No not like that

I've been watching the oil and coolant temps in my CD lately and I've noticed something I find odd that I'm wondering if someone can explain.

I notice that the coolant heats up much quicker than the oil. This makes perfect sense to me and my limited understanding of thermodynamics (e.g. it's more fluid so it will pick up the heat faster).

What I'm finding odd though is that the coolant seems to retain the heat longer and this doesn't make sense to me. What I'm seeing is that on a <40 degree day I'll drive into the office which is long enough for both to get up to the 200 degree mark. Then 3 hours or so later when I go out for lunch I'll see that the oil is 20-60 degrees colder than the coolant.

My first thought was maybe just the placement of the temp sensors, but they both then follow the expected time lines to warm up rather than a sudden spike once the fluids get circulating.

Does anyone have a logical explanation of this that I'm missing?

 

Your going to drive yourself crazy!

 

Nah we already have a 996. Such a thread title over in that forum would incite panic and possibly riots

I'm just curious about why the basic thermodynamic principle works in one direction (oil heats slower than the coolant), but apparently not in the other (oil loses its heat faster). I'm positive there is a simple and reasonable explanation, its just eluding me and I don't like that

 

Think in terms of how the fluids are heated and cooled. Both are cycled through the engine when being heated. There is work going into the heating and distribution of the fluids over time is pretty even. When they are being cooled upon shut down, the process is much different depending on the fluid in question.

Most of the oil goes to the pan and cools as a bulk liquid by heat transfer through conduction to the finned, aluminum oil pan and then through convection to the air around the pan.

The water on the other hand, is distributed through the head and block, various coolant hoses, and the radiator, etc when shut down and it stays there. The coolant must wait for the engine long block and other components to cool as well before it can come down to ambient temperature. That process is much slower than the heat transfer from the oil pan. Make sense?

 

Perfect. Thanks

Where the fluids end up was my next best guess, but I guess I attribute more volume to the coolant outside the block while the majority of the oil being closer to the core residual heat than is really the case.

 

I think the oil is pretty well isolated from the heat of the block. Only the pan itself touches the block, and the heat transfer to the pan is insulated by a gasket. Then, I think you'd be surprised at how abrupt the heat gradient is between the flange of the oil pan and the part of the pan that the oil actually touches. This is considering a conventional oil pan setup. Since our vehicles use a dry sump system, it may differ a little. I think the concept probably still applies for the most-part.

Also, I think there is more coolant in the block, head, and associated than you would think. The sensor is in close proximity to the flow coming out of the head/block as well, and the coolant it sees is that which has been held in the block (by the closed thermostat) in order to warm-up quickly.

I'm mostly just throwing out ideas here, but I feel it is fairly justifiable!

 

The process may play a small part in the phenomenon but the main reason is that water has a higher specific heat capacity (SHC) that almost any other fluid. The money shot is that anything which takes longer to heat up....also takes longer to cool down.

What doesn't make sense?

However during heat-up the coolant is being re-circulated past the the hottest parts of the engine without being cooled until the thermostat opens up whilst the the oil is sitting, for the most part, in the cooling air flow beneath the car.

 

He's saying it heats up faster, but takes longer to cool down, which is not what you just said at all. Specific heat capacity is not part of the equation in this instance.

 

gnat, i think youre eating lunch too early

 

mcbit, lox has it right. What you describe is exactly why I was confounded since the oil is cooling faster (odd) but heats slower (expected). The provided explanations of where the majority of the fluids end up makes sense in answering the puzzle.

To be fair I don't eat breakfast and don't get to the office before 9:30 so a few hours is the "normal" noontime thing. Today was my last day at that job though and the new one revolves around PST for the HQ (my office is EST) so hopefully I can get back to the 1300 or 1400 time range for lunch!

 

To be fair i wrote my last reply while I was pissed!

Unfortunately i'm in the same condition whilst writing this!

Whilst the coolant is heating, it is in a closed circuit with the thermostat closed; heats up very quickly due to the small volume of coolant surrounding the block. When it cools down, the former is also the case.

Oil heats up in an airstream and cools down in a sump open to atmosphere, All things being equal, coolant heats up and cools down slower than oil.

 

to put it simply, the coolant sensor is typically located in or near the head and there is a lot of thermal mass, the metal of the engine. some cars have multiple sensors. the one at or near the radiator will change more dramatically.

it really depends on where the oil temp sensor is located.