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Eutectic Microsphere Engine Coolant

Enhance the properties of engine coolant with eutectic-cored PTFE microspheres
  [vote for,

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.

bs0u0155, Apr 21 2017

PTFE https://en.wikipedi...tetrafluoroethylene
...for anyone else who doesn't know what that is. [2 fries shy of a happy meal, Apr 22 2017]

Eutectic system https://en.wikipedi...iki/Eutectic_system
...for anyone else who doesn't know what _that_ is. [bungston, Apr 23 2017]

Heats of Fusion https://en.wikipedi.../Enthalpy_of_fusion
I sort of know what this is, but still have to look up numbers. [bungston, Apr 23 2017]


       Would 1µm spheres survive? Or would they be squished/crushed at a few points in the engine?
MaxwellBuchanan, Apr 22 2017

       But are they going to taste nice in Atlantic Salmon?
wjt, Apr 23 2017

       I found the heat of fusion for water to be 334 j/gram. That is better than what is listed for sodium acetate and probably comparable for any of the eutectic acetate salts - certainly cheaper.   

       I take it that this phase change aspect for water is hard to use because of the difficulty circulating solids. But you have made a workaround with the microspheres. Why not put water in the microspheres? You could circulate the spheres in some low freezing solution (hydrocarbon, methanol + water etc). You get your phase change benefit.   

       Or maybe I misunderstand - maybe this is to get the phase change benefit at the higher temperature where engines are hot but should not get hotter?
bungston, Apr 23 2017

       //Would 1µm spheres survive? Or would they be squished/crushed at a few points in the engine?//   

       I'm going for much more than that, 20+ Otherwise the content/containment ratio reduces badly and they would perform mostly like plain PTFE balls. Anyhow, I think they'd be fine. The water in an engine isn't like the oil or fuel, it's not driven by a gear pump, there's no tremendous pressure changes, no tiny holes. Everything is on the scale of mm or cm and the flow can be nicely controlled. We have some fluorescent beads that we have had stirring in a cuvette for days, they've been through a flow cytometer 100's of times, they're fine, and PTFE is tougher.   

       //I wonder if when we are talking about exotic materials whether the lubricant could also be the coolant.//   

       It is already, there are a many variants but the oil is always considered part of the cooling. My motorcycle manual goes to great lengths to explain that the engine is "Air/OIL cooled" The oil circulation is used to move heat around so that the parts are all at similar temperatures, also, there's an oil cooler, this functions in the same way as a water "radiator". Turbocharged cars use an oil circuit to and from the turbocharger which operates at temperatures that would trouble water. They often have features such as oil jets pointed at the underside of the pistons to cool an otherwise isolated component.   

       //maybe this is to get the phase change benefit at the higher temperature where engines are hot but should not get hotter?//   

       Exactly this. Water has excellent enthalpy of fusion/specific heat capacity properties, but for many reasons, around 100C has been the operating temperature of engines for a century. Much hotter and emissions start to move in undesirable directions regarding nitrous oxides and the difference between starting temp and running temp gets larger creating problems with uneven expansion. Aluminium is pretty useless as a structural material above 200C as well, and thats the engine material of choice nowadays. Much cooler and there are problems with fuel vaporization and the rate of excess heat rejection becomes very problematic, imagine if you set your engine to work well at 65C and left it on a black parking space in full sunlight in Nevada. Your car would be overheating before you started it.   

       The whole point is to create a dramatic increase in energy absorption and rejection moving above and below the operating temperature. The sphere cores absorb huge energy while melting in the engine and reject huge energy while re-solidifying in the radiator.
bs0u0155, Apr 24 2017

       Well, this sounds like an excellent idea. You have my full and unfettered permission to test this out on [8th]'s Ford Anglia. Or, for that matter, on [8th].
MaxwellBuchanan, Apr 24 2017

       I think the collective would be very interested in such technology. I'm sure they are eager to increase the cooling capacity of various blood systems to handle the more interesting and higher powered "upgrades"
bs0u0155, Apr 24 2017

RayfordSteele, Apr 24 2017


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