Adding air compressor to bus

[Dietrichfarms]

My 78 mci air will leak down after a few days of sitting. I'm always chasing small air leaks, don't know if I'll ever get them all. So I'm looking at mounting a small 110volt air compressor in the engine bay to use when parked while on trips to keep air pressure up. Is there any problem with this idea? When parked in the barn she just settles down on the bumpers after a few days.

[itspaidfor]

Great idea! mine is in compartment where the ac compressor used to be....Dan

[sparkplug188]

I don't see any problems.  Plumb the compressor discharge into the wet tank with a one way check valve at the tank (Bendix SC-3).  The compressor discharge air will be very hot-- consider using a stainless steel braided teflon hose.  Also, move the air pressure switch to the wet tank.  If you have room in the engine compartment for an air compressor with the tank attached, you won't need a high temperature hose and the pressure switch can stay on the air compressor tank.  You should still use a one way check valve.  Also, an automatic drain for the wet tank would be a good idea if it doesn't already have one.

Oasis makes some very high quality 12-24 volt DC compressors.  XD3000 and XD4000  They aren't cheap, but you get what you pay for.

[gumpy]

Strongly recommend you put it in a bay where it's shielded from road spray. All that spray will cause your electric compressor to corrode and eventually
fail. I had a small one in my steering gear compartment under the driver. It's purpose was to pressurize my bay door locks when parked. It only lasted about
a year and seized from internal corrosion. Got another and put it in the front bay where it was protected. It's lasted about 12 years so far in there.

You can plumb it directly into the compressor discharge line (use the drain ling next to the tag switches). This will fill the entire bus system, and also runs it through the air dryer. However,
the dryer will not discharge with the electric compressor, so you'll want to run the bus periodically just to get the moisture out of the dryer. How often will
depend on how much the 120v compressor runs.

[lvmci]

I think 2 days is more than most, lvmci...

[sparkplug188]

gumpy- The desiccant inside the air dryer absorbs a small amount of moisture every time the compressor cycles. At the end of each cycle, that small amount of moisture is purged from the desiccant.  I am concerned about the long term affects of saturating the desiccant for days or weeks at a time.  There must be a way to plumb purge air to the dryer with relay valves of some sort...  I just can't think of how to do it.  Running the electric air compressor air through the air dryer would be much better than sending it directly to the wet tank, as long as the air dryer purges at the end of each cycle.

Edit: Just a thought... what if you controlled the electric compressor with a normally closed pressure switch (closed circuit at 0 psi, open circuit at > 0 psi) on the purge line from the governor and "T"ed both compressor discharge lines together at the air dryer supply port?  When the reservoir pressure exceeds the governors set cut out pressure (usually 120 psi) purge air is sent to the air dryer until the reservoir pressure drops below cut in pressure (usually 90 psi).  You could use the pressure switch on the purge line to shut off the power to the compressor at cut out pressure and resume power at cut in pressure.  As a bonus, the electric air compressor would unload through the engine driven compressors after each cycle.  The air governor operates even if the engine driven compressor isn't running.

For anyone unfamiliar with the air governors: https://www.youtube.com/watch?v=9Vb32PL1PVU

[chessie4905]

   My 4905 has a sears oilless 1 or 1 1/2 hp compressor with about a 12 gallon tank back in bay with holding tanks. Don't have to worry about comp pump oil with it. Apparently the previous  owner rarely or never drained tank as there was about 5 gallons of water in it when I bought it. It is powered by a switch on panel left of driver's seat. It does have the cutout switch on the comp tank. You can hear it run inside the coach, but it is pretty quiet. Yrmv. If yours is leaking down that fast, you need to repair it soon if feasible. The compressor is nice item to have though, in case engine driven system fails. It can get you off the road to a safe spot. Although it wouldn't be legal to operate the coach using this to supply brake air pressure in an emergency.......... It is nice to be able to build up air before departing and not having to run engine for an extra 10 minutes, especially in cold weather and having a block heater.

