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train acceleration vs horsepower

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train acceleration vs horsepower
Posted by gregc on Monday, January 28, 2019 5:35 AM

I'm trying to get a better understanding of horsepower and train acceleration.   looking for quantitative confirmation.

the plots shows train speed v. time for 4 loco horsepower values: red 3000 HP, orange 2000 HP, cyan 1500 HP and green 1000 HP.  Raw results are also listed.

 

horsepower is a measure of work, lb-ft, per unit of time, minutes. This is equivalent to the product of force, lb, and speed, feet per min.    My understanding of this is the tractive effort, the tangential force of the wheel on the rails is equal to the horsepower / speed.   This means the force decreases with speed (the work is the same because a greater distance is traveled in the same amount of time).  (The rectangular hyperbola mentioned in previous threads).

 

the plots also account for a maximum tractive effort of 70,000 lbs assumed for 150 ton locomotive

the plots indicate speed (vertical axis) in mph vs time in minutes (horizontal axis).   The red curve, 3000 HP case, quickly reaches >30 mph w/in 5 min.   ~7.5 mins. for 2000 HP (orange) and ~27 mins. for 1500 HP (cyan).    The 1000 HP case (green) never exceeds ~24 mph because of insufficient HP.

 

the simulation also accounts for train resistance due to friction and aerodynamic drag as described in Armstrong's book, "The Railroad".   His tables indicate that resistance increases from 2.3 lbs/ton @ 10 mph to 10.4 @ 70 for fully loaded cars and 4.5-19.8 lbs/ton for empty cars.  This sim uses resistance for a fully loaded cars.

since the resistance increases with speed and the force from a constant horsepower decreases with speed, there can be a point where both are equal and no further increase in speed is possible.  That is why the 1000 HP case does not reach 30 mph.

 

while the numbers may not be completely accurate, I believe the trends are.  They show that acceleration is not constant.   For limited horsepower, as in the 2000 HP case, it took just a few minutes to reach 15 mph, but an additional 20 mins. to reach 30.    I think it would be interesting if our DCC throttles implemented this behavior instead of constant acceleration rates.

 

the following lists values for each point of each curve.   Assuming a starting speed of 1 foot/sec, acceleration is calculated for each time period (1 min) and the speed is adjusted.

the mass is in slugs (lb / 32.2) for what Armstrong reported as an average tonnage of 4760 tons for a train.

lb/ton is the resistance for the current speed and lbFres, lb-force resistance, is the product of resistance and tonnage (e.g. 1.6 * 4760) and increases with speed

lbFloco, lb-force for the locomotive is the tractive effort (33000 * hp / (60 * fps)) limited to 70,000 lbs

the net force is lbFloco - lbFres

acceleration, ft/sec^2, is the mass / lbFnet

ft/sec is the accumulation of the product of acceleration and time: fps += 60 * fps^2

mph is fps * 3600 / 5280

 #min   mph ft/sec  fps^2 lbFnet lbFloco    HP lbFres lb/ton    mass tonnage
    1   9.3  13.61  0.210  62156   70000  1000   7843   1.65  295652    4760
    2  13.4  19.64  0.100  29690   40399  1000  10708   2.25  295652    4760
    3  15.6  22.87  0.054  15926   28004  1000  12077   2.54  295652    4760
    4  17.1  25.15  0.038  11235   24047  1000  12812   2.69  295652    4760
    5  18.3  26.88  0.029   8537   21867  1000  13330   2.80  295652    4760
    6  19.3  28.25  0.023   6734   20457  1000  13723   2.88  295652    4760
    7  20.0  29.35  0.018   5434   19468  1000  14034   2.95  295652    4760
    8  20.6  30.26  0.015   4450   18736  1000  14285   3.00  295652    4760
    9  21.1  30.99  0.012   3627   18177  1000  14549   3.06  295652    4760
   10  21.5  31.60  0.010   2980   17745  1000  14764   3.10  295652    4760
   11  21.9  32.10  0.008   2464   17406  1000  14941   3.14  295652    4760
   12  22.2  32.51  0.007   2047   17134  1000  15087   3.17  295652    4760
   13  22.4  32.86  0.006   1706   16915  1000  15209   3.20  295652    4760
   14  22.6  33.15  0.005   1427   16737  1000  15310   3.22  295652    4760
   15  22.8  33.39  0.004   1196   16591  1000  15394   3.23  295652    4760
   16  22.9  33.60  0.003   1004   16470  1000  15465   3.25  295652    4760
   17  23.0  33.77  0.003    845   16370  1000  15525   3.26  295652    4760
   18  23.1  33.91  0.002    712   16287  1000  15575   3.27  295652    4760
   19  23.2  34.03  0.002    600   16218  1000  15617   3.28  295652    4760
   20  23.3  34.14  0.002    506   16160  1000  15653   3.29  295652    4760
   21  23.3  34.22  0.001    428   16111  1000  15683   3.29  295652    4760
   22  23.4  34.30  0.001    361   16070  1000  15708   3.30  295652    4760
   23  23.4  34.36  0.001    305   16036  1000  15730   3.30  295652    4760
   24  23.5  34.41  0.001    258   16007  1000  15748   3.31  295652    4760
   25  23.5  34.46  0.001    219   15982  1000  15763   3.31  295652    4760
   26  23.5  34.49  0.001    185   15961  1000  15776   3.31  295652    4760
   27  23.5  34.53  0.001    157   15944  1000  15787   3.32  295652    4760
   28  23.6  34.55  0.000    133   15929  1000  15796   3.32  295652    4760
   29  23.6  34.58  0.000    112   15917  1000  15804   3.32  295652    4760

