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Surging ahead

Trying to pick up that last drop of fuel out of the tank is a challenge, especially if the car is developing high g-forces while cornering or under acceleration. However, ensuring complete evacuation of the fuel tank just as the race is finishing not only gives the race engineer a sense of satisfaction but minimises the start weight of the car, giving the equivalent of just that little bit extra engine performance. In a sense therefore, fuel that can’t be picked up out of the tank is a particularly inefficient form of ballast – and nobody likes excess weight in a racecar

But getting to that situation where all the fuel can be scavenged is an issue that has tasked engineers for many years, because as soon as the end of the fuel pick-up pipe is uncovered, the efficiency of the fuel pump falls and it will far sooner try to pump air than fuel.

Traditionally there are a number of ways of attacking the issue. The first might be to introduce a kind of foam plastic into the fuel tank. Using foam with an open-cell structure will reduce the mobility of the fuel around the tank, but under longer lateral g-loads the fuel will still move across the tank, and the foam – which is designed principally to prevent slosh and reduce the rate of spillage in the event of a tank rupture – will be largely ineffective in battling fuel surge.

Another, far more effective, way is to use a system of trap doors around a much smaller ‘fuel surge tank’ within the main tank. This allows fuel to move freely towards the pump pick-up but closes a trap door when the fuel tries to drain away. Having these trap doors arranged such that enough fuel is always trapped is highly effective and highly popular in many track applications.

Yet another way is a sort of swinging pick-up. Attached to the fuel pump at one end and having a fine mesh filter at the other, moveable end, the loose end is allowed to move freely in the tank. If the fuel pump is positioned centrally in the fuel tank, the arc of the loose end will follow the movement of the fuel side to side during surge conditions and – so the theory contends – fuel pressure will be maintained throughout. While surprisingly effective, systems such as these do take a little bit of fine tuning to get them working to their best. 

However, the most sophisticated way of avoiding fuel starvation is to have multiple fuel pumps positioned around the tanks and in those areas where the fuel eventually resides under the high-g conditions. Switched in and out according to pre-programmed software, fuel tanks can now be made in all manner of shapes and sizes to suit the space available on board the vehicle.

It may not be the easiest of tasks or the most glamorous, but scavenging the last drop of fuel out of the tank is well worth the effort.

Fig. 1 - Typical fuel cell incorporating a surge tank

Written by John Coxon

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