As most people know, the engine temperature gauge sits happily on 'normal' until the coolant reaches 115C. From that point on, things happen very quickly indeed. Should an overheat occur, the needle rockets up almost instantaneously into the red and the warning light pops on. In a K16, that's not good and the engine must be stopped immediately to avoid serious problems. An overheat is often accompanied by ejection of coolant from the header tank cap, which is set to relieve the over-pressurised system. So far, I've seen no data as to what temperatures are involved when these things happen.
One of these can be determined by looking at the properties of ethylene glycol(EG) solution (the basic ingredient in OAT antifreeze). These are well documented and vapour pressure curves can tell us how hot the system is when the cap blows. The standard pressure cap is rated at '140'. That's the relief pressure in kiloPascals. It's about 21psi in old money. The physics is simple enough, the valve opens when the coolant vapour pressure reaches 140 kPa. Look at this diagram:
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Using the standard 50% OAT as coolant mix, you find that the coolant is barely above 'normal' when the cap starts to open. It's about 118-120C taking my red lines as near enough.
This explains why the events happen so quickly once the overheat starts. It also illustrates how useless the gauge is for anticipating a potentially serious situation. Of course this graph doesn't tell you where the red line is on the gauge, but I reckon 120C is about right.
Another point to bear in mind is that some engines don't have a 50% EG mix in them. This is especially so where the owner has to keep topping up a leaky system and tends to use tap water. As the EG content drops, the pressure cap will open at progressively lower temperatures. This means coolant is ever more easily ejected, leading to an even more rapid spiral of events. Note where the red line crosses the 0% EG curve (about 115C).
TC
One of these can be determined by looking at the properties of ethylene glycol(EG) solution (the basic ingredient in OAT antifreeze). These are well documented and vapour pressure curves can tell us how hot the system is when the cap blows. The standard pressure cap is rated at '140'. That's the relief pressure in kiloPascals. It's about 21psi in old money. The physics is simple enough, the valve opens when the coolant vapour pressure reaches 140 kPa. Look at this diagram:
Using the standard 50% OAT as coolant mix, you find that the coolant is barely above 'normal' when the cap starts to open. It's about 118-120C taking my red lines as near enough.
This explains why the events happen so quickly once the overheat starts. It also illustrates how useless the gauge is for anticipating a potentially serious situation. Of course this graph doesn't tell you where the red line is on the gauge, but I reckon 120C is about right.
Another point to bear in mind is that some engines don't have a 50% EG mix in them. This is especially so where the owner has to keep topping up a leaky system and tends to use tap water. As the EG content drops, the pressure cap will open at progressively lower temperatures. This means coolant is ever more easily ejected, leading to an even more rapid spiral of events. Note where the red line crosses the 0% EG curve (about 115C).
TC
