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
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.
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
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 point
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.
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!
doctorwayneAl Krug did a nice article on tractive effort vs. horsepower. Unfortunately, I can't seem to find it on-line.
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."
thanks for the comments
rrinkerYou 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.
rrinkerThe 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.
doctorwayneAl 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 fiddlerWow that's one large burst of technical data
the plots summarize the data
Track fiddlerYour 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.
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
rrinkera 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.
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.
My You Tube
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.
OldEnginemanRunning 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.
Dave H. Painted side goes up. My website : wnbranch.com
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
Conductor.
Summerset Ry.
"Stay Alert, Don't get hurt Safety First!"
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
.
Thanks for that link, Jim. All that was coming up for me was dead-ends.
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.
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
rrinkerRealistic 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
dehusmanThe 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.
selectorBEMF 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.
rrinkerMy 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
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
gregcin 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
dehusmanNow 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.
dehusmanIf 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
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.
...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.
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
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
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.
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
selectorThese 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?
ATLANTIC CENTRALWhat 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
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
ATLANTIC CENTRAL What about steam vs diesel?
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
ATLANTIC CENTRALhttp://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)?
gregcif 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.
gregci 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.
dehusmanCorrect. 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.
dehusmanAlso 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
dehusmanThe 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)
dehusmanThe 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
dehusmanThe 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.
dehusmanIt doesn't.
are we looking at the same calculator?
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.
less dramatic reduction in TE for steam. > hp and TE for steam at higher speed.