Engine Cooling Water Tank Sizing

What Harold posted is good for a starting point. But there is much, much more to it.

When you ignite the fuel/air mixture you produce heat. Heat produces power. In a perfect world we would insulate the engine and keep all of it. Heat also makes stuff melty. I know, the technical terms like “melty” are hard to follow.

Between melty and cold is a point that makes the most horsepower . . . without melting. To keep it at this temperature we have to transfer the excess heat out of the engine.

Temperatures in the combustion chamber of the engine can reach 4,500 F, so cooling the area around the cylinders is critical. Areas around the exhaust valves are especially crucial, and almost all of the space inside the cylinder head around the valves that is not needed for structure is filled with coolant. If the engine goes without cooling for very long, it can seize. When this happens, the metal has actually gotten hot enough for the piston to weld itself to the cylinder. This usually means the complete destruction of the engine.

The Volkswagen engine, and others, simply use air. Most use water.

The heat in the engine has to transfer through the water jacket to the water and from the water to the air.

It’s perfectly simple: q = m(DT)Cp
amount of heat transferred = mass x change in temperature x specific heat.

If your head didn’t explode looking at that formula, do a Google search on heat transfer to learn more. Way more than can be discussed here.

To transfer the heat from the engine to the water, we need to know how well different materials transfer heat.

Thermal Conductivity
Thermal Conductivity is the amount of heat a particular substance can carry through it in a unit of time.
Copper   401
Gold   318
Aluminum   237
Brass (37/15 Cu/Zn)   159
Iron, pure   80.4
Carbon Steel   54
Bronze   50
Lead   35.3
Titanium, pure   21.9
Stainless Steel   16.3
Glass   1.2 - 1.4
Concrete   1.1

It’s no wonder we don’t make blocks out of concrete! It makes a big difference whether your block and head are aluminum or cast iron when it comes to cooling. A copper radiator sheds heat better than an aluminum one, but you pay a weight penalty. Not to mention a copper radiator would be massively expensive.

Once we transfer the heat through the jacket, we have to carry that heat out of the system.

Specific Heat Capacity
Specific Heat Capacity is the amount of heat a particular substance can hold.
Water   4.18
Methanol   2.55
Ethanol   2.48
Glycol, Antifreeze   2.38
Liquid Nitrogen   2.04
Steam (at 110°C)   1.97
Benzene   1.72
3M Flourinert FC-43   1.47
Air (at 100°C)   1.00
Freon 11   0.87

It’s no wonder we use water to move the heat out of the system. It by far carries the most heat. Note the difference between water, steam, and air. Air cooling requires a massive flow of air. If your system overheats and turns the water to steam you lose half of the cooling capacity instantly. Note that the antifreeze lowers the capacity of water to carry heat.

But it is more than just heat.
What we want from the cooling system liquid? Water and antifreeze does this:
1.   Carry heat out of the engine
2.   Lower the freezing point.
3.    Raise the boiling point.
4.   Resist corrosion.

Water alone will carry heat out of the system. Additives reduce the capacity of water to carry heat.
Antifreeze lowers the freezing point. Antifreeze also raises the boiling point. (Anti-boil would be a better term. Compounds that achieve both are called colligative agents.)

A 50/50 antifreeze mixture will lower the freezing point to -35F and raise the boiling point to 223F. A 70/30 mix will lower the freezing point -67F and raise the boiling point to 235F.

The boiling point of a liquid is not only the temperature, but the pressure of the liquid. Raising the pressure raises the boiling point. Water boils at 212ºF at atmospheric pressure. A steam boiler at 3,200 psi produces 705ºF steam. At slightly lower temperatures you could have 650ºF water.

The engine is producing temperatures far in excess of 212ºF. If the water is allowed to turn to steam all of the heat capacity goes away and the engine melts. Raising the boiling point in a pressurized system allows us to transfer more heat without boiling the water.

10 psi raises the boiling point to 240ºF. 20 psi raises the boiling point to 260ºF. 30 psi raises the boiling point to 275ºF. Automobile systems run from 9-15 psi. Racing radiator caps run as high as 28-32 psi. The radiator cap serves as a pressure relief valve in case the pressure exceeds the cap rating. Can you run higher pressures? Sure. You can design any system pressure you want, just keep it in mind that you are building a pressure vessel.

The highest point in the system must be a bleed valve. There must be no air in the system.

For racing, the antifreeze isn’t as necessary as you would think.

Corrosion isn’t a problem if you use distilled water. Distillation involves boiling the water and then condensing the steam. Distillation produces very pure water. You know that salt water corrodes everything. That’s because the salt with water allows a galvanic reaction between dissimilar metals. Pure water doesn’t have anything in it to allow a galvanic reaction, or minerals to cause buildup.

What about those freezing days at El Mirage and Bonneville. Antifreeze might have been useful here. You always wanted to heat that trailer, right? A block heater would also be an answer. Draining the system and refilling before racing is another option.

What temperature should the water be?
Wrong question. What temperature should the engine be? Easy! What temperature will it melt at? Because you have to solve for all of the equations in a heat system, it may be something the F1 guys do all the time with unlimited engineers and computers. Some of the high buck LSR guys may be doing it. Dr. Goggles posted: “No science is better than bad science” That’s great if you understand the science, have the tools and computers to analyze all of that and come to a conclusion. In between rocket science and “It ain’t melted so it must be good.” is where most of us are stuck.

The actual answer would be for the particular system you are running. The Hayabusa guys and the BBC guys have no doubt worked out solutions based on trial and error and the number of similar engines out there.

I would hazard a guess and say that there are as many running too cold as too hot. Should you heat the water before the run? Yes, yes, yes! The heat that it takes to warm up the water is power the engine will never see.

