Diesel screw compressor idle down/performance issues

[TVA]

Hi everyone!
This is the first time I am messing with screw compressors. I am working on Atlas Copco XAS 185 (JD 7).
The problem is it seems to activate idle down cylinder earlier then supposed to.
It will unload, idle down and maintain the pressure if I abruptly close the discharge valve.
But it will start to activate idle down cylinder and reduce speed and power right around 85 psi. It is set for 125 psi.
Very often it will kill the engine close to rated pressure.
Atlas tech support said that spread between unloaded max. pressure and going back to full RPM should be 15-20 psi.
So if it is unloads at 125, I supposed to be back to full speed at 110-105 psi.

Don’t think it is an engine performance issue, and don’t really understand why idle down system reduces the engine power right when it needed the most.

This is NOT solenoid unloading machine.

Anybody have any takers?!
Thanks in advance!

 

[John C.]

I've been out in the weeds today so I'm struggling to remember how the ones I've had to work on acted. I seem to recall that they work more on CFM instead of pressure. If they are set at 125 PSI, they should maintain that. If the load requires less air the engine should slow down but not drop the air pressure on the line. Does that sound right?

 

[TVA]

No this one actually closes the intake valve to unload. The thing is the same chamber/channel in pressure regulator has both hoses going to it - the unloader ( which cuts the intake air) and idle down cylinder feed ( which mowed fuel rail to idle position).

But for some reason it is either not done abruptly, but ramps up slowly. Or my another theory that unloader is lagging behind the idle down.
I disassembled all three ( meaning pressure regulator, unloader - or otherwise known as capacity limiter, and idle down cylinder), and everything looks good and almost brand new!

 

[John C.]

How does it know to put out more or less air?

 

[TVA]

How does it know to put out more or less air?

It is LOAD/UNLOAD controls, constant run.
At set pressure, pressure regulator crack open the valve and introduce pressure to both unloader and idle down cylinder.
Pressure introduced to the unloader closes the intake valve depriving the screws intake air feed, somewhere around that idle down pneumatic cylinder is pulling fuel rail to idle stop.
So pressure regulator controls both from the same chamber in its body, but the drain orifice ( to drain air pressure when pressure regulator goes back to LOAD) is only inside of unloader/intake valve body.

It does not look like someone stupidly drilled out that orifice.
This system is finely balanced with springs inside the valve and cylinder and drain orifice, and it is my first time trying to diagnose it.

 

[TVA]

BTW, forgot to mention: there’s little bit of pressure “hunting” going on, both at load close to 100-110 psi ( in instances when engine didn’t get killed), and at unload. If I close valve abruptly, pressure would jump to 150 psi, then slowly go back to set 125.

Not used to see this on reciprocating compressors, so don’t know if it is normal for this type of unloader, or it is a problem and the clue.

 

[John C.]

The couple that I worked on would maintain outlet pressure at any flow. If a jackhammer was being used, the engine on the compressor would ramp up to only what the jackhammer needed to operator. On drills it worked the same. When you shut off the flow the engine would ramp down. So it seems that something is plugged or leaking somewhere to cause the mismatch between the compressor ramping down and the engine throttle ramping down.

 

[mg2361]

Atlas Copco XAS 185 (JD 7).

Just curious, you mention JD7. Does this unit have a 4045 or a 4024 Deere? If it is the 4024 then I strongly suggest chasing an engine issue.

 

[TVA]

Just curious, you mention JD7. Does this unit have a 4045 or a 4024 Deere? If it is the 4024 then I strongly suggest chasing an engine issue.

As far as I see it has 4024 ( need to double check).
Air and fuel filters and fuel lines are new!
Don’t see anywhere where I can tap in to check boost pressure.

Is there any particular performance issues these engines are notorious for?

 

[TVA]

The couple that I worked on would maintain outlet pressure at any flow. If a jackhammer was being used, the engine on the compressor would ramp up to only what the jackhammer needed to operator. On drills it worked the same. When you shut off the flow the engine would ramp down. So it seems that something is plugged or leaking somewhere to cause the mismatch between the compressor ramping down and the engine throttle ramping down.

This one strictly pressure controlled.
Unloads and idle downs when set pressure is achieved.

 

[Vetech63]

I agree with MG. The 4024 has a funky fuel/governor system that is prone to failures and issues. Ide bet money its an engine problem. The special tooling and governor set up alone will drive you to using drugs.

 

[TVA]

I agree with MG. The 4024 has a funky fuel/governor system that is prone to failures and issues. Ide bet money its an engine problem. The special tooling and governor set up alone will drive you to using drugs.

I think I will disable idle down and see if the engine is up to its task without it, might even leave it like that.
Screw fuel saving when not in use

 

[TVA]

Hey guys?! Should I tee in to aneroid line to read boost pressure on this engine?

Another question: i didn’t read the pressure on the idle down cylinder - I just measured the rod length on it when RPM start to drop too early. Is there a possibility that it wasn’t the idle down but engine fuel controls doing it?

 

[mg2361]

Post an engine serial number please.

Is this an issue that has been happening for a while or just started happening?

Should I tee in to aneroid line to read boost pressure on this engine?

Yes, but with the engine dying you probably won't see spec.

Also check the fuel pressure. If it is a mechanically injected engine (which I'm betting it is) it should not go below 10 psi.

Is there any particular performance issues these engines are notorious for?

The list is too long to write here.

 

[TVA]

Well!
After the day of going through everything, cleaning some dumbasses hardened type of gasket maker from O-ring sealed surfaces, putting thin layer of the electrical tape under the lip seal on the piston of the unloader valve ( was not about spent $200 and wait few days with uncertain results), double checking and re tightening everything - the compressor fully unloaded and at idle at 125 psi (per manual) and back off the idle down and unloading around 107-110 ( again - per manual). Idle down and unloader start activating simultaneously!

Didn’t find anything big, could be slightly loose two bolts on the unloader valve sucked a little extra air, and that slightly under sized lip seal caused all this trouble.

Engine does slow down a little, around 100 psi just from the load ( idle down disabled). Don’t know if it is normal, or this is a slight performance issue though.

But compressor starts and runs beautifully now, maybe tiny bit not 100% on the performance!

 

[abedsunna]

hello dears

i have atlas copco ax 137 and the air pressure is not going to more than 2.5 bar evrn if i make the rase of engine full