Eutectic Microsphere Engine Coolant

The internal combustion engine requires a cooling system. There are several ways to accomplish this but most common road vehicles use a closed loop of circulating liquid coolant composed largely of water. Typically, a mechanical centrifugal pump forces water through channels in the cylinder head and block where it is heated by waste heat from combustion. The water then exits the engine and is passed through a "radiator" * to cool down, completing the circuit.

The point of all this is to stabilize the temperature of the engine under varied conditions, preventing problems with thermal stresses on materials and to optimize combustion. The temperature selected depends upon the role and construction of the engine but is typically between 75-120 C.

Over the years a refinements have been included. A "thermostat" acts as a valve to short circuit the cooling system when the engine is below optimal temperature, decreasing the warm-up time. Clutch equipped mechanical water pumps and radiator fans remove the direct relationship between engine speed and cooling capacity and later, electric fans and pumps enabled finer control. Similarly "water" cooled engines now circulate various coolant blends containing water laced with corrosion inhibitors, pH stabilizers, anti foaming agents and substances to depress the freezing point.

Water is a great choice for moving heat around, it has a high specific heat capacity, 4.2 J/g/K reasonable density and is liquid at pleasing temperatures. During warm up however, these qualities play against water. It's often the case that a cold start can lead to an engine circulating water at -20C which actively slows the progress toward ideal operating temperature. To reduce this, strategies include restricting flow rate, reducing coolant volume or possibly reducing the specific heat capacity of the coolant. However, this must be balanced by having enough thermal capacity to cool the engine at maximum load.

It is possible to improve both aspects, we can use the tremendous energy changes observed during solid-liquid phase change in eutectic substances. Water absorbs 4.2 J/g/K, but phase changing sodium acetate trihydrate absorbs 289 J/g at its melting point of 58C**. Now, liquids and solids are usually handled very differently, a traditional water pump would not function too well trying to circulate solidified sodium acetate.

So, lets encase our eutectic material in polymer (e.g. PTFE) spheres and make the coolant a suspension. The spheres should be about 20-30um in diameter, a wall thickness of 1- 2um seems feasible, and PTFE can be adapted to be a little better at thermal conductivity. The density of the liquid and the spheres should be roughly equal so that the suspension is nice and stable.

Say the suspension is 5% spheres in what is essentially water. The specific heat capacity is down to ~4, 5% less than water, right up to the melting point where it suddenly jumps 350%. Now, sodium acetate trihydrate is a known example used in some hand-heating packs, sodium acetate is also good but has a melting temp too high. I looked at some potassium, magnesium and calcium acetate derivatives and saw figures of 350, 80 and 160C, which suggests there should be a Na/K/Be/Mg/Ca acetate/propionate +/- hydrate variant that has an appropriate melting temperature.

So, if you have 2-3.5 thermal capacity, you can downsize the liquid part of system, oh and the heater would be amazing.

* Very little radiating takes place, while obviously designed for maximum surface area, most of the elements are flanked by other equally hot elements, very little net radiation. Instead heat is largely transferred by conduction to the air flowing through them. So when your mechanic falsely refers to the "radiator" be sure to reprimand them. They will be glad to correct such a glaring blemish in their knowledge.

** although a domestic heating system would be fine with this, it's just below the pain threshold so the temperature of radiators would fall into a less burny range, but their heat capacity would increase.. if you can get the transfer working a bit faster.