How Efficient is Steam?

[broadgage]

As far as I know steam is just a power transmission method, its in how that steam is produced that the efficiency factors come in. Although one was probably never officially built a small nuclear reactor powering a steam turbine would have interesting efficiency figures. I can't believe given their huge loading gauge and distances that the Soviets didn't at least consider this.

The smallest nuclear reactors of significant power output are those used to power nuclear submarines, still much to big for even a generous railway loading gauge. They also need a lot of cooling water, easily achieved at sea but not so easy on a locomotive.

 

[michael769]

Wikipedia says "Electric locomotives benefit from the high efficiency of electric motors, often above 90%", and although there's no reference to a primary source, its close enough to the 85% I found elsewhere that I find it credible. If they routinely exceed 90%, it would be reasonable to expect that the best were 95% efficient.

Note that the article is talking about the efficiency of the motors not the train itself.

There are now motors that on their own are more than 96% efficient. But when measuring the efficiency of a train one must consider the efficiency of a whole system. Once such a motor is fitted to a train a mass of additional sources of energy loss will come into play and the overall efficiency will decline very rapidly. Major sources of loss in a train will include friction of moving parts, rail/wheel friction and air resistance. I suspect that more than 50% is an impossible dream unless we start running maglev trains in vacuum tubes.

 

[76020]

As the rest of his post has "train" in it a few times it doesnt take a genius to work it out does it! :roll:

I think steam LOCOs are between 7 and 15% efficient (and very labour intensive), a diesel electric loco is about 80% and a straight electric loco 95% on pure fuel in to power out measures.

25KV overhead is a very efficient way of power distibution and needs a substation every 50-75 miles (more where there are lots of trains running).
I think that is right, its been ages since I looked into this.

750V DC is very inefficient and needs a substation every 3 miles (again more where there are lots of trains running).

Thanks for the info 455, I will have to do some TRAINing so I can improve my gramma!

 

[HSTEd]

Note that the article is talking about the efficiency of the motors not the train itself.

There are now motors that on their own are more than 96% efficient. But when measuring the efficiency of a train one must consider the efficiency of a whole system. Once such a motor is fitted to a train a mass of additional sources of energy loss will come into play and the overall efficiency will decline very rapidly. Major sources of loss in a train will include friction of moving parts, rail/wheel friction and air resistance. I suspect that more than 50% is an impossible dream unless we start running maglev trains in vacuum tubes.

Is it really appropriate to include air resistance and rail wheel friction?

Those are nothing to do with the efficiency of the machines.

And if you think about it, if you include such resistive losses, on level track all trains would have zero efficiency stop-to-stop.

 

[Bill EWS]

Likwise, a steam loco required two in the cab, not to forget the massive ground staff, while, in most cases, diesel and electric trains only require the one driver. This also needs to be taken into consideration when comparing the overall expense of comparing one against the other. Then there is the time out of ervice when not required and the figure I remember being state was £90 a day per mainline steam loco just ticking over on depots awaiting their next turn of duty. Diesel & Electrics are simply switched off in these and many other idle time situations.

 

[The Decapod]

I think I read somewhere that over 80% of the heat generated in the firebox goes up the chimney and so less than 20% goes into creating steam.

So let's say a steam engine is 19% efficient maximum.

Diesel engines can exceed 40% thermal efficiency.

I think steam LOCOs are between 7 and 15% efficient (and very labour intensive), a diesel electric loco is about 80% and a straight electric loco 95% on pure fuel in to power out measures.

The 80% efficiency figure I think refers to a diesel-electric's transmission system - i.e. 80% of the power input to the generator by the diesel engine reaches the wheels, via the traction motors, to move the train. But that 80% figure would be calculated from what power is left AFTER 60% of the heat produced by burning diesel fuel has gone up the exhausts! So if a diesel loco has an engine that is 40% efficient and an electric transmission that is 80% efficient, the loco overall would be 32% efficient.

With electric trains, are the power losses at the generation stage taken into account - I mean at the power station? I didn't know the answer so I Googled and found that an advanced gas-cooled nuclear reactor can be up to 40% thermal efficiency.

 

[Ploughman]

What all these posts need to consider rather than Air resistance, Power transmission grids etc is that to compare systems fairly you need to consider like for like.
Which to me means you ignore how the power gets to the loco just consider how it is used once it is there.

 

[jopsuk]

no, for a proper comparison of efficiency- by which, in this day and age, we're really most worried about enivromental impact- you really should be considering how the power gets there- which for steam and diesel means making allowances for delivery of the fuel (and water for steam), extraction from the ground, refining in the case of diesel. All these are relevent to electrics as well- though there you have a fossil fuel/biomass/hydro/wind/nuclear mix of sources.

