Atifreