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WHY USE A FUEL CELL?
The stock fuel tank in the BMW E46 has both good and bad properties. Good: it is strong, compact, mounted very low in the chassis, just aft of the front seats. Under the rear seats, under sheet metal, completely sealed away from the cabin. It crashes well being in the rear middle of the car, too.
On the bad side the saddle shape makes for nasty fuel slosh in lateral loading and the "tunnel" between the two sides is too tight to fit dual 3" exhaust pipes and still keep good ground clearance. It is also plastic which means in a fire it can melt, rupture, etc. That isn't super common in crashes, but it is a possibility. Also, one side of the E46 saddle tank is much smaller than the other, as you can see above left. It is cheap and easy to just re-use this tank - which is what I did on
my personal E46 330 race car.
In use a fuel cell provides a minimal anti-slosh effect due to the foam that is inside the bladder but can be designed to always pick up fuel. A fuel cell bladder is VERY tough and made to hold up very well in a motorsports crash. The bladder is mounted inside of a can that is made of steel or aluminum, with one end (usually the top) that can be unbolted, to extract the bladder for service. The bladder has to be replaced every 5 years to maintain FIA certification, so keep that in mind. The fuel cell foam can also degrade, especially when exposed to (and soaked /stored with) ethanol.
For many decades an FIA approved fuel cell was required for many road racing and hill climb (like the
Pikes Peak Subaru STi above) classes, but safety improvements in modern OEM plastic fuel tanks - and their common, optimized placement under the car behind the front seats - has allowed these to be permitted in many Wheel-2-Wheel racing classes. We are building this E46 M3 for NASA Time Trial and Optima competition, so a fuel cell was not required, but it is never a bad idea. They do tend to be safer than almost any OEM fuel tank.
We talked about a custom fuel cell early on. Trunk mounting is the easiest and probably smartest move. There are many rectangular shaped fuel cells with an aluminum enclosure and kevlar bladder, like the ATL below in the
69 Camaro track build. This is mounted behind the axle just forward of the rear bumper.
I usually forego this in my own cars because I don't do wheel-to-wheel racing, but for serious builds a fuel cell is pretty standard. If packaging had permitted, we would have used an off-the-shelf fuel cell / shape and stuck it in the trunk. If possible, try to use an off-the-shelf cell. You will save mountains of packaging hassles and costs by sticking with common fuel cell shapes.
This car, being built for Optima competition, needed to be more optimized. Visible "race car" things (like a trunk mounted fuel cell) can sometimes spook the D&E judges. Besides, having a big chunk of weight in the form of 16-20 gallons of fuel mounted way out back makes for some weird polar moment issues.
MAKING A CUSTOM FUEL CELL CAN
Pretty early in this project we decided to make a custom fuel cell, to gain the exhaust clearance room in the tunnel, add safety, and keep the fuel load as low and centrally/rear located as possible. Putting it in/under the back seat would also leave the trunk floor area open for a
diffuser - another really useful addition to get additional downforce in an Optima car (
rear wings are severely limited in that series).
This custom cell was modeled somewhat around the lower shape of the right/passenger side of the OEM saddle tank, just made significantly taller. It actually holds 16 gallons all on the one side, which equals the OEM dual saddle tank's volume. We figured it would be easier to do a couple of sessions in a row on track feeding a hungry 800 hp engine if we had a full 16 gallons on board. Ryan made the "can" out of aluminum plate that was TIG welded together, with a flange on the top to be able to access the bladder inside.
The shape of the can is funky - not rectangular - and this will cost us later when we have a custom bladder is built, but the alternative is a compromise of one sort or another. Of course there is a hole in the rear seat floor to let the taller portion poke through - to get the fuel volume we needed. This hole will need to be sealed up from the cabin, and an enclosure around the fuel cell can will be needed as a firewall. That work is shown below in another sub-section.
The fuel cell can is 15" tall and sits about 5" above the rear seat floorpan, but sits no lower under the car than the OEM saddle tank. The fuel cell can is mounted inside of a mounting structure or "cage" made of 1" square steel tubing. The upper portion can be unbolted to remove the can.
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