Fuel Starvation at Watkins Glen

[304065]

At The Glen I discovered that my re-engineered fuel system wasn't worth a dog's rear end. When I went to a fuel cell, I had to ditch the factory fuel pump, as it was located on the crossmember back by the port heat exchanger, and wouldn't pull fuel out of the top-loading cell.

As you can see from the picture, I mounted the pump in the smuggler's box, which I thought was a good idea from a safety standpoint, and assumed that the capacity of the pump would be sufficient to pull fuel out the top of the cell and back to the engine, even though there was an approximately 3' run of hose from the pickup to the pump inlet. The trouble is that the pump's application notes call for it to be no further than 12" from the tank, and below the level of the fuel. Looking back (and looking at the website of Aircooled Racing www.aircooledracing.com the guys who invented the 911 cell for ATL) I should have put the pump low on the starboard side with the supply line running forward from the fill plate. http://www.aircooledracing.com/atl-3.jpg

The fuel cell has a three-door tank with a pickup in the bottom, which I thought would be adequate to keep the fuel pressure up under high g conditions at the Glen.

But it wasn't, and as a result, in the middle of the esses at the glen, the fuel pressure dropped from 2.5 bar (I had it cranked up a little bit) to ZERO, and the engine began to stutter. The only way I was able to finish the enduro was by lifting off at the end of the straights and letting the plenum refill itself, allowing me enough power to make it through the corners.

So I now have sitting here another ATL three-door tank with an integral Walbro GSS340 gerotor style in-tank pump, and a spare pump just in case.

The application notes for this pump:
PSI l/Hr
0 295
10 283
20 264
30 244
40 229
50 209
60 194
70 174
80 143
90 105
100 43
110 4

OK, and the system pressure is between 1 bar (15 psi) and 2 bar (30 psi). According to ze little white book, the "E" motor wants 125 liters per hour. At a testing pressure of .8 bar +/- .2 bar, the pump should flow a minimum of 900-1000 cc with the power on for 30 seconds. This we will test. But at an operating pressure of 1 bar, the pump should flow 274 liters/ hour (interpolated), or 220% of capacity. THAT should do it. (But that's what I said BEFORE! http://forums.pelicanparts.com/showthread.php?threadid=61592&highlight=fuel

As a final check what does the motor want at WOT?

HP = pounds per hour/ BSFC
assumed BSFC =.50 (rule of thumb)

155 = pounds per hour /.50
= 77.5 pounds per hour or 13 gallons/ hour or 50 liters/hour.

Does this look right to you guys? If we go back to this thread http://forums.pelicanparts.com/showthread.php?threadid=113702&highlight=bsfc and pull the BSFC of 290g/HPh. . . (which is .64 pounds/hour per horsepower)

Premium gas = .780 kg/l density (from BOSCH Auto. Hndbk. 5th ed p. 244) or 780g/l. 290g/780g = .37l/HPh .37 x 155 = 57.35l/hour.

Okay, so the factory used about a 100% margin in sizing their fuel pump.

I'll let you know how it works out. On paper it looks fine but the real test will be at Gingerman. . .

[dad911]

I have seen cars set up with 2 pumps, with a pick up on each side of the fuel cell. Not an issue with a full tank, but the second pump can be switched on with a toggle switch in the cockpit if you encounter low fuel/starvation conditions.

BTW, you have a spare fuel cell now? Let me know if its 4 sale.

[304065]

Alan,

I don't have a spare fuel cell at the moment, only the one in the car.

Thanks for the idea about multiple pumps, I had considered running them in series for redundancy. The pump I have should draw about 7 amps at 2 bar pressure. 7 amps x 14v =98 watts but I have a Carrera 90 amp alternator that puts out 1260 watts. I also have a couple of SPAL fans for the oil coolers that draw 6.5 amps each or 91 watts. With dual pumps and the dual fans that's a cool 378 watts of STUFF running in up front. . .

As far as putting them on opposite sides of the tank, if the tank had dual pickups in the corners that might not be a bad idea. The three-door surge tank with integral pickup is supposed to prevent fuel starvation by being mounted in the center of the tank and there is only a single outlet.

[304065]

Update: spent all day (now 0422 and I just got home) fixing the fuel problem. I drained the cell, flushed it with water, then removed all 26 bolts from the fill plate, pulled out all the foam, then cut away the three-door surge tank that was molded to the bladder and replaced it with the ATL "Black Box" with integral Walbro pump. After rerouting the lines to make a shorter path and rewiring the pump, also fabricating an aluminum bracket to mount the fuel pressure regulator, everything worked great. I pressure tested the system for a couple hours at 4 bar, no leaks, and the new pump is very quiet, almost hard to hear it running.

I disconnected the return line and put it in a gallon jug. The gallon jug filled in 70 seconds, so that's an unladen flow rate of 3.32 liters per minute, or roughly 1.5x the factory spec of 900-1000cc in 30 seconds.

Now let's see if it works in the race!

[daloff]

I have the same fuel cell and run dual pumps mounted on the passenger side of the firewall in the trunk compartment above the old a/c condensor ("smugglers box"). Seems to work fine as I have had no such problems.

David