Why is there a MIN and MAX marking on a car coolant bottle?

Question

This question was initially posted in physics stack exchange, then the mechanics stack exchange but I think it's more appropriate for engineering section instead.

I've always thought the minimum and maximum mark on the coolant bottle were quite pointless as it doesn't seem to make much of a difference to the function of the vehicle if I overfill a little bit or under fill, I really only used it as a rough guideline to show me if I'm low on coolant need to top off. Recently though I've had to deal with a few coolant leaks which made me curious.

My understanding is that liquid cooling systems in cars operate under pressure to increase the boiling point of the liquid (glycol) which allows it to absorb more heat from the engine and dissipate it to the environment making the system overall much more efficient. I suspect the engineers have designed system to have a certain amount of air inside of it as air compresses unlike liquids and therefore the system needs a little bit of "cushion" to stop hoses and pipes from blowing up (or collapse due to vacuum), but not too much "cushion" as then the system wouldn't be able to reach correct operating pressure.

Also it seems that the same bottles are used by many car makers across the industry for all kinds of models so I'm guessing there's very few important variables that determine required air-to-liquid ratio. (The ubiquitous VW bottle https://i.stack.imgur.com/rXie6.jpg)

Is my suspicion somewhere in the right ballpark or am I dreaming?

Answer 1

First the volume change of the coolant is low compared to the vgolume of coolant in the system.

Then the flexible pipes do expand a bit due to the increase in pressure afforded by the pressure cap - used, as you seem to understand, to increase the boiling point of the coolant. This is where the danger can come from for those who ignore the "Do NOT open" until cold warnings. (Mechanics have to deal with systems under pressure and work around that).

There are many designs of "bottle" - each manufacturer often makes one for each model of car they produce. Usually because the underbonnet space is at a premium. With older cars (some 30 years ago) there was more space, or height, by the radiator and expansion bottles were often fixed to the side of the radiator. Some of those had radiator pressure tanks which "overflowed" into a separate tank - on cooling they would draw coolant back into the system as needed.

As for the heat transferred, that is down to the amount of heat carried by the coolant between its hottest point (in the engine cylinder head) and the coolest point (in the radiator), that difference is likely to be about 27 degrees 9based on 115 deg C engine and 88 deg C radiator. So the heat transferred is based on the specific heat capacity of the coolant, the delta T and the mass flow rate. It is not the same as a system which uses a fluid phase change to transfer heat.

So, the min and max marks are mainly to allow users to fill sufficient ie min and allow to put a bit more by accident.