color=cyan
next
 #min   mph ft/sec  fps^2 lbFnet lbFloco    HP lbFres lb/ton    mass tonnage
    1   9.3  13.61  0.210  62156   70000  1500   7843   1.65  295652    4760
    2  16.2  23.74  0.169  49889   60598  1500  10708   2.25  295652    4760
    3  19.2  28.15  0.074  21744   34753  1500  13009   2.73  295652    4760
    4  21.3  31.26  0.052  15294   29305  1500  14011   2.94  295652    4760
    5  22.9  33.60  0.039  11553   26395  1500  14841   3.12  295652    4760
    6  24.2  35.43  0.031   9027   24553  1500  15526   3.26  295652    4760
    7  25.2  36.90  0.024   7222   23283  1500  16061   3.37  295652    4760
    8  26.0  38.09  0.020   5869   22358  1500  16489   3.46  295652    4760
    9  26.6  39.07  0.016   4822   21659  1500  16837   3.54  295652    4760
   10  27.2  39.88  0.014   3993   21117  1500  17123   3.60  295652    4760
   11  27.7  40.55  0.011   3327   20687  1500  17360   3.65  295652    4760
   12  28.0  41.12  0.009   2786   20343  1500  17557   3.69  295652    4760
   13  28.4  41.59  0.008   2341   20063  1500  17722   3.72  295652    4760
   14  28.6  41.99  0.007   1973   19834  1500  17861   3.75  295652    4760
   15  28.9  42.33  0.006   1667   19645  1500  17978   3.78  295652    4760
   16  29.1  42.62  0.005   1411   19488  1500  18077   3.80  295652    4760
   17  29.2  42.86  0.004   1196   19357  1500  18160   3.82  295652    4760
   18  29.4  43.07  0.003   1016   19247  1500  18231   3.83  295652    4760
   19  29.5  43.24  0.003    863   19155  1500  18291   3.84  295652    4760
   20  29.6  43.39  0.002    734   19077  1500  18343   3.85  295652    4760
   21  29.7  43.52  0.002    625   19012  1500  18386   3.86  295652    4760
   22  29.7  43.63  0.002    533   18956  1500  18423   3.87  295652    4760
   23  29.8  43.72  0.002    454   18909  1500  18455   3.88  295652    4760
   24  29.9  43.80  0.001    387   18869  1500  18482   3.88  295652    4760
   25  29.9  43.87  0.001    330   18836  1500  18505   3.89  295652    4760
   26  29.9  43.92  0.001    282   18807  1500  18524   3.89  295652    4760
   27  30.0  43.97  0.001    241   18782  1500  18541   3.90  295652    4760
   28  30.0  44.01  0.001    205   18761  1500  18555   3.90  295652    4760

color=orange
next
 #min   mph ft/sec  fps^2 lbFnet lbFloco    HP lbFres lb/ton    mass tonnage
    1   9.3  13.61  0.210  62156   70000  2000   7843   1.65  295652    4760
    2  17.5  25.65  0.201  59291   70000  2000  10708   2.25  295652    4760
    3  21.6  31.62  0.100  29447   42890  2000  13442   2.82  295652    4760
    4  24.3  35.65  0.067  19836   34784  2000  14948   3.14  295652    4760
    5  26.3  38.64  0.050  14732   30856  2000  16124   3.39  295652    4760
    6  27.9  40.97  0.039  11471   28469  2000  16997   3.57  295652    4760
    7  29.2  42.83  0.031   9173   26851  2000  17677   3.71  295652    4760
    8  30.2  44.34  0.025   7462   25684  2000  18221   3.83  295652    4760

color=red
next
 #min   mph ft/sec  fps^2 lbFnet lbFloco    HP lbFres lb/ton    mass tonnage
    1   9.3  13.61  0.210  62156   70000  3000   7843   1.65  295652    4760
    2  17.5  25.65  0.201  59291   70000  3000  10708   2.25  295652    4760
    3  24.5  35.98  0.172  50893   64335  3000  13442   2.82  295652    4760
    4  28.6  41.99  0.100  29645   45865  3000  16219   3.41  295652    4760
    5  31.6  46.32  0.072  21316   39293  3000  17977   3.78  295652    4760
 

greg - Philadelphia & Reading / Reading

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Posted by rrinker on Monday, January 28, 2019 10:08 AM

