Any benefit to FASS fuel/air seperation?

[drury]

I have the Glacier Diesel Fuel pump, but before I got it, I did some soul searching because I was concerned about giving up the fuel air separation function of the FASS. I was still fretting about it this morning, and wondering how I could add that function back in, and dreaming of ways to combine the nice quiet fuel-cooled Walbro pump that I like so much with the FASS filters. Then it hit me. What's the point of separating the air out of the fuel anyway?



Consider:

1) The whole point is to mix the fuel with air in the cylinder, so there's nothing intrinsically bad about air being in the fuel.

2) The only downside I can see is if the air is displacing the fuel being shot through the injectors.

3) The injected fuel is under around 10,000 psi or more of pressure, which is about 1,000 times its pressure when it was mixed with air in the tank.

4) Since the fuel is non-compressible, and the air is, the volume of the air will be about 1,000 times smaller relative to the fuel when it is put under pressure in the injection rail. So if the fuel started out as 10% air by volume in the tank, it will be . 01% air by volume in the rail.

5) A 0. 01% reduction in the amount of fuel shot through the injectors is not going to be measurable.

6) It seems like the little tiny high pressure air balls would assist in fuel atomization as they expand in the lower pressure of the cylinder chamber.



Am I missing something, or is there really no point to fuel/air separation?



The only possible benefit I can see is that if you are running your CP3 to the limit and draining the rail, a fuel/air separator might help the CP3 push a few % additional fuel into the rail, but if I was that close to the edge, I'd rather upgrade the CP3 than than mess with adding the fuel/air separation.

 

[bzingre]

Cavitation = Fuel System Wear

 

[cerberusiam]

1) The whole point is to mix the fuel with air in the cylinder, so there's nothing intrinsically bad about air being in the fuel.



Where you are mixing the air/fuel outside the combustion chamber that is true. Not so with a direct injected system. The air is alreay there so we don't need a mix we need a measured amount of fuel, preferably atomized enough to promote complete combustion across a specified area. The key is atomization not mixing.



2) The only downside I can see is if the air is displacing the fuel being shot through the injectors.

3) The injected fuel is under around 10,000 psi or more of pressure, which is about 1,000 times its pressure when it was mixed with air in the tank.

4) Since the fuel is non-compressible, and the air is, the volume of the air will be about 1,000 times smaller relative to the fuel when it is put under pressure in the injection rail. So if the fuel started out as 10% air by volume in the tank, it will be . 01% air by volume in the rail.

5) A 0. 01% reduction in the amount of fuel shot through the injectors is not going to be measurable.



Yes, the entrained air is compressed and takes up minimal room in the pump and rail. However, the amount of air originally present is still there so if you have 10% going in you have 10% going out thru the injectors. In addition, once the pressure is released thru the injector the air will expand to its original size or as close as possible given the compression in the cylinder. What this does is leave gaps in the in the injection stream and causes different patterns in the stream that may interfere with the combustion event. Depending on the amount of air held in the fuel you may also be upsetting the stoichiometric balance in the cylinder which has an impact on mileage, power, etc.



6) It seems like the little tiny high pressure air balls would assist in fuel atomization as they expand in the lower pressure of the cylinder chamber.



Atomization has to do with orfice size and injection pressure so the air does not help. In fact it is a hindrance to getting a pre-measured combustible stream of fuel that wil l burn evenly and as completely as possible.



Since the regulator is built into the FASS unit return fuel is not subjected to engine heat like the typical setup of a regulator under the hood. That can be remedied with repositioning the regulator but it is more work and plumbing.





Now the real question, can you tell the difference between a FASS system, GDP, or any other type? Probably not with the equipment most of us have access to or in 99% of our use cases. The FASS obviously provides some features abov and beyond the others but at an additional cost. EVERYBODY has a different answer to the cost\benefit ratio so there is rght or wrong.



As far as worrying about the GDP for its lack of features, don't.

 

[TAbbott]

Detroit Diesel markets a product for it's 60 series engines called a Pro Chek that is a fuel air separator. I think it's actually manufactured by Davco. This is an item that can be ordered from the factory when a Detroit Diesel engine is specified in a truck. They are small and relatively inexpensive and don't really look like they'd work, but they do. I have one on my big truck which is running 600 hp + on diesel only (855 cubic inches, factory set at 575 hp) so they'll pass enough fuel for any sane application in a 5. 9. They de-aerate the fuel which makes for a more uniform injection by making the timing more precise. They definitely smooth out the engines operation. These are off the shelf items at most Detroit Diesel shops.

 

[Mark Craig]

drury,



Cerberusiam gave a great reply, on a more simpler note look at a hydraulic system. If you have air in it you get sloppy operation and if it is bad enough the system just won't function at all, like your brakes with a little air in the lines too! The common rail or any pressurized fuel system works better and gives a higher fuel charge per injection cycle etc when there is NO air in the system. Diesel as a fuel is one of the worst to en-trap air especially in the colder times of the year as it thickens up etc.



Mark @ DPPI