Fuel efficiency DMU compared to loco-hauled

[AlastairFraser]

A rectifier converts AC into DC, not the other way round. The Electric Train Heating standard for BR coaching stock allows for the supply down the train to be DC or AC at quite a wide range of frequencies, but this requires each coach to generate its own AC for whatever might need it. Traditionally this was done by a motor-alternator set, which may be what you are thinking of.

But a DMU would have a similar issue. The engine would drive a generator producing DC or an alternator producing AC, but the frequency of the latter would vary with engine speed. So it would still need a M-A set or in more recent designs electronics to produce whatever voltages and frequencies are needed for auxiliaries.

Yep, sorry the motor alternator set is what I am thinking of. My branch of science is not physics I'm afraid!
The modern power electronics are much more efficient than the older MA sets, no?

 

[43096]

A rectifier converts AC into DC, not the other way round. The Electric Train Heating standard for BR coaching stock allows for the supply down the train to be DC or AC at quite a wide range of frequencies, but this requires each coach to generate its own AC for whatever might need it. Traditionally this was done by a motor-alternator set, which may be what you are thinking of.

But a DMU would have a similar issue. The engine would drive a generator producing DC or an alternator producing AC, but the frequency of the latter would vary with engine speed. So it would still need a M-A set or in more recent designs electronics to produce whatever voltages and frequencies are needed for auxiliaries.

Or you fit auxiliaries that can cope with a range of frequencies, as was done with the HST fleet. These use 415V three phase AC on the stock, so don't need motor-alternators.

North American practice had been to either fit Head End Power (HEP) gensets or to 'lock' the loco in a particular power notch to give a fixed frequency for the 480V three phase supply - this approach was used on Amtrak's P42 locos, for example. The P32AC-DM locos for Amtrak and Metro North were the first to use an HEP inverter to produce a three-phase supply from the main engine which means they don't need to be in a fixed engine power notch.

 

[edwin_m]

Yep, sorry the motor alternator set is what I am thinking of. My branch of science is not physics I'm afraid!
The modern power electronics are much more efficient than the older MA sets, no?

I don't think the MA sets were particularly inefficient, but I imagine a modern solid-state replacement would be better.

 

[Bald Rick]

I don't think the MA sets were particularly inefficient, but I imagine a modern solid-state replacement would be better.

They were pretty good at flashing over!

 

[Richard Scott]

Sure, my understanding is that old diesel electrics have to produce power for their DC motors, but the auxillary power systems like e.g. lighting use AC power, so you need a rectifier.
Electric tech has moved forward a lot in the last 50 years, so the older rectifiers lost quite a chunk of power converting from DC to AC.

The 3 phase alternators fitted to majority of ETH locos were highly efficient and the rectifiers converting the output to DC were of the semiconductor type so also very efficient and reliable. Only issue with early 1970s conversions were the use of early electronic AVRs (automatic voltage regulators) and they weren't that reliable. Certainly in a 45/1 they were a source of trouble.
The one thing that did need changing was the engine idle had to be higher when ETH being provided to avoid stalling the engine at idle. This obviously did increase fuel consumption but same likely to be true of a DMU if air conditioning fitted?

 

[Halish Railway]

Dare I say it, this does significantly improve the case for LHCS eventually replacing Class 159s out of London Waterloo.

If it is determined that there isn’t adequate power supply to enable a bi-mode locomotive to use the Third Rail in electric mode, the carriages could have shoes fitted to provide hotel power, negating the need for the locomotive to provide ETS, as well as powering radiators to keep the locomotive’s engine warm if shut down at Waterloo to negate the issues caused by the engine heating up and cooling down, providing that there is a high-voltage bus line between the carriages and locomotive.

If there is sufficient power supply, shoes on the carriages can supply power to the locomotive, preventing the locomotive being ‘gapped’.

 

[Meerkat]

Dare I say it, this does significantly improve the case for LHCS eventually replacing Class 159s out of London Waterloo.

If it is determined that there isn’t adequate power supply to enable a bi-mode locomotive to use the Third Rail in electric mode, the carriages could have shoes fitted to provide hotel power, negating the need for the locomotive to provide ETS, as well as powering radiators to keep the locomotive’s engine warm if shut down at Waterloo to negate the issues caused by the engine heating up and cooling down, providing that there is a high-voltage bus line between the carriages and locomotive.

If there is sufficient power supply, shoes on the carriages can supply power to the locomotive, preventing the locomotive being ‘gapped’.

The 159s split and join, and run in various lengths. I don’t think they would want to be converting that to loco hauled.
But if they did then just sort the power supply out rather than making the coaches really complicated!

 

[AlastairFraser]

I don't think the MA sets were particularly inefficient, but I imagine a modern solid-state replacement would be better.

OK, must have been other areas of power electronics where the improvement was more significant!

The 3 phase alternators fitted to majority of ETH locos were highly efficient and the rectifiers converting the output to DC were of the semiconductor type so also very efficient and reliable. Only issue with early 1970s conversions were the use of early electronic AVRs (automatic voltage regulators) and they weren't that reliable. Certainly in a 45/1 they were a source of trouble.
The one thing that did need changing was the engine idle had to be higher when ETH being provided to avoid stalling the engine at idle. This obviously did increase fuel consumption but same likely to be true of a DMU if air conditioning fitted?

