Adding an electric air compressor

[Iceni John]

I would like to connect a small air compressor to my accessories tank, so I can maintain pressure in the accessories air system while the bus is parked.   This will allow me to always have a supply of air independent of the braking system's, that I can use to inflate tires and run air tools.   In an emergency I will also be able to connect this air to my wet tank, so I could release the brakes if the engine won't start.   This electric compressor will NOT be a substitute for the engine's compressor, i.e. I definitely won't be driving the bus with only the electric compressor functioning!

My question is  -  if I feed air back into the accessories tank, and the brakes' two tanks and the wet tank are empty (such as after sitting for a week or more), will the air then flow back into the rest of the air system?   There is a PR-3 valve at the inlet to the accessories tank that is set to 85PSI, but I want to be sure that it functions as a non-return valve in these circumstances.   I think it does, because long after all the brakes' air has bled off I still have enough pressure in the accessories tank to operate the door and driver's seat and wipers.   According to Crown, the PR-3 will only admit air to the accessories tank after the other three tanks have reached 85PSI, and when that tank has 85 PSI it will allow the entire air system to finish filling up to cut-out pressure.

I just want to be sure that adding an outside source of air to the accessories system won't in any way affect the operation of the brakes.   I'm probably not the first person to have tried something like this  -  how have other folk done it?

Thanks for your advice and collective wisdom,
John

[DaveG]

Probably need to look at the Bendix big book and the diaghram of the PR-3 and see if it functions as a check valve...you could pull the line off and air the tank and find out...if need be you could add a check valve too.

[bevans6]

I looked at the diagram of the PR-3 (No, see edit below)

Now, I have never backfed a pressure protection valve, so I don't know that it will backfeed for sure, it just looks like it will to me!

Edit - it's too late now, but I looked at a PR-2 valve, not the referenced PR-3.  The PR-3 does have a check valve and is designed to retain full pressure in the downstream tanks.  It's designed to NOT  backfeed, in other words.

Brian

[buswarrior]

Short answer, as long as it is closed to begin with, you should be able to build air pressure just in the auxiliary.

See this document from Bendix:

http://www.bendixvrc.com/itemDisplay.asp?documentID=4574

It depends which pressure protection valve you have, whether it is working properly, and at what pressure you start adding air.

From a design standpoint, it is desirable for the auxiliary to sacrifice itself back to the brake tanks under a catastrophic air failure in the brake circuits, which they do, up to a point.

happy coaching!
buswarrior

[rv_safetyman]

I plumbed mine directly to the wet tank.  I did not run it through the air dryer.  That is about as simple as you can get.  That way your brakes have pressure when you are ready to leave and the accessory tank gets air as well.  No worry about back feeding tanks and the regular system check valves protect all of the systems.

The issue you will run into is what compressor to use.  Most small ones are pretty noisy and don't have much volume.  The oil less units are really noisy.  I think they sound worse than running the engine to air up.  I bought a Sears oil type and it has terrible volume and gets hot.

My long term plan is to drive a York (now CCI) AC type air compressor via my generator.  A good reference is: http://www.kilbyenterprises.com/faq.htm.   Lots of off road folks use them and I have one I use on my '56 Chevy for the air suspension.  One version (on many trucks) had 10 cubic inches.  They talk about 3-4 cfm at idle.  Rigged up with at power switch and pressure switch, it could be a sweet system.

Jim

[Just Dallas]

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[rv_safetyman]

Dallas, I had thought about using a bus or truck type air compressor.  About everything I see is gear driven (at least modern units).  Many are water cooled and have an oil system plumbed into the engine oil system.  It just seemed to me to be pretty complicated.

The AC type compressors are belt driven, and have an electric clutch.  That really makes it easy to fabricate a simple system.  I have worried a bit about long running periods since they use the freon in the AC application to cool the compressor.  However, the Kilby folks don't seem to be concerned.  Indeed, if you think about it, we would not be running them for maybe a hand full of minutes for normal building of air or running an impact wrench.  I think I would limit it to 5-8 minutes max.  There are a ton of these systems on Jeep type off road vehicles and I do not hear of any problems.