 Hmm, I don't see why this couldn't be implemented with DCC. But not int he throttle. The decoder can measure the load ont he motor via BEMF, and a limit could be established to simulate both the maximum horsepower as well as tractive effort (often the limiting factor when starting out). For older locos you would need feddback to the throttle to light up the wheel sli indicator, whereas in many modern locos you can just (assuming indestrucbile drawbars) just open it wide up and the control system will push as much power to the wheels as it can without slipping. 

 Hmm, good reason to build a DIY decoder, so you cna experiment siwht using the BEMF data, and a couple of the available CVs to enable/disable this feature and to set the limit. You'd basically want to program in somethign that reprsents toe HP of the loco, as well as the maximum TE - a small 1200HP switcher can move a huge cut of cars that might require a pair of 3000HP road units - but the switcher can't get that cut much above 15MPH while the pair of 3000HP road units can move it along at 60MPH. Low gearing and a higher TE to HP rating help the little guy do its work. 

 I think you could also do this in the throttle, but it would be more of a preset simulation and you'd need to change it if you drop the entire train and then move the same engine(s) light. You can do it today in the loco by implementing a speed curve that is not linear, but again then it is fixed no matter what the loco is going - moving light, moving a small train, or moving a train close to its maximum capacity. Actually, this has been somewhat mentioned in the past, using 3 step speed curves (or even the full 28 step tables) to configure some engines for faster response at the low end of the throttle and others for slower response at the low end. Uually discussed in terms of a drag freight loco vs a passenger loco. One example - lots of railroads used Trainmasters on commuter trains because of their high power and ability to get the train up to sped quickly after each station stop - critical in an all stops local. 

                             --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by doctorwayne on Monday, January 28, 2019 5:11 PM

Al Krug did a nice article on tractive effort vs. horsepower.  Unfortunately, I can't seem to find it on-line.

I have a copy of it, but it's rather lengthy and perhaps it's not suitable that I re-post it, as it's not my material. 

A more thorough on-line search may get you better results than my quick look.

Wayne

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Posted by Track fiddler on Monday, January 28, 2019 6:08 PM

Wow that's one large burst of technical data, ... I will give you that.

In scale modeling, I will just call it momentum.  My brother and I had this capability in the later part of the 70s when we were still teenagers. 

You get set up with the right Transformer, or build one like we did.   Then you just crank the knob on your power pack.  Your locomotive or locomotives start to creep very slowly and accelerate up to your command in a realistic time frame. 

Careful though,  they stop doing the same in a more prototypical fashion like your data in reverse I suppose. It takes some getting used to.  They have them with a break option but would be unrealistic at this pointWhistling

If you ever over shoot your destination,  you might as well back up and try again.

Locomotives take awhile to start and take even longer to stop.

Just my thoughts.

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Posted by OldEngineman on Monday, January 28, 2019 10:47 PM

You wouldn't want to use a dcc setup that replicated the real acceleration rates of locos hauling long and heavy trains. You'd be pulling your hair out waiting for the thing to get up to normal speed -- if you'd get there at all.

Running the Conrail SEOP (Selkirk-Oak Point) some nights, with a longish train (say, 120 loads), on the table-top flat Hudson line going south, the engines would be in the 8th notch continuously and it would take 15 miles to begin to edge up close to track speed (50mph). That was with 3 B23-7's, all they gave you for that run.

After dropping cars at Poughkeepsie and Croton -- down to, say, 60 cars -- they'd run much better!

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Posted by "JaBear" on Tuesday, January 29, 2019 1:10 AM

doctorwayne
Al Krug did a nice article on tractive effort vs. horsepower.  Unfortunately, I can't seem to find it on-line.

QuestionQuestion

http://www.n0kfb.org/rail/railphs.htm

"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."

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Posted by gregc on Tuesday, January 29, 2019 7:00 AM

thanks for the comments

rrinker
You can do it today in the loco by implementing a speed curve that is not linear, but again then it is fixed no matter what the loco is going - moving light, moving a small train, or moving a train close to its maximum capacity.

yes, a more logarithmic speed curve, where each speed step gets smaller might model this behavior better than constant acceleration rate programmed into CV3.

My understanding of the use of acceleration rate is to delay each speed step change by an amount proportional to CV3.

The plots show a relatively sharp rise in speed and then each increase is substantially delayed for lower HP locos.

 

rrinker
The decoder can measure the load ont he motor via BEMF, and a limit could be established to simulate both the maximum horsepower as well as tractive effort (often the limiting factor when starting out).

BEMF isn't necessary unless you want to account for grades.   The current speed step is known to the decoder.

