I use the above TE chart and plugged in weights to figure out limit of adhesion. I used the 630,000 lbs for the DE and 280,000 for the steam engine. I could have sworn the steam would be heavier but I guess not. Then again I guess this is only taking into account weight on the drivers and not the leading/trailing trucks or the tender. The results do not reach the same conclusions that merely looking at TE do as I suspected when I said looking at one thing would favor one engine and looking at another thing would favor the other.
In terms of Limit of Adhesion, each engine is equal at 0 mph. That’s not a useful speed though. Above 0 mph, in terms of Limit of Adhesion, the diesel walks away from the steam engine and downright destroys it as speed increases. At a mere 35 mph, the DE has 343% more Adhesion which is basically where the peak advantage is! Over most of the speed range the diesel has 3 times the adhesion limit that the steam engine does. That means the steam engine can’t get nearly as much of it’s power to the rails without wheel slippage or another way to think about it is that the DE can get more of it’s power to the rails before slipping. TE didn’t tell us everything and certainly not everything we need to know when it comes to pulling a train. We need to know TE and adhesion to make a final conclusion but each has to be compiled as the results of each on their own isn’t enough either.
Now we need to look at this information further. If one engine has the power to pull a train of a certain weight a certain speed, does it have the traction to do so? Ultimately you are limited by both power and traction. If an engine has the adhesion to put all of it’s power to the rails, at what point does it not have the power to pull anymore?
With all due respect, I must ask again: What is comparable about the two locomotives? You are comparing a steamer to a diesel with the steamer having higher maximum horsepower and higher tractive effort than the diesel. And then you conclude that the steam locomotives in general are fundamentally better than diesels in general at producing tractive effort.
Both locomotive types develop their highest tractive effort at lower speeds and then exhibit a fall-off as their speed increases, even though accelerating the train to higher speeds requires increasing tractive effort.
Here is the comparison that I would like to see. This is a table you posted earlier. It showed the steamer producing a maximum of 8,400 hp at 55 mph. If I change the comparison so the steamer develops a maximum of 5,600 hp at 55 mph, what are the other numbers? And then what is the tractive effort for each locomotive at each speed?
The whole problem is the question. You can’t determine “better” until you define “at what” and “where”.
It’s kind of like asking whether a Clipper ship or a modern day oiler is “better.”
The “at speed” part of the question helps - but only a little bit.
The tractive effort vs. speed curves for each machine are what they are by virtue of their nature.
Tractive effort x speed = power always per Newton!
Different routes, trains, schedules and specific locomotive are going to give different results.
Worse yet, diesel locomotives changed RR operations. We don’t even have good trains and routes to compare. How about a 9000 ton train eastbound on the B&A or a eastbound 125 car Powder River coal train going to a plant in GA? These simply would not and could not exist in the steam era. Are these good or bad?
No. You need pick a speed. At lower speeds, the Steam engine starts out as a lower TE/ lower hp engine. The Diesel-electric has plenty of both – it’s just that the train doesn’t need it there.
At the speed at which the engines have equal TE – and the same ability to move the identical train – above that point the Steam engine develops a higher TE than the Diesel-electric.
At the speed at which the engines develop equal HP, above that speed the Steam engine continues to develop horsepower and the Diesel-electric doesn’t.
Maybe another way to phrase this is no matter what the metric, at each point where those metrics are in fact comparable – that is, at which the engines are exactly comparable – above those points the Steam engine develops a greater ability to move a heavier train faster.
Let me get back to your comment from one other perspective.
You have wanted to compare outputs of one motive power type at 20 mph with the output of another at 60 mph and call them comparable, even though one is at the maximum of a power curve and for the other motive power, it is near the end of its power curve.
Say your Ford pickup develops 150 hp at 20 mph. And for comparison, you can find a vehicle that develops the same hp at 60 mph. Indeed, you look around for a vehicle that specifically can develop no more hp at 60 mph than your pickup first does at 20 mph out of some sense of “fairness”.
You are probably comparing your heavy Ford pickup with a Volkswagen and saying that they
TE is really nothing more than torque at the wheels. It is not horsepower though. Horsepower does work and torque does not. However torque tells us nothing about traction. You could have all the TE in the world but without traction, you are spinning your wheels. Limit of adhesion is the theoretical limit of traction on the rails. TE and weight on wheels determine this. Divide weight by TE to figure out the theoretical Limit of Adhesion.
I always figured that a steam engine was a much heavier device, power for power than a diesel. However much of this weight is not on the drive wheels which I never thought about before. Even though the total weight of the engine and tender may equal one number, the amount of weight that contributes to limit adhesion is apparently much less.
I keep running more numbers and keep learning more. I thought I had it until this last revelation and then it was pretty much “duh!”
