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Difference in braking performance between freight car types (technical questions)

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Difference in braking performance between freight car types (technical questions)
Posted by Zaphr on Monday, October 26, 2020 9:46 AM

First time posting here so I'm not sure if this question was best placed in the locomotives subforum, if not, perhaps the admins can move the thread.

 

I'm trying to improve the realism of freight trains in a simulator and have some specific questions about running trains on mountain grades.

I know that there is a rule of thumb when descending the mountain grade near Tehachapi pass that prescribes 1 pound of air set for every 10 tons of operative brakes. This would imply that a heavy grain train (143 TPOB) would need around 14 pounds of air while a lighter intermodal may only need a minimum set or 10 lb (around 80-100 TPOB).

I realize this rule of thumb is a generalization but it confuses me slightly because reading the diagrams from BNSF the "net braking ratios" are relatively similar comparing a grain car and an Autorack for example.

From my understanding, two cars having equal net braking ratios would have equal air brake retarding forces as fractions of their weights (so equal deceleration from the air brakes) *if* they use the same brake shoes (same friction relationship between brake shoe and wheel).

Obviously one car or train might have more rolling drag (un-aerodynamic cars for example) or curve drag (longer trains) but is this the full reason for the discrepancy in the TPOB rule of thumb?

I also have a bonus question that may or may not be difficult to answer:

For two trains of similar weights (and similar number of dynamic brake axles), discounting stuff like curve drag (i.e the trains are reasonably similar in length), would more air need to be used to hold the speed for a coal train, tank train or grain train? All these trains would have similar tons per operative brake numbers.

/William

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Posted by daveklepper on Tuesday, October 27, 2020 3:54 AM

Depends on what you mean by ?more air."

The actual air quantity that applies the brakes on each car comes from the air reservoir, air-tank on each car.  The train-line air, the air from the locomotive, has two functions, first to replace the air in the cars' air-tanks after brake releases following applications, as well as any leaks, and the second, control the application in each car's own brake system.  Except for the Independent brakre on the locomotive, brakes are applied by reducing air pressure in the train-line air, the reduction proportionally controlling the brake system in each car, and full emergency brakes applied by exhausting the train-line ar to zero pressure.

With everything adjusted perfectly, and with no leaks, the differences in car types and even train lengths should not make any difference in the amount of pressure reduction for a given braking effect ---except that a long train may still have part going uphill while the front is on downhill or part on or off a curve.  But things aren't perfect, and an important part of a locmotive engineer's skill is rapidly getting the "feel" of the train, the way it responds to brake commands, and to throttle settings, as well.

Yes, each train is different, even with the same car types.

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Posted by oltmannd on Tuesday, October 27, 2020 9:17 AM

You have the idea of the braking ratio understood correctly.  You get a certain force for a certain reduction. Cars with high load/tare ratios often have low braking ratios to avoid sliding wheels when the car is empty - typically open top hoppers, stack cars, etc.  Box cars, covered hoppers, gondolas are all going to have higher braking ratios, generally.

The "tons per operative braking" rules are usually "on the safe side" rules to have safe operation.  There are often more rules to deal with specific circumstances, like loaded unit trains, etc.

So, to your "bonus question", yes.  Loaded unit trains have very low braking ratios (when loaded) and need more air to develop a retarding force equal to force of gravity on the grade.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by Overmod on Tuesday, October 27, 2020 9:27 AM

daveklepper
Depends on what you mean by "more air."

By 'more air' I am pretty sure he means a deeper set; a heavier application.

The genius of the one-pipe Westinghouse brake is that it accomplishes a great many things using just one fairly crude air pipe: it is both a source of reasonably-conditioned power air and a variety of control signals, both pressure and pulse.  Many of the problems and issues of the one-pipe Westinghouse brake also come from the characteristics and features that, over the years, have refined the operation.

One characteristic is that you can increase a brake application once started, but you can't release even a bit of it without completely releasing the brakes and then recharging the line ... which has lags associated with it.  It should come as no surprise that brakes with graduated release are preferable in passenger service, and this is a major stated advantage for ECP braking; it should also come as no surprise that train control via dynamics, and extended-range dynamics where available, is preferred to air when that is legal and safe.

