Tractive Effort for Diesels

How to do you calculate the starting tractive effort for a diesel. If you know the weight, horsepower, and number of axles can it be done? What about continuoue tractive effort? Adhesion limit?

Thanks.

let's compare the engine weight with the tractive effort:

you can see, that with AC propulsion technology the ratio can reach up to 0.35 (0.39 at AC 6000 CW) and with DC up to 0.27

The axle count has nothing to do direct with tractive effort,
but with a limited axle load you have to distribute the engine weight on more axle to reach accepting tractive efforts.
(But for the calculation you didn't need the axle count)


With more horsepower the engine can hold the maximum tractive effort over a higher range of speed.
In a tractive effort - speed - diagram you will see a hyperbola that marks the maximum horsepower of the enigne.
And if the engine has more HP, the hyperbola will move/shift to the right.

Hope that helps a bit

I guess I see a relationship to weight. But I was wondering if you could find the traction effort of an engine if you didn't have it?

Also hoping to explain or understand why two engines with same horsepower and same traction motors and the one that wieghs more has a lower traction effort. Seems off to me. Their genatrators are different, but not sure if that is the key.

Ok, next try:

Horsepower has nothing to do with tractive effort !

You get the maximum theoretical tractive effort by multiplying the engine weight with the "Coefficient of friction"; this coefficient is 0.35 for steel on steel (the theoretical coefficient of friction is 0.25, but train wheels aren't polish and you can use sand between wheel and rail, to increase the coefficient of friction up to 0.35)

But to use this theoretical coefficient of friction you need modern electrical drive and control technology! This is the AC three-phase drive!
So the acronym 'AC' *) in a engine type designation doesn't mean that the engine use only an other type of generator! It use inverters to create an three-phase volltage/current that feed the asynchronous machines.

SD 60 ___ - weight 390,000 lb - TE 100,000 lb - cof 0.25
SD 60 MAC - weight 390,000 lb - TE 117,000 lb - cof 0.30

So the only different between these two engines are the type of electrical drive engineering.


*) In the past there where types that use the acronym 'AC' to indicate an AC generator, for example MP 15 DC and MP 15 AC. But the MP 15 AC doesn't use three-phase inverters!


result:

Their genatrators are different, but not sure if that is the key.

No, the different is bigger!
It's quite another type of electrical drive technology.
It use inverter and feed the traction motors with a three-phase current!
And with modern control technics the engine can operate everytime in the best possible adhesion.

This is why all AC-Units produces more tractive effort compared to DC-Units with the same weight.


----


To answer your question:
You can create the maximum tractive effort by multiplying the engine weight (resp the weight on the driving wheels) with the coefficient of friction ( < 0.35 )
My diagramm shows in the column 'ratio' a coefficient of friction close to reality.

Ok. I think I have it now. So the horsepower doesn't tell you how much. It is weight and friction. Thanks. Sorry I am not math major.

I think I see what you're getting at, in the bigger picture, since you originally asked about continuous effort as well. Horsepower isn't related to STARTING tractive effort, but.. well...

Let's take (since your pic is the ALCO emblem) an example of the ALCO-GE 1600 HP units built at the same time with four axles, and with six axles and with the same generators and traction motors. RS-3 and RSD-4 models, that is.

Both are 1600 HP, both use the same generator; we'll give both the same traction motor gear ratio of 74:18. The four-axle unit has a continuous tractive effort rating of 52,500 lbs at about 9 MPH, while the six-axle unit has a rating of 78,750 lbs at about 5 MPH. Note that with identical motors, and with other equipment not limiting (generator, cabling) the continuous tractive effort value PER AXLE in pounds force remains the same. For this example, it's 13,125 lbs per axle.

The six-axle unit develops more continuous effort since it has more traction motors through which to spread the power. It develops that higher effort at a lower speed, since the real limit is load current (heating, actually, due to load current) which increases exponentially with decrease in speed.

So, we see that the six-axle unit has a considerably higher tractive effort and thus drawbar pull at lower speeds. It also weighs more - say, up to 360,000 lbs versus around 250,000 lbs. Using a thumbrule of 25% of adhesive weight on drivers as starting tractive effort, we get a STE of 90,000 lbs versus 62,500 lbs.

If we knew the values per axle for one unit we might be able to guess them for the other as you can see. But you'd have to be sure nothing else were limiting in either case.

One general, broad rule is to take the horsepower, multiply that by 308 and divide that result by the speed you're moving at and you will get the tractive effort developed at that speed. You MUST keep in mind though that "308" is a rough number and that if you take the actual manufacturer ratings for various locomotives you'll find, if you do the math, that the result is a number OTHER THAN 308. They can go from just under 300 to over 310 in fact. But, ROUGHLY, for many purposes it's a good number.