Thanks for the technical explanation - I would note there was a crossover period when LHCS had air con and DMU did not e.g Mark 3s and Sprinters.
So DMUs of that era would be more fuel efficient (theoretically, according to your explanation.)

 

[Ken H]

The 3 phase alternators fitted to majority of ETH locos were highly efficient and the rectifiers converting the output to DC were of the semiconductor type so also very efficient and reliable. Only issue with early 1970s conversions were the use of early electronic AVRs (automatic voltage regulators) and they weren't that reliable. Certainly in a 45/1 they were a source of trouble.
The one thing that did need changing was the engine idle had to be higher when ETH being provided to avoid stalling the engine at idle. This obviously did increase fuel consumption but same likely to be true of a DMU if air conditioning fitted?

Is that why yiu heard a change in engine note when the ETH jumper was plugged in? I assumed it was just the load on the alternator.

 

[edwin_m]

Thanks for the technical explanation - I would note there was a crossover period when LHCS had air con and DMU did not e.g Mark 3s and Sprinters.
So DMUs of that era would be more fuel efficient (theoretically, according to your explanation.)

You need to compare like with like, and if considering a new train it would almost certainly have aircon.

Even air-conditioned trains generally have separate heaters, and with a DMU these can use engine waste heat so are essentially free to run. With a loco-hauled set they would have to be run electrically, so increasing the load on the engine.

 

[Richard Scott]

You need to compare like with like, and if considering a new train it would almost certainly have aircon.

Even air-conditioned trains generally have separate heaters, and with a DMU these can use engine waste heat so are essentially free to run. With a loco-hauled set they would have to be run electrically, so increasing the load on the engine.

I doubt waste engine heat is sufficient to heat a carriage so would have to be supplemented by an additional heater. Having said that any contribution is better than nothing.

 

[edwin_m]

I doubt waste engine heat is sufficient to heat a carriage so would have to be supplemented by an additional heater. Having said that any contribution is better than nothing.

An underfloor DMU engine is around 500kW and it's probably no better than 50% efficient, so I would have thought there was scope to scavenge the 30kW or so needed to heat a coach saloon.

 

[Meerkat]

An underfloor DMU engine is around 500kW and it's probably no better than 50% efficient, so I would have thought there was scope to scavenge the 30kW or so needed to heat a coach saloon.

The 175s seem to have plenty of spare thermal!!

 

[AlastairFraser]

The 175s seem to have plenty of spare thermal!!

So do the 180s….

 

[Gloster]

I hope that this question isn’t too far off subject: what was the fuel consumption, preferably in mpg, of BR diesel locos?

The specific details refer to an article I wrote about ten or fifteen years ago about BR blue era loco duties (Class 24 and 26) on the lines north of Inverness. One detail that I was interested in was whether the locos would do a whole diagram, possibly of three days away from Inverness, without refuelling or whether they would be refuelled during short layovers back at Inverness. Nowhere could I find an answer, not even the Railway Performance Society could help.

 

[sprinterguy]

I hope that this question isn’t too far off subject: what was the fuel consumption, preferably in mpg, of BR diesel locos?

The specific details refer to an article I wrote about ten or fifteen years ago about BR blue era loco duties (Class 24 and 26) on the lines north of Inverness. One detail that I was interested in was whether the locos would do a whole diagram, possibly of three days away from Inverness, without refuelling or whether they would be refuelled during short layovers back at Inverness. Nowhere could I find an answer, not even the Railway Performance Society could help.

I've seen it said that there was a rule of thumb that BR diesels would do about a mile per gallon of fuel.

This includes the big Type 4s such as class 45 and 47, so you might expect to get a bit more out of the smaller Type 2 engine in a class 26, with a comfortable margin for running round, and idling at beginning and end of journey.

The class 26s had a 500 gallon (2,300 litre) fuel tank: I've seen a figure of a 575 mile range for the larger class 37s that superceded them on what I assume were similar diagrams around the Far North, so that would support my theory of "a bit more" than a mile per gallon for the 26s.

Seems a bit of a low range for the 37s, mind, given their much larger fuel tanks: I might have some eighties Far North diagrams I can check once I get back in the house.

 

[Richard Scott]

An underfloor DMU engine is around 500kW and it's probably no better than 50% efficient, so I would have thought there was scope to scavenge the 30kW or so needed to heat a coach saloon.

But a lot of that is lost in exhaust and radiated heat from engine block. What is left isn't going to all be usable to heat the coach. Also that assumes engine running at maximum power, which won't always be happening. Trains spend a lot of time coasting!

Is that why yiu heard a change in engine note when the ETH jumper was plugged in? I assumed it was just the load on the alternator.

