The “can steam make a comeback” thread has included the inevitable confusion about the supposed advantage the conventional steam locomotive has over a diesel locomotive at, say, 50-60 mph and up.
“…ton for ton a steam locomotive can and does pull more tons at higher speeds than a diesel locomotive can.”
“The H.P. of a diesel is less effective at faster speeds because more electricity is needed to keep the traction motors spinning at the higher speeds.”
“hp per hp, the steam engine still passes the d/e in tractive effort quite early in speed.”
“The tractive effort of a steam locomotive increases as the speed increases…”
“This is exactly the opposite of what is clearly shown by the published and generally accepted TE/HP curves for the respective motive power types.” [The “this” that he was responding to was “the Diesel should always pull at least as much tonnage as a steam engine over the entire practical speed range for a given maximum dbhp.”]
“At 19 mph, a reciprocating Steam engine that is putting out 5600 hp will generate 50% more TE than a Diesel-electric locomotive(s) generating 5600 hp at that speed.”
In the last one he probably didn’t say what he intended to say, and in the next-to-last he must have misread the other guy’s claim, but it all adds to the confusion. This spinoff thread is an attempt to straighten out this one aspect of the argument-- no discussion here about fuel costs or capital costs or air pollution or mining practices.
Start with the diesel locomotive. The diesel itself (the prime mover) is happy to produce full horsepower at any locomotive speed, and clearly our aim in designing the locomotive is to arrange a transmission that can transmit that full power to the wheels over a range of speeds, the wider t
There are 2 things that need to be known to determine which can pull what amount. Those 2 things are TE (tractive effort) and Limit of Adhesion. Neither on their own tells us anything useful as you can have all the tractive effort in the world but no adhesion and you will go nowhere. You can also have all the adhesion in the world but without tractive effort you don’t have the necessary force required to pull the train. Therefore you need to know both. This is easy.
Weight on drive wheels / Tractive effort = Limit of Adhesion
To figure out TE, we need to look at steam and diesel/electric separately. To figure out TE of a steam engine we need to know:
Boiler working pressure, piston bore, piston stroke, drive wheel diameter, and then we need a constant that allows for losses incurred in the real world.
To figure out TE of a diesel/electric engine, we need to know:
The torque of the traction motors, the gearing, and the diameter of the wheels. The power of the generator is not a factor.
Notice that I haven’t listed weight on driver’s as being important in figuring TE. That’s because it isn’t. TE is Tractive EFFORT. Effort doesn’t mean traction. It just means available force. We need to use weight on drivers in conjunction with TE to determine traction or limit of adhesion. Even then we are trying to determine a theoretical maximum limit of adhesion. Real world can be very different and hence the reason for technology to control adhesion more effectively.
You quoted me as saying “hp per hp, the steam engine still passes the d/e in tractive effort quite early in speed.” This statement is in fact true. However if you don’t think this is so it is because you are forgetting limit of adhesion. Remember adhesion is traction. Effort isn’t. Just because tractive effort on one is higher doesn’t mean it can necessarily pull more. You need to also know adhesion. Keep in mind my above statement could in fact also be
Actually, the chart I produced for the thread was specifically based on the accepted limits of adhesion utilized at that time (1957) for road motive power of the two motive power types, and was specifically keyed to the weight of the respective machines on the driving wheels because it was the weight on the driving wheels that provided a consistent common denominator to compare across the very different motive power types and reflected the practical results of literally thousands of dynamometer tests on American railroads on both Diesel-electric and Steam power.
And although I utilized some fairly generic TE’s as measured, and compared each against the resistance force of a hypothetical train, the relationship of the TE curve for the Diesel-electric to the Steam TE curve is in fact the measured relationship taken from generally accepted, published, power curves.
The data has long been available, and I reproduced it on the last thread as follows, upgraded for the very high hp diesel-electric example used by another, and a very large steam engine of the kind planned before the transition ended. A person can use any hp they like, but the relative relationship for each motive power type at the respective speeds is well established:
It seems to me that for as difficult as it is to understand this subject, the greater difficulty is communicating it with words in a discussion forum. I have some understanding of this subject, but I am also learning things as these threads progress. So I would like to lay out some thoughts for review and to check my understanding.
I have heard the term “tractive effort” many times over the years, but apparently have not correctly understood it. I always interpreted it to mean drawbar pull. As I think about it now, I don’t recall if drawbar pull is a standard term of measure, or not. In any case, my interpretation of drawbar pull is the actual pulling force of the locomotive exerted at the pulling coupler. It would not only depend upon the pulling force that locomotive could apply to the wheels, but also to the traction of the wheels on the rails. That traction is what I would call adhesion. It is affected by the weight on the drivers and by the slipperiness of the rail/wheel contact.
