OBA/pneumatic/compressed air questions. (1 Viewer)

[Mandrake]

Gentlemen.

I have completed a York OBA setup on my rig, and I'm having some control issues. My setup is probably quite a bit different than the usual in that I designed it to perform both air down and air up tasks automatically, so please allow me to give a brief rundown of how it works before I get to the issues.

From the compressor, everything is as you'd see it anywhere else up to the check valve- copper line to deal with the heat, filter to pull the oil from the air, pressure relief valve, and finally the check valve. But downstream of that, I have a solenoid controlled dump valve that's used for airing down. The line crosses the top of the firewall and as it passes over the brake booster, there are two pressure switches tee'd in- one set to 16 PSI, and one set to 35 PSI. The line then goes down to the drivers side frame, tees off to run fore and aft to quick disconnects. On the male side of the QD, I have a tee with two ball valves and coiled hose that connects to the valve stems. On the electrical side, a repurposed rear heater fan switch sends power to the air up and air down components, depending on if I press "HI" or "LO." The HI and LO pressure switches are intended to allow the dump valve to release air during the airing down, and then once 16 PSI is reached, the LO pressure switch closes the dump valve. When I air up, the compressor fills the tires and once 35 PSI is reached, the HI pressure switch shuts off the compressor and actuates the pressure release valve to decompress the air line between the check valve and compressor outlet. Once I set the rear heater fan switch back to neutral, the entire system de-energizes and I can remove the tees and coiled hose from the QD and valve stems.

The problem I'm having is that while it all works as I intended, I'm having issues with pressure spikes causing the pressure switches to fluctuate. I attached a gauge to the system to see what was going on and during airing down, the LO pressure switch will close the dump valve when it reaches 16 PSI, but then there's a spike in the pressure, so the switch opens the dump valve again, reducing the pressure, causing the switch to close the valve again, ect ect ect... The same thing happens while airing up, but I suspect it's the pressure pulses from the compressor. When the pressure switches are electrically bypassed or if the system is more than 4 PSI away from the target pressure, it operates normally. The pressure spikes and subsequent feedback loops could best be described as sustained water hammer.

I'm going to install some pulsation dampeners between the pressure switches to see if that helps, but they're about a week out so I figured in the mean time, I'd post this up to see what you guys thought. Other solutions that came to mind were to move the pressure switches further away from the dump valve (currently, they're approximately 4' away), or add a two gallon tank to act as a surge tank and mount the pressure switches on that.

If there's interest, I can post pictures, schematics and do a full- blown writeup once I get things worked out. I will say this- with the pressure switches electrically bypassed, it airs down all four from 35 to 16 PSI in a little under two minutes, and airs them all up in a tick under three minutes... with the engine idling. It'd go faster if I used my hand throttle, but I haven't tried that yet.

 

[NLXTACY]

Sounds like the issue RVs have with pressurized water systems and why they use accumulators. You have nothing in your system to buffer the air at all. Maybe installing a small air tank is all you need?

 

[maxamillion2345]

Id try a tank

 

[Mandrake]

Nope, adding an air tank and also reducing the dump valve orifice diameter didn't cure the problem, but it did slow down the on/off oscillations. I'll try the pulsation dampeners when they come in and update this thread.

Sooooo close...

 

[Mandrake]

I bought both an adjustable pulsation dampener and two fixed rate dampeners. My first experiment had a three gallon tank with multiple ports on it tee'd into the system with about 30' of hose between it and where the pressure switches originally were. Those ports were capped off, and I put the adjustable dampener between the tank and a tee that held both switches, and installed a dial pressure gauge on one of the other ports on the tank. I connected all four tires to the system and hit the air down button. The problem persisted, albeit at a lower oscillation rate than without the dampener.

After depressurizing the system, I installed the additional fixed rate dampeners between the switches and the tee, so that the pulsations had to go through both the adjustable dampener and the fixed rate dampeners. Again, the oscillations still persisted, but the time between them increased.

I then removed the two fixed rate dampeners and put them inline between the adjustable dampener and the tee. There was no change between this and the previous configuration.

