Amskeptic wrote: ↑Wed Oct 03, 2018 10:00 am
It is a great mystery to me why ambient temperatures seem not to have as great an effect on CHTs as I might have surmised.
I have done delta-T analyses in my head that suggest that a drop in ambient temperatures from 115* to 43* should improve heat transfer off the heads by a nice ridiculous amount, but when you add to the equation that the engine is a big huge heat sink that has stored up a big pile of excited atoms, the cooling air that only briefly passes by the hot heads is limited in its absorption of heat. That means that 43* air can only absorb "X" calories of heat barely different than 115* air that briefly passes by. Does this make sense?
Colin
Hmm.
Newton's Law of Cooling states the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings while under the effects of a breeze. The constant of proportionality is the heat transfer coefficient.
q=h*A*(delta T) where q is the rate of heat transfer, h is the heat transfer coefficient, A is surface area.
You're implying the heat transfer coefficient increases as air temperature increases, such that hot air is so much better at removing heat than cold air that it makes up for the decreased delta-T.
I don't think so.
If we assume the head temp is 400 degrees, let's compare a 115 degree day with a 43 degree day.
400-115=285, 400-43=357. 357/285= 25% greater delta-T on a cold day. If h is the same, that's a 25% increase in heat transfer. In 16 credit hours of Thermodynamics, Heat Transfer and Fluid Mechanics, a change in h with temperature was never mentioned. But I can see hot air has different density, more energetic molecular movement, greater water vapor capacity, etc. So maybe a a few percent greater h. Low enough to be ignored in most cases, but something. Let's give it 5%.
I think another 20% could be explained by:
- cold air is denser, leading to greater air resistance to our rolling billboards, making the engine work harder
- colder temps reduce air pressure in the tires. If we forget to adjust, it makes the engine work harder
- colder temps make the rubber in the tires stiffer, making the engine work harder
- colder temps tell the fuel system to enrichen the mixture, perhaps beyond what stoichiometry would be
whaddya think?
Truth is like poetry.
And most people fucking hate poetry.