However, power output is also important. For the hugely powerful BR Standard 9F, I can find a quoted tractive effort of 176.45kN- I can't find if that is continuous or maximum. For the Class 37, a design only a few years newer, Maximum: 247kN, Continuous:156 kN. A modern Class 66 will do Maximum: 409 kN, Continuous: 260 kN. Both those diesels weigh about the same or less than a 9F.

Electrics are tricky to compare, all the mainline electrics barring the 89 are BoBo.

 

[TDK]

Is it really appropriate to include air resistance and rail wheel friction?

Those are nothing to do with the efficiency of the machines.

I would argue that fact, less air resistance the machine if it is moving must be more efficient.

 

[matchmaker]

Mostly its natural gas that supplies the "juice" these days.

And hopefully, if the madness abates, it will be uranium.

Between them, Drax and Longannet (Britains two largest) power stations get through over 13,500,000 tonnes of coal per year. Plenty of "juice" must come from them!

 

[dysonsphere]

As far as I know steam is just a power transmission method, its in how that steam is produced that the efficiency factors come in. Although one was probably never officially built a small nuclear reactor powering a steam turbine would have interesting efficiency figures. I can't believe given their huge loading gauge and distances that the Soviets didn't at least consider this.

The US tried something similar the infamous nuclear powered bomber. I think they got as far as flying a reactor in a B36 I think it was then common sense preveiled and the idea was dropped.

 

[jopsuk]

the key with electrics is that the transmission system is very efficient, and it means that you're only moving around the drive system- you're not moving the power generation system. Diesels electrics in effect carry small power stations everywhere; steam has to carry a huge fuel supply and a massive boiler. These parts are in effect dead weight.

A nuclear reactor with adequate shielding to protect the crew would be very heavy- as the americans discovered with their flying program.

 

[LE Greys]

the key with electrics is that the transmission system is very efficient, and it means that you're only moving around the drive system- you're not moving the power generation system. Diesels electrics in effect carry small power stations everywhere; steam has to carry a huge fuel supply and a massive boiler. These parts are in effect dead weight.

A nuclear reactor with adequate shielding to protect the crew would be very heavy- as the americans discovered with their flying program.

There have been some impressive attempts with turbo-electric locomotives. Here, the LNER tested the Reid-MacLeod Steam Turbine, which was not entirely successful, and ended up being replaced with a geared drive turbine. Several US Railroads also tested turbo-electric designs, including one of the biggest locomotives ever, Jawn Henry (data page is in German, but the pictures make it worthwhile). In general, one major problem is that turbines work most effectively when hooked up to a condenser, so that they exhaust into a near vacuum, and this adds weight and cost, hence the size of these monsters. Geared turbines such as Stanier's Turbomotive were more successful, mainly through being less complicated. However, that produced a few problems, not the least of which was that she couldn't climb Camden tender-first with a train. The inefficient, rugged Stephenson engine was much better suited to the rough-and-tumble of everyday railway service.

I seem to remember a nuclear locomotive appearing in a book of Eagle comic diagrams from my local library. Fascinating, but I'd rather not imagine the consequences of a Ladbroke Grove situation with an active reactor on board.

 

[jopsuk]

The US has a bit more leeway to experiment- a large loading gauge helps, and very large distances between population centres makes electrification less practical and efficient than in a relatively densely packed island like ours

 

[HSTEd]

It was discovered that nuclear reactors scale very well.
While they are not practical at the scale of the B36 there were other options considered.

Like enormous aircraft that could stay up for weeks and would have reactor vessels rated to survive flying into a mountainside at 400 knots intact.

The 50s and 60s really were nuts for stuff like that.

 

[Kali]

I think quoted efficiency figures for diesel electrics just cover power losses in the transmission, so a 70 can use 90%ish of it's power unit's output vs about 81 for older types. I have no idea how efficient a 70s power unit is at extracting energy from diesel.

The 5AT project has projected overall thermal efficiency figures of 14% which is considerably bigger than any other steam engine I've seen, although I can't find numbers for others right now; as a comparison I think this might be a useful number to try and calculate for other types of power. Current electric trains are effectively steam powered with electric motors replacing recyprocating cylinders, the heat source changes of course.

 

[broadgage]

The main drawback of steam turbine drive for a locomotive is that turbines cant be readily reveresed.
In the case of electric transmission reversing is easily achieved electricly, but for mechanical drive it is a serious drawback.

The "turbomotive" refered to earlier had TWO turbines one for forward and a much smaller reverese turbine.
When running forward, the reverese turbine was static, being dissconnected from the wheels by a one way slipping clutch.
In back gear, the forward turbine simply spun backwards.

Back gear was therefore only suitable for low speed, low horsepower, limited time use, and was not suitable for running tender first with a train of significant weight or up an incline.

A traditional steam locomotive can of course produce about the same HP in either direction, though running boiler first was invariably preffered on main lines due to the improved sighting.