Engine bay temperatures - a matter of degrees

[Wuey]

Just came back from work with today's max ambient temperature reached just a tad over 37C at around 3pm. I take out my little infrared thermometer for my rc toys to measure the SE engine bay temperatures one minute after shutting down the engine.

The bonnet measures 76C which is just hot enough not to linger my hand on it for any longer than a second or two, and the bonnet stick is almost 80C while I mutter 'ouch ouch ouch' to have it hooked up. The cam cover measures just over 100C, too hot to touch of course. The intercooler inlet and outlet pipes are about the same in the mid eighties with the former a tad cooler. The inlet manifold measures about the same as the bonnet while the radiator cap is a cool 54C. The outlet manifold measures a sizzling 146C. I can't see the snail but I reckon its temperature should be much higher. Interestingly the dip stick handle measures in the high eighties which is not something one wants to trifle with.

The measurements are all done with the probe pointed at the components as close as possible.

It never gets too hot for me to go and fly my toys as long as it's not too windy so off to the flying field I go.

[bruce]

I wanted to do that but the numbers would scare me.
I went for a drive when it was only 27 degrees, and I couldn't keep my face over the extractors afterwards (turbod, etc).

[Cus]

My inner nerd appreciates this post.

Can you follow it up next winter so my inner nerd can appreciate it more?

[davekmoore]

As usual mine is being worked on at the moment but I promise to do the same measurements on a hot day to see if large holes int he bonnet make any difference.

[Wuey]

My inner nerd appreciates this post.

Can you follow it up next winter so my inner nerd can appreciate it more?

Will do. I'm just 'nother nerdy old fart.

[StillIC]

...while the radiator cap is a cool 54C

No it isn't. But because the cap is shiny its emissivity is much lower, thus giving a false (low) reading. The cap is as hot as your coolant, which will be >85C if running normally.

Thermal cameras and infra red temperature sensors rely on the (infra red (IR) spectrum) emissivity of the surface being measured to match the emissivity the sensor was calibrated at. Some sensors allow the emissivity to be set to match the surface being measured. We have some thermal cameras where I work. Ours are worth between ~$5000 and ~$30000, depending on accuracy and functionality. My guess is that the cheap IR sensors are calibrated at high emissivity values and will therefore work well with rubber, almost as well with other dull surfaces, gradually losing accuracy with increasing shininess of surface.

Experiment: Try measuring the temperature of a sheet of glass. If it is perpendicular to you it should read about 30+C, whatever its actual temperature is, as it reflects the warmth of your body back at the sensor. The infrared emissivity of the glass is virtually zero, but it is a great infra red reflector (of you).

[Wuey]

...while the radiator cap is a cool 54C

No it isn't. But because the cap is shiny its emissivity is much lower, thus giving a false (low) reading. The cap is as hot as your coolant, which will be >85C if running normally.

I concur absolutely. While I was flying my toys in the field, I was thinking the coolant must be much higher than 54C as otherwise the engine will be over cooled, the thermostat will never open and there will be more engine sludge than I can poke a stick that causes premature engine wear. Thanks for the education on surface emissivity which explains why I always read higher temperature when I aim the temp probe at my rc motor wire winding compared to the reading taken from the shiny outrunner rotor.

The red SE cam cover temperature measured at slightly over 100C could be 15C-20C cooler than the normal engine oil operating temperature.

[manga_blue]

I did this once after a moderate run on a 25 deg day. I think I was hunting for cool air at the time ...

[Cus]

...while the radiator cap is a cool 54C

No it isn't. But because the cap is shiny its emissivity is much lower, thus giving a false (low) reading.

Would little matt black stickers on things make the temp readings more accurate?

[Hoolio]

...while the radiator cap is a cool 54C

No it isn't. But because the cap is shiny its emissivity is much lower, thus giving a false (low) reading.

Would little matt black stickers on things make the temp readings more accurate?

Yes, as long as the sticker itself is thin and not a thermally insulating material.

The best way to test the sticker would be to place it on something which is already matt black and a reasonably uniform temperature (coolant hose, away from branches or ends) and measure both the sticker and the region next to the sticker.

Then it would be interesting to do the same on surfaces which are not matt black, such as the inlet manifold (again, away from branches and ends).

[StillIC]

Would little matt black stickers on things make the temp readings more accurate?

Better than a sticker is a thin layer of matt black paint. We do this quite a bit when using our thermal cameras. We have tried black marker pen, but it can still be a bit shiny.