vehicle
  • Class 158: 7.04 miles per gallon per vehicle
  • Class 156: 6.18 miles per gallon per vehicle
 
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Fawkes Cat

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If I have read your first sentence and last bullet point correctly, you have got more or less the same answer from 2 different sources for each 156 vehicle. Which therefore suggests your other calculations also accurately translate the opening figures. But I know nothing about whether your opening figures are right.
 

philthetube

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I think your maths are wrong somewhere , multiply litres by 4.5 to get gallons, therefore the 195 would come out at

0.81 x 4.5 gives 3.645 mpg multiplied by two gives a consumption of 7.29 mpg

Sorry if I have it wrong :D
 
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If you want to be very accurate you should then multiply your final answers by 1.xx to simulate real life loadings and weather conditions. I wouldnt have a clue about what .xx would realistically be though, I'd guess .05-10 on average. Also as car lengths increases units should become more fuel efficient due to a reduction in drag per vehicle I believe (need to check my maths books about this though)
 

Dai Corner

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A gallon is about 4.5 litres.

If a 156 uses 0.68 litres to go a mile, it will use

0.68 / 4.5 = 0.15 gallons per mile

Take the reciprocal to turn it into miles per gallon

1 / .15 = 6.67 miles per gallon.

Edit: That's Imperial gallons rather than US ones

Or, more simply, divide 4.5 by litres per mile to give miles per gallon.

For a 170, 4.5 / 0.97 = 4.6mpg which those used to mpg as used for road vehicles would recognise as worse than the 156 .
 
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Rhydgaled

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If I have read your first sentence and last bullet point correctly, you have got more or less the same answer from 2 different sources for each 156 vehicle. Which therefore suggests your other calculations also accurately translate the opening figures. But I know nothing about whether your opening figures are right.
Yes, I ended up with similar values for the class 156, but to do so I had to treat the Northern data (which is captioned as litres per mile) as miles per litre. In other words, to get the end result to come out right I had to assume the source data was in the wrong units. I have no other reason to doubt the accuracy of the Northern data, which leads me to suspect I'm doing something wrong.

I think your maths are wrong somewhere , multiply litres by 4.5 to get gallons, therefore the 195 would come out at

0.81 x 4.5 gives 3.645 mpg multiplied by two gives a consumption of 7.29 mpg

Sorry if I have it wrong :D
Let's test this on the class 156:
0.68 litres per mile​
0.68 x 4.5 = 3.06 gallons per mile​
Note: gallons per mile, not miles per gallon. The value is similar to that given in the other thread, which would suggest the equation is correct, but the units are different.

If you want to be very accurate you should then multiply your final answers by 1.xx to simulate real life loadings and weather conditions. I wouldnt have a clue about what .xx would realistically be though, I'd guess .05-10 on average. Also as car lengths increases units should become more fuel efficient due to a reduction in drag per vehicle I believe (need to check my maths books about this though)
It's impossible to be all that accurate here. The Northern figures are, I believe, an average across the whole fleet in real-world conditions, but include both two and three carriage units (hence my average unit lengths when trying to estimate fuel consumption per vehicle). Also, longer trains aren't simply an extra load of weight which must be shifted around since the frontal area does not increase, giving a fuel saving for longer trains due to less air resistance for a given tonnage of vehicle moved. I'm just trying to get a rough idea of fuel consumption for the class 158 in particular so I can make an argument similar to this:
If I use a 158 for X years, I will consume Y amount of fuel​
If I use a U (something more economical than a 158) for X+Z years, I will also consume Y ammount of fuel​
If I use a U for longer than X+Z years, I will therefore consume more fuel overall than if I used a 158 for X years (and then electrified the route)​

A gallon is about 4.5 litres.

If a 156 uses 0.68 litres to go a mile, it will use

0.68 / 4.5 = 0.15 gallons per mile

Take the reciprocal to turn it into miles per gallon

1 / .15 = 6.67 miles per gallon.

Edit: That's Imperial gallons rather than US ones

Or, more simply, divide 4.5 by litres per mile to give miles per gallon.

For a 170, 4.5 / 0.97 = 4.6mpg which those used to mpg as used for road vehicles would recognise as worse than the 156 .
philthetube above multiplied by 4.5 rather than dividing. Using your direct convertion (4.5 / lpm = mpg) I get:
  • Class 195: 5.556 mpg
  • Class 170: 4.639 mpg
  • Class 158: 6.250 mpg
  • Class 156: 6.618 mpg
Which is more what I expected (195s and 170s have a longer average unit length and are heavier than sprinters, so I would expect them to be poor). However, it means a class 156 is roughly twice as economical as the other topic suggested (around 3mph remember). I'm so confused, maths problems have never been my strong suit.
 