A point associated with this is that, on particularly steep grades, there is a point where a train with a given momentum cannot be stopped with air brakes with composition shoes: they smoke and outgas so much that enough force to slow the train, let alone skid the wheels, cannot be developed.  We have had threads on this, for example concerning Seventeen Mile Grade on the ex-B&O; I suspect BaltACD has specific knowledge.  The magic "Vne" here is about 23mph, not fast at all, and quickly reached in the absence of braking.  This is one reason why it is not legal to rely on dynamics for part of the necessary braking to keep trains safely below such speeds: should the dynamics kick out, or fail to engage properly for any reason, the train speed might increase to effective runaway before proper action can be taken.

Meanwhile, "TPOB" is usually an average; these cars each have vagaries in the brake system and running gear, and the braking ratio between loaded and empty can vary substantially.  So the initial 'recommendation' of a certain number of lb. is just that: an initial guide.  You'd then take a progressively deeper set if train speed increases beyond what experience tells you is correct for that part of the railroad -- another of the many good reasons why engineers are, and should be, required to know their territory.  You don't want to take 'more' of a set than required, especially if you have momentum grades or potential stalling to contend with, but you will want to remain well shy of critical speed on any heavy downgrade.

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Posted by SD70Dude on Tuesday, October 27, 2020 1:03 PM

Certain cars of the same design may have their braking ratios adjusted differently.  

CP currently has a bulletin out instructing crews to charge some coal trains to 105 PSI instead of 90, if the train contains at least a certain percentage of cars in certain number series.  These cars are leased, and while their brakes comply with AAR interchange requirements they do not brake as well as CP's own cars, and their braking performance on heavy and mountain grades has been found wanting.

Cars with truck-mounted air brakes brake poorly compared to other cars of the same weight.  I don't know why, they just do.  

FYI - it takes about a 17-20 PSI reduction from a 90 PSI charge to hold a short loaded aluminium coal train (263k lb cars) at 15 mph on a curvy 3% grade in warm, dry weather.  Moderate DB should allow that same train to be held at 15 mph with about a 14 PSI reduction.

Greetings from Alberta

-an Articulate Malcontent

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Posted by Erik_Mag on Tuesday, October 27, 2020 2:26 PM

Overmod

A point associated with this is that, on particularly steep grades, there is a point where a train with a given momentum cannot be stopped with air brakes with composition shoes: they smoke and outgas so much that enough force to slow the train, let alone skid the wheels, cannot be developed.  We have had threads on this, for example concerning Seventeen Mile Grade on the ex-B&O; I suspect BaltACD has specific knowledge.  The magic "Vne" here is about 23mph, not fast at all, and quickly reached in the absence of braking.

I suspect the "Vne" is a function of both of TPOB and the down grade. "Vne" for a train of 315k cars on a 3.3% downgrade would be a lot lower than a train of 265k cars on a 1% downgrade.

SOP for the Sacramento Northern freights bound for Oakland (4.7% downgrade) was for the train crew to measure and record piston travel for each car before starting descent to Bezerkeley/Oakland.

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Posted by Overmod on Tuesday, October 27, 2020 2:42 PM

Erik_Mag
"Vne" for a train of 315k cars on a 3.3% downgrade would be a lot lower than a train of 265k cars on a 1% downgrade.

It certainly would.  Note that here, I suspect the composite-shoe contact area is about the same but the dissipated heat per 'square inch' of contact face would be dramatically higher both because of the greater inertial mass and the higher gravity force.  I don't remember the actual "interfacial" temperature at which outgassing exceeds applied brake-piston force but it shouldn't be that difficult to find.

It is also nonlinear with increasing downgrade.

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Posted by mvlandsw on Tuesday, October 27, 2020 9:41 PM

Most cars with a high load/empty weight ratio are equipped with brake equipment that senses whether a car is loaded or empty and reduces the braking force to avoid sliding the wheels on empty cars. This allows more braking force to be applied to loaded cars.

Mark Vinski

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Posted by oltmannd on Thursday, October 29, 2020 7:45 AM

mvlandsw

Most cars with a high load/empty weight ratio are equipped with brake equipment that senses whether a car is loaded or empty and reduces the braking force to avoid sliding the wheels on empty cars. This allows more braking force to be applied to loaded cars.

Mark Vinski

 

Compared to my career (started in 1978), this is a "recent" development! Mechanical load/empty valves were more trouble than they were worth.  I guess as it got hard to keep the loaded braking ratio high enough without sliding lots of wheels on empties, they became necessity.

The problem was when they would stick on "loaded" - flat wheel city!

I would rather use a strain gauge or similar feeding a new generation of ECP equipment (along with "anti-lock" braking sensor) to improve braking and safety.

 

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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