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Unlike the example in the article the use of PCM for block cooling would not be a simple heat-sink, but would be an intermediary, with the wax itself in liquid-state pumped into a specially designed radiator. On current liquid-cooled engines the mass of the fluid has the effect to even-out the temperature load across the driving cycle (ie. it is sinking variations in heat, not only at local hot-spots on the head, but from a moment of hard-driving) - This is why it is not an immediate effect for an electric rad-fan to switch on if you start pushing it.
The intermediate-PCM stage would allow several effects.
1. It could mean a much smaller mass of coolant in the system (although what there was would need to circulate quicker or have higher heat-transfer characteristics).
2. Because the specific heat of the PCM is equivalent to many times that of the same mass of water, the length of time that the engine would be able to be pushed hard for without overheating would be much longer. Looked at another way, if a driver is driving more-or-less normally then the size of the radiator (and it's aero impact) gets smaller.
So, I'm not saying this idea is workable for a mass-produced car of today (unless it's hybrid) due to it having limitations for continued heavy driving. But for the eco-minded driver it is, I believe, a realistic proposition - Remember, just like in the intercooler example in the article the driver is borrowing heat-capacity now to be paid-back later.
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