Another Science Question

[Woodie]

Another Train/Science question has come up on my Science Forum:

From: Dale 29/05/2002 11:27:03

Subject: trains post id: 51069

I have a question about trains. If you have one engine pulling some train carriages and you add another engine in series, do you double the pulling power? Or do you add a lot of power but not quite double? I'm stuck on the thought that either train would be pulling more weight than the other. Either the first is pulling the second and so the second is not effectively used OR the second is opulling the wieght behind it or the train in front is not required to pull as much. Can someone who understands physics help me with this??



Anyone with a good "scientific" answer? Click here to go straight to the thread, and click "reply", or if you post an answer here, I'll pass the info on.
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Thanks, if you can provide me with a good answer!

 

[sumpter250]

Woodie,
If the second engine is identical to the first, then, theoretically, you should have twice the pulling power. The horsepower doubles, the tractive effort should double ( there are twice the driven axles), but in reality there is probably less than twice the ability to pull. I do not know the exact numbers, but would guestimate adding the second equal engine results in 175% of the original pulling power. This is based on the assumption that some of the second engine's power is used to help move the weight of the first engine. Energy is niether created, or destroyed, but energy is used to overcome friction in bothe engines.
Pete

 

[Vic]

Tractive Effort

Good Morning Woodie and Sumpter! I agree with Sumpter's evaluation on the question with respect to diesel or electric engines that can operate as multiple units but in the case of doubleheaded steam engines its just a little bit different.

In the case of steam engines the lead engine is the primary source of power. The trailing engine is used to provide extra tractive effort as needed. For example starting the train from a dead stop or pulling a grade wherein extra tractive effort is required. The engineer on the lead engine would whistle signal the engineer on the trailing engine to "cut in" or "cut out" as needed. Of course both engineers needed to be familar with the "road" so they could anticipate when the extra power would be needed while underway.

We've all seen doubleheaded steam fan trips where both engines are running "flat out" but that's just for show. In the cost concious real railroad world it wasn't done that way. Even in the case of diesel or electric engines used now the engineer has the ability by MU connections to cut in or cut out units as needed.

I may have digressed a bit from the orginal question but just thought I would throw in this little "tid-bit"

 

[rockislandmike]

Good points, everyone. One additional thought - additional power would also be lost because the original diesel has to pull part of the weight of the second engine (corollary to sumpter's point of the reverse).

 

[Virginian]

Hi Y'all
A tangent:...I don't really know re: current practice, but I remember my Grandaddy telling me that the 2nd diesel ( or 3rd, 4th, 5th, as needed were used for braking as well as pulling in the mountains of Va./W.Va. I've only seen one or two photos of a single trainmaster... and that was doing yard duty. The standard tandom use, coupled cab to cab, allowed for the quick reversal at the end of a run, but also better braking on downgrades and curves, with the extremly heavy tonage of mine run consists. My understanding is that the throttle control/dynamic braking using multiple engines was almost an art, engineering science merely being the tools the crew used.
Think about it...190 hopper and gondola cars, loaded with coal (average..70 tons each), plus the weight of the engines... then take a look at a contour map of the Alleghenies...they follow the river valleys/canyons,lots and lots of hairy curves and the grades are impressive...unbelievalbe!!
Even without pure technical/scientific expression...the basic equation of weightxgradexdgree of curvextractive effortxgravityxweatherxHPXdynamic braking powerxnumber of engines to adjust and monitorxtimetablexgrade crossingsxmany other variables...
It's mind boggling that any coal every made it to the Norfolk docks!!
VGN cool:

 

[YakkoWarner]

It is my understanding (and I could very well be wrong) that when lashed, locomotives are slaved to a single control. As power is applied to each axle as needed, the gp-35 just to cite one example effectively becomes an eight axle locomotive instead of a four axle loco. Each axle is driven by it's very own electric motor. Each motor is powered in accordance with throttle demand/train performance. All motor response and input/output are computer controlled through a logic controller or other device.

Basically you have all the power available minus the drag of the second engine. In effect, I agree with Sumpter.

 

[cidchase]

My amateur analysis of the "2 loco problem" is this:
1. Consider a single loco pulling a string at its max tractive capability.
2. Now consider a second train, identical to the first, running at the same speed and on the same track (grade and other conditions identical).
3. Now locate the second train directly behind the first with couplers engaged but not closed. Each loco is pulling its own weight plus the weight of its cars.
4. Now close the couplers. Does this have any effect on the system?
5. My answer is no. Therefore, I conclude that the theoretical load capability of two locos is exactly double that of one. Any part of the weight/mass of the 2nd engine which is actually pulled by the 1st engine would decrease the load requirement of the 2nd and increase its available power.

Now, please heed that "theoretical". I'm sure that in practice Pete's estimate of 175% is much closer to the limit. The 2 locos probably cannot be that closely synchronized. At any rate, it seems unlikely that any loco or consist would be required to pull its theoretical 100% load. Like I said, an amateur opinion.
That's my 2 cents worth from Mississippi.
Cid

 

[Drew Toner]

Re: Tractive Effort

Originally posted by Vic
Good Morning Woodie and Sumpter! I agree with Sumpter's evaluation on the question with respect to diesel or electric engines that can operate as multiple units but in the case of doubleheaded steam engines its just a little bit different.

In the case of steam engines the lead engine is the primary source of power. The trailing engine is used to provide extra tractive effort as needed. For example starting the train from a dead stop or pulling a grade wherein extra tractive effort is required. The engineer on the lead engine would whistle signal the engineer on the trailing engine to "cut in" or "cut out" as needed. Of course both engineers needed to be familar with the "road" so they could anticipate when the extra power would be needed while underway.


>>Vic, what were the actual signals for 'cut in' and 'cut out'?
Just curious!

Drew


We've all seen doubleheaded steam fan trips where both engines are running "flat out" but that's just for show. In the cost concious real railroad world it wasn't done that way. Even in the case of diesel or electric engines used now the engineer has the ability by MU connections to cut in or cut out units as needed.

I may have digressed a bit from the orginal question but just thought I would throw in this little "tid-bit"

 

[Drew Toner]

Double the power

I have walked from one loco to another, at speed, many times! And 99% of the time, the two coupled drawbars are just happily going up and down, over the terrain, like there wasn't even a load at all, allmost coasting.
But, when lifting, thats when it really matters. In fact, the second loco could push the lead loco and pull the entire train, untill the lead loco picks up speed. Remember, we could be talking only 1, maybe 2 mph. It could also be the other way around, were the lead loco pulls everything. If all diesel locos were tuned up perfectly, and all the wheels were the same size, I'd have to agree that you would have twice as much power.

IMHO

Drew

 

[Vic]

Drew, Thanks for the information. Good to have someone here with actual railroad experience.

About the whistle signals for cut in and cut out. I think they probably varied by road...sitting here trying to remember what I read about the D&RGW narrow gauge. HA! I may have it backwards or totally wrong but....

o _ = cut in
_ o = cut out

 

[kf4jqd]

Mistakes in calulating power

Hello All:

I have delt with power equations in electronics and to find out how much more gain I will have on my radio signal when I add and amphiler.

The rule of thumb is, you must multiply your power by three to double the out put. In other words, you would need 3 locos to get the double output of horse power.

Here is a example in radio: Lets say I am transmitting a 100 watts to a person in England. I want to increase my signal by one "S" unit ( this is a form on how to measure how strong your signal is.) I would have to increase the transmitter power to 300 watts to do it.

The laws of Phyics don't change, so I believe this is right. I could be wrong! I haven't had college Physics in 10 years!

Andy