Curriculum Vitae ~ Michael Farrow

 

MEng Aerospace Engineering Graduate

Formula Student

 

FS09 Fuel System Redesign

The main design consideration behind the SURTES09 (Surrey Racing Technology and Engineering Solutions) car was quite simply mass; as on average, the lighter the car, the more competitive it is. Therefore all vehicle components were designed as light weight as possible.

 

It was supposed to go without saying that each component also had to actually function. Unfortunatly, my predecessor in fuel tank design negated the fact that liquid fuel will slosh around the tank, and the first tank simply didn't work. Anything more than a slight right hand turn would cause the fuel pick up (mounted on the RHS) to draw air, and the fuel pump to cavitate. As if this wasn't bad enough, the tank also failed the rules in three places, and clashed with it's own fixings. Definite Fail!

Existing poorly designed Aluminium fuel tank.

So I was tasked with the job of completely redesigning and manufacturing the entire fuel system within one month. I was also given an ambitious target mass for the tank of 800g, whereas the previous tank (above) was 1500g.

Upon investigating the design brief, I identified two key problems. The first was that the geometry of the tank was largely fixed by the chassis and firewall, which had already been manufactured. Secondly, in order to keep the tank rigid, I would be unable to meet my mass limitation by using aluminium fabrication.

The effect of the restricted space envelope was twofold. Not only does it limit the maximum capacity, but also it completely negates the normal fuel pickup method, which involves one smaller vertical walled chamber sunk below the floor of the main tank. With this condition imposed, I would have been left with the choice of a heavily baffled normal tank (as above), or a reduced capacity tank with a lowered chamber. However, due to my work on offroad vehicles and armoured cars, I was aware of another solution; the multi-point fuel pickup.

Walbro multi-point fuel pickups

Putting two normal fuel pickups in a tank would result in terrible performance, as the fuel pump would draw air when cornering in either direction. With a Walbro multi-point pickup, the fuel inlet is controlled by a fine mesh screen attached to a valve. When the mesh is completely submerged in fuel, fuel can flow through it freely; however, when the fuel sloshes away from the inlet, surface tension in the wet mesh forms a screen, and the pressure differential closes the valve. Fuel will then always be drawn by the submerged pickup, or both at once, and never from a pickup exposed to air.

The new tank was therefore designed with one of these pickups at both extreme sides of the tank, so that lateral accelerations would not cause the fuel system to starve.

PICTURE

In order to meet the meet the mass restriction, it was nesisary to construct the tank out of composite materials, instead of standard aluminium. In order to ensure good impact resistance and crash survivability, whilst maintaining excellent low mass, LTM-26 Kevlar Epoxy composite was used. The reason for using LTM (low temperature) curing prepreg was that it enabled a CNC milled polystyrene foam core to be used, which in turn allowed the tank to be made in one piece (with only two access holes)

Walbro multi-point fuel pickups

The finished tank was then coated internally with a fueproof sealing compound. The kevlar wall thickness was 1.2mm, and the tank is quite capable of taking my (considerable) bodyweight jumping up and down on it. The final tank was weighed at 650g, thus exceeding the design requirement.

Other changes to the fuel system included the purchase of a lightweight fuel filter, substitution of all brass pipe fittings with their nylon counterparts, and reorganisation of the fuel line to use minimum pipe. In total, a 4kg fuel system was almost halved to 2.4kgs.

Finished Tank being Fitted into Vehicle [Firewall Removed]

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