TE doesn’t tell us how much a train can pull. It tells us torque at the wheels at any speed and that’s it. It doesn’t tell us how much traction we have or if we are going to spin away all day going nowhere. Adhesion tells us how much weight an engine can pull before losing traction on it’s drive wheels at any speed. Notice neither of these say anything about horsepower. Horsepower is a measure of work. It is the ONLY measure of work. The answer then gets quite simple.
If a steam engine and a diesel/electric engine have the EXACT same horsepower at the rails, the engine with the most traction will pull the train the fastest. If each engine has the same amount of horsepower at the wheels and the same amount of traction at equal speeds, they will pull an equal amount. Ultimately horsepower does work. Torque doesn’t and neither does traction. We just need to know the other stuff to determine how much available traction we have as well as using the information to determine useful wheel size and gearing.
Lots of TE with no adhesion doesn’t pull trains very well. Lots of adhesion with no TE doesn’t pull trains very well. Lots of TE and adhesion with no horsepower doesn’t pull trains very well. HP tells us how much power we have available to do the job. TE tells us how much torque is at the rails which is necessary to know for gearing purposes/wheel diameter and is a variable in Limit of Adhesion and Limit of Adhesion tells us how well we can get the power to the ground within our traction limits. That pulls it all together. The one that wins is the one that has the HORSEPOWER to do the job AND the TRACTION to grip the rails.
No. You need pick a speed. At lower speeds, the Steam engine starts out as a lower TE/ lower hp engine. The Diesel-electric has plenty of both – it’s just that the train doesn’t need it there.
At the speed at which the engines have equal TE – and the same ability to move the identical train – above that point the Steam engine develops a higher TE than the Diesel-electric.
At the speed at which the engines develop equal HP, above that speed the Steam engine continues to develop horsepower and the Diesel-electric doesn’t.
Maybe another way to phrase this is no matter what the metric, at each point where those metrics are in fact comparable – that is, at which the engines are exactly comparable – above those points the Steam engine develops a greater ability to move a heavier train faster.
Let me get back to your comment from one other perspective.
You have wanted to compare outputs of one motive power type at 20 mph with the output of another at 60 mph and call them comparable, even though one is at the maximum of a power curve and for the other motive power, it is near the end of its power curve.
Say your Ford pickup develops 150 hp at 20 mph. And for comparison, you can find a vehicle that develops the same hp at 60 mph. Indeed, you look around for a vehicle that specifically can develop no more hp at 60 mph than your pickup first does at 20 mph out of some sense of “fairness”.
You are probably comparing your heavy Ford pickup with a Volksw
The fact that the steamer in that chart has more horsepower doesn’t mean it can pull as much. It’s got the HORSEPOWER to pull more. However if the steam engine runs out of adhesion (traction) and spins it’s wheels before the diesel runs out of power and adhesion, that extra steam engine power didn’t do anything useful. You have to know TE, ADHESION, and HORSEPOWER to same a comparison as to which will physically pull more down the rails in the real world. If the steam engine with it’s higher horsepower can maintain adhesion past where the diesel runs out of power and/or adhesion, then in fact it can do more work. Work over time and distance is what we want done.
I think that the calculation is performed the other way around – multiply weight by factor of adhesion (expressed as a percentage); and the product is TE. And this calculation is only valid at low speeds, where TE has not yet become a function of speed.
For example, as a standard AC4400CW decelerates, it will reach its design maximum TE of 180,000 lbs at about 9 mph, assuming that rail conditions are ideal. If the unit weighs 412,000 lbs, that equates to adhesion of almost 44%. However if rail conditions are poor (i.e. during a light rain or with contamination by grease or leaves), the adhesion will be considerably reduced; and when the unit slows to 9 mph, it will produce a maximum TE of less than the 180,000 lbs. In other words, the unit’s adhesion is a function of the unit’s traction-control system and rail conditions.
As the unit accelerates above 9 mph, its TE becomes increasingly less dependant on adhesion and increasingly more dependant on speed. It’s as if there’s only so much horsepower to divide between the production of TE and the production of speed. So for any given horsepower level (i.e. throttle notch), TE has to decrease if speed increases. And throughout this entire process (i.e. at or below 9 mph and then during an acceleration above that speed), the adhesion can remain constant (i.e. 44% if rail conditions are ideal or considerably lower if rail conditions are considerably less than ideal). In fact, if we took our hypothetical unit and increased its weight from 412,000 lbs to 432,000 lbs but left all other factors (i.e. rail conditions and speed) the same, the unit’s adhesion would not change. However it would produce more TE (especially at low speed and up to the design max
I understand exactly what you are saying about adhesion being a necessary factor to consider in order to determine what a locomotive will do in the real world. But we can still compare locomotives on the basis of horsepower and tractive effort alone just in the interest of discovering how much horsepower and TE they each produce at various speeds. Then if we want to go further, we can factor in adhesion for both locomotives to refine the comparison.
