Power vs Tractive Effort

[NekoMaster]

When Im playing openttd sometimes I come across a powerful locomotive (from 4000 HP and up) that have lower TE then some less powerfull locos (3600 HP and down) SO im wondering, is the trade off for tractive effort worth the extra power or would I just be better doubling up less powerful but high traction locos, even when the power of another loco might be more then 2 less powerful locos?

[Voyager One]

I usually find it more useful to "double" a weaker loco with higher TE. It accelerates faster with realistic acceleration turned on (I always play like that).

[NekoMaster]

I usually find it more useful to "double" a weaker loco with higher TE. It accelerates faster with realistic acceleration turned on (I always play like that).

Yeah, thats what i normally do. There are some times where a higher TE loco also has a higher max speed, but the cars it huals limit its speed to close to a lower TE\Higher Power Loco. Still doubling up is helpful if you can afford the money and space

[FooBar]

Tractive Effort (I'm talking real life now) is a measurement for the amount of power that a train can put onto the track. It defines maximum speed depending on train weight. It also defines the acceleration rate (again depending on train weight) and it defines the maximum slope gradient for a specific train (also depending on train weight).

[Eddi]

rule of thumb:
tractive effort is more relevant when travelling at low speed (freight trains) or start/stop a lot (local passenger) and power is more relevant when travelling at high speed (express passenger)

[NekoMaster]

rule of thumb:
tractive effort is more relevant when travelling at low speed (freight trains) or start/stop a lot (local passenger) and power is more relevant when travelling at high speed (express passenger)

SO higher powered trains are better for Express where as High TE Trains for best for Frieght and Drags?

[Voyager One]

FooBar & Eddi are right. Take a look at wikipedia about TE, it's pretty explanatory.

[michael blunck]

[tractive effort ./. power]

I´ve written much about this topic on this forums. E.g. this post gives you a simple recipe to calculate TE and/or power needed for a given task.

BTW, using the seach function beforehand would have answered this part of your question immediately.

OTOH, whether to use one or more engines for a specified task is just a different question, namely one of economic and/or organisational conditions.

HTH
regards
Michael

[Paploo]

Tractive Effort (I'm talking real life now) is a measurement for the amount of power that a train can put onto the track. It defines maximum speed depending on train weight. It also defines the acceleration rate (again depending on train weight) and it defines the maximum slope gradient for a specific train (also depending on train weight).

Tractive Effort is a measurement for the amount of *force* that a train can put on the track. This is in contrast to power, which is a measure of the amount of, well, power.

This may seem like a trivial distinction, but to a physicist power and force are very different quantities; and, unfortunately, I'm a physicist.

More specifically, Power = Force * Velocity. This is, of course, because Work = Force * Distance and Power = Work / Time.

[NekoMaster]

Tractive Effort (I'm talking real life now) is a measurement for the amount of power that a train can put onto the track. It defines maximum speed depending on train weight. It also defines the acceleration rate (again depending on train weight) and it defines the maximum slope gradient for a specific train (also depending on train weight).

Tractive Effort is a measurement for the amount of *force* that a train can put on the track. This is in contrast to power, which is a measure of the amount of, well, power.

This may seem like a trivial distinction, but to a physicist power and force are very different quantities; and, unfortunately, I'm a physicist.

More specifically, Power = Force * Velocity. This is, of course, because Work = Force * Distance and Power = Work / Time.

But, if you have lots of power but low traction, IRL you'd get lots of wheel slip along with low acceleration which can be seen in openttd if you have realistic acceleration on and you'r using a powerful but low TE train.

[Paploo]

But, if you have lots of power but low traction, IRL you'd get lots of wheel slip along with low acceleration which can be seen in openttd if you have realistic acceleration on and you'r using a powerful but low TE train.

Yes. That is true, and is not--insofar as I can tell--contradicted by what I said.

The point I was trying to make was merely that one cannot use the words "force" and "power" interchangeably (which is unfortunately very common). They are indeed related quantities, but are different and used to measure different things. (The same is true of power and energy, which are not the same thing, but are highly related, and often freely switched in lay-conversation.)

Put simply, TE is the measure of the amount of *force* that the locomotive has available to pull the train, quoted on a 0% grade. If you multiply that by velocity--by definition--you get the current power output of the locomotive. Hence, a train that is on a grade, standing still, and pushing as hard as it can to keep from being pulled backwards down it, is exerting full TE, but zero power.

Of course, at this point, it'd probably be better to let the famous article by Al Krug, an old BNSF engineer, explain it, as he is much better at it than I am: http://www.alkrug.vcn.com/rrfacts/hp_te.htm

[michael blunck]

The point I was trying to make was merely that one cannot use the words "force" and "power" interchangeably [...] Put simply, TE is the measure of the amount of *force* that the locomotive has available to pull the train [...]

These topics have been discussed numerous times on this forum. People are only too lazy to use the search function and/or to read.

http://www.tt-forums.net/viewtopic.php? ... 80&start=0
http://www.tt-forums.net/viewtopic.php? ... 9f#p573658
http://www.tt-forums.net/viewtopic.php? ... 48#p751674
...

regards
Michael

[Voyager One]

Well said Michael.
As far as I understood NekoMaster, he was only trying to get some opinions about double-headed trains with fewer HP and higher TE vs. one engine with high HP and smaller TE.

[FooBar]

The point I was trying to make was merely that one cannot use the words "force" and "power" interchangeably (which is unfortunately very common).

At a given speed, one can. Hence the equation P = F * v holds true. So Tractive Effort (a force) can simply be translated into a "Tractive Power", not common, but not wrong either.

