my understanding is air-brakes affect the brake cylinder in the locomotive.
what happens when independent brakes are applied while the air brakes are also applied?
can the independent brake setting release as well as increase the locomotive brake cylinder pressure?
what would need to be done to return control to the air-brakes? Does the “Relay Valve” shown in the diagrampass whichever pressure source is higher?
As I understand it (lots of reading and some playing with the real thing – newer 26L brake setups with self-lapping independents) the independent brake valve literally lets the locomotive brakes work independently of the rest of the train (controlled by the automatic brake valve).
You can apply the locomotive brakes independently of the train brakes (by moving the independent handle with the train brakes released), or release the locomotive brakes while the train brakes are applied (by pushing down on the IBV handle – “bailing off”). The latter could happen, I believe, if the operator was trying to stretch the train by applying both power and some train brake.
What would happen if you made an IBV application with the train brakes applied? I believe if the pressure commanded by the IBV exceeded that from the ABV, you’d simply get more locomotive brake. It’s been a LONG time since my last cab ride, but I believe I saw engineers do this to bring the train to a halt. Train brakes cannot be partially released, only completely released, so using locomotive brakes would let you modulate your stops a little better, as they can be released and reapplied quicker than the train brakes.
I am not a locomotive engineer so take what I say with a grain of salt. 
Aaron
More on this – it’s not so straightforward. (And forgive/ignore me if I’m telling you something you know.)
The independent brake valve does, I believe, directly control the movement of air into the brake cylinders. The automatic brake valve works differently. There is a line of air that runs throughout the train – the brake pipe (consisting of all the lines, pipes, etc). Each car has its own air reservoir, a valve (triple valve, I believe it’s called), and a brake cylinder.
The locomotive charges the brake pipe, and from this pipe the individual cars pump up their individual air reservoirs. When the brakes are to be released, the pipe is pumped up to a certain pressure (I believe it used to be 90 psi for freight trains, 110 for passenger). This can only happen once the air reservoirs, etc. are full. The pressure moves the triple (?) valve to the release position, causing each car to release air from its own brake cylinder.
When the engineer wants to apply the brakes, she makes a reduction in brake pipe pressure – say, venting the pipe from 90 down to 80 psi. This causes the triple valve on each car to move to an apply position and admit some air from the individual air reservoir to the brake cylinder. When the pressure is restored to the brake pipe, the valve on each car causes the brake to release, and the reservoir restores its pressure from the pipe. (This is why it takes a while to release the brakes on a train.)
If the engineer puts the brake in Emergency, the rapid reduction in pressure causes each car to apply its brakes immediately, IIRC.
The beauty of this system is that if the train comes apart and the brake hoses separate, the pipe loses pressure and the train goes into emergency. It also allows emergency application from simple valves in the loco or caboose. BUT – if a car is left to sit and all of the air bleeds out
thanks
does the “Relay Valve” simply pass the greater of the (2) pressures from the independent brake valve and air brakes control valve?
can the IBV reduce the brake cylinder pressure to less than what the air brake control (triple) valve provides? (i think no)
The independent brake valve does not directly control the pressure in the brake cylinders. It controls the pressure sent to the relay valve on each locomotive in a consist. The relay valves are calibrated to then send the proper pressure to the brake cylinders on each locomotive. This is done because of the different cylinder pressures required by various braking setups on locomotive trucks. As I recall 72, 45, and 36 pounds are used. If 72 lbs. were sent to a loco designed for 45 or 36 lbs. the wheels might slide or the brake rigging be damaged.
The IBV can reduce the brake cylinder pressure on the locomotive below what the automatic brake calls for by what is called “bailing off” the brakes. This is done by depressing the IBV handle. All or any portion of the automatic application on the locomotive can be released by bailing off.
Mark
but if the IBV only controls the relay valve, can there be brakes without the air brakes being applied
The IBV can apply or release the locomotive brakes entirely independently(thus its name) of the automatic train brakes.
An application of the automatic train brakes will also apply the locomotive brakes, but that application of the locomotive brakes can be released, either fully or partially by bailing off the IBV.
Mark
can you describe IBV positions that would increase and decrease brake cylinder pressure with and without air brakes being applied?
On modern equipment (24, 26, 30, and the newfangled electronic “valves”), there are no real “positions” other than release (all the way left) and fully applied (all the way right).
Hold down the handle (regardless of position), and the engine brakes are “bailed off” (from brake pipe application), pressure can be increased/decreased/released regardless of brakes on train.
You could be just rolling on a light engine, hold the independent brake valve down in the release position… and then… put the auto brake valve into emergency. Brake pipe will go to zero, but engine will keep rolling because you “bailed off” loco brake cylinder pressure from building up. At least that’s how it worked on “real air” (when moving the brake handles actually controlled air pressure directly).
Nowadays everything goes “through the computer”, and there’s a slight “lag” between where you put the handle and how cylinder pressure responds. Maybe they’ve gotten it to work better since I was there.
does this mean that the independent brakes only have effect – overriding the air brakes – when the IBV is pressed down?
