Some injector trivia

[franklin2]

I was discussing the function of the return line system on our trucks in another forum. I thought it was for cooling the injector, and then maybe lubrication also. He corrected me and said the return line system is just to carry the fuel away that leaks around the pintle, and that you can sort of tell the condition of your injectors by how much flow you have in the return lines. I don't think I have ever heard you guys discuss this over here, I thought it was interesting.

Originally Posted by Franklin2

You must be registered for see images attach

So what is the offical reason for the return flow on these injectors? They put it there for something. It does make it nice, since it purges the air out of the injector line, but I doubt that is why they put it on there. Lubrication is my only other guess.


matthewq4b

You must be registered for see images attach

Postmaster

Join Date: Feb 2008
Location: St Albert, Alberta
Posts: 2,932

You must be registered for see images

You must be registered for see images

You must be registered for see images

You must be registered for see images

You must be registered for see images

It is not put there, it is a function/byproduct of the design. Below is a cut away of Bosch injector nozzle.

To really understand this we should cover some of the internal components and how the injector functions. .

Bosch style mech Diesel injectors are in short a pressure regulating valve. When the pressure reaches X the valve opens.

Fuel is fed by the injection pump to the base of the needle valve which is sealing the Nozzle body's outlet orifice by mated machined surfaces.
When the Injection pump forces fuel in the to annular fuel gallery eventually the pressure will reach the point that it will lift the needle valve off it's seat against the pressure of the valve spring holding the needle valve against it's seat. The pressure at which this occurs if generally called the "pop pressure" As the injector makes a "pop sound as the pressure is released. This "pop" pressure is set and adjusted by adjustment shims between the valve spring and injector body.

Since the needle valve has to able to move freely move the there needs to be clearance between the nozzle body and the needle valve to allow this.
Since this required clearance leads directly to the annular fuel gallery it is subjected to the full pop pressure and may allow for leakage of fuel from the annular fuel gallery. This fuel needs to go some where and is routed back to the fuel tank via the return fuel system.

This return fuel flow is a result of clearances between the nozzle body and needle valve. . Now there will always be some minute of return flow even on brand new injectors, but ideally the clearance should be so small that the mere presence of fuel in the clearance should be enough to seal this clearance leading to basically no spill flow to the return line.

Now lets look the return flow it's self and the impact it has on engine operation. The spill/return flow is taken directly from the preset amount of fuel delivered to the Injector from the injection pump. Now as long all injectors are spilling the same or near same amount of fuel this is not an issue as the fuel is not lost but returned to the fuel tank. Where this does become an issue is when the spill flow is not equal between injectors. The injection pump delivers a preset volume of fuel if a larger percentage of fuel is lost through spill flow this may delay the opening time of that particular injector the and this may impact the power balance on that particular cylinder

The return flow is a good measure of injector wear and balance, new injectors should have little to no return flow. It is something to check when you install new injectors and make sure they are all relatively balanced in regards to return flow I have on occasion returned brand new injectors that had excessive spill flow.. Now new injectors have their pop pressures adjusted to be the same regardless of spill flow. But injectors with a higher spill flow will initially wear faster and increase their spill flow compared to tighter injectors this could potentially lead to that cylinder being out of balance in both timing and and power.

The spill flow is a function of the design of injectors, perfectly manufactured injectors will have basically no spill flow, now this does not say that the spill flow does not help lubricate or cool the injection but this is more a by product of function rather than the reason for the spill flow. Some injector designs purposefully have bypass flow to cool and lubricate the injector and some.have no spill flow at all.