 

rrinker
 I think you could also do this in the throttle, but it would be more of a preset simulation and you'd need to change it if you drop the entire train and then move the same engine(s) light.

I think there should be separate values for tonnage and HP.  HP could be programmed into the decoder.   Tonnage should be set for each train in the throttle and would need to be communicated to the decoder.

If this were implemented in a decoder, the loco would presumably start from zero if it lost power, a glitch that causes the processor to reboot.   If this were implemented in a throttle, if the loco lost power it would start at the speed dictated by time since the throttle was increased.  A keep alive would help in both cases.

 

doctorwayne
Al Krug did a nice article on tractive effort vs. horsepower.

I posted a link to the rectangular hyperbola curve showing tractive force vs speed.  

the plots show the effect of tractive effort on speed over time (i.e. acceleration).

 

Track fiddler
Wow that's one large burst of technical data

the plots summarize the data

 

Track fiddler
Your locomotive or locomotives start to creep very slowly and accelerate up to your command in a realistic time frame.

the plots show that a train actually accelerates (gains speed) quickly from being stopped, but acceleration diminshes and it takes longer to get to full speed.

 

http://www.n0kfb.org/rail/railphs.htm

thanks for posting.   It actually mentions F=ma.  it touches several issues: max speed, grades, max tractive effort, ... w/o really summarizing

I think Randy's comment was succinct

rrinker
a small 1200HP switcher can move a huge cut of cars ... but the switcher can't get that cut much above 15MPH while the pair of 3000HP road units can move it along at 60MPH.

i've read more than once that horsepower is speed

interest in the ProtoThrottle suggests modelers are interested in more realism.   Not sure if that interest is in looks or performance.    The ProtoThrottle includes code to specify tonnage that could be used to model acceleration as the plots suggest.   They indicate it's a future feature.

 

 

greg - Philadelphia & Reading / Reading

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Posted by mbinsewi on Tuesday, January 29, 2019 7:22 AM

I've watched and listened to operators using the Proto Throttle, and they say the start up, moving, acceleration, breaking, all feel much more realistic.  One of the participants was an engineer.

I've never operated one, and probably won't, but the way they describe it, it reminded me of the IC Hogger power pack with teathered walk around throttle.  It had momentum and breaking features that took some time to get used to.  I still have it, haven't used it since the last plywood central.

It was nothing like the conventional power pack.

With my small layout, I'm happy with what I have, but watching the demo of the PT was impressive.

Mike.

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Posted by rrinker on Tuesday, January 29, 2019 7:30 AM

 My reasoning for using BEMF is that the decoder could automatically know the 'tonnage' of the train it was pulling. Versus the somewhat unrealistic action of keying it in the throttle (which could just program the decoder)

 Realistic yes, but there's going to be a limit. Imagine you had unlimited room and really could put 50, 100 scake miles of track between towns. How many people would actually do such a realistic op session where at proper scale speed it takes you an actual 8 hours on your feet walking along with the train to get from the starting point to the destination? That would get old, real fast.

 Hence a fast clock, even a moderate one, compressing the 15 miles of a real train getting up to speed to fit within what can be realisticlly built - even then it's too much to try and simulate that accurately. At a 4:1 clock ration, it woult take 1/4 the time, 1/16 the distance - but how many layouts have even that much room even. At best, you'll just reach speed when it's time to brake for the next stop. At worst, you'll be in the next town before even getting close to speed. Which is why I think using momentum, and maybe a non-linear speed table, is about as good a simulation as you are going to get int he confines of model space.

 That does result in decidely non-linear acceleration. While the accel and deccel CVs are linear, setting a static time for changing from one speed step to another, if the speed curve is non linear so that each step is not an equal increment in speed, then you have made a non-linear acceleration curve. Because of the single factor nature of the momentum CV, the speed curve might have to be more exaggerated than it would be if no momentum were in use. Short of adding a second table that controls the momentum for each speed step, that's the only current option and probably close enough, given all the other limitations in the model world. 

                                   --Randy

 


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Posted by dehusman on Tuesday, January 29, 2019 7:39 AM

OldEngineman
Running the Conrail SEOP (Selkirk-Oak Point) some nights, with a longish train (say, 120 loads), on the table-top flat Hudson line going south, the engines would be in the 8th notch continuously and it would take 15 miles to begin to edge up close to track speed (50mph). That was with 3 B23-7's, all they gave you for that run.

The largest layout in my area is ironically part of this territory and has maybe about 10 scale miles of track.  That means that with "prototypical" acceleration, the train would NEVER reach full speed.  There is less than 15 miles, plus at some point the train would have to start to decelerate because it would run out of model railroad and would have to stop.