No it means that the independent brake applies the brakes on the engine when it is applied, but it can release the brakes independent of the automatic brake setting. The brakes on the engine can be controlled independent of the train brakes.
how?
it’s not clear what position the IBV must be in for the air-brakes to solely affect the loco brakes and what position(s, including holding it down the IBV affects the loco brakes regardless of the air brake application: some or no braking?
doesn’t the loco control (triple?) value maintain the loco brake cylinder pressure based on the drop of the brake line pressure?
if the IBV isn’t increasing (apply) or decreasing (release) pressure in brake cylinder, would the control valve restore it?
can the IBV maintain the brake cylinder pressure at less pressure than the control value initially set it to?
I think you’re missing the bigger picture here: The IBV is a seperate lever from the ABV.
Here’s a pic that shows 26L brake gear with two handles (and describes their position): http://www.sdrm.info/faqs/brakes/control/26stand.html
The IBV does exactly what it says it does – controls the loco brakes independently of the train brakes. Push it forward and it applies; pull back and it releases, and if the train brakes are applied, pushing down on the handle releases the locomotive brakes.
Positions depend a bit on the equipment. Newer self-lapping indepenent brakes work a bit like the brake pedal on your car. The father you move it forward, the more braking effort you get.
Older non-self-lapping brakes have an apply position, which gradually applies the brakes, a lap position, which holds the application, and a release position that (duh) releases them.
IIRC (and others here can correct me), with a self-lapping brake, let’s say you’re running a light locomotive and want a moderate loco brake application. You’d move the independent handle to its half-way-or-so mark, and that’s that. To release, you move the handle back to the release position, and maybe press down (bail) for good measure.
With a non-self-lapping brake, you’d move the handle forward to Apply and watch the brake cylinder pressure gauge until it reached the pressure you wanted, then move it back to Lap, which would lap off the flow of air. To release, you’d pull the handle back to the Release position.
How the independent and automatic interact depends on what you want. If you want to apply both power and t
yes, i understood that (IBV is on right)
yes, this completely releases the air from the brake cylinder
yes, presumably the IBV supplies greater pressure to the relay valve and the relay valve passes the greater pressure to the brake cylinder
and i understand that if there are no air brakes, the IBV can release pressure from the brake cylinder thru the relay valve (as shown)
what i’ve read is that the IBV can
but can the IBV be used to “partially” reduce the brake cylinder pressure due to air brakes, other than “bailing off”? (no)?
what happens when the IBV is in the release position when air brakes are applied? (no effect)?
[:)]greg,
The illustration above shows an OLD brake valve assembly, obsolete for many decades (although you might still find them in use on old engines).
Same with the piping. It looks NOTHING like more recent equipment.
26 brake equipment (underneath the engineer’s seat) would have a “rack”, with a control valve, “J-type” relay valve, A-1 charging cutoff valve, P2a switch, etc.
The brake “equipment” on new engines is all electronic, not sure what they’re like.
The most “modern” freight engine I ran was an SD50. I did climb up on an SD60 once at Selkirk, to see what it was like…[:D]
The use of this diagram is very misleading because it is incomplete not showing the independent system.
does that prevent answering the question of can the IBV “partially” reduce the locomotive brake pressure when the air brakes are applied without releasing the brakes complete by “bailing off”?
No, but, you don’t seem to be able to comprehend how the two systems work together in applying and releasing the locomotive brakes and that diagram doesn’t seem to be doing anything but confuse you more and more.
of course i don’t understand how they work, that’s why i’m asking questions. (as an EE, i don’t assume i understand, i try to verify that i completely understand)
i had the impression from someone else, that the IBV has both a quick and slow Release position. i’m guessing this is wrong? brakes can never be partially released
the above quote suggests that the loco brakes can be released (i.e. completely) by moving the IBV to the release position to release the brakes due to the IBV but that the handle has to be depressed to release the brake due to air brakes.
I’m somewhat familiar with the #6 ET brakes used on a USRA Mikado.
The independent brake lever had a “running” position that the handle was left in for nearly 95% of the time. Upon a reduction in brake pipe pressure using the automatic valve the engineer would push the handle of the independent further past and into the release position (quick release) this was a spring return so the handle came back to the running position.
IF you wanted to “partially” apply the engine and tender brakes you could move the independent toward the apply position. There really wasn’t a detent for a lap position but you could leave the independent handle in a neutral position between apply and release and your ET brake cylinder pressure would stay where you wanted it even if that was “partially applied”.
Likewise, on the apply side there was a “quick apply” that would then provide increased air flow into the locomotive and tender brake cylinders. This position was also a spring return back to the apply position.
It didn’t take long to apply full pressure to the brake cylinders therefore you really didn’t have to hold the handle in quick apply for very long. You had to use your experience and feel to know where that sweet spot was to get maximum stopping force (if that’s what you wanted) without sliding the wheels. Normally, running the light engine you certainly anticipated where you were going to stop and you made incremental applications to the independent.
While running with a train you “bailed off” the independent (placed the lever in the quick release, spring return position) several times during the automatic reduction. You could hear the brake cylinder exhaust from the independent so you knew (