Dai Corner

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philthetube above multiplied by 4.5 rather than dividing. Using your direct convertion (4.5 / lpm = mpg) I get:
  • Class 195: 5.556 mpg
  • Class 170: 4.639 mpg
  • Class 158: 6.250 mpg
  • Class 156: 6.618 mpg
Which is more what I expected (195s and 170s have a longer average unit length and are heavier than sprinters, so I would expect them to be poor). However, it means a class 156 is roughly twice as economical as the other topic suggested (around 3mph remember). I'm so confused, maths problems have never been my strong suit.
Is the 0.68 lpm per unit or per vehicle?

If it's per vehicle and the 156 is 2 vehicles then the mpg for the complete train would be

4.5 / (2 x 0.68) = 3.3 mpg

which is close to the 3 mpg quoted.

(I'm old enough to remember working out the cost of 2lb 8oz of flour at 1/6 a lb and that 'a litre of water's a pint and three-quarters' . My school aritmetic and algebra has stayed with me all these years!)
 
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superkev

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I'd take the above figures with a pinch of salt as it depends on where and how they are used. I suspect the figures are an average from the leasing companys portfolio.
For example a big difference for stop start on Northerns hills to some of the flat lands elsewhere.
When Northern first deployed 170s on the 60mph max Harrogate loop they where reported to be using vast quantities of fuel as they never got out of low gear. So much so Northern next introduced them on the Sheffield Hull Bridlington line.
K
 

Vespa

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Big question here, is it cheaper to run EMU trains than DMU based on operating cost per mile ?
 

NoMorePacers

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I'd take the above figures with a pinch of salt as it depends on where and how they are used. I suspect the figures are an average from the leasing companys portfolio.
For example a big difference for stop start on Northerns hills to some of the flat lands elsewhere.
When Northern first deployed 170s on the 60mph max Harrogate loop they where reported to be using vast quantities of fuel as they never got out of low gear. So much so Northern next introduced them on the Sheffield Hull Bridlington line.
K
They still have to do mid-day set swaps on Scarborough-Sheffield services from time to time at Hull in order to refuel the 170s (so I understand it).
 

Rhydgaled

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Is the 0.68 lpm per unit or per vehicle?

If it's per vehicle and the 156 is 2 vehicles then the mpg for the complete train would be

4.5 / (2 x 0.68) = 3.3 mpg

which is close to the 3 mpg quoted.

(I'm old enough to remember working out the cost of 2lb 8oz of flour at 1/6 a lb and that 'a litre of water's a pint and three-quarters' . My school aritmetic and algebra has stayed with me all these years!)
I've provided more details of the source below, my interpretation of it is that the litres per mile are for the complete train (an average across Northern's fleet, hence the business with the average vehicles per unit to try and calculate an average per-vehicle score).

I'd take the above figures with a pinch of salt as it depends on where and how they are used. I suspect the figures are an average from the leasing companys portfolio.
For example a big difference for stop start on Northerns hills to some of the flat lands elsewhere.
When Northern first deployed 170s on the 60mph max Harrogate loop they where reported to be using vast quantities of fuel as they never got out of low gear. So much so Northern next introduced them on the Sheffield Hull Bridlington line.
K
The Northern data at the top of the thread comes from an FOI response dated March 2020, which says this:
Under Section 1 (1)(a) of the FOI Act we can confirm that we hold some of this information. We do not have breakdowns of fuel consumption by numbers of carriages, only the overall consumption, and these figures are in litres per mile rather than miles per gallon

When did they take the 170s off the Harrogate loop? I had read about this 'stuck in low gear' problem and therefore when the 170 came out as the most-efficient of the four types (at 13.23 miles per gallon per vehicle versus 7.04 miles per gallon per vehicle for a 158) I suspected my calculations were incorrect.
 

NoMorePacers

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When did they take the 170s off the Harrogate loop? I had read about this 'stuck in low gear' problem and therefore when the 170 came out as the most-efficient of the four types (at 13.23 miles per gallon per vehicle versus 7.04 miles per gallon per vehicle for a 158) I suspected my calculations were incorrect.
They never took them off the Harrogate Loop. They started running on Hull services in the May 2019 timetable change though, which might be the information you're looking for.
 

superkev

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They never took them off the Harrogate Loop. They started running on Hull services in the May 2019 timetable change though, which might be the information you're looking for.
Yes the first delivery remains on the Harrogate loop but as more were delivered I seem to remember they were destined for somewhere else than Sheffield Hull etc. Bit slow off the mark too for hilly routes.
K
 

NoMorePacers

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Yes the first delivery remains on the Harrogate loop but as more were delivered I seem to remember they were destined for somewhere else than Sheffield Hull etc. Bit slow off the mark too for hilly routes.
K
I remember Northern were originally going to use them on Leeds-Southport services although that hasn't materialised (and said service does not exist anymore).
 

PG

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Big question here, is it cheaper to run EMU trains than DMU based on operating cost per mile ?
I'd speculate, as I have no actual knowledge, that EMUs will be cheaper.

No heavy engine or fuel tank to lug around for starters plus a DMMU (as opposed to a DEMU) has a less efficient method of transmitting power to the wheels.
 
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