There is also the consideration of the the dynamics of the actual model train.  A model car weighs lets say 5 oz.  If it represents a load it weighs 5 oz.  If it represents an empty it weighs 5 oz.  A real car weighs maybe 30 tons empty and 130 tons loaded, roughly 4x the weight of an empty.

The speed curves are neat, but they only work when you match them to the weight of the train.  What weight are you going to use for the train?  If John runs 3 SD60's on the point of his 15 car "loaded" grain trains because that's the sizee for his layout space and Jim runs 3 SD60's on the point of his 25 car "loaded" grain train because that's the size for his space, are you expecting them to operate the same?  They both represent the SAME train, a loaded unit grain train.  What train weight are you going to tell the system to use to figure the acceleration?  15 model cars?  25 model cars?  15 real cars?  25 real cars?  100 real cars?

John's layout has only 3 miles of main track.  The grain train will meet 2 other trains along the way.  That means it will only go about 1 mile before it has to stop.  That means that one third to one half of each leg is deceleration. If it takes a real train 10 miles to accelerate to 50 mph, and John only has one half to 2/3 mile to accelerate, what speed will his trains reach?

If you compress these curves to account for shorter model running distances or fast clocks what you will end up with is essentially a square wave, all the acceleration in the first little bit, essentially a jack rabbit start.  That totally defeats the purpose. 

The only way to make it "prototypical" on a model railroad is to make something not match the model scenario.  Either the speed curves have to be changed, the "horsepower" of the engines altered or the "weight" of the train altered to fit the specific situation.  At that point its art not science, the acceleraton of the curve is adjusted to match whatever the observer thinks looks "prototypical".  It has NOTHING to do with the actual physics of the situation.

 

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Posted by BRAKIE on Tuesday, January 29, 2019 8:06 AM

Jst for a fun fact..The majority of the engineers I worked with was more concern over locomotive  tonnage rating vs. tonnage of the train.

I suspect they figure they could get the speed up once they begin moving.

On my ISLs I use momentum and speed step.

Larry

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Posted by BigJim on Tuesday, January 29, 2019 8:34 AM

doctorwayne

Al Krug did a nice article on tractive effort vs. horsepower.  Unfortunately, I can't seem to find it on-line.

I have a copy of it, but it's rather lengthy and perhaps it's not suitable that I re-post it, as it's not my material. 

A more thorough on-line search may get you better results than my quick look.

Wayne

Al Krug's Home page: 
https://web.archive.org/web/20150205123806/http://www.alkrug.vcn.com/home.html

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Posted by doctorwayne on Tuesday, January 29, 2019 12:30 PM

Thanks for that link, Jim.  All that was coming up for me was dead-ends.

Wayne

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Posted by selector on Tuesday, January 29, 2019 12:40 PM

OP wrote: "...BEMF isn't necessary unless you want to account for grades.   The current speed step is known to the decoder..."

This is incorrect.  BEMF is very much needed for our decoders to meter out the proper voltage to the drive mechanism so that acceleration and deceleration are prototypical, and so that the drive responds accurately to the speed table's parameters as the drive mechanisms running characteristics change with lubrication, operating temperature later in the session when the drive loosens, and to account for gear lash and wear over time.

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Posted by gregc on Wednesday, January 30, 2019 6:46 AM

i had asked for confirmation of the plots.  No one seems to dispute their accuracy.   Thanks for the link to the Krug page.   I believe the plots are consistent though Krug is more focused on a train going up a grade which is a bigger deal.

the goal is to recognize how prototypical trains actually behave

rrinker
Realistic yes, but there's going to be a limit. Imagine you had unlimited room and really could put 50, 100 scake miles of track between towns. How many people would actually do such a realistic op session where at proper scale speed

dehusman
The largest layout in my area is ironically part of this territory and has maybe about 10 scale miles of track.  That means that with "prototypical" acceleration, the train would NEVER reach full speed. 

of course most modelers can't realistically model the distances and train lengths of real railroads.    but the ProtoThrottle suggests that many want to model the behavior more realistically.   I don't believe constant acceleration is realistic.

in terms of time, simply rescale the time axis in seconds instead of minutes: ~5 sec for the 3000 HP loco to get to 30 mph, 8 sec for the 2000 HP loco and ~20 sec for the 1500 HP loco.

on the following plot I added curves using CV3 settings of 5, 9, 25 and 40 where the x-axis for these curves is in seconds.   If modelers are willing to use CV3 value similar to these, then the more realistic plot times in seconds should be acceptable on many layouts.

but again, it's not the length of time, but the speed profile that makes it more realistic, depending on hp and tonnage.

selector
BEMF is very much needed for our decoders to meter out the proper voltage to the drive mechanism so that acceleration and deceleration are prototypical

while BEMF can accurately measure speed, it is not necessary for more realistic behavior unless there are severe mechanical issues with the model loco or you're interested in a very accurate speed profile.  

Simply setting the motor voltage for more realistic speed at a more realistic time should be more than adequate.

