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
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
Postmaster
Join Date: Feb 2008
Location: St Albert, Alberta
Posts: 2,932
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.
Originally Posted by Franklin2
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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
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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.
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