Cooling efficiency and coolant flow rates

[18Fiatsandcounting]

OK, since I know some of you are interested in arcane stuff such as this, let's have a go at this question that keeps coming up: Is it better to have higher flow rates of coolant in an engine, or do you have more cooling efficiency when the flow rates are lower? Rather than derail existing threads, I figured I'd get your thoughts with this post. Disclaimer: I am not a heat exchanger engineer nor do I play one on TV, online, or anywhere else.

I think the applicable equation is Q= m times Cp times dT, that is, heat flow is the mass flow rate times the heat capacity of the fluid times the temperature differential. If you double the coolant (mass) flow rate, you double the removal of heat from the system, all else being equal. So, doubling your coolant flow rate should cool your engine better, both on the engine block side as well as the radiator side (same idea, just different directions of heat flow).

However, and this is a big HOWEVER, life ain't that simple. There are several reasons why a higher coolant flow rate might be undesirable:

1. Increasing the coolant flow rate essentially means a bigger water pump, which sucks more power from the engine as it has to move coolant around more quickly.

2. Due to some of the small cooling passages in both the block and radiator, increasing the flow rate will result in a pressure increase, which could stress the system in terms of seals, gaskets, radiator cap, etc.

3. Cavitation could become a problem. You want turbulent flow in your coolant (keeps things mixed up), but if it's moving too fast, you might end up with cavitation around some of the intricate passageways, which is a very good way to quickly erode metal. Or maybe you'd chew up water pump impellers at a faster rate.

So, I'm guessing the engineers designed the system with Goldilocks and the Three Bears in mind: Fast enough coolant flow for efficient cooling, but not too fast to cause other issues.

OK, guys, hack away at my analysis.

-Bryan

[dinghyguy]

Which Bear are you Bryan, cause we know you ain't goldilocks?

So were you to remove more heat out of the engine implies you would need more radiator cooling or all that will happen is the overall temperature will be controlled by the radiator dissipation …...hmmm.....kinda like the having a thermostat in the system!

Oh and internal combustion engines work better when they are warm......if you take to much heat away the efficiency would go down.....perhaps not a good idea either.

If you are bored, you might see if you can measure the head loss/pressure drop in the engine on the water pump side and on the radiator side. That might hint at what is the control on the system. ie if the rad is the high resistance and if you put in a low resistance rad your water pump will increase flow through the system. That will still happen regardless if the engine is the constraint because it is the total resistance in the system that counts, but the effect would be bigger if the rad is the constraint.

Of course the fluid properties matter as well, perhaps using heavy water would help keep your nuclear pile cool!

cheers
dinghyguy abandoning the too small garage for the tent while construction of the luxury garage proceeds.

[18Fiatsandcounting]

Ahhh, Dinghy, temporarily in a tent but about to move into the Taj Mahal of garages in the not-too-distant future, you have given me much to chew on.

And yes, you are right, the best efficiency for an engine running on a Carnot cycle is when it is hot. Really hot. However, oils, seals, gaskets and such don't like being at 1000 oF, so compromises had to be made. Personally, I like my engine to run at 190 oF by the temp gauge, although I have no idea what the flame temperature inside the combustion chamber is. 1000 oF?? Gotta get it hot enough to burn off the carbon deposits, at least...

And no, I am not Goldilocks. Don't even look remotely like her. Teddy bears? Winnie the Pooh? Hmmmm, maybe....

-Bryan