You must be registered for see images attach

 

[Thewespaul]

That’s really interesting, I appreciate you sharing this. I think using the return to measure injector wear could be a semi accurate way to read into what’s going on in an injector, not really into its condition I think, especially in our injectors. When you are rebuilding injectors or diagnosing a nozzle issue, you always pay attention to what’s returning when the injectors is fed fuel pressure. In our injectors, fuel will return with or without the injector firing, and really is mostly unaffected by that. Say you have a stuck pintle, generally you won’t have any fuel returning, and when you disassemble the nozzle the pintle will be difficult to remove. Now if you got a lot of fuel returning, but no injector fire, then that would point to too high of pop pressure, or binding issue with the spring. The clearance of the pintle/nozzle isn’t jammed so the path for fuel to return isn’t deadheaded, but the pintle can’t lift to inject the fuel so it all goes out the back hatch. Thinking about the real world application for this, it wouldn’t be very feasible to take a fuel return reading between each injector with the engine running, and just taking a reading of the whole return flow wouldn’t be good data because the injection pump has dozens of reasons to vary it’s fuel return. Still a very interesting discussion to have, I hope the author considers making an account here so we can continue the discussion.


As for your explanation of the return system, you’re not wrong. With a clogged return our fuel system won’t function, and our injectors are designed precisely to lubricate and cool the springs and clearance between the pintle/nozzle. If there is no fuel cooling/lubricating that tolerance the pintle would overheat and seize in the bore, and likely open.

 

[Clb]

Nice bring on the engineering sessions!

 

[PROFG]

Hope this is not a useless bump. So, in between injections (POPs) is there any constant pressure in the injection lines and thus constant flow in return? Not just flow on injection pulse. Why is injection stopped if return blocked, seems like only would retard time, maybe a lot? So smoke, no start, but injections.

 

[SLC97SR5]

Hope this is not a useless bump. So, in between injections (POPs) is there any constant pressure in the injection lines and thus constant flow in return? Not just flow on injection pulse. Why is injection stopped if return blocked, seems like only would retard time, maybe a lot? So smoke, no start, but injections.

All I know is that if you pinch off the return leaving the top cover of the IP the engine will shut down.

For such a "simple" mechanical design our fuel system really does have a lot of action going on.

 

[saburai]

Thanks Franklin2.
It'd be great if you would invite that gentleman to join our discussions over here.

 

[Thewespaul]

Hope this is not a useless bump. So, in between injections (POPs) is there any constant pressure in the injection lines and thus constant flow in return? Not just flow on injection pulse. Why is injection stopped if return blocked, seems like only would retard time, maybe a lot? So smoke, no start, but injections.

There are is no fluid coming from the pump after the injection ends, the pump has a delivery valve set to around 1300 psi pop off pressure, so it maintains that pressure in the steel line all the time. Once the plungers roll onto the injection lobe of the cam ring and that pressure rises enough to exceed the delivery valve setting then injection begins again. The pump itself is always returning fuel as long as fuel is being supplied to it, and it’s turning.

 

[Macrobb]

Why is injection stopped if return blocked, seems like only would retard time, maybe a lot? So smoke, no start, but injections.

Because, on the pump, the "pumping elements" have no spring to push them open. They just sit there, freely moving, until they get pushed closed by the cam ring. To open again, they need pressure on the inside of them(which comes from the transfer pump). If the housing(and therefor pressure on the outside of the elements) is at a similar pressure to the transfer pump pressure on the inside... plungers won't open, so the cam ring can't push them together and make pressure.

 

[PROFG]

"Similar pressure" kind of implies near transfer pump pressure but seems like plugged injector return system o rings would not sustain anywhere near that before IP is shut down, unless TP pressure low at idle (20-50psi WAG). Ok, it does not take much of a blockage of IP housing return to prevent dual pump plungers from resetting, in time, for a full injection stroke. Return back pressure less than this just retards time (3 deg/psi I recall) AND reduce injected amount. So as injectors and IP wear, or RPM increases, or fuel temp drops, or duckbill stiffens, or LP ages, or filter clogs, etc. everything tends to RETARD timing and or reduce HP. Great.

 

[Thewespaul]

Pretty much, but the thing we love about our IDIs, is they don’t care and keep trucking along regardless. You can run a really worn pump and still be able to perform your necessary truck duties, even if they are missing out on 25% of performance.