The Swirl Pot
Car manufacturers don’t like to waste money fitting unnecessary parts, so why doesn’t our Jetta simply have a high-pressure fuel pump, gravity fed from the tank? What it actually has is a low-pressure pump in the fuel tank, feeding a fuel reservoir which the highpressure pump sits in. Surely one pump would do the job? Well yes, it would, but not with this type of fuel tank.
Remember, lesser versions of the Jetta and Golf had a carburettor. On those models the low pressure in-tank pump on our car would be connected straight to that car’s carburettor float chamber – job done. But even if VW had simply replaced that low pressure in-tank pump with a high-pressure unit suitable for fuel injection, it still wouldn’t have worked properly. The reason comes back to that carburettor float chamber. When a carburettor-equipped car is low on fuel, the fuel pick-up in the bottom of the tank can sometimes momentarily suck in air while the car is cornering and the small amount of fuel in the tank sloshes over to one side. But because the float chamber acts as a reservoir for the carb, the engine’s running is unaffected and as the fuel sloshes back the other way, the pick-up sucks up more petrol and the float chamber gets topped up.
Fuel injection systems have no float chamber, so designers need to make provision for the fuel pick-up in the tank to always be sitting in petrol. This is generally done by building what’s known as a swirl pot (also known as a surge tank) into the fuel tank – a cylinder about the size of a baked bean can welded into the tank’s lowest point. It has a series of small holes in the side which allow petrol to flow into it, and the fuel pick-up sits in the very bottom of this cylinder. The holes obviously allow petrol to flow out of it too, but not quickly enough for it to empty when the car is cornering. This baked bean can sized swirl pot hence makes sure the fuel pick-up is always surrounded by petrol, and so the injectors don’t get starved of fuel. VW didn’t bother designing a whole new fuel tank for the injected versions of their Jettas, they simply added a separate fuel reservoir, or swirl pot, for the high-pressure fuel pump to sit in, and mounted it separately from the fuel tank. The low-pressure pump constantly circulates fuel around the reservoir, and the high pressure pump circulates this fuel around the high pressure fuel rail, which feeds the injectors.