Fuel flow to cool injectors?

[TMoe]

Issue 54 of TDR magazine page 44 states "The HPCR system uses fuel to cool the injectors" and goes on to say why the Cummins needs more flow than the Duramax. Is that true and how does the excess, above the burnt fuel, cool the injectors? Doesn't the excess from the CP and rail go back to the tank? I realize the Cummins injectors flow more fuel because there is only 6 vs 8. I think the flow is to cool and perhaps lubricate the CP not cool the injectors. Someone please straighten me out. I am asking simply to understand what flow we need and why since I want to add a 2 micron filter prior to the CP. Decreased flow in cold ambient may be fine if cooling CP is the only concern not lubrication, etc. Someone mentioned 40 GPH as minimum, why? Thanks.

 

[nickleinonen]

when the fuel spills through the injector solenoid cartridge [cartridge opens a pilot valve, then pressure differential opens needle in the nozzle]. all the fuel that is spilled when the cartridge opens, flows around the injector bodies and out the back of the head. from there i am not 100% sure how it flows, but eventually goes back to tank.

 

[CarlsonR]

There is no tank return line once the fuel is routed to the injectors. Once it's on its way to an injector it's a oneway trip to the tail pipe. I to was wondering what the author of that article was thinking when he wrote that erroneous statement Sounds like he was confusing the HPCR with some other system to me.

 

[TMoe]

The second gens had some sort of return that connected at the back of the head for fuel that leaked past injector. Is that gone with the 3rd gens? It wasn't for a high flow of fuel anyway. If what you say is correct then cooling injectors with fuel makes no sense at all. Cummins injectors obviously need more fuel than Duramax as there are only 6 vs 8 thus more fuel per injector at same power. I think that is where the confusion lies. I didn't understand the comment on pressure either other than there is more restriction in cummins than duramax somewhere. Thus for the same flow you need more pressure. The flow must only be to maintain correct operation, not starve the CP under high hp with some lubrication and cooling for CP. Now we just need to find out exactly what is causing some of the injector failures like sticking open, scary. If it is faulty design were out of luck. If finer filtration is the answer then there is hope. I am going to add a 2 micron filter post stock filter and that's all.

 

[nickleinonen]

see #10 in attachment

 

[AMassaro]

I knew that the fuel would be returned from the rail... it only makes sense, especially when you are looking at the rail front to back... thanks for the diagram N.

 

[TMoe]

Thanks for the picture. Very similar to 2nd gen layout. OK, so how much fuel is actually flowing in the fuel drain manifold line, very little I believe and not for cooling? I think there is leakage between injector and injector tube that is returned to the tank. If that is true the only part that needs excess fuel is the CP similar to the VP in 2nd Gens. 40 GPH flow is required to not stumble at high HP. If you never operate near peak HP you can live with reduced flow that results from the additional filter? There will be additional stress on fuel pump also. I think 3/8 lines from tank forward may be a good idea for cold weather operation. Next purchase factory manual set or CD.

 

[cerberusiam]

OK, so how much fuel is actually flowing in the fuel drain manifold line, very little I believe and not for cooling? I think there is leakage between injector and injector tube that is returned to the tank.

IIRC, there is flow test that is performed to determine if the injectors are bad. A flow rate above or below spec would indicate issues that need to be looked at. A number of 400 ml sticks in my mind but not sure if that is valid for return. Leakage between inector and injector tube is where you get a crankcase full of diesel and a oil undercoating. My understanding of the system is the injectors are cooled and lubed by the fuel so there is a fairly substantial return to the tank to bleed hot fuel out of the rail.



Intersting to note that the Dmax's don't use an LP and they seem to have/had a lot more injector failures than the CTD. This leads to thinking the injector return flow is a critical area and a design with the LP might be a bit "better", even with its problems.

 

[BRandolph]

I like to see under 100ml in 1 min idling on injector return on a normal running truck. Mine is about 80ml. If the tube leaks between the injector it will run back into the injector return and you will have a high return like 200ml. When the injector leaks out around the o rings that is when you have a crankcase full of fuel and a big oil leak out the draft tube. If you take the valve cover off and run the engine at idle when you shut the engine off you will get a puff of white smoke out of the leaking injector.

 

[TMoe]

Now I am beginning to see the light. So how does the flow change with engine load? Is it related to number of pulses/ rpm, pulse duration or both? Is this designed in to cool the injectors? This fuel is hot and responsible for heating up the in tank fuel. This return could get significant since at idle both rpm and duration are minimal. If flow triples under moderate load that is around 300 ml per minute, 18 liters or 4. 75 gallons per hour. Wow. This definitely needs to be considered in flow requirements especially if it gets higher. Nothing flows back from the rail unless it exceeds the pressure limiting valve setting. Is all this explained in factory service manual?

 

[cerberusiam]

This definitely needs to be considered in flow requirements especially if it gets higher. Nothing flows back from the rail unless it exceeds the pressure limiting valve setting. Is all this explained in factory service manual?

Don't forget what is returning from the CP-3 as well.





If it is in the FSM I haven't found it yet. The whole diesel fuel system is missing from the electronic copy I have.