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The air apparent

When I was young (TV shows in black-and-white only, and so on) vehicle radiators were much taller than they were wide. Designed to make maximum use of a natural phenomenon called the ‘thermo-syphon effect’, when the coolant was introduced into the top of the radiator it proceeded to fall to the bottom as it cooled, only to be replaced by more hot fluid from the engine. The advantage of the tall radiator maximised this effect and at the same time, being raised above the engine, offered a header tank to ensure the radiator was full with coolant at all times. In addition any air entrained in the system would eventually vent into the void high up in the header and thus cooling systems of this type tended to be self-bleeding. 

As the years passed, however, we learned that vehicle bonnet lines needed to be low, and with the universal inclusion of the water pump to forcibly circulate the coolant, radiators with the same frontal surface area could be low and wide. Instead of having an increased number of short but vertical tubes leading from top to bottom of the radiator, it would make sense for the coolant to flow from side to side, with a small collector tank at each side. The downside was that with the radiator mounted so much lower in the chassis, a totally separate header tank would be needed elsewhere higher up on the vehicle. So when cooling systems would more or less automatically vent air when being filled or when in use, the new cross-flow systems would not be quite so accommodating. And when we come to high-performance engines of course, the problems become worse.

In essence, the problem is all down to air and/or steam becoming trapped inside the engine or radiator during use, displacing the engine coolant. In a carefully designed cooling system, since the coefficient of thermal expansion of the coolant ( at 207 x 10-6/K) is greater than the volumetric equivalent of aluminium (69 x 10-6/K) or iron (33 x 10-6/K), the coolant expands greater than the water jacket containing it. As the engine warms up, the system is therefore pressurised and the coolant will be ejected past the relief valve/cap and into the catch tank. If perhaps at some point, say when making a long pit stop and the cooling effect of the radiator passing through the air is lost, then as a result of heat soak the coolant would expand even more – perhaps even boil – and push out even more fluid into the catch tank. Returning to the track afterwards and with less coolant in the system, the inevitable is just around the corner – literally!

The obvious remedy is to make sure the header tank is large enough and placed high enough in the chassis and linked into the bottom hose of the radiator. In addition, if the total volume of the cooling system void is greater than that of the coolant at its maximum running temperature, provided that little air is circulated with the coolant then no further coolant should be expelled. If air or steam is carried around then some kind of swirl pot may be needed. This will centrifuge the coolant at some point in the system, separating air from the coolant and bleeding the air into the low-velocity zone at the top of the header tank.

Fig. 1 - Header tank mounted on top of the engine with the top hose inlet tangential, thus creating a centrifuge effect and separating any air

Written by John Coxon

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