WEIGHT tells you how much force you can apply since for any given type of transmission and wheelslip control, when you use a thumbrule of "percent adhesion" (like our 25% in the example above.) HORSEPOWER tells you how fast a locomotive can move what it's pulling. CONTINUOUS EFFORT is a function of things NOT related to weight but rather to equipment design, gear ratio and wheel diameter in the vast majority of cases.

Vast majority, you say? Let's go back to our ALCO-GE 1600 HP locomotive example. They also built 1600 HP units with A1A-A1A wheel arrangement and thus only four motors but six axles. A good weight value for these units is about 237,000 lbs. That gives us 158,000 lbs on drivers. Starting tractive effort for such units would thus, at 25% adhesion be 39,500 lbs. Now, this design had four of the same traction motors as the others, and same engine / generator too and so theoretically the continuous effort would have been 52,500 lbs. But notice - that's ABOVE the available tractive effort as limited by that light weight on drivers! You thus theoretically cannot overload these units electrically since they'll slip continuously before they'll overload. For units such as this, there really isn't a useful "continuous tractive effort" rating since they cannot overload - but they can't pull a whole lot, either.

So, NO, in starting tractive effort horsepower is out of the picture. Once you're moving, it comes into play at all times.

-Will Davis

Wow Typewriters that was extension and informative post.

Iwas trying to compare the tractive effort of the C-430 and U30B. But thwe numbers listed didn't make sense.
C-430
Hp 3000
Weight 272,000
Traction motors GE 752 (four)
Gear ratio 74:18
Main Generator: GE - GTA9
Alternator: GE Exciter GY27
Tractive Effort (starting) 68,220 lbs @ 25%
Tractive Effort (continuous): 57,200 lbs @ 8 mph
http://www.thedieselshop.us/DataC430.HTML

U30B
HP 3000
Weight: 254,800 lbs
Traction Motors: GE 752 (Four)
Gear Ratio: 74:18
Main Generator: GE - GTA-11AC
Alternator: GY27
Tractive Effort (starting) 70,000 lbs @ 25%
Tractive Effort (continuous): 64,000 lbs @ 10.7 mph
http://www.thedieselshop.us/DataU30B.HTML


Given what I can see they have the same tractive effort.

Surely the size of the engine must have something to do with it ?

Well, look at the figures there -- you've been misled by some poor math, for starters! In the data for the U30B, I see listed an engine weight of 254,800 lbs and the listed starting effort there is given as 70,000 lbs @ 25% adhesion. But those don't match up - if it were 70,000 lbs then engine weight would have to be four times that or 280,000 lbs. Note: 254,800 is on the light side for a U30B; most roads had 'em a lot heavier than that. The New York Central's spec for both types, C-430 and U30B was 275,000 lbs plus or minus a couple percent, with the largest fuel tank offered and with dynamic brake.

Now, as far as continuous tractive effort goes, you have to note a few things here. First, if I recall correctly the standard gear ratio for the C-430 and the U30B wasn't 74:18 but 81:22 although I think some of them were built or modified to 74:18 later. However, if we look just at four axle units with 74:18 gear ratio, there was a progression in improvements in generator / alternator and traction motor design that allowed an increase in continuous effort during production. In other words, earlier units with four traction motors had a continuous effort rating of 53,000 lbs (like the ALCO RS-11, RS-32 for example) and later on 57,200 lbs (C-425 for example) and even later 60,400 lbs (say, well into U23B production.)

I'd have to bet that the given "64,000" there SHOULD be "60,400" and it's a typo.

The horsepower of the locomotive determines the speed at which the continuous effort is achieved. So, you can look at an 1800 HP ALCO RS-11 rated 53,000 lbs at 10 MPH and a 2500 HP GE U25B rated 53,000 lbs at 14.7 MPH and at once see the difference. The higher horsepower of the newer unit pushes the continuous SPEED higher even if the VALUE in force expressed in pounds remains the same.

So, as we see, uprating and improving of equipment can cause a change in continuous effort while a given type of unit was in production - and for the U30B that was rather a long time. However, again, I should note that the standard ratio for the U28B and later on the U30B was 81:22. This gave a higher top speed, but it caused a reduction in continous effort and an increase in the speed at which it was achieved.

As an example, the U30B and C-430 units owned by the New York Central were ALL rated at 55,100 lbs continuous effort at 17.0 MPH. They were built during roughly the same time. Had these units been built with a 74:18 gear ratio instead of 81:22 (81:22 allows a top speed of 75 MPH) then the continuous effort would have been 57,200 lbs with this type traction motor.

Later motors allowed an increase in current and so the continous rating in four axles with 74:18 ratio went up from 57,200 to 60,400 lbs. The continuous speed for any type unit with this gear ratio and equipment depended upon its horsepower rating.

