Why are steam locomotives less efficient at low speeds?

[Overmod]

lenzfamily

And that is precisely what I did recognize....

Well, no.  The subject wasn't the 'starting under load' scenario.  And that, indeed, was the point I intended to be making, and did make, I think, regarding the off-topic drift..

Unlike some, I apologize when mistaken.  I see no reason to defend, or change, my behavior in that respect, or for that matter to comment on certain other aspects of your response that do not address the thread topic.

I am quite certain that Juniatha recognizes the spirit in which I commented -- on topic or otherwise -- and would expect to hear from her directly regarding any details of my comments.  Your intrusion is condescending to her, at best, in my opinion.

If you have experience and qualifications concerning the topic of low-speed steam efficiency (which you may well have), as I said, I look forward to further discussion.   (Perhaps including, even though not strictly on topic, how you think reciprocating steam locomotives are regularly started without being under load.)  But this is neither the place nor the subject for any further discussion of other matters; it belongs off-list.

[lenzfamily]

Overmod

Unlike some, I apologize when mistaken.  I see no reason to defend, or change, my behavior in that respect, or for that matter to comment on certain other aspects of your response that do not address the thread topic.

I am quite certain that Juniatha recognizes the spirit in which I commented -- on topic or otherwise -- and would expect to hear from her directly regarding any details of my comments.  Your intrusion is condescending to her, at best, in my opinion.

Sir

Your opinion is yours to hold. Mine is mine to hold. That's how i see the thread progression here.

I do indeed know when and how to apologize on this forum as I have done on a number of occasions previously.

I will not continue further discussion with you.

Charlie

Chilliwack, BC

[cx500]

Ulrich

I've read that steam locomotives are less efficient when running at low speeds, why would that be? And how does steam locomotive efficiency correlate with speed in general?

To return to the original question, I think the lowly pedal bicycle provides a reasonable analogy that may help understanding.  The steam locomotive is like a bicycle in a tall gear ratio.  How tall a ratio is the equivalent of the driver diameter.  It is a struggle to start from a standstill if in top gear, but once on a good roll the pedaling becomes easier.

The diesel equivalent is a bicycle fixed in the bottom gear.  Starting is easy, but soon you are going too fast for your legs to apply much pressure on the pedals..

The analogy is highly oversimplified since the mechanisms are very different.  But there are some surprising equivalencies.

John

 

[gap920]

Juniatha, See emdiesels.com. EMD turbochargers are gear driven at lower speeds. See Products, Power Products for torque curve info. This gives it a much fatter torque curve at low speeds. gap920

[CSSHEGEWISCH]

rrandbb

in m. baldwins day there were no answers, only questions that were only proven or disproven by trial and error. some engineering department guidelines were those of one mans decisions. the camelbacks a case in point.The FEC/flagler systems chief engineers need the most powerful fastest engines money could buy? were they lees efficient than there heavy 0-8-0 switch engines? it all is a matter of your perspective on the historic facts?

It's not a matter of perspective, it's a matter of how the locomotives were designed and built.  An FEC 4-8-2 would be quite efficient on the head end of a fruit block or the "Havana Special", it would be decidedly inefficient if it was used to kick cars in Bowden Yard.  An FEC 0-8-0 would be reasonably efficient in low-speed yard or transfer duty, it would incur the wrath of the M/W department if it was assigned to a local freight.

[GP40-2]

cx500

Ulrich

I've read that steam locomotives are less efficient when running at low speeds, why would that be? And how does steam locomotive efficiency correlate with speed in general?

The diesel equivalent is a bicycle fixed in the bottom gear.  Starting is easy, but soon you are going too fast for your legs to apply much pressure on the pedals..

That is not a good analogy. The diesel engine doesn't directly turn the wheels. The electric traction motors do, and electric motors are an infinitely variable rate transmission.

The notion that diesel-electrics somehow lose lots of power at speed that I have seen mentioned in this thread is not only amusing, but it is completely wrong.

If you look at the power curve of a modern diesel-electric, it is almost flat as a pancake over its entire operating speed range. For example, the 6,250 HP GEVO CSX AC6000 is a constant force, rising HP machine from 0 to 11 mph. This is due to the adhesion software maintaining a constant TE as the speed increases. Above 11 mph, it becomes a constant HP machine to its software limited speed of 75 mph. The only losses to drawbar HP as speed increases is from air drag and rolling resistance. This isn't much, it only takes around 300 HP to move an AC6000 at 75 mph on level track.

The bottom line is an AC6000 is a 5,600 to 5,700 drawbar HP locomotive from 11 to 75 mph. That is about as far from a fixed gear bike analogy as you can get.

[Juniatha]

To quote Overmod :

 

>> Well, no.  The subject wasn't the 'starting under load' scenario. <<

Well , yes , that’s at least how I saw it , too .   Idle starting mode wouldn’t point out such a significant difference between steam and IC reciprocating engines .

 

>> Unlike some, I apologize when mistaken. <<

Well , I take your word for it ...

 

>> I am quite certain that Juniatha recognizes the spirit in which I commented <<

I’m not sure at all ..

 

>> and would expect to hear from her directly regarding any details of my comments. <<

?? Dunno what makes you expect that ?

 

>> Your intrusion is condescending to her, at best, in my opinion.<<

Ho-ho-ho , strong medicine .. intrusion .. condescending .. and that from one who has been quite self-righteous at times .

No insult intended , yet I think this had to be clarified .

Juniatha

[Overmod]

Likewise for clarification only:

This was the salient point, and its response.

