*** SPAM *** Re: "Bottling the Air"

[NW Mailing List]

"In a full service application of the brakes from a 70 psi brake pipe (the value commonly used in technical manuals), the pressure in the auxilliary reservoir equalizes with the pressure in the brake cylinder, giving a nominal 50 psi cylinder pressure (with 8" standard piston travel) because of the relative volumes, but in an emergency application the pressure of both reservoirs equalizes with the pressure in the brake cylinder, giving a nominal 60 psi cylinder pressure, again because of the relative volumes.  This 20 percent increase in brake cylinder pressure will shorten the stopping distance in an emergency application of the brakes.

I know there are some good air brake people among us, so I would appreciate any comments to correct anything I set forth above, or to update my comments to apply to present equipment or practices."

Gordon,
    One way to visualize what is happening here is to put three barrels (cylinders) side by side, #1 (Auxilliary or Service) - #2 (emergency) - #3 (Brake Cylinder). 
    Connect barrel #1 with a pipe from the bottom with a shutoff valve to the bottom of barrel #3. 
    Connect Barrel #2 with a pipe from the bottom with a  shutoff valve to the bottom of Barrel #3.
    Fill Barrels #1 & #2 with water.
The water in barrels #1 & #2 will represent the air going into the brake cylinder, barrel #3, thus applying the brakes to the train.
    To make a "Service" brake application, open the shutoff valve on barrel #1 for a short time and then close it. Notice how water level lowers in #1 and rises into barrel #3.  Open and shut it again and see the water lower in #1 and rise in #3 for more braking effort. 
    Now open the valve from #1 and leave it open. Notice how the level of water in barrel #1 is now even with the level of water in barrel #3. This represents a "Full Service" brake application. The pressures are equalized. There is absolutely no more brake that can be applied with a "Service" application, so close the shutoff valve.
    Now that the levels have equalized, notice that the level in barrel #2, the emergency side, is still full. To obtain an emergency application, open the shutoof valve from barrel #2. Watch as the level in barrel #2 forces more water into barrel #3 until they equalize at a higher level than the service application. This is how an emergency application applies more braking effort to the train.
    Now if you have been paying attention, you will see that a series of steps have to take place in order to obtain an emergency application of the brakes. An emergency application is not a sudden, instantaneous, "Slam - Bam - Thank you Maam" happening. Something like that would cause parts to go flying everywhere.
    Instead it is a series of events that happen inside of and controlled by the "Control Valve" (Triple Valve, if you must) that takes a certain amout of time to develop into an full emergency brake application.
    Notice also that I have used no specific pressure amounts in my example. It is life's "General Therory of Relativity" (not Einstien's) at work. Equalizing pressures are relative to what the engineer has the feed valve on the locomotice set. If set at 75 psi, the brakes equalize at a lower pressure than if the feed valve were set at 90 psi. This can be observed by only filling the barrels #1 & #2 in my example up say 3/4 of the way to the top.
    Hope this has helped.

Jimmy Lisle
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