 

rrinker
My reasoning for using BEMF is that the decoder could automatically know the 'tonnage' of the train it was pulling.

i agree that specifying tonnage is a headache.   But I think using BEMF is not straight forward

  • when should it be measured to determine tonnage?
  • how does it distinguish between an increase in the length of the train vs the train going up/down a grade? (delta threshold)
  • how accurately can it determine tonnage in terms of # of cars accounting for varying wheel resistance on cars (how big an effect is it on BEMF)?

i can see how using BEMF in a yard during switching might be necessary (maybe an option)

 

if the plots are recognized as being accurate and there is a desire to improve realism, then i believe it will take more than just a tweak to the CV3 setting (assuming you means to implement something either in a decoder or cab).   

The profiles are the result of limited HP resulting in decreasing tractive effort with speed and train resistance that increases with speed -- their difference

it looks like there are 3 interesting profiles

  • excessive HP where the profile is basically linear (3 and 2000 HP)
  • less excessive HP where speed increases rapidly initially but reaching full-speed is delayed (1500 HP)
  • limited HP such as in a switcher where max speed is significanly limited depending on tonnage (1000 HP)

greg - Philadelphia & Reading / Reading

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Posted by dehusman on Wednesday, January 30, 2019 9:02 AM

gregc
in terms of time, simply rescale the time axis in seconds instead of minutes: ~5 sec for the 3000 HP loco to get to 30 mph, 8 sec for the 2000 HP loco and ~20 sec for the 1500 HP loco. on the following plot I added curves using CV3 settings of 5, 9, 25 and 40 where the x-axis for these curves is in seconds. If modelers are willing to use CV3 value similar to these, then the more realistic plot times in seconds should be acceptable on many layouts.

Now graph those same speed curves on the SAME time scale.  You end up with a square wave.

The other thing to realize in all this is that railroads power their trains up to certain horsepower per trailing ton based on the TRAIN not necessarily an engine.

If there is a 5000 ton grain train it might get 2500 hp.  If its a 5000 ton manifest train it might get 5000 hp.  If its a 5000 ton intermodal train it might get 12,000 hp.

If you put 2 GP40's on a 5000 ton manifest train, and 8 GP7's on an 5000 ton intermodal train, while a single GP7 has a slower acceleration curve, I guarantee that the intermodal train will out accelerate the manifest train.  The acceleration curve of a TRAIN is the sum of the horsepower of the engines (actually its more complicated).

If you program a 1500 hp engine to top out at 30 mph, that's baloney.  Early 4 axle units were (F3-F7-GP7-RS2-RS3-FA1-FA2) were nominally 1500 hp and easily handled trains at speeds up to 60-70 mph.  The ATSF used 1500 hp engines on its premier transcontinental passenger trains at very high speeds with rapid acceleration.

A railroad powers a TRAIN to the achieve the acceleration it needs for that TRAIN. 

Some railroads have gone away from horsepower/trailing ton (hp/tt) and have gone to a tons per powered axle (TPA).  A "powered axle" represents a specific amount of tractive effort and engines are rated at how many "axles" they have.  A C44AC might represent 10 axles, while an SD40 is 6 axles.  Power on a TRAIN is managed by TPA.  A grain train might get 500 TPA and an intermodal train might get 125 TPA.

Dave Husman

recovering Director of Locomotive Utilization

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Posted by gregc on Wednesday, January 30, 2019 9:46 AM

dehusman
Now graph those same speed curves on the SAME time scale.  You end up with a square wave.

i don't understand why

you mentioned a square wave in your frist response where all the plots were on the same time scale: minutes.

dehusman
If you compress these curves to account for shorter model running distances or fast clocks what you will end up with is essentially a square wave, all the acceleration in the first little bit, essentially a jack rabbit start.  That totally defeats the purpose. 

if you used a 60:1 fast clock, the scale of the plots would be seconds.

if you used a 1800:1 fast clock - 1 sec == 30 mins, then all trains would immediately jump to full speed (i.e. 23 mph for the 1000 hp case).   Is this what your suggesting.

not withstanding the case above, I don't see how time scale affects the profile.

 

i'm not suggesting modeling prototypical increases in speed that take minutes.

i'm suggesting that instead of using constant acceleration to full speed over the number of seconds resulting from the DCC CV3 value, that for hp limited trains, the speed should increase rapidly at first but then much more slowly.

 

that profile depends on the hp of the loco (or combined hp of the consist) and the tonnage of the train.   I've mentioned 3 profiles

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Posted by selector on Wednesday, January 30, 2019 12:42 PM

gregc

 

...while BEMF can accurately measure speed, it is not necessary for more realistic behavior unless there are severe mechanical issues with the model loco or you're interested in a very accurate speed profile.  

 

Simply setting the motor voltage for more realistic speed at a more realistic time should be more than adequate.

