This will be the first winter my car will spend in a cold climate in a few years, and I realized that my cooling system is in danger of freezing if I don’t add some anti-freeze. This brought me to look up the ideal water to coolant for cold weather usage, and then lead to much more research on cooling systems and how coolant works. I thought it might be a good idea to share results on how the boiling point, freezing point, and heat transfer ability of coolant change depending on the mixture. Also, this would have been easier if my thermodynamics textbook wasn’t in a storage unit 1000 miles away.

Anyhow, because the whole point was to look up freezing points I’ll start there. The freezing point of water as we all know is 0C (32F). You might presume that the freezing point of ethylene glycol, the main ingredient in most automotive coolants, is very low, and that a system filled with only anti-freeze would be the ideal choice for cold winters in places like Alaska and North dakota and Canada. In reality, it is only -13C (8F). That’s not very cold! If you were to only use coolant in a cold climate, the possibility of it freezing is very possible (although it’s not water and would not expand as such, so damage to the engine block and cooling system would not be a worry). The key is the mixture of coolant and water. When done in the proper proportions, the freezing point gets much lower:

From that you can see that the ideal mixture for extreme cold weather is about 65% anti-freeze and 35% distilled water. Now, unless you live in a very cold climate, you might be concerned with what happens when the coolant is hot. Anti-freeze alone has a pretty high boiling point of 197C (387F), which is much higher than you would see in an automotive cooling system. Water, of course, boils at 100C (212F). When mixed. we end up somewhere in the middle:

The interesting thing here is that it takes quite a bit of anti-freeze to get the overall boiling point of the coolant to increase by a significant amount. Much like the freezing point, a 50/50 mix doesn’t split the difference, and at that ideal mixture for cold of 65%, we’ve only increased the boiling point by about 15C (27F).

When it’s hot out and the car is under a lot of load (say going up a hill in the desert or doing laps around a race track), it’s important to keep the coolant from boiling, because boiling coolant doesn’t do it’s job very well, and the increase in pressure will result in much of it being dumped on the road.

Most engines operate at around 90-105C (190-220F), which is very close to the boiling point of this water/coolant mix. Thankfully, there is something else that can be done besides varying the mix- increase the pressure. This is the important part, and the reason why cars aren’t constantly overheating and boiling over. Increasing pressure increases the boiling point of a liquid. For water, it works out to about 2.5 extra degrees F per psi. So if you have 10psi of extra pressure, water will boil at 237f instead of 212. Go up to 15psi, which is normal for a car, and straight water boils at around 121C (250F). This is above the normal operating temperature of most engines so there actually isn’t much reason to worry about the composition of our coolant in a pressurized system.

When we’re on the hot end of the temperature range, the much more important value in the cooling system is what’s called the specific heat capacity. The heat capacity is basically how much heat energy the coolant can carry out of the engine and to the radiator. As far as carrying heat goes, water does a really good job. It’s actually one of the most effective coolants that could possibly be used. The specific heat capacity of ethylene glycol is only about 65% of water at the temperatures seen in a cooling system. That means that to cool the engine by one degree, you would need about 50% more of it than straight water. So even though you could use pure antifreeze as a coolant, or any other liquid for that matter, you would need a bigger radiator and more robust cooling system compared to one that used straight water. Here is what happens to heat capacity as our coolant mixture changes:

From a cooling point of view, it seems ideal to have the system filled 100% with water. At that 65% mixture we’ve lost 20% of the cooling capacity of the system, and when the cooling system is being used at near capacity, such as on a race track, canyon drive, or simply a hot day with the a/c on, it could mean the difference between a fun time and being parked on the side of the road with blown headgaskets.

While pure water has the highest possible heat capacity, it’s not quite the ideal automotive coolant. Aside from freezing, water can also corrode the inside of engine blocks, radiators, and water pumps. We need to have something in there to keep iron blocks from rusting up and water pump bearings and seals from failing. Anti-freeze contains these necessary additives and most auto manufacturers have pretty specific requirements on the types of coolant you can use. So, for the most part it’s a good idea to put some of the recommended coolant into the system, however, if done at a lower ratio, the cooling capabilities of the car will improve. If you live in a warm climate, it’s not a bad idea to use a little more water than coolant to improve cooling capacity without losing the anti-corrosion and lubrication properties (although it might be necessary to change the coolant more often if you do this).

There are also additives on the market specifically meant for systems that run 100% water, like redline’s water wetter and motul mocool. These products also claim to improve the cooling capacity of straight water to reduce running temps. This is because there’s more to cooling than just the heat capacity of the coolant. What occurs on the boundary between engine parts and the coolant is also important. This is where the additives come it: they reduce the surface tension and “wetting” capacity of the water, which will make a slight improvement in how well heat can be transferred from the block to the water.

Most marketing and testing for these products compares a 50/50 water and ethylene glycol mix to pure water with the additive, and show dramatic reductions in coolant temperatures. Most of these gains actually come from the added water content, and not from a magical additive. However, there is still a benefit to use them because in addition to improving the heat transfer ability they have anti-corrosive and lubricating properties.

So, in conclusion, you should only use as much anti-freeze as is necessary for your climate and operating conditions, and those coolant “wetting” additives are generally a good thing to use.

Coming eventually- some tips on improving the cooling system to keep a track or modified car cool.

sources:
Don’t fill her up with antifreeze, Alaska Science Forum
Ethylene Glycol product guide (pdf)
CSU engine web page
Ethylene Glycol heat-transfer fluid
Increased cooling power with nucleate boiling flow in automotive engine applications