I've concluded that air is a compressible fluid, which may come as a surprise to a lot of you, and thusly, pressure will always fluctuate when one end of the system is either open to air down or pressurizing the system to air up. The only way I see this working is to have switches that have adjustable on/off pressure settings. They would allow me to set the stop pressure at my target pressure and then the lower pressure setting below the threshold of the oscillations (which have consistently throughout this process been approximately 4 PSI), which would allow the pressure in the system to stabilize without functioning the switch, causing the dump valve or compressor clutch to actuate. As of this writing, I've yet to look if a switch even exists.

For now, one of the original pressure switch ports and the tee in the line for the air tank have been capped, and I installed the dial pressure gauge in the other original pressure switch port located above the brake booster. Airing up all four tires from 14 to 35 PSI takes about 2:30 with the engine idling, and airing them back down takes about three minutes. I haven't timed that task yet. From getting out of the truck, setting it up, doing what I have to do, putting everything away and getting back in the truck, I'm less than five minutes for either airing up or down. That's a huge, huge, massive improvement over the roughly half hour that it took to air up with an MV-50 electric compressor and the 10- 15 minutes for the brass automatic air down valve stem caps.

Off to go find mythical pressure switches...

 

[Rivman1243]

Put the "dampener/accumulator" on the electrical side of your system. Or a 1 to 2 second delay in the pressure switch response. I don't know enough about electrical engineering and circuit design/theory to explain it with the correct terms.

 

[Dork]

As @Rivman1243 suggested, timers. In electrical controls we normally either use or install an on-delay timer operated from a pressure switch or level switch. This shouldn't be any different really.

Set the on-delay time to say 2 seconds, and then use the timer output to control the operation of your clutch on the York (or turn an air compressor on). This should act as a input filter on your low pressure switch: e.g. the short pressure drop needs to last 2 full seconds before your control system starts so that short term dips are ignored.

If you want to control the other side of the equation: e.g want to keep your compressor running even if the pressure spikes suddenly for a split second, you would also add an off-delay triggered by your high pressure switch. Set it for 2 seconds as example so that you ignore short bursts.

With both timers, it would need to see low pressure for the set time before starting, and see high pressure for the set time before shutting off, preventing your cycling issue. Could probably even get away with 1 sec or 1/2 sec delays.

 

[Rivman1243]

^^^^^^ What @Dork said^^^^^^^

 

[Mandrake]

Of course! I was so focused on the input side of the pressure switches, I never even considered the output side, eg- electrical. I even have a hand full of programmable timer delay relays in my electrical parts drawer!

Thank you both for getting my train of thought going down the right track. If I have time this weekend to add the delay relays and I'll let you know what I find. If not, it won't be until next weekend.

 

[Mandrake]

I had a few minutes to throw together a simulation circuit on the bench and lo and behold, the delay relay solves the issue for the compressor side of the wiring. For the airing down side of the wiring, it's a little more complex and will require more thought than I'm willing to put into it tonight.

I'm sure it'll come to me at 3AM...

 

[Mandrake]

Well, it didn't come to me at 3AM... more like 730ish AM. I rushed to get some paper and a pen before I lost it, scribbled it out and went about my day. I just had some time just now to throw it together on the bench and it works! Integrating it with the wiring in the truck will take a bit of doing, but that's for this coming weekend... if I don't end up flying to Oregon to buy a car I've got my eyes on.

Rivman and Dork, thank you again for the nudge in the right direction.

 

[Rivman1243]

Glad it worked out!

 

[Mandrake]

Well, I done flew to OR and brought a car home, and I've had a really hard time prying my ass out of it what with little free time I've had.

Anyway, I wired up the delay relay on the air up side and after some tweaking and adding a resistor, it works. Airing down is proving to be more difficult, but V3.0 of the circuitry might solve the issue. When I get things ironed out, I'll post up some wiring diagrams.

And in case you were wondering... the car in question.

 

[Rivman1243]

Tesla?

 

[NLXTACY]

Tesla?

Jag

 

[Mandrake]

2010 Jaguar XKR.