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heat rejection rates

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PostPosted: Thu Nov 26, 2009 14:16    Post subject: heat rejection rates Reply with quote

Can anybody provide me with heat rejection rates for any of the water cooled V12 engines? Im particularly interested in racing applications, but would also like to see data on these engines used in normal service.

Actual flow rates and delta T across the radiator would be best, but Ill settle for BTU per minute if its derived from hard data.

Thanks in advance!

Aaron Burhoe
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Joined: 15 Aug 2003
Posts: 140

PostPosted: Thu Apr 22, 2010 21:07    Post subject: Reply with quote

I read that specific heat rejection rates were in the 15 BTU/Min/BHP,
less than 10% of fuel energy to cooling. For some liquid cooled WWII aircraft engines. No specifics though, like power settings etc.

You see alot of information about liquid cooling offering more power because of greater heat rejection on the net. I would think the opposite.
That is the reason for cummins experimenting with its Adiabatic Diesel Engine in the 70's. Heat rejection means lost energy. Its best to use all the heat you can, and why the Turbocyclone was getting the excellent fuel consumption numbers it did.
Yes low heat rejection in a gasoline engine means more susceptibility to detonation, and thus loss of power, and possible failure.
The biggest advantage of liquid cooling is better control of the engine temperature. It is much more difficult to control cylinder and head temperatures in an aircooled engine, especially something like the Wasp Major. Sorry to get off topic.
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PostPosted: Wed Feb 07, 2018 13:14    Post subject: Reply with quote

It sounds like a good idea to isolate the combustion chamber walls, but in most cases, it won’t help to reduce heat losses. Air offers good isolation and heat losses in a combustion engine are already quite low. You should note, that most of the heat losses take place during the exhaust stroke. Tests with adiabatic engines mostly resulted in lower efficiency due to increased heat losses (sic!). If the wall temperature is above 300°C, the fuels burns directly at the wall, thus giving very high heat losses during combustion. In conventional engines, there is no combustion directly at the wall and thin layer of air is giving quite a lot of isolation.

Most air cooled engines needed very rich mixtures at full power to keep the temperature down, whereas liquid cooled engines are less prone to overheating.

The best way to reduce heat losses during combustion are: large cylinders, long stroke, high supercharging, small compression ratio. Besides that, a short exhaust port with small surface (e.g. by using only one exhaust valve) is also very effective to reduce heat losses but it will not increase engine effectivity a lot.
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