Juniatha

>> Well, no.  The subject wasn't the 'starting under load' scenario. <<

>Well , yes , that’s at least how I saw it , too .   Idle starting mode wouldn’t point out such a significant difference between steam and IC reciprocating engines .<

All the rest ... you can put your popcorn away.  Digression ends here.

  (Except to note that a far better pun would have been "Sliver Cloud"...  ;-} )

[blue streak 1]

GP40-2

The bottom line is an AC6000 is a 5,600 to 5,700 drawbar HP locomotive from 11 to 75 mph. That is about as far from a fixed gear bike analogy as you can get.

 

 GP30-2::   ===  what all these persons seem to forget is that HP is constant.  The tractive effort is inversely related to speed due to applying the HP over a longer distance in relation to time.

So if a 5600 HP loco can move a certain train at 15 MPH it takes 2  ==  5600 HP locos to move that train at 30 MPH or 3 - 5600 HP locos to move that train at  45 MPH disregarding any speed related parasitic drag.

[Overmod]

blue streak 1

GP40-2

The bottom line is an AC6000 is a 5,600 to 5,700 drawbar HP locomotive from 11 to 75 mph. That is about as far from a fixed gear bike analogy as you can get.

 

 GP40-2::   ===  what all these persons seem to forget is that HP is constant.  The tractive effort is inversely related to speed due to applying the HP over a longer distance in relation to time.

So if a 5600 HP loco can move a certain train at 15 MPH it takes 2  ==  5600 HP locos to move that train at 30 MPH or 3 - 5600 HP locos to move that train at  45 MPH disregarding any speed related parasitic drag.

He knows that.  What he's doing is addressing the fixed-gear bicycle analogy ... -- as in the curve for an ordinary reciprocating steam locomotive, which if you think about it acts JUST like a single-geared pedal bike across its speed range. 

Clearly he was addressing a diesel-electric which is inherently limited by the constant maximum horsepower of its prime mover, but the specific (I think) point he he is making regarding the particular example locomotive is that other losses, such as counter EMF, that might reduce *drawbar* horsepower also remain proportional to prime-mover/generator hp all the way to the 'artificial' imposed speed limit.  That is different from what occurred on older locomotives, where the effective hp would fall off the hyperbola at higher speed.

(A peripheral point, which GP40-2 knows well, but didn't think he had to mention in his context, is that the requirement for horsepower to keep a train moving at fixed speed is different from that to ACCELERATE the train to that speed, or then to a higher speed.  This was a critical issue, say, when early Geeps replaced Hudsons (the example is in Staufer's Thoroughbreds) in some NYC services.  The Geep could run the train up to the same speed... and fairly happily keep it there with diesel economy... but it took 'until the next town' or longer to accelerate the train up to that speed with the fixed horsepower available...) 

(The speeds you give in your example appear to be balancing speeds, with some kind of GHA having made the train go that fast when you make your measurement.  Isn't that a bit like the old math problems that had the solution inherent in the terms of the problem?  ;-} )

[EDIT -- can the moderators please tell whoever is coding the Forum that they have now made the same software mistake that Yahoo Groups did, in parsing HTML I think, and all the carriage returns/line feeds I use to separate my paragraphs are being stripped mistakenly...]

[cx500]

I recognize the bicycle analogy fitted steam better than diesel.  The main point I was thinking of is how the diesel's superior tractive effort when starting from a stop or grinding slowly up a hill matches low gear on a bicycle.  Gear ratios in the traction motors of course also have some influence for those who wish to get picky..

Having said that, I seem to recall an article, possibly in TRAINS, about 10 or 20 years back describing a mainline steam excursion in the eastern USA.  Apparently at a key point on the early part of the grade speed was relatively low.  In the experience of the host road's diesel era officer (with freights) that meant the train would stall on the steeper grades ahead.  Much to his surprise quite the opposite happened and they were going faster by the summit.  The helpers he was organizing were never needed.

John

[Juniatha]

Hmmm -

amazing --

classic reciprocating direct drive type of steam performing *better* the *steeper* the grade I haven't yet heard of .

Maybe it was a matter of crew not having fully responded to the grade in the beginning , so when it was realized what kind of work it asked for the engine still had reserves enough to fight her way up and even gain speed .

About the inevitable 'anxiety diesels' included in steam specials :  considering what happened to # 844 , at least UP had to learn they not always solve problems or prevent them , but they can *cause* problems , too , avoided had they not been part of the consist at all !

Regards

Juniatha

[Firelock76]

Hi Juniatha!

"Anxiety diesel", hey I like that!  Can't think of any other reason to put a diesel on 844's tail unless it's a "just in case"  set-up.  At any rate, look what happened when there was no-one on board the "just in case" diesel  when the wrong "just in case"  happened!

I've been told another reason for the diesel is to provide dynamic braking  Now I'd never dream of telling professional railroaders how to do their jobs but 844 didn't need dynamic braking back in the old days.  The train brakes were more than sufficient.

Oh well, I'm heading over to the "High Drivers"  thread to look at those pictures of those kool German steamers you posted.

Wayne

[NorthWest]

The only case where I like to see a diesel is when the train is long enough that the locomotive might be damaged by slipping and stalling. They're getting old, let's let them enjoy retirement and not beat them up!

(Now, tell us about the German 1-0 . Congrats on the win!)

[cx500]

You don't "need" dynamic braking, but it greatly reduces brake shoe wear.  And that saves a lot of cost and labor in frequent replacement of brake shoes on a dozen or more coaches where long steep grades are a common feature of the territory.  The mechanical staff already have their hands full keeping the steam engine ready to go.