Well, an engine which reaches a max hp output at 4,000 rpm just isn’t comparable with one that reaches its maximum hp output at 8000 rpm, even if the final hp is the same. They reach their maximums at completely different points which, by definition, means they are not “comparable.”
In essence, what you are looking for is an engine that matches your max output at 4000 rpm, you just don’t want it to match it at 4000 rpm because a particular argument falls apart. So, you specifically reject any engine that actually matches your engine at 4000 rpm. In the name of “comparability” you impose your engine’s 4,000 rpm limit on another engine at 6,000 rpm, so that you can say “see, this engine performs no better at 6,000 rpm than my engine at 4,000 rpm!” Well, that’s because that’s how you defined it; but it has nothing to do with the fact that an engine like yours at 4,000 rpm might very well outperform it at 6,000 rpm; your engine just happens to max out at 4,000 rpm so you want to impose your engine’s performance limit at 4,000 for all other engines at all rpms above that, and exclude all engines that might outperform it. You do this by imposing the additional limit that it can’t match your engine at 4,000 rpm; and that is just in case there is an engine out there that can match your engine at 4,000 rpm, and can do it better at 5,000 rpm since yours has maxed out at 4,000 rpm.
Well, your set of rules pretty much protect your engine and that fact that it peaks pretty early on the performance curve. And by imposing its limits at 4000 rpm on all alternatives at all rpms, you pretty much assure the result you want no matter what.&nbs
For a diesel, that’s done by assuming a given factor of adhesion and graphing a TE-versus-speed curve for the various throttle settings on the locomotive, each of which represents a given horsepower level. The result is a presentation of how, for a given locomotive model with a given weight and adhesion, speed and TE are functions of horsepower.
You were right, that graph did originate in Johnson’s book-- in fact it’s a bit worse there. I guess he gave the job of drawing the illustration to some draftsman without explaining just what it needed to show, and the draftsman didn’t know from horsepower, and Johnson didn’t check it. If anyone had pointed the error out to him he would have immediately tossed the drawing in the wastebasket.
I was hoping somebody could catch what I’m referring to, but maybe we’re in worse shape than I thought. The graph as you reproduced it (from Brown?) shows two locomotives whose horsepower curves cross (i.e. are equal) at 19 mph but whose TE curves cross at 6 1/2 mph.(The graph in Johnson’s book shows power equal at 19 mph and TE at 5 mph.)
You remember what horsepower is, and you should know that’s absurd-- if two locomotives have the same rail horsepower at the same speed they have the same TE, by definition.
Who knows who figured what out. They didn’t bother to publish anything so obvious, is all we know.
Who’se “us”? I’ve shown you two examples; one in three printings by a major publisher over a forty year period – I do in fact assume that implies “wide usage”, and one which was produced in the largest circulation engineering journal of its time and which has been reproduced multiple times including when I first saw it in 1973.
For those who don’t have that one handy, don’t imagine you’re missing any info crucial to this discussion. Bruce preceded the graph with this:
"The greatest advantages of the diesel-electric locomotive over the steam locomotive of today are the number of multiple-unit driving axles that may be concentrated under the control of one operating crew and the fact that full engine-power output is much more quickly available for transmission to these driving axles for train acceleration. These characteristics make the diesel-electric locomotive a far superior unit for undulating profiles or frequent stops.
"A typical 484 [Bruce liked to leave the hyphens out] steam locomotive of 6,000 ihp with an axle loading of 70,000 lb will have a weight of about 280,000 lb on drivers with a tractive effort of perhaps 70,000 lb maximum. A 6,000 bhp diesel-electric three-unit [A1A-A1A] locomotive with an axle loading of 53,000 lb will have a weight of about 630,000 lb on drivers and a tractive effort of perhaps 157,500 lb.
"A typical 4884 steam locomotive will have a weight of about 540,000 lb on drivers with a tractive effort of 135,000 lb. A 6000-bhp four-unit diesel-electric locomotive will have its entire weight of about 920,000 lb on drivers with a tractive effort of 230,000 lb. On the basis of the above figures alone, the superiority of the diesel-electric is obvious, but there are additional advantages.
"The wide acceptance of the diesel-electric is, of course, largely due to the absence of the boiler and its servicing requirements at terminals and also to the fact that generally maintenance requires only the replacement of worn parts by spare units. In operation the diesel-electric is
Not true. Wheel size alone can change that. TE is basically torque at the rails. Torque is not horsepower although it is mathematically related. If you keep the same horsepower at the rails but increase wheel diameter, you now have a larger wheel that is turning slower at the same speed. It’s tractive effort will in fact go up. A smaller wheel with the same horsepower at the rails at the same speed will have a lower tractive effort. Go plug in the numbers into the TE formula. Your statement is only true if each locomotive has the same rail horsepower at the same speed AND the same wheel diameter. When wheel speed goes up, torque goes down. Wheel speed goes down, torque goes up. They are inversely related mathematically and very easy to prove.