 

 

These might help you:

https://dccwiki.com/Back_EMF

http://www.sumidacrossing.org/ModelTrains/ModelTrainDCC/DCCDecoders/BEMF/

http://www.members.optusnet.com.au/nswmn1/y_qsi-back_emf.htm

 

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Posted by ATLANTIC CENTRAL on Wednesday, January 30, 2019 12:57 PM

doctorwayne

Al Krug did a nice article on tractive effort vs. horsepower.  Unfortunately, I can't seem to find it on-line.

I have a copy of it, but it's rather lengthy and perhaps it's not suitable that I re-post it, as it's not my material. 

A more thorough on-line search may get you better results than my quick look.

Wayne

 

This one:

http://hm.evilgeniustech.com/alkrug.vcn.com/rrfacts/hp_te.htm

Al's site is down, this guy has mirrored it.

I don't really have an opinion here since I don't use DCC.

Even with the momentum feature turned off, my Aristo throttles have an "acceleration rate" that is not instant. Being a push button throttle you must hold the "FAST" button for a period of time to achieve full speed.

I have never mapped it to see if it is linear or not, but my natural sense is that it is not. And that by default, the pulse with modulation circuit of the Aristo throttle already provides the effect Greg is looking for.

Sheldon

    

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Posted by ATLANTIC CENTRAL on Wednesday, January 30, 2019 1:31 PM

A few last thoughts:

I have never cared for using any throttle with momentum, DC or DCC.

And one of my dislikes of many DCC throttles are encoder wheel throttle knobs.

So maybe pulse width modulated DC radio throttles, with its ability to provide constant brighness headlights before motion, a reasonable acceleration curve, and reliable slow speed is not so "dated" after all.

But I have never been concerned with having the "latest thing", just with having stuff that works well for my needs.

Sheldon

    

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Posted by ATLANTIC CENTRAL on Wednesday, January 30, 2019 1:47 PM

I forgot one other question/thought?

What about steam vs diesel? In my world I model both, yet in the real world the difference between electric traction, and mechanical traction is vast.

As commented above, a diesel may start a train and run out of power to accelerate it.

But generally, a steam loco can pull at speed any load it can start, limited only by its boiler capacity and its mechanical design tolerance that limits maximum speed.

Great, just what a DCC throttle needs, one more button labeled "steam or diesel".

More on tractive effort here:

http://webspace.webring.com/people/ib/budb3/parts/ste.html

Sheldon

    

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Posted by gregc on Wednesday, January 30, 2019 2:30 PM

selector
These might help you: https://dccwiki.com/Back_EMF http://www.sumidacrossing.org/ModelTrains/ModelTrainDCC/DCCDecoders/BEMF/ http://www.members.optusnet.com.au/nswmn1/y_qsi-back_emf.htm

i believe i understand BEMF, it is the counter current/voltage generated by an armature turning in a magnetic field proportional to motor rpm.   I understand that it can be used to determine if a motor is turning, its direction, as well as speed.

dcc momentum is currently implemented by simply increasing the speed step after a delay specified in CV3.   

why do you think the speed profiles i ploted can only be implemented using bemf instead of simply assuming a linear change in speed with each speed step and varying the delay with each change in step?

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Posted by gregc on Wednesday, January 30, 2019 2:36 PM

ATLANTIC CENTRAL
What about steam vs diesel?

i'm curious too.

i'm trying to get a handle on horsepower, TE and acceleration.   I believe i understand most of the Krug note, at least it makes sense to me.

i assume one difference with steam is that the leverage between the piston and wheel can vary (Johnson bar?).     I don't know if horsepower is constant on a steam locomotive.   that's another study

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Posted by ATLANTIC CENTRAL on Wednesday, January 30, 2019 3:49 PM

gregc

 

 
ATLANTIC CENTRAL
What about steam vs diesel?

 

i'm curious too.

i'm trying to get a handle on horsepower, TE and acceleration.   I believe i understand most of the Krug note, at least it makes sense to me.

i assume one difference with steam is that the leverage between the piston and wheel can vary (Johnson bar?).     I don't know if horsepower is constant on a steam locomotive.   that's another study

 

OK, first, a Johnson bar is the manual control that controls the cutoff rate, and puts the locomotive in reverse. Along with the throttle it controls the application of power after the locomotive is moving.

In real general terms, HP is not so important with a steam loco. Tractive effort and factor of adhesion mean much more in terms of its abilty to start a load and reach a given speed.

You can start learning more here:

http://steam.wesbarris.com/misc/tractiveEffort.php

 

    

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Posted by gregc on Wednesday, January 30, 2019 4:18 PM

ATLANTIC CENTRAL
http://steam.wesbarris.com/misc/tractiveEffort.php

i understand that max tractive force, which depends on coefficient of friction, is simply a fraction (~25%) of the weight on the drivers

i don't understand why changing the cutoff affects the weight on the drivers?   (how does it change the weight of the locomotive)?

and tractive effort is proportional to cutoff (why not always set cutoff to 100%)?    (I thought there was an optimal setting depending on speed)?