When ETH switched on some locos run in higher notches, think 50s do this so rev up when ETH switched on. Some are set to run at a higher idle at all times, believe 45/1s and possibly 47/4s do? Issue is if no or low ETH demand then engine still runs at higher idle wasting fuel.
Some foreign locos were/are really noticeable that they increase engine idle when ETH (ETS nowadays) is switched on, notably SNCF 67300/67400 and 72000s, also HZ 2044s. Probably others too.

 

[Bald Rick]

Trains spend a lot of time coasting!

Indeed - and when coasting the diesel engines are 0% efficient, ie all of it ends up as heat, so plenty to get into the heating.

 

[Richard Scott]

Indeed - and when coasting the diesel engines are 0% efficient, ie all of it ends up as heat, so plenty to get into the heating.

But at idle very little power produced, most of the heat will be lost to exhaust and radiated heat from engine block, be next to nothing left for heating interior of the coach.

 

[Bald Rick]

But at idle very little power produced, most of the heat will be lost to exhaust and radiated heat from engine block, be next to nothing left for heating interior of the coach.

That depends on how much comes through the heat exchanger in the cooling system, surely?

 

[Richard Scott]

That depends on how much comes through the heat exchanger in the cooling system, surely?

Yes, but it'll be very little with an engine at idle. A significant amount of heat gets lost through exhaust gases and the engine block itself, the remainder is then removed by the cooling system and at idle that won't be very much. If you leave an engine idling and see how long it takes for the cooling fan to kick in you'll wait quite a while.

 

[158747]

I doubt waste engine heat is sufficient to heat a carriage so would have to be supplemented by an additional heater. Having said that any contribution is better than nothing.

Waste heat from the engines is used for heating on DMUs, the heat exchangers in the heaters are supplied with coolant from the engine.

 

[heathrowrail]

All depends on the load being carried surely? Having a loco designed to haul 10 coaches but only hauling 3 compared to a 3 car 166s is bound to have a difference in fuel burn.

 

[hexagon789]

I hope that this question isn’t too far off subject: what was the fuel consumption, preferably in mpg, of BR diesel locos?

The specific details refer to an article I wrote about ten or fifteen years ago about BR blue era loco duties (Class 24 and 26) on the lines north of Inverness. One detail that I was interested in was whether the locos would do a whole diagram, possibly of three days away from Inverness, without refuelling or whether they would be refuelled during short layovers back at Inverness. Nowhere could I find an answer, not even the Railway Performance Society could help.

1mpg, as mentioned, was assumed for diesels for the purpose of calculating diagrams. This was also the figure quoted to enginemen on their traction course. A further caveat - it was assumed that the last 75-100 gallons in a tank was unusable (being mostly sludge and detritus) so that was deducted from mileage calculations.

DMUs and later HSTs had different assumptions, the HSTs were the first to get mode precise fuel consumption figures utilised but it was the advent of the Class 60 that saw modern computer modelling used fully in this field.

There were tables showing the mileages for each class, for instance

Class 40, steam heat on, load 16 - (Mk1s): 560 miles
Class 45/46, steam heat on, load 16 - 525 miles
Class 45/46, heat off, load 16 - 630 miles
Class 45/1 - 630 miles. (Curiously this figure is given as identical for the ETH being on or off)
Class 47, steam heat off - 600 miles
Class 47, steam heat on - 500 miles
Class 47/4 - 600 miles (again as per 45/1, the figures are the same for ETH on or off)
Class 47/7 (additional tanks) - 1110 miles

Hopes that's of interest.

I may be able to find some data for the 26s...

 

[edwin_m]

Waste heat from the engines is used for heating on DMUs, the heat exchangers in the heaters are supplied with coolant from the engine.

This is the same heat that goes into the engine block, and thence to the coolant.

 

[Richard Scott]

This is the same heat that goes into the engine block, and thence to the coolant.

No it isn't. Not all heat in the engine block goes to coolant, some will be radiated to atmosphere and it's not an insignificant amount.

 

[edwin_m]

No it isn't. Not all heat in the engine block goes to coolant, some will be radiated to atmosphere and it's not an insignificant amount.

Obviously not all of it will go to the coolant, but any IC engine bigger than a lawnmower needs cooling so this is obviously pretty significant in removing heat. #52 confirms this is used for interior heating, although it's possibly supplemented in some cases. The first-generation DMUs had separate diesel-burning heaters, mostly responsible for them commonly smelling of diesel I think, but I'm not sure if this was their only source of interior heating.

 

[Richard Scott]

Obviously not all of it will go to the coolant, but any IC engine bigger than a lawnmower needs cooling so this is obviously pretty significant in removing heat. #52 confirms this is used for interior heating, although it's possibly supplemented in some cases. The first-generation DMUs had separate diesel-burning heaters, mostly responsible for them commonly smelling of diesel I think, but I'm not sure if this was their only source of interior heating.

Yes it does but at idle (which will be a reasonable proportion of time) very little heat will go to coolant and hence to heating the saloons so supplementary heating is almost guaranteed to be required. This is the point I'm trying to make.
Under load a lot of heat will need to be removed by coolant so using waste heat for heating the saloon will be a sensible use of waste heat.
First gen DMUs did use separate heaters, which were diesel fired hence smell. Running them on kerosene can remove some of this issue.