[gumpy]

  It can get you off the road to a safe spot. Although it wouldn't be legal to operate the coach using this to supply brake air pressure in an emergency..........

Great! Here we go again! You just had to open that can of worms up, didn't you!   

[digesterman]

You can get water eliminators for shop compressors , just add one of those before the bus connection, then dry air would be going into the buses air system. We have them in the shop on painting gun supply lines and on plasma cutting equipment


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[eagle19952]

  .... Apparently the previous  owner rarely or never drained tank as there was about 5 gallons of water in it when I bought it. I would wonder why you even bought it........

It is nice to be able to build up air before departing and not having to run engine for an extra 10 minutes, especially in cold weather and having a block heater.

I never understood this......how would you know if the engine compressor was failing if you don't use it to air up the coach....get 2 miles down the road and find out it isn't safe...that would kinda suck....especiallly if it takes 10 minutes to air up your bus, mine takes about 1 minute 30 seconds.....

[bevans6]

Re- the air compressor.  I inject my air from my external air compressor into the ping tank on my MCI, and I set the compressor pressure lower than the cut-out pressure on the air governor so the air dryer doesn't purge.  I don't run mine often so I don't worry about over-saturating the air dryer, but that is a worry and an air dryer as used for painting would probably work very well.

Re- the engine compressor not working.  Opinions vary on using a small electric air compressor for your brakes on an emergency basis and it's been well debated here.  What I did the few times I tried for a "stealth" getaway from a KOA type stopping place is use the electric compressor to air up the bus, start and leave immediately, and park just outside the campground on the side of the road to let the engine warm up a bit and to do the daily inspection, which for me consists of the compressor recovery test (fan the brakes down to 80 psi, time the rise from 85 psi to 100 psi), and all the lights and wipers.  I'll have done the tires, blown out the wet tank and checked the oil level the night before.

Brian

[chessie4905]

  Should have said .5 to 5 minutes  . This doesn't, of course, replace pre-run systems check over before going out on highway. I  use this for pulling coach out of my shop so it doesn't stink like diesel smoke and avoid long term residue build up on the interior. I have an exhaust removal hose to connect to tail pipe if I run it while sitting in shop. Regardless, having an auxiliary air compressor can be beneficial for many reasons if you can spare the space. Bevans6's remark about campground use is also a useful.

[sparkplug188]

Good news everyone!  The setup I described earlier works flawlessly!  I put a Haldex 13255 pressure switch on one of the plugged governor unloader/purge ports.  The pressure switch is wired to a contactor on the electric air compressor.  Both compressor discharge lines are braided stainless steel teflon hose "T"ed together at the air dryer supply port with a one way check valve on the electric air compressor supply line.  The electric air compressor runs up to cutout pressure then shuts off and the air dryer purges. When the reservoir pressure drops below cut in pressure, the electric air compressor starts.  I also wired a normally closed relay to the ignition switch.  The relay will stop the electric air compressor from running while the engine is running.

If anyone wants to copy my setup, use the Haldex 13255 pressure switch or a similar 1/8"-27 NPT pressure switch.  The threads on the D2 governor are 1/8"-27 NPT, not 1/4"-18 like most pressure switches.  The pressure switch can not control the compressor motor directly-- it needs to be wired to a contactor.

Use this setup at your own risk.  It is the drivers responsibility to make sure his/her vehicle is safe to drive.

[gumpy]

That's a cool idea. So now, when you're parked, your electric compressor is automatically cycling to keep the bus system up, and every time it shuts off, it purges the dryer, just like the
when the motor driven compressor is unloaded! Yeah, I like it.

[Oonrahnjay]

... the daily inspection, which for me consists of the compressor recovery test (fan the brakes down to 80 psi, time the rise from 85 psi to 100 psi), ... 

     Brian, what's the time you usually see and what's your "Fail" criterion on that test?   Thanks,  BH