Now, there is ONE more thing to note. Both types of unit were available with optional "power matching" equipment. (The NYC units didn't have it.) What this equipment basically did was reduce power output at low speeds so that the units could operate with older units that had continuous speeds down near 10 or 11 MPH. If you ran units of say C-430 or U30B with old EMD F-units, heavily loaded at low speeds with the throttle wide open the newer units would either be slipping and spinning, or else overloading (to put it VERY generally, and the other guys would call me out on this if I didn't note GENERALLY) and so this optional equipment was available which lowered power at low speeds. THAT would be one reason why you might see a unit like a U30B (or U33B or U36B) with a rated continuous speed of something lower, around ten or so. The setting was adjustable with resistors in the control compartment. It did this even with the throttle wide open.

(Keep in mind that with standard equipment and no automatic power matching the minimum continuous speed for C-430 / U30B was 17 MPH as noted above, with 81:22 gear ratio - if you used 74:18 you would almost be FORCED to apply power matching to reduce torque at the driving wheel rim at low speeds or else spin like crazy.)

Hope that helps!

-Will Davis

Typewriters Yes that did help!

I appreciate your taking the time to explain all of this. I was looking at the figures for different engines and it wasn't making sense. I will print all of this out and read it in detail. Thanks again.

Finally got a chance to read through all of this. Fascinating stuff, gents, thanks to all for the information; I learned a lot!

Typewriters or anyone. I was wondering if starting traction effort was the same as adhesion limit? At least that was what I gathered from everything here. I sure hope I didn't miss the boat.

Another thing I just can't grasp.

Typewriters said:.

As an example, the U30B and C-430 units owned by the New York Central were ALL rated at 55,100 lbs continuous effort at 17.0 MPH. They were built during roughly the same time. Had these units been built with a 74:18 gear ratio instead of 81:22 (81:22 allows a top speed of 75 MPH) then the continuous effort would have been 57,200 lbs with this type traction motor.

How do you calculate the 57,200 continuous effort with the 74:18 gear ratio?
What about the 55,100 lbs continuous effort at 17.0 MPH with 81:22 gear ratio?
Thanks

Adhesion is really a fancy word that often leads to confusion. Starting tractive effort is the force a locomotive can exert at the rail in a direction to move the train (ie parallel with the rail) and it is convenient for comparsion to express that as a percentage of a locomotive's weight on driving wheels.

So, a locomotive weighing 280,000 lbs with all weight on drivers, built back in the late 60's say can exert about 70,000 lbs starting tractive effort. That's 25% of the weight on drivers, or "25 per cent adhesion."

Now, as to continuous effort: One model of generator / alternator with a given model of traction motor and a given gear ratio will have a given continuous tractive effort. Sometimes generator / alternator isn't the limit, and it's just the traction motors which are. For example, taking the ALCO line of locomotives from late 1967 as an example we can pick out:

C-420 / 2000 HP / B-B / 74:18 / Continuous speed 10.2 MPH
C-425 / 2500 HP / B-B / 74:18 / Continuous speed 12.8 MPH

Both of those units are developing 57,200 lbs continuous effort at the speeds indicated. Now look:

C-420 / 2000 HP / B-B / 81:22 / Continuous speed 11.4 MPH

At this continous speed the effort is 55,100 lbs. Every four axle unit in the line at this time had the same effort continuously for the same gear ratio - but the higher-HP units were running at a higher speed at the continuous rating. Change gear ratio, and you change continuous speed and continuous effort. Continuous effort and continuous speed are functions of traction motor current limit and gear ratio.

There are complicated ways to calculate these things, I suppose (actually having been a nuclear engineer I can guarantee there are) but you are FAR better off to assemble for yourself a good pile of technical materials and glean the information from these. You will begin, shortly, to see certain numbers crop up over and over and begin to recognize them and associate them with locomotives and eras. You'll know, eventually for example that the C-420 listed above was first built with lower-rated motors that only allowed 53,000 lbs with 74:18 ratio and later on the higher rated motors came along that allowed 57,200 lbs with 74:18 ratio - so that if you see data for a C-420 that gives "53000 lbs CTE" you can say immediately to yourself "oh, that's an earlier one, not a later one." See?

OR, if it is a C-420 built right at the end of production you can say "wait a minute, that info is probably wrong since they were rated higher than that before the end" and then you can look into it. As we already know, there's a lot of wrong information out there.

After seeing all this (and NOT to insult anyone) I believe that our upcoming book will by necessity include some information to cover these kinds of topics. I cannot believe that this isn't already covered in good detail, accurately, SOMEWHERE findable on the net or in print but I must assume that it's not. Graphical presentation will certainly help (and we have LOTS of that stuff, too.)

Work on this book progresses quite well, by the way.

-Will Davis

Thanks Typewriters.

If I may be so bold as to ask what is the book about? And can you any kind of estimate as to its release date?

Thanks again.

These descriptions are great for how tractive effort is a function of weight, speed, efficiency, and horsepower. However, there are advertisements in Trains for the new gensets and others that claim 50 to 65% improvement in adhesion efficiency. How can that be since it does not conform to the formula? Are they held to the track by some electromechanical force, or are the wheels and gear ratios different, or....? Thanks for any information you can give.