As I understand it now, according to Fred, tractive effort does not include the affect of adhesion, so it is not drawbar pull. Instead, tractive effort is pounds of force developed at the running tread of the drive wheels. It is not affected by slipperiness of the rail or the weight on the drivers. In fact locomotive tractive effort could be measured without even having a track. I guess the expression of tractive effort would be the total of the drive wheel force of all drivers. So a B-B diesel producing 100,000 pounds of TE would be exerting 12,500 pounds per wheel, at the running tread of each wheel.
In order to compare the fundamental abilities of steam and diesel locomotives, we need to compare locomotives of equal stature for a fair comparison. To determine if two locomotives are of equal stature, we could consider their attributes of horsepower rating, the actual production of horsepow
We’ve using different definitions of “data” here. Your table isn’t “data” in the sense of the results of some sort of measurement-- or is it? I’m guessing you (or someone) calculated the horsepower for a hypothetical steam locomotive, based on certain necessarily simplified assumptions. We wouldn’t mind knowing the assumptions, but in any case you’re not claiming these are measured horsepower for any actual locomotive, are you?
If we are going to discuss the table further, you might as well give us the details of your assumptions and calculations.
To kill two birds with one stone (yet again) here are the actual drawbar horsepower measurements of a Class A:
I can not copy and paste the graph here because it is in an expensive book which will remain intact.
At 10 MPH-3,000 Drawbar Horsepower
At 15 MPH-4,000 Drawbar Horsepower
At 20 MPH-4,800 Drawbar Horsepower
At 24 MPH-5,000 Drawbar Horsepower
At 30 MPH-5,400 Drawbar Horsepower
At 40 MPH-5,600 Drawbar Horsepower
At 50 MPH-5,500 Drawbar Horsepower
At 60 MPH-5,200 Drawbar Horsepower
As can be seen the A reaches 5,000 Drawbar Horsepower at 24 MPH and continually remains above this number until the final measurement point at 60 MPH is reached. The power curve at 24 MPH and above is relatively flat through out this wide speed range.
So then the ability of a standard steam locomotive to produce a high rate of almost continuous horsepower over a wide speed range is clearly evident.
I guess there’s no official definition, but when I say “tractive effort” I mean the force exerted at the rim of the drivers (averaged thru 360 degrees of driver rotation, if necessary). Drawbar pull … not much mystery about that.
Sounds like you’re at a dead end. Far as I’m concerned you’re worrying too much about making your comparisons “fair”. Go ahead and make the comparison, doing your best to make it fair; if somebody says it’s unfair, leave it up to the reader to decide.
Adhesion is mostly or entirely irrelevant to this discussion-- we’re discussing TE/drawbar pull at speed, when either locomotive, steam or diesel, is producing TE far below its maybe-adhesion-limited maximum.
AFAIK nobody’s disputing the A’s dbhp figures. The only question is whether that dbhp was enough to pull 7500 tons at 64 mph on the level-- and, if it was, why isn’t it now. (I’m going out on a limb and guessing it isn’t.)
It is probably pointless to discuss the issue further. I get the impression it isn’t for enlightenment. A gentleman asserted on the other thread that a Diesel-ellectric locomotive he once knew generated 5,600 hp. He refused to provide a power curve for the machine when asked.
Given the published horsepower curves for the respective motive power types with the same weight on the drivers, there is an equivalent steam engine for that size and rating. At the time the data was obtained, the models of each at those particular selected horsepower outputs did not exist, however, the gentleman wanted to talk abolut his 5600 hp DE, and experience already had shown that scale changes did not and do not appreciably change the statistical relationships. The “data” is contained in the numbers as those represent the specific proportionate changes with speed for each motive power type according to the published data and, in an Excel spreadsheet, the numbers recalculate according to the predetermined relationships taken from dynamometer tests for any given hp at a specific given speed. If you wished to take each succeeding hp at each succeeding speed, and subtract the preceeding and then determine the percentage change, you will see the
Your figures are based on published curves? Published where, and when?
The (H.F. Brown?) diagram you showed in the last thread is misdrawn, and doesn’t purport to represent actual locomotives, does it? I don’t recall any other curves being mentioned there, except the figures for the N&W A-- but I didn’t go over that thread carefully.