[michael blunck]

The point I was trying to make was merely that one cannot use the words "force" and "power" interchangeably (which is unfortunately very common).

At a given speed, one can. Hence the equation P = F * v holds true. So Tractive Effort (a force) can simply be translated into a "Tractive Power", not common, but not wrong either.

Nah. "Force" and "power" are two very different things. Firstly, "force" is a vector. It´s a measure for a mechanical effect on a point mass or body by means of other bodies or fields.

(Mechanical) "power" is a scalar. It defines the rate at which work is performed or energy is converted. I.e., power is the derivative of "work": p = dW/dt, with "work" being the line integral over the scalar product of force by the displacement vector (arc length differential). Thus, the amount of work depends primarily of the direction of those vectors.

The equation P = F * v is a very special case linking power and force. It doesn´t mean that those two values are interchangeable, nor that they´re of equal importance. Force is the primary phenomenon. Exerting it on a body mass over a certain path in a given time "generates" a certain amount of power.

regards
Michael

[Paploo]

The equation P = F * v is a very special case linking power and force. It doesn´t mean that those two values are interchangeable, nor that they´re of equal importance.

To extend on Michael's point, it might be meaningful to think about a similar looking equation: x = t * v, which relates distance and time via velocity. So then the question is, would you measure the time until you need to change the channel to watch your favorite show in meters, or the distance to the store in seconds?

Distance and time are different quantities, but they are related to each-other by the velocity. You can define the distance to the store in terms of the time it takes you to get there, but it doesn't mean the same thing. The distance between your home and the store is always the same, but the time is dependent on a lot of factors.

This gets even weirder when you think about the distance until you change the channel, as you aren't going anywhere. You'll have travelled zero distance because your velocity is zero, even though the time is well defined!

By the way, this last one is identical to noting that the power can be zero even when a force is applied.


Anyway, I think that we've done what we can to explain that power and force are different quantities, but we still haven't answered the original question. However, there is now enough information sitting on the table to say what we'd expect should happen: You want to chain together a bunch of high TE engines to get to the power you desire, as TE doesn't stack, but HP does.

[ostlandr]

In my games, I only "doublehead" when absolutely necessary. With NARS, I will sometimes doublehead faster but low TE engines on "time freights" in mountainous districts. I find myself doing this a lot with early steam locomotives (like the early 4-4-0 and 2-6-0 in NARS 2) and early diesels. I tend to doublehead heavy diesel passenger trains since gearing them up for high speed (in NARS 2 again) means high speed but low TE, and acceleration is slow. That's also an issue when I have freights and passenger trains sharing the same track- I need the freights running at their best speed to keep out of the way. Of course, when I can get a single locomotive like the Mountain, Challenger, Trainmaster or SD45, I use those- putting all the HP and TE in one package makes things cheaper.

I also do it for fun sometimes- I just like the look of a coal train with a pusher/helper/banker on the back end climbing a grade, or an AB set of hood units on a '50s freight or a pair of GP38-2s on a hot shot intermodal. I think I need to start a game with longer platforms. . .

[NekoMaster]

In my games, I only "doublehead" when absolutely necessary. With NARS, I will sometimes doublehead faster but low TE engines on "time freights" in mountainous districts. I find myself doing this a lot with early steam locomotives (like the early 4-4-0 and 2-6-0 in NARS 2) and early diesels. I tend to doublehead heavy diesel passenger trains since gearing them up for high speed (in NARS 2 again) means high speed but low TE, and acceleration is slow. That's also an issue when I have freights and passenger trains sharing the same track- I need the freights running at their best speed to keep out of the way. Of course, when I can get a single locomotive like the Mountain, Challenger, Trainmaster or SD45, I use those- putting all the HP and TE in one package makes things cheaper.

I also do it for fun sometimes- I just like the look of a coal train with a pusher/helper/banker on the back end climbing a grade, or an AB set of hood units on a '50s freight or a pair of GP38-2s on a hot shot intermodal. I think I need to start a game with longer platforms. . .

Heh, for the helper thing, I wish there was a order for trains so that when they goto a depot they will automatically purchase and place a new loco the same as the one in the front at the back and then another order to remove it, it would cost money every time but it might be helpful and kinda realistic (though, its not like IRL you buy and sell a new loco for pusher service each time one is needed)

[Eddi]

would you measure [...] the distance to the store in seconds? [...] The distance between your home and the store is always the same, but the time is dependent on a lot of factors.

i actually do measure certain distances in minutes instead of kilometers, like distance from home to the city, or to the work place. because the route is travelled frequently and the average speed is fairly constant [unless extreme conditions kick in], the time to get there might be more meaningful than the actual distance travelled.

other things like the exchangability of "weight" and "mass" are also very common, because it actually does not make any difference in the life of the average person.

[FooBar]

So then the question is, would you measure the time until you need to change the channel to watch your favorite show in meters, or the distance to the store in seconds?

Note the very important

At a given speed

in my previous post.

You can't express the time to change channel in terms of distance, as there's no speed involved.

You can however express the distance to the store as a time value, given a certain speed.

And something similar applies to power and force at a given speed. In fact, one can calculate the maximum power at train can output at a different speeds. Together with the train weight, a friction coefficient and the efficiency factor of the drive this gives you the maximum tractive effort (a force) at that speed. So there you have it, a force expressed in terms of power.

Now don't tell me that I can't do that. I know I can. I actually study "Transport Tycoon" at university level and I've done the tractive effort calculations.

Bottomline is that when explaining something to people who aren't into physics you need to do that in language they can understand, without using jargon that only complicates things for them. Unfortunately this is something a lot of scientists fail to understand; I don't want to be one of those. Decide for yourself if this applies to you, I'm not here to judge people