Quote:
Originally Posted by Ecky
What got me thinking along these lines was the experience of living in a very leaky house in winter. I heavily insulated the attic, and under the floor, and it would still lose all of its heat in minutes rather than hours, especially if the wind was blowing, because all of the seams around the windows and doors had high levels of convection cooling. Until I went around and caulked all of the seams, that insulation was next to useless.
On my car, even after driving for hours, the hood is not discernably warmer than ambient. I'd wager that insulating my hood would make little difference in terms of heat loss through it until I close up the engine bay and keep fresh air out adequately for the hood to get warm. It stands to reason that you engine bay already has its airflow limited enough.
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Ahhhh! This makes much more sense, and is pretty reasonable to boot. Okay, let's go back to the data. I've attached a couple of choice Torque Pro logs. The first is the coldest temperature I could find, which shows a 12F+ temp difference between the outside and air intake (inside the engine compartment). This is with the constricted inlet, covered front, etc. as sealed as it normally is, and after the run (stationary).
However, the second is more interesting one for our purposes. This shows a 7F+ difference at higher temp, *but* it was with the nose of the car completely taken off, and at speed. (I replaced the AC condenser on my car and drove it to the shop au natural). So the temperature gain with maximal air infiltration is at least 50% that with minimal air infiltration. Note that all this is post radiator, so what we are really measuring here is the temperature raise across the radiator.
Okay. I believe we can draw some conclusions. The first is that your air replacement model is very reasonable and is definitely at play. If the engine temperature is 185F but the engine compartment air is near ambient, the air must be being replaced frequently. In fact, I've seen this happen in reverse in real time, i.e. when I stop and see the temp of the engine compartment go up. [Hmmm... this also means that testing the hood temperature by hand is time dependent.]
However, that's not the only factor going on. It's also the case that we have a *heat flux* occurring as well, from the engine, through the radiator, to the flushed air. Unfortunately, the temperature is only a rough proxy measure for the heat flux. But even here we can make some tentative conclusions. The most important of which is that we should expect insulation changes temperatures *on the order of a few degrees*, but that's sufficient to significantly alter performance characteristics.
Now onto the mechanism. I'm proposing the insulation works by actually blocking heat flux, while you are proposing it could also be simply by blocking airflow. We could figure out precisely how much each factor contributes by putting a heat conductor in place instead of insulation. For example, wadded up aluminum foil. Something like that should transmit the heat but occupy the same space / block the same airflow.
Would you be up for an experiment to test it out?