What if I can’t get rid of enough heat?
As you put more heat into the system, easy with a turbo or supercharger, you have to take a higher volume of heat out. If you are at the maximum temperature and pressure designed into the system you need to take more heat out with a larger radiator, or increase the flow rate so the water can carry more heat to the radiator.

What are the cooling media options? (Rules not considered.)
This is all I could find under any type of cooling. Industrial, aerospace, overclocked CPU’s, you name it. The reason for mentioning it, is there is no reasonable non-flammable substitute for water.

Air
Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity.

Water
Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it's easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point. It can also be corrosive if the pH (acidity/alkalinity level) is not maintained at a neutral level. Water with a high mineral content (i.e., "hard" water) can cause mineral deposits to form in system plumbing.

Glycol/water mixtures
Glycol/water mixtures have a 50/50 or 60/40 glycol-to-water ratio. Ethylene and propylene glycol are "antifreezes." Ethylene glycol is extremely toxic. Most glycols deteriorate at very high temperatures. You must check the pH value, freezing point, and concentration of inhibitors annually to determine whether the mixture needs any adjustments or replacements to maintain its stability and effectiveness.

Hydrocarbon oils
Hydrocarbon oils have a higher viscosity and lower specific heat than water. They require more energy to pump. These oils are relatively inexpensive and have a low freezing point. The basic categories of hydrocarbon oils are synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils. Synthetic hydrocarbons are relatively nontoxic and require little maintenance. Paraffin hydrocarbons have a wider temperature range between freezing and boiling points than water, but they are toxic. Aromatic oils are the least viscous of the hydrocarbon oils.

Refrigerants/phase change fluids
These are commonly used as the heat transfer fluid in refrigerators, air conditioners, and heat pumps. They generally have a low boiling point and a high heat capacity. This enables a small amount of the refrigerant to transfer a large amount of heat very efficiently. Refrigerants respond quickly to solar heat, making them more effective on cloudy days than other transfer fluids. Heat absorption occurs when the refrigerant boils (changes phase from liquid to gas) in the solar collector. Release of the collected heat takes place when the now-gaseous refrigerant condenses to a liquid again in a heat exchanger or condenser.

Silicone oils
Silicones have a very low freezing point, and a very high boiling point. They are noncorrosive and long-lasting. Because silicones have a high viscosity and low heat capacities, they require more energy to pump. Silicones also leak easily, even through microscopic holes.

Fluorinert $770 for ¾ gallon. Nuff said.

Water pumps
Electric or mechanical driven pump? Mechanical pumps take horsepower to run. Electrical pumps generally don’t have as much flow. Does it have to be a racing pump? Not necessarily. There are tons of commercial pumps of every flow capacity out there. Racing pumps are usually lighter. Calculating the correct flow and the correct pump is another area that is very tough to calculate.

On an open system, like a pond, placing the pump at the lowest point is necessary to use all of the water. On a closed system in an engine, the liquid level never changes so it isn’t as important to have the pump at the absolute lowest point. Most automotive systems don’t. Once you fill and bleed the system the pump is going to have water in it and priming it isn’t a problem. The pump does need to be feed with a large enough line so that it doesn’t starve and start cavitating. Because of the pressure in the system from heat, cavitation isn’t as much as a problem. But you can still starve it.

How much “head” should I have? Head or discharge pressure, is the difference between the output pressure and the inlet pressure. That differential pressure exists no matter what the system pressurization from the radiator cap might be. The pump is attempting to push water (never pulled.) through the engine. The choke point in the system will determine what that pressure will be. If you change to a larger pump that pressure might go up without much increase in flow. The water is heated by the engine and then cooled by the radiator and that will create a small pressure differential by itself. Once the water is outside of the engine the system has to be large enough to not restrict flow to the inlet of the pump. Bigger is always better. The differential pressure doesn’t matter as much as having the correct flow.

Tank or radiator?
A tank isn’t really required if the system will remove enough heat. The tank would act as a thermal reservoir to delay overheating. If overheating isn’t a problem then you don’t need a tank.

Do you need a radiator? That depends on the cooling needs of your system. Some participants are running a tank with no radiator because they need ballast anyway. Aero factors come into play also. The air has to come into the radiator from a pressure point and that can cause drag. The air has to exit at a low pressure point, and that may not be easy. Can you run a radiator without putting outside air through it? Sure. The heat is going to build up in the engine bay, but on a short run that might be possible.

A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.
Most modern cars use aluminum radiators. These radiators are made by brazing thin aluminum fins to flattened aluminum tubes. The coolant flows from the inlet to the outlet through many tubes mounted in a parallel arrangement. The fins conduct the heat from the tubes and transfer it to the air flowing through the radiator.
The tubes sometimes have a type of fin inserted into them called a turbulator, which increases the turbulence of the fluid flowing through the tubes. If the fluid flowed very smoothly through the tubes, only the fluid actually touching the tubes would be cooled directly. The amount of heat transferred to the tubes from the fluid running through them depends on the difference in temperature between the tube and the fluid touching it. So if the fluid that is in contact with the tube cools down quickly, less heat will be transferred. By creating turbulence inside the tube, all of the fluid mixes together, keeping the temperature of the fluid touching the tubes up so that more heat can be extracted, and all of the fluid inside the tube is used effectively.

Top fuel engines do not have coolant. The entire cooling is provided by the incoming fuel charge.

If you really need more here's a class:
Engine Cooling Design: A System Engineering Approach
Provider: Society of Automotive Engineers – $725
Prerequisites
Prior exposure to thermal sciences at the undergraduate level is recommended.

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