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Fuel System Question

Flying Pig

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I operate a business that deals mostly in new and used racing parts and supplies. I also sell some things for the big trucks. I recently picked up the Pure Flow Technologies line that has Airdog fuel/air separation and Raptor fuel pumps.(similar to the FASS products) I want my customers to get the best value for their dollar they can and dont want them spending money they dont need to.

My questions are then, how is the fuel supplied to the injector pump? Is it more like a Cat or Cummins? How much fuel is bypassed back to the tank?

Cummins engines benefit quite a bit from getting the entrained air out of the fuel. Im told that the air pretty much passes thru the Cat piston style pumps as the injectors fill from the bottom of the stroke rather than the top.

I have a customer with both an older Mack 300 straight truck and a newer Freightliner with a Detroit 60 asking for info. Im not your typical salesman that just says "it always works-trust me."

You'll never go wrong by doing right.

Who do you call when the lawmakers ignore the law?

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OK, what engine we talking here. The later E7/EA7 have unit injectors on the side of the block that are fed from a single high pressure pump. Earlier engines use an inline injector pump fed from a piston type lift pump. And any airin a diesel fuel system is bad and will cause problems no matter what the make. Cummins were more susceptible to it with the PT (pressure timed) fuel system than others simply by design. Tiny amounts of air would make them run like a hairy goat. Fuel return to the tank simply is what's not required for the engine to produce required power at that time. More returns at idle while less is returned under full load climbing hills for example. Series 60 use a gear type pump to pressure feed fuel galleries in the head, where unit injectors are fitted. Most of the modern electronic engines (except E7/EA7) are the same. One thing these "bolt on" parts must not do is restrict fuel flow. The gear type pumps will differ from make to make, but they all need an unrestricted supply from the tank.

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I did quite a bit of looking for information on the air/fuel separation systems. I found an SAE paper written by a Parker engineer that showed when diesel is put under a vacuum (when being pulled up to the engine from the fuel tank) entrained air is separated out and makes larger bubbles that cause problems. If fuel is fed to the injector pump by gravity(like in a farm tractor) the air doesnt make the larger bubbles and goes through the injectors with no negative issues.

I know on my Cat the plungers are filled through a port in the bottom of the cylinder and dont have many of these bubbles to interfere with the spray pattern. Im assuming that the older Macks are the same way.

You brushed up against the question with your answer, but its still not quite clear for me. In these newer electronic systems, (common rail?)do they bypass the fuel before it gets to the injector and therefore bypass the air or is the fuel that still may contain bubbles sent to the injectors and bypassed after going past the injector body?

If Im not being clear on my question please let me know. Ive tried to get this information right from the engine dealers but apparently there isnt a "book" answer so I have to look elsewhere.

Ive tried in vain to find the Parker paper. The Parker website seems to be "busy". I did find these quotes tho:

Parker - Racor Technical Support confirms:

"Fact #1; There is Air entrained in diesel fuel."

Milwaukee School of Engineering, Handbook of Hydraulics agrees:

"Hydraulic oils also contain varying degrees of entrained air."

Caterpillar®, Special Instruction 651-1250 points out:

" Normally No. 2 Diesel Fuel contains about 10% air in solution, although the air is not visible."

Cummins® Engine Co., Service Topic 5-135 states:

"Like water, fuel contains a certain amount of dissolved air depending upon the fuel temperature, pressure on the fuel , specific gravity and the amount of aeration to which the fuel has been subjected."

Those came from the Airdog website. Ive put them here not to sell product but to give you an idea of the answer Im looking for. I hope someone can find the SAE paper Id seen before. Im only trying to find out more detail on the routing of the fuel from the tank to the engine and the return of the bypassed fuel.

Thanks for being patient.

You'll never go wrong by doing right.

Who do you call when the lawmakers ignore the law?

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Agreed yes, there is entrained air in all fuel supplies. But this is negligible and diesel fuel systems are designed with this in mind, as most have a minimum of 1/2" supply lines, but of late more like 5/8" or bigger to reduce vacuum restriction in the fuel line. As for the returned fuel, current engine technology isn't true "common rail" yet, although the new Series 60 Detroit is supposed to be, but I can't comment as haven't looked into it yet. What is normal situation with unit injector type setups is a fuel gallery in the cylinder head which feeds to the injectors. The injectors sit in a machined "hole" in the head ( usually in an injector tube or sleeve) and fuel under pressure is fed directly to them via the fuel gallery. The "out " end of the fuel gallery is usually smaller or fitted with a restrictor of some type to allow pressurization of fuel in the gallery, thus supplying pressurized fuel to the injectors. This fuel comes directly from the ift pump, usually a gear type either directly, or through a filter. Sometimes the filter is positioned on the inlet side of the lift pump, and sometimes there are 2 filters, one before and one after the lift pump. So to answer your question, returned fuel to the tank is what is not used by the injectors. Earlier Injector pump and individual injector type fuel systems are basically the same except the fuel gallery is in the actual injector pump, which has its own spill return line and some systems have spill return off the injectors as well. I will try to get into one of the engine manufacturers sites and send you a link that will give a diagram, easier tpo understand when you have a picture I find.

I hope this clears it up a bit

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This is a link that shows the old Detroit 2 stroke diesel fuel system The injector fuel is supplied to the injector via pipes external of the injector, but supplied from, and returning to galleries in the head, in this case 2 galleries, one for supply, one for return. Take out the external pipes and you have basically todays modern unit injector setup.


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