Elvas Tower: Bug or Not - Steam Tractive Effort Issue - Elvas Tower

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Bug or Not - Steam Tractive Effort Issue Tractive Effort jumps to Theoretical TE and very low speed Rate Topic: -----

#1 User is offline   joe_star 

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Posted 23 November 2021 - 08:40 PM

Hi,

I am not sure if the observation below is a bug, or perhaps actually how the steam model is supposed to work. When crawling at very slow speeds up a grade, I noticed that at around 2 mph, the Tractive Effort starts jumping between the actual TE based off the steam chest pressure available, to the Theoretical Tractive Effort of the locomotive. This naturally has the positive effect of keeping the train going, but I wonder if this is realistic behavior or not.

Video below shows the effect in action. You can see the TE going from around 23klbf (which matches the available chest pressure (120 psi) vs max operation pressure of the chest pressure (160psi). However at low enough speed,it starts switching between this TE value, and the Theoretical TE value of 30.3klbf


https://youtu.be/pjGdj-e5JiA

#2 User is offline   steamer_ctn 

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Posted 24 November 2021 - 07:29 PM

This is not a bug, as at zero speed (and very small speed values) OR does not produce any TE from the "boiler" calculations. So it uses the maximum TE until it starts moving.

#3 User is offline   joe_star 

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Posted 24 November 2021 - 09:58 PM

View Poststeamer_ctn, on 24 November 2021 - 07:29 PM, said:

This is not a bug, as at zero speed (and very small speed values) OR does not produce any TE from the "boiler" calculations. So it uses the maximum TE until it starts moving.

Is there somewhere I can understand the real physical principle for such a condition? It seems a little counterintuitive that MaxTE is available when the boiler pressure is low or close to nothing.

#4 User is offline   copperpen 

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Posted 25 November 2021 - 03:38 AM

View Poststeamer_ctn, on 24 November 2021 - 07:29 PM, said:

This is not a bug, as at zero speed (and very small speed values) OR does not produce any TE from the "boiler" calculations. So it uses the maximum TE until it starts moving.


That is a "work around" that looks like a bug. Given pressure in the boiler, when controls are operated to admit steam to the cylinders the code should start generating TE, not using a theoretical maximum to get moving, neither should it be using theoretical TE to keep moving at low speeds.

#5 User is offline   joe_star 

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Posted 25 November 2021 - 07:22 AM

View Postcopperpen, on 25 November 2021 - 03:38 AM, said:

That is a "work around" that looks like a bug. Given pressure in the boiler, when controls are operated to admit steam to the cylinders the code should start generating TE, not using a theoretical maximum to get moving, neither should it be using theoretical TE to keep moving at low speeds.

Hmm, that was my concern as well. Frm what I understand TE in OR is calculated off the MEP, the value of which must be lower than boiler pressure.

Unless running with a full head of steam, I don't understand how the starting TE (or at low speed) could be equal to the theoretical TE. Unless the "workaround" here is meant to represent some aspect of the actual steam power delivery to rail that is too difficult to it implement in code.

#6 User is offline   joe_star 

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Posted 27 November 2021 - 06:40 PM

View Poststeamer_ctn, on 24 November 2021 - 07:29 PM, said:

This is not a bug, as at zero speed (and very small speed values) OR does not produce any TE from the "boiler" calculations. So it uses the maximum TE until it starts moving.


From your excellent CTN reference page, I understood below:-

Quote

Theoretical Tractive Effort
To allow the comparison of locomotives, as well as determining their relative pulling ability when the locomotive starts to move, a theoretical approximate value of tractive effort is calculated using the boiler gauge pressure and includes a factor to reduce the value of M.E.P.

Thus our formula from above becomes

TE = Cyl/2 x (0.85 x BP x d2 x s) / D
Where:
BP - Boiler Pressure (gauge pressure - psi)
0.85 - factor to account for losses in the engine, typically values between 0.7 and 0.85 were used by different manufacturers and railway companies.


The Theoretical TE is driven by the boiler gauge pressure. If the Boiler pressure is lower than max, it does not seem correct that at stationary a locomotive will generate its rated theoretical TE. This is also the same as noted on locomotive spec sheets I have seen, which give different tractive force apabilities and different boiler pressures.E.g. in document below:-

https://babel.hathit...eq=37&skin=2021

#7 User is offline   steamer_ctn 

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Posted 27 November 2021 - 07:15 PM

View Postjoe_star, on 27 November 2021 - 06:40 PM, said:

The Theoretical TE is driven by the boiler gauge pressure. If the Boiler pressure is lower than max, it does not seem correct that at stationary a locomotive will generate its rated theoretical TE. This is also the same as noted on locomotive spec sheets I have seen, which give different tractive force apabilities and different boiler pressures.E.g. in document below:-
OR uses the formula that you have quoted to calculate the "Max Theoretical TE" (MTTE).

The 0.85 factor reduces the boiler pressure to allow for steam loses, etc. Hence in reality the MTTE is not using the full boiler pressure of the boiler.

When would you expect the locomotive to reach its rated theoretical TE?

#8 User is offline   copperpen 

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Posted 28 November 2021 - 11:45 AM

The video shown by joe-star is clearly travelling up a stiff grade with a heavy consist in tow. In that situation, I would not expec tto see maxTE being applied to keep moving simply because of the nature of the steam locomotive. It will either move along at a steady pace with minor fluctuations in TE as each piston stroke is active, or it will stall on the hill, unable to haul the load. There should be zero hint of the code keeping the engine alive just because it can.

Tractive effort is the figure often quoted when comparing the powers of steam locomotives, but is misleading because tractive effort shows the ability to start a train, not the ability to haul it.

Back when Al Krug had an active website, he had a calculator that would work out the required HP to haul a given weight up a given grade at a required speed. From that you could work out how many locomotives you would need to get the train over the hill

#9 User is offline   R H Steele 

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Posted 28 November 2021 - 12:52 PM

View Postcopperpen, on 28 November 2021 - 11:45 AM, said:

Back when Al Krug had an active website, he had a calculator that would work out the required HP to haul a given weight up a given grade at a required speed. From that you could work out how many locomotives you would need to get the train over the hill
Apologies http://www.elvastower.com/forums/public/style_emoticons/default/offtopic.gifI always found that calculator useful, does anyone have the proper formula that the calculator was based upon?


#10 User is offline   joe_star 

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Posted 28 November 2021 - 08:37 PM

View Poststeamer_ctn, on 27 November 2021 - 07:15 PM, said:

OR uses the formula that you have quoted to calculate the "Max Theoretical TE" (MTTE).

The 0.85 factor reduces the boiler pressure to allow for steam loses, etc. Hence in reality the MTTE is not using the full boiler pressure of the boiler.

When would you expect the locomotive to reach its rated theoretical TE?

Yes, plugging in the values for my locomoyivr as below:
Cylnder diameter x stroke = 22" x 26"
Driving wheel diameter = 56.5"
Rated Boiler Pressure = 160psi

Into the equation above gives exactly the tractive force of 30.5klbf, which in my understanding would only be delivered when the boiler is actually at 160psi, not at lower pressures.....

And the delivery of power to the wheels as noted by copperpen above, rather than the large changes seen today.

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