And you really ought to make some slight effort to explain just how you transformed the published curves into the figures in the table above. We agree that they are what they are, but so far none of the rest of us knows what that is.
The N&W officially reported that it did so in testing, in daily service their time freight tonnage rating was 6000 tons which were pulled everyday at 50 to 60 MPH, so there you are. Either the N&W simply lied about the test results or it didn’t.
The point about posting the actual horsepower ratings was to give real world data to support the argument about Steam’s ability to produce high horsepower over a wide range of speeds and to give credence to the trends supported in the other comparisons above.
There’s the truth, and there are lies, and then there’s everything else-- my feeling is the last category is maybe 90% of the total, but there’s room for dispute on the exact figure.
Silly me for chiming in way out of my element, but… I agree that adhesion is a necessary, but irrelevant, factor in comparing the engines unless the adhesion is ever overcome in a side-by-side comparison of any two engines of any type. In other words, if one has 10K ponies and the other has a modest 1200 in a switcher’s body, but the 10k spins all the time, which engine can actually to the work? So, forget the adhesion for now. Just deal with two “really heavy” engines that have a less than 0.0001 probability of spinning in any one test, and deal with how much resistance they can overcome at the tire tread. Let’s say they will always be evenly matched for adhesion. So, let’s now concentrate on getting tonnage down the linear axis of the rails.
We already had this discussion the previous thread. The curve was posted there. It shows the relationships that exist within the motive power types and between the motive power types. These relationships are defined by physics and, unless you can identify a heretofore unspecified change in the physics – such as adhesion improvements – the relationships will always hold true.
The chart uses the word “comparative” and that is designed to offer a specific piece of information: because the relationships are tested and true – from there you can play with individual numbers all you want, but the relationships have to remain.
I’ve seen similar curves in various publications but that was long ago. The Steam Engine, by Ralph Johnson, published by a regular railroad publishing house, Simmons-Boardman, shows the same chart.
The interesting thing is that if we focus solely on TE and ignore adhesion, the results will be biased towards one type of engine. If we were to focus only on adhesion and ignore tractive effort (which would be hard to do as it’s a required variable in adhesion), then we may get a result that favors the other engine. I think the original intent of this little debate/discussion is to know which one can physically pull the most and at what speed in the real world. Everybody can bench race with only certain variables and anyone can win this way with some creativity. The unfortunate part as has already been pointed out is that there are so many different variables between each type of locomotion that a true equal comparison becomes very difficult if not impossible.
I do feel that just because there are those who disagree with anothers perspective, they shouldn’t just automatically write them off as completely wrong altogether and then demean them publicly for their views. Debates can be friendly and when done properly everyone involved can hopefully benefit from it in the end. The point of this shouldn’t be to try to prove everyone else wrong but rather to get all the facts out in the clear so the fundamental physics behind it are understood. Let’s face it, everyone could completely understand it but there would still be opinions as to which one would be beneficial to the world of today. Keep opinions to yourselves for this thread and let’s get all the facts out. If anyone’s “facts” disagree with others, let’s figure out why. Everything I have typed I have learned in the past couple of days and I studied it because of this forum. the debate has had merit for me. Let’s make it have merit for everyone.
Well are we only comparing individual locomotives to see which one does what? Or are we comparing locomotives to discover some fundamental difference between steam and diesel locomotives?
Well, it is, but that goes to the idea of equivalent weight on the drivers, and if they are different, the lighter engine can’t put as much of its theoretical TE to the rail. If you impose a weight difference, you impose a penalty on the lighter engine, regardless of its power.
Look at the general equation posted above a little differently.
Tractive effort = Weight on drivers/Limit of Adhesion
There are some things missing to make it accurate, that show the curve with changes in speed, and that each power type has different coefficients, but it is the basic principle.
What is the fundamental determining factor of tractive effort: weight on the drivers.
Therefore, to obtain the benefits of the fundamental principle in terms of understanding the concept, you can’t go off and use a metric that is not, in fact, relevant to the fundamental principle because then you are not comparing “tractive effort” on the comparable basis that defines tractive effort. Because it is the fundamental principle, how could you compare tractive effort of two motive power types with different weights on the drivers?
Lowering the weight on the drivers on one motive power type until it finally matches the output of another motive power type doesn’t really tell you anything except that, sure enough, lower weight will reduce tractive effort. That’s not a conclusion: that’s an assumption already built into the formula!! You lose the benefit of any meaningful comparison by ab
Okay, I think I got really close to getting it at the end of your reply, Michael. I’ll continue to ponder it and re-read to see if it will click. It’s close.