On the '56, my unit is pretty old (not sure what displacement and it will air up two pretty big air bags ~~8 " from dead empty to 100 in a couple of minutes at perhaps 1000 RPM on engine.  I figure that I will run the air compressor each time I run the generator (with pressure switch) and that will keep things topped off and not ask the compressor to run for long periods of time at least under boon docking conditions.  Since I don't have air springs, it makes the job easier.

I got a couple of units from the local truck wrecking yard for something like $50 but have to put the two together to make one good unit (bad clutch on one)

Jim

[Just Dallas]

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[gus]

I think most bus aux air systems are designed to provide air to the brake system, maybe not all, but most.

My external air connector is on the aux air tank. Yes, it will bleed back to the main brake system and won't air the aux system until the main system is up to appx 65ps,i so it really doesn't matter where you hook it it will operate the same way.

It will not, however, work very well between the comp and the master check valve. It mostly makes the check valve flap back and forth while allowing very little air into the tank.

If you use a small electric compressor, especially on DC, it will be a big power drain because most small ones pump out a lot of pressure but very little volume. Your bus system requires a pretty big volume if it leaks down all the way. If it doesn't leak much you will be good to go.

I have a small DC pump but I only use it for tire inflation, I wouldn't even attempt to use it to air the bus. I have a large shop comp at home for that purpose but it is too large to carry along.

[robertglines1]

Have added a elect compressor to work accessories and air bags on 89 prevost 4 yrs ago runs about 2 minutes a day depending on how many trips they make in and out of doors..use bags for leveling system.

[PCC]

Mine is going straight into the wet tank, but not on the compressor side of the valve in that line to the tank. That line has no pressure on it unless the engine is running, and the compressor is making more air for the system. I think that is so the compressor starts with no back pressure.

I found that you can hook straight into the wet tank, which bypasses all the OEM check valves, but I recommend adding a check valve at the tank, so if the line from the electric compressor gets damaged in any way, you do not lose your system air pressure for brakes, suspension, etc.

It is real nice to be able to start and get out of an overnight park quickly, so as not to disturb your noisy neighbors.

[TomC]

Take a look at this site http://www.silentaire.com/silentaire/index.asp.  Good Luck, TomC

[luvrbus]

Best to keep the air leaks fixed then there is no need for a aux compressor.
I know that my bus doesn't have as many places to leak as a air bag unit but I can set for 2 weeks and still have over 90# of air on my Eagle rust and all lol. 



good luck

[Iceni John]

Thanks for your replies.   I will be buying an electric compressor anyway in order to run tools, but your advice is steering me towards connecting it to the wet tank instead of only the accessories tank  -  I'll probably do that, but I'll also try the other way out of curiosity to see what the PR-3 valve does if there is more pressure in the accessories tank than the rest of the system.   Either way will be simple, because I already have couplings at both tanks.

Some of you mention air bags.   No, I have some of those funny bits of bent metal underneath the bus by each wheel  -  I believe they're called Sprongs, or something like that.   As far as I know, they don't need air to work, but I'll try connecting my air to them and see if they move.   I'll let you know what happens!

I plan on using one of those generic cheapo oil-lubricated twin-tank 120V compressors that are sold everywhere under a variety of different names.   Whether it's Home Despot or Harbor Fright or any other name on it doesn't matter  -  whether I can easily exchange it if/when it stops working is more important.   They seem to work OK as long as they aren't running continuously.   It will neatly fit above the front axle between the frame rails in a space I haven't yet appropriated for anything else, and it will get very little use compared to my other shop compressor.

Thanks for your advice,
John

[PP]

John, I have one of those no-name twin tank oil-filled compressors mounted up front beneath the driver and plumbed into the aux tank for leveling. Except for the regulator valvle blowing off  , the compressor has been  working well for about 2 years now. I went through a CH oiless in less than a year. My aux tank will bleed back into the brakes, but very slowly. I have another fitting back at the wet tank that I use with a long air hose for filling the entire system before firing her up when we get ready to roll. The compressor kicks on for under 5 minutes a day to keep the bus leveled. The front bags never lose air, but I still have slow leaks in the rear LOL  Good luck, Will