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Posted by dehusman on Wednesday, January 30, 2019 5:09 PM

gregc
if you used a 1800:1 fast clock - 1 sec == 30 mins, then all trains would immediately jump to full speed (i.e. 23 mph for the 1000 hp case). Is this what your suggesting.

Correct.  If you compress the curves due to shorter distances and times, the curve "disappears".

Also I think you are misunderstanding "full speed".  The top speed of a 1000 hp road engine is somewhere in the 50-70 mph range (depending on gearing.)  The top speed of 1000 hp switch engine is typically about 45 mph, that's due to truck suspension, not horsepower.

  i'm suggesting that instead of using constant acceleration to full speed over the number of seconds resulting from the DCC CV3 value, that for hp limited trains, the speed should increase rapidly at first but then much more slowly. that profile depends on the hp of the loco (or combined hp of the consist) and the tonnage of the train. I've mentioned 3 profiles

The problem is how do you vary the speed curves depending on the train?

The curves really don't have anything to do with the locmotive, it has to do with the hp/tt of the train.

You have two sets of 3 SD40's.  Same hp, same gearing, same tractive effort.  You put one set on a 10,000 ton grain train.  Its going to accelerate more like the  "2000 hp" curve.  It will max out at about 50 mph.  Take the other set and put it on a TOFC train.  It will accelerate like the "3000 hp" curve.  

Same engines, same power, differnt curves.  Put 3 GP40's on the intermodal train and it will accelerate even faster.  Same hp, better acceleration.

If you put six 1500 hp GP-7's on the intermodal train it will accelerate like the "3000 hp" curve.

The only way to vary the acceleration and make it look right is to assign engines to a specific train and adjust the acceleration for that train. 

If you have an ISL, then it matters even less because below 20 mph the acceleration rate is more determined by the electrical gear and turbocharger than the hp.  For example GE's accelerate really slow.

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Posted by dehusman on Wednesday, January 30, 2019 5:31 PM

gregc
i don't understand why changing the cutoff affects the weight on the drivers? (how does it change the weight of the locomotive)?

It doesn't.

You are over simplifying.  There is a factor of how much power the locomotive can transmit to the rails without slipping.  That is a function of speed, weight on drivers and wheel slip.  That's why a high speed, high horsepower engine is a pig at low speed (until AC engines).  That's why the PRR T-1 4-4-4-4 wa a "slippery" engine.  It had so much power it could easily exceed its adhesion and spin the drivers.  That why AC engines and modern DC engines are so much more powerful than equivalent hp engines of decades ago.  They have so much better wheel slip control.

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Posted by gregc on Wednesday, January 30, 2019 5:44 PM

dehusman
Correct.  If you compress the curves due to shorter distances and times, the curve "disappears".

of course the curves are going to compress if i use an 1800:1 fast clock.   

I've run trains with so much momentum that they take 15+ seconds to get to top speed.   All profiles rapidly get to half top speed in < 5 secs.

dehusman
Also I think you are misunderstanding "full speed".

not suggesting max loco speed but the desired top speed of the train.  of course different trains run at different speeds.   

But the 1000 HP profile demonstrated that the max speed was limited due to the limited horsepower and train tonnage

dehusman
The problem is how do you vary the speed curves depending on the train?

i'm not suggesting changing the speed curves.

i'm suggesting that the current DCC implementation of momentum with constant acceleration is not realistic.

it's not obvious how it can be improved (changing the speed curves for different train seems impractical)

dehusman
The curves really don't have anything to do with the locmotive, it has to do with the hp/tt of the train.

I thought it was made clear that each plot (profile) depended on HP and train tonnage

dehusman
The only way to vary the acceleration and make it look right is to assign engines to a specific train and adjust the acceleration for that train.

it was suggested that that it could be implemented in a throttle if the HP were specified for the loco(s) and tonnage specified for the train.   The throttle sends the appropriate speed step to follow the profile.   This could work as long as the loco did not loose power since the profile would only depend on time.

the ProtoThrottle code has a feature to enter train tonnage as a future feature.

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Posted by gregc on Wednesday, January 30, 2019 5:51 PM

dehusman
It doesn't.

are we looking at the same calculator?

http://steam.wesbarris.com/misc/tractiveEffort.php

changing the cutoff changes the field that says "Weight on Drivers"

i understand that for a given weight, there is a max TE that can't be exceeded due to adhesion.

i expected increasing the cutoff to change, possibly increase the TE up to some max.

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Posted by gregc on Wednesday, January 30, 2019 7:40 PM

less dramatic reduction in TE for steam.  > hp and TE for steam at higher speed.

greg - Philadelphia & Reading / Reading

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