Cooling down aluminum using cold materials.

[stewfoo]

I have a racing engine that is air-cooled. It operates most efficiently when it is cool (250 degrees Fahrenheit. When it exceeds 300 degrees Fahrenheit, performance begins to fall drastically. It has an aluminum engine mount. By the end of a 20 minute session, the mount is heat soaked from all of the heat that is absorbed. I am going to redesign this mount so I can insert a frozen (via dry ice) object into a cavity within it. I was thinking of steel.

My question is : Which material can be most useful in storing the most possible Coldness (pardon my hacking of your technical jargon)so as to reduce the overall temperature of this engine mount for up to 40 minutes.

Thanks for your help!

STEWFOO

 

[IRstuff]

None that are practical. Joule heat alone is insufficient; just consider that your entire engine block exceeds your desired temperature. Small chunks of metal won't contain sufficient coldness to make much difference. By adding weight, you load down the car and make it even more inefficient. Phase-change materials are ore likely to be useful for something like this, but, again, the amount of material you can add is miniscule compared to the total thermal mass of the engine.

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[stewfoo]

Thanks for the input IRSTUFF! To be clear, the engine I am speaking of is approximately 20 lbs. It is a small 2 stroke engine...

 

[stewfoo]

..and I could used up to 5 lbs of additional material to add to the mount.. So, if I were to use a 5lb steel (or whatever material) block that was brought down to the temperature of dry ice, you believe that would it have little to no effect on the overall temperature for this brief period of time?

 

[racookpe1978]

How long does the engine have to run?
Total race time?
How long before the engine heats up to what would be "good" (adequate or useable) temperature? (Include typical practice and starting and waiting times, please?)
How long during the race does it take to get the engine "too" hot?

 

[stewfoo]

Typically races are 8-14 laps. Each lap is approximately 1 minute. Ideal temperature is attained at lap2-4 (200 deg Fahrenheit). Often times the tires are not at an ideal heat (too cold) by the time the engine is ready...

 

[IRstuff]

I think the best you can do is about 250, but it's a bit of a swag, and assumes things not in evidence, e.g., that the motor mount thermal pathway is efficient. Another distinct possibility is that the added heat sink will distort the temperature profiles within the engine, possibly warping structures and changing tolerances.

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[racookpe1978]

No pit stops, right?

Why not have the inlet cooling air flow around the solid CO2 via a 3 ounces of sheetmetal duct and a hand-operated/radio-controlled air bypass damper to divert colder air to the outside of the oil tank?

Flip the damper when temperatures get too hot, get another two or three laps of cooler temperatures ... until the CO2 sublimates away.

 

[IRstuff]

I think if you do the math, you'll find that the cooling capacity of any plausibly-sized hunk of dry ice will be miniscule. I get something on the order of 50°C cooling for about 24 sec for 5 kg of dry ice; that's discounting the Joule cooling and only considering the heat of sublimation, i.e., phase change.


I think you need to be realistic about this. People have been trying to game these sorts of things for nearly a century, and most of them are not any less intelligent than you. If they did succeed, it was not for the lack of trying or technology. Every possible way of doing something has a cost or trade-off. Whatever you are using now is probably as good as it gets.

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[chicopee]

Aren't some engines packed in ice before drag racing?

 

[MintJulep]

Interestingly, aluminum has a higher specific heat than steel.

http://www.engineersedge.com/properties_of_metals.htm

.24 * 5 * 300 = not enough BTU to be significant in light of the total heat rejected by an air-cooled engine and the dominant paths of heat rejection.

 

[dvd]

It may be helpful to understand the relation between Charpy impact energy and temperature for various materials before cooling them too low.

 

[Compositepro]

Keep in mind that steel is denser than aluminum, so its heat capacity per cc is greater.

But really, using energy storage to cool a racing engine instead of improving the radiator is silly. Put any extra mass in increasing radiator area or air flow.

 

[Compositepro]

Or just take off the radiator cap and let the water boil off. Just keep the water from spewing out with the steam.

 

[Compositepro]

Okay, never mind. Air-cooled engine.

 

[1gibson]

Still, increase: airflow, fins, maybe even surface roughness, of the engine.

Maybe you can use "heat pipe" technology to get some more surface area for cooling at high temps. Depending on the rules and interpretation thereof, you might be able to argue that it is still just "air cooled."

 

[btrueblood]

Put water in the hollow, and a wick running up to the base of the fins? Water has a lot more J/kg (both specific heat and latent heat of vaporization) than any other readily available material.

 

[btrueblood]

Should have added: I doubt, given IRstuff's post, that even water will give you any big benefit.

 

[ratrod1]

Build an external water jacket around the engine's fins, fill it with water. The water will keep the engine fins at 212 deg F max, until the water boils off. Add enough water for one race.

 

[IRstuff]

Seems to me that having a boiling water interface is not particularly conducive to heat transfer. Water's heat transfer characteristics are excellent, but mostly only when it's a liquid

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