How do engine coolants work?


How do engine coolants work?

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You mean how do they cool or how do they not boil or just all of it in general from how they flow and so on?

Engine coolant is a liquid that gets heated up by the engine block and other components, and is then passed through a radiator to cool it down with the outside air.

This is important because if an engine overheats it will become damaged and could fail.

Liquid coolant is required for most car engines because air cooling (passing air over the engine) just isn’t adequate to cool down most car engines. It can’t get the engine heat out quickly or efficiently enough.

Coolant is circulated through the engine by a coolant pump that is spun by the engine belt.

Car coolant is primarily water, and in fact replacing your coolant entirely with water would work depending on the conditions. However this is not recommended other than as a temporary fix.

Coolant is typically mixed with anti-freeze like propylene glycol that as the name implies prevents the coolant from freezing in the winter. If you use water as coolant without antifreeze it will turn to ice and destroy your radiator and coolant system.

Antifreeze also increases the boiling point of the coolant, albeit not by much.

The coolant is kept under pressure within the cooling system, and it’s this pressure that prevents it from boiling and turning into steam. However if the pressure becomes too much a relief valve will pop to prevent the system from exploding. This will let off steam and the coolant will boil off. This is what you sometimes see in movies when people are driving through the desert, they let their engines get too hot and the boiled over. But they can’t drive away until they re-fill the system with coolant.

Racing cars like Formula 1 are known for running ‘waterless coolant’ esoteric chemicals that are more efficient at removing engine heat and can operate at higher pressures.

The basic function of a coolant is to take heat from where it is to somewhere else. In the case of a car, that means taking the heat out of the engine block. Of course, the heat still has to go *somewhere*, and it can’t just keep going into the coolant because that coolant will get super hot, and then it can’t carry heat anymore.

So, the coolant carries the heat to a radiator. The radiator exchanges the heat with the air. The engine can also directly exchange heat with the air, but it isn’t very good at it. The engine block is a pretty big, solid chunk of metal. To efficiently exchange heat you need a lot of surface area. Even if you were to attach the radiator directly to the engine, the heat will take time to flow from the inside of the block to the radiator, and there’s not a good, easy way to get air to flow through those parts of the engine.

Coolant is the solution. You pump a liquid through the engine block, where the coolant gets very hot. Then, the coolant goes through a radiator where it gets divided up amongst hundreds of tiny, thin fins that make a ton of contact with the air. The coolant cools down and circles back to the engine.

Just about any liquid can do this, but you do want it to have at least one property which is *being liquid at the temperatures and pressures inside of the system*. Another great property is to be cheap and abundant. Given that car and truck engines have to take a ton of abuse, it doesn’t make a lot of sense to fill up the engine with some super-efficient coolant that you’re going to lose to leaks. Water has both of these properties. And, indeed, car and truck radiators are full of mostly water.

Water does have one unfortunate property that is undesirable, which is that it freezes at temperatures that are very common around the world, at least during some part of the year. So, we add chemicals to the water, which we call *antifreeze*, that prevents the water from freezing. The chemicals have some added benefits, like also helping the water stay liquid as it heats up, helping to lubricate the water pump, and prevent corrosion normally caused by water.

Engines do produce a lot of heat. First as the compress the air which makes it hot, and then when the fuel is burned the temperature increases even further. If the engine gets too hot it will fail. The metal will warp or even melt. The lubricating oil will decompose or burn turning it into charcoal. Metal will expand so pieces will no longer fit together. Rubber hoses and such will melt. Even things like spark plugs may end up melting so they stop working.

So the engine can not get too hot and it needs to be cooled. Some smaller engines can be air cooled. Just letting enough air flow over it and fill the outside with cooling ribs for more surface area will cool them down enough. But this is not enough for larger engines like most of the ones you find in cars, and even medium sized bikes. Air is not good at transporting heat so we need much larger surface area. This is what the radiator is for. Tiny fins in the radiator makes for lots of surface area to carry the heat away. But to get the heat from the engine to the radiator we use a liquid. So the engine block have lots of coolant passages going through it and a coolant pump to pump the liquid from the engine block to the radiator and back. Some vehicles might have other things connected to the cooling loop. The liquid is basically water, but it is best practice to add things like glycol to it in order to prevent it from freezing if you park in winter and various other things to prevent it from rusting out the metal.

There are lots of little details to this system. The radiator cap is designed to let steam vent if the radiator can not keep up and the coolant boils. There is often an expansion tank so the coolant can expand when it is hot and shrink as it gets cold, although this tank is usually integrated in the radiator. The engine have freeze plugs to let the water out if it would freeze in winter. There is a thermostat to allow coolant to bypass the radiator if it is cold in order to heat up the engine to operating temperature faster and keep it at this optimal temperature. Some of the coolant is sent to the cabin heater to heat up the cabin if it gets cold. Certain cars have electrical heaters in them to allow for easier cold start, or they may have connectors to combine the cooling loops of a running car to a parked car to heat it up before starting it. There are lots of these details and concepts around the cooling system of a car.

Liquid cooled engines have channels that go through them that allow liquid to travel via something called a water pump. As the liquid goes through the channels, it picks up heat generated from the engine and then it is deposited at the top of the radiator. The liquid then drains to the bottom while the fins in the radiator spread the heat to a large area and is blown on by the radiator fan and air coming in from driving. The cooler liquid is then shot through the engine again with the water pump and the whole process starts again.

A common problem with engines is sludgy oil / coolant, there is a failure somewhere (all the moving parts in an engine sit in what is essentially a bath of oil except for the combustion chambers) which is allowing the coolant that moves through the engine to mix with the oil.

Not all engines are liquid cooled, Lycoming and Continental produce 4 and 6 cylinder horizontally opposed engines that are entirely air cooled. They can do that because there is a huge prop that acts as a fan *and* as the airplane goes up in altitude the air cools significantly, making liquid cooling mostly unnecessary.