Hi all. Most of us are probably aware that real diesel-electric locomotives, at least the transition era ones, control their speed via eight notches of power. As I understand it, basically the notch corresponds to how much horsepower the prime mover is sending to the traction motors. I expect the details may be more complicated.
So my question is, why don’t decoders allow you to control your model train in this manner? It seems like it should be easy to implement. Simply convert the eight throttle settings into eight corresponding wattages to send the motor, and let physics do the rest. Say notch 4 gives your engine 1.5 watts. It might be going a decent speed on the flats, then pulling cars up a grade it’ll slow down and you’ll have to notch up, just like the real thing. The manufacturer could have set a default notch to watts conversion, and allow the user to adjust it if wanted.
I googled this a bit to see if some decoders already support this. The closest I got was “manual notching” that seemed to only manually adjust the sound of the prime mover. The idea being, you’re going up a grade, you can put the prime mover at notch 8, and dial down the speed step to what your best guess of train behavior with be. My solution seems both easier and more prototypical.
Are there decoders that can do this, or does anyone see a reason why it’s not feasible? If not I’m thinking I’ll make my own, though it’s just another thing on a long list of projects.
Yet another one of those things that comes up all the time. The answer is pretty simple - physics does not scale.
Back at the beginning of DCC, the original spec had only 14 speed steps, 28 was proposed, and a few held out for an option of 128. The 14 step people were always going on about how real locs only have 8 notches, and 14 is almost double.
Well - physics doesn’t scale. Momentum is what keeps a real train moving. A finer control than 8 notches isn’t really needed (but there were some with 22 notch controllers, and Baldwin switches had an effectively infinitely variable air throttle, but those are special cases). When the engineer gets his train up to speed, and backs off the throttle, the train doesn’t really slow down. Not for a long time. Unless it’s going upgrade. By varying the throttle through just a limited number of sections, the engineer can keep the train moving at whatever speed is dictated by rule. I won’t say it’s ‘easy’ - it’s a lot harder than it looks, just try one of the train simulator programs, and then realize even those are not 100% realistic. It’s nothing like a car where you have near instantaneous control over speed.
The other half of the equation is brakes. You can certainly do the acceleration in 8 steps using momentum, and maintain the train’s progress with momentum, but you also need momentum to keep the train moving, or even gain speed, with the throttle reduced. ANd to actually stop, you need brakes to actually actively slow you down.
Most of the newest DCC Sound decoders have these things. Switching between them all with buttons to activate features and functions isn’t the easiest thing int he world. However, there is a solution - check out the Proto Throttle from Iowa Scaled Engineering. Watch some videos of people running locos with it. It works (and even looks) like the real thing. 8 speed notches, brakes, etc.
I know of at least one very prominent model railroader (I’m sure there are others) that are currently using the 14 speed steps in their decoders / command stations. Only the first 8 steps are actually used and steps 9 through 14 are set to the same as 8.
The trick is that the acceleration / deceleration (momentum) are both set very high (if not maxed). Thereby the speed transition between steps is very smooth.
I’ve tried this out of curiosity, and it IS very effective. However, like operating a real train, it really takes a lot of getting used to. There are NO quick starts and stops by no means. You really have to develop a feel for the control and really plan out your starts and stops. An added bonus - and almost a necessity - is that you will really need to learn how to operate that brake feature as well.
I did get used to it and knew precisely how much time / space I needed for what I was doing, but there was no way possible I was going to hand it off to any of my regular operators and expect them to run it ! But then … you don’t give a greenhorn the “keys” to a real train and expect him to get from A to B without destroying something either !
If you’re bored some time, set one up and give it a try. Give it more than a few minutes though. You might find it rather interesting.
Besides the eight notches common to many diesels there was transition to consider as well.
Once momentum got the train rolling and the amperage fed to the traction motors in series dropped off the engineer would reduce the throttle, switch the transition lever to series/parallel (later engines had automatic forward transition and manual downward transition) then to parallel shunt. So the engine RPM didn’t always exactly correspond to the running speed.
The ESU “Full Throttle” feature allows you to select either the engine RPM or the motor RPM independently. Other decoders use “notch up/down” and some rely on BEMF to mimic load.
I do like the “Full Throttle” feature but it can get bothersome after a while.
GG-1s had 22 notches, if I remember (it’s been a LONG time since I ran one of those).
Seems to me that the very early GEs like U-25s had “half-notches” along with the “full” ones. I only recall being on one U-25, ever (hostling it around in Danbury yard).
The AEM-7’s had a single “notch”, just off idle. Beyond the first notch the throttle was stepless to maximum.
The HHP-8s and Acelas have completely notchless throttle levers.
Prototype diesel electric use the electric power part of the drivetrain as a torque converter in an automatic car transmission would do. In fact, all those electrical gubbins are “just” the transmission, electric power only need just the traction motors. Diesel electric also carry a portable electrical generating power plant, just as a hybrid car does, sort of. There are Diesel railcars that have been built with mechanical gearboxes, like a bus on rails, but no mechanical gearbox can efficiently handle the massive torque delivered by a prototype locomotive.
Torque delivered to the rails maybe as low as zero rpm (zero speed) there and as high as maximum rpm at the Diesel engine. Those steps are not speed steps but torque steps (equivalent to “power” at the Diesel engine output but not at the rails, the locomotive has to move for there to be power there.)
Apart from the physics issues, which are not insurmountable, you can’t model this with a direct geared drive.
What a sound decoder can and does now do is model the sounds of the Diesel engine running at different rpm (and loudness!!) while the locomotive does not change speed, as per prototype. Acceleration and free running or deceleration effects can also be modelled by the sound board, the motor cannot.
Model locomotives do not have variable ratio transmission.
decoders are given a speed value and provide a proportional voltage to the motor. momentum determines the time between in/decreasing the speed one step at a time. but for constant HP, acceleration varies with speed.
the relative power of an electric motor in a model is vastly greater than a prototypical locomotive, such that it can accelerate a model train much quicker
but even the small processor in a throttle can calculate the drawbar force and resulting acceleration and speed given the tonnage (# cars) of the train (~50 ton/car) knowing some simple frictional forces. the throttle would give the speed to the decoder.
imagine the difference in performance between pulling the tonnage of 10+ fully loaded cars and then just pulling one car to switch in an industry
since the knob or lever on the throttle doesn’t dictate speed, a separate braking control is necessary to slow/stop the train. most modelers are unfamiliar with westinghouse air brakes which can’t be partially reduced like car brakes. so both increasing and decreasing the speed of a train would be different from what most modelers are use to.
the ProtoThrottle has an unused tonnage setting that suggests they were thinking of doing this. my understanding is many ProtoThrottle enthusiasts are fond of the need to use brakes.
It is hard for me to imagine that you cannot model a diesel-electric locomotive, whose final drive consists of an electric motor permanently geared to wheelsets, with a model driven by an electric motor permanently geared to wheelsets.
The complication is in simulating the power output of an engine with, say, an 8-notch Woodward governor commanding crankshaft output speeds, directed through a generator/alternator with variable excitation, with DC traction power being periodically switched series/parallel or field weakening being applied to motors. This is a complex thing but can certainly be simulated; the question is not ‘if’ but ‘how much extra will it cost’ and ‘will model railroaders want it’. We had a recent thread on this in which the preponderance of ‘vocal’ commenters did not like the idea much, either in general or in how one manufacturer, ESU, appears to be directing their product evolution.
AC locomotives of course control their transmission much differently, and their motor output characteristics are very different from DC designs. But there, too, the motors are permanently geared to the axles, so emulating the torque commanded to the motor over time could duplicate the ‘real’ locomotive’s performance well enough on a model.
Model locomotives only have the fixed gear ratio traction motor part of the drive. With only that you cannot model diesel electric or diesel hydraulic drives. You can model electric and steam power nicely.
Apart from wondering why anyone would want to model diesel electric accurately I am quite sure it could be done.
To model it as we do for model railroads all that is required is a DCC with sound. Prototypical locomotives do not accelerate in discrete steps (physics doesn’t allow this anyway) just because the drive system has a step type controller. So, you would be modelling a progressive stepless acceleration using a stepped power plant controller (DCC already offers three ranges of step control, being digital it has to be stepped in some fashion).
The only prototypical reason for diesel electric drives is the advantages of the portability of the electrical power plant. Otherwise it makes no sense.
First, in building a model of a prototype, it is certainly not inappropriate to model its prototype behavior. And that is what the very complex PWM synthesis of the behavior over time of the ‘rest’ of the diesel-electric powertrain accomplishes … for those who appreciate the fidelity.
Yes, the waveform that is sent to the motors will be complicated, and yes, many parts of it will be deemed unneeded or even undesired by many model railroaders. Likewise the single electrical signal out of a high-end hi-fi system contains far more information about its prototype ‘sound’ than, say, a MIDI electric piano reproducing the musical score would. The result at the ‘output device’ is not constrained by the relative mechanical simplicity of the power transducer used in either case … and to the extent the result is unpropirtional ‘as a result’ we can apply the equivalent of Dolby correction or wave shaping.
Second, there are in fact well-known examples of motor-electric drive in larger scales, although none with the pretense of miniaturizing a ‘scale’ 645 or traction alternator…
For a model railroad model it makes no sense to model the prototype diesel electric drivetrain. It can be modelled much more easily by mere representation.
In case you are wondering, that is what I got done saying when you tried making that point a different way.
The discussion is about the other ‘half’ of the “representation” which is allowing model controls to be made and manipulated in ‘prototypical’ ways that produce the ‘prototypical’ simulated behavior in the model.
As noted, not all modelers will find this attractive, or interesting, or necessary for anyone to do … all that is in previous comments. On the other hand if you were building a training simulator for real railroaders, this would provide it… or any more ‘forgiving’ adaptation for early training or just ‘more prototypical’ fun.
Well, there are the Kato P42’s with axle mounted coreless motors in HO, simulating traction motors.
But no internal cumbustion prime mover turning a generator inside.
Hornby did live steam in HO, but it’s not exactly practical when you get down to that size. The most common diesels in larger ride-on scales seem to be gas-hydraulic, mainly because appropriate hydraulic pumps and motors are pretty easy to source. Ones that size with electric motors tend to be battery powered, no engine of any sort.
Plus the Rapido ex-New Haven FL9s where the prime mover is shut down and the traction motors are powered off the DC third rail. About as close to model railroad simulation you can get.
That’s just simulated in the sound decoder though, those locos have a typical electric motor and gears driveline. I don’t think the 3rd rail pickups on the Rapido models even actually work.
Reading about the Hornby live steam project on UK Hornby site it seems it may have failed largely because it was not understood. The speed control was actually quite good but difficult to understand. I nearly bought one just to have such a tiny marvel but it seemed too impractical. The difference between a display model and a real steam locomotive was insufficient to prompt the purchase. Also, at the time it was about 3x the price of an all electric plastic bodied steam locomotive.
My opinions on this subject are influenced by my perception of what this hobby is about. It’s more art than science. More theatre than engineering.
When you get up to 1 gauge live steam then you transition from art and theatre to science and engineering. Smaller gauges are mostly representational art rather than accurate engineering. The Hornby project at least served to illustrate that.
"Maybe just me … I’ve read this three times and still don’t understand what you’re driving at. "
Prototypical diesel electric locomotives may have 8 step controllers but they don’t react in 8 audible or motion steps.
Engine rpm is not connected to or even related to speed.
The real problems with scaling physics is scaling friction is not possible.
Momentum scales exactiy: “mv” scales exactly, and the size of the model is irrelevant. If the mass is scaled and the velocity scaled the momentum effects will be the same, in an airless vacuum in the absence of gravity and friction.
Friction does not scale. In particular static friction really does not scale well.
Scale model locomotives cannot haul scale sized trains. Period.
