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August 1997 - Page 53

p ressing the independent brake handle and holding i t down-sometimes cal led "bailing t h e air. " As I mentioned, locomotives also have an indepen dent "straight air brake," so called because air pressure goes directly from the locomotives' compressor reser voirs to the locomotives' brake cylinders. This brake is controlled by the independent brake handle and is gen erally used to a p p l y t h e locomotive bra kes d u ri n g switching o r for holding a stopped train o n level track. You are now an expert on train brakes. There will be a quiz on Wednesday. Quiz. ( I s it Wednesday already?) Question: You have made a 1 0 psi brake pipe re duction on a fully charged 1 1 O-car train. The brakes have applied, but one car has a leak in i t s service reservoir. What happens to the brakes on that car? What happens to the brakes on the entire train? Answer: (no peeking) The key is to remember how the triple valve works. It senses the difference between the brake pipe pres sure and the service reservoir pressure. If you have made a 1 0 psi reduction, from 90 to 80 psi, and the brakes have set, the reservoir and brake pipe are both now at 80 psi. As the reservoir slowly leaks, the pres sure drops, from 80 to 79 to 78 to 7 7 e tc. As soon as the reservoir pressure leaks from 80 to 7 9 psi the triple valve "sees" that the brake pipe (80 psi) is high er than the reservoir ( 7 9 psi) a nd will release the brakes on that car. Whoa ! That is not good ! But you still have brakes on the other 1 09 cars of your train. Hopefully.

C ontrolling Slack
B ecau e all the brake pipe air has to vent through the locomotive's control valve during a normal service ap p l i ca t i o n , it t a kes a l o n g time for the brakes set t h roughout the train: The pressure first drops near the front of the train and then drops further and fur ther towards the rear. This causes the free-rolling rear end to run into the slowing front end-slack action. So some smarty came up with the idea of modify ing the triple valve on the cars. ow when a car first senses a drop of pressure, it opens a passage from the brake p i p e to a small reservo i r (a t h ird reserv o i r, called a "quick service reservoir" ) . This reservoi r is sized in such a way that fi lling i t with air from the brake pipe reduces the brake pipe by 7 psi . When I make a 1 0 psi reduction, 7 psi worth of it is reduced at each car, resulting in a faster ( but not fast enough to trigger emergency) and more even application of the brakes through the train . It only works the fi rst t i m e , since, once fi l l e d , t h a t reservoi r w i l l rem a i n filled until t h e brakes are released. Because of the long brake pipe of a t ra i n and a l l t h o s e cut-off valves at the e n d s of e a c h car and other restrictions , it takes time to pump a i r back t h rough the train to release the brakes. As a m a t ter of fac t , as each c a r goes i n t o release, i t begin s re-charging its reservoir from the brake pipe, consuming air and further slowing the build-up of pressure toward the rear. This results in more slack action problems if



lectro n i c control is t h e latest development i n a i r brakes. a n d the systems b e i n g tested now may well hold the key to a giant lea p forward i n brake response t i m e and reservoi r depletions.

A n Electron ic Futu re

With conventional a i r brakes. the delay from when the engi neer starts a brake pipe re d u ction to

when the brakes actua lly a p p ly can ra nge from five seconds to nea rly a m i n ute. depe n d i n g o n the temperature. length of the tra i n . state of charge. and a ny restrictions i n the brake p i pe. With an electro n i ca lly controlled syste m . a n wi re extends the length of the tra i n . and a s i g n a l to a p p ly o r re lease the brakes travels at the speed of light. rather than at the speed of cha n g i n g a i r press u res. To set the brakes on this system . the engi neer leaves h is brake valve in the release position a n d presses a button on the electro n i c control box. A n electrical com mand tells circuits on each c a r how m uch braking to a p p ly. Air p ressu re from the car's reservoi r is connected to the car's brake cyli n der until the proper cyl i n der p ressu re is obtained. Although the air is taken from the rese rvo i r. keep in m i n d that the reservoi r i s constantly being recharged because the eng ineer left his valve in release. which supplies air pressure from the locomotives to the brake p i pe. S i nce the e n g i neer never re duced the brake p i pe pressure. the cars' triple va lves rema i n i n the release position . So. as a i r i s used from the c a r reservo i r to a p p ly the brakes. it is s i m u ltaneously replaced from the brake p i pe . Since t h e reservoi rs rem a i n charged . t h i s system eli m i nates the peri od of t i m e j ust after re lease when t h e reservoirs a re p a rtia lly e m pty. In that res pect. the safety of the b ra k i n g system is g reatly i m p roved . When t h e e n g i neer is ready t o release the b rakes. he a g a i n pushes a b utton o n the control box. a n d the box sends a release co m m a n d to a ll the cars s i m u ltaneously. The i nstant a p p ly and release signals eli m i nate the slack run ins a n d run outs caused by the slower react i n g a ll-a i r syste m . Electro n i c control a lso a llows brakes to be g ra d uated off-partially released t o reduce b ra k i n g effort i f the g ra d e lessens o r to refine the sto p p i n g poi nt. (With conventional a i r b ra kes yo u can g rad uate the brake i n crease b u t the release is all o r noth i n g .) Several ra i lroads are cu rrently testi n g electronic air brakes. The conventional a i r brake compo nents remain on the cars as a safety backup a n d to permit the cars to be used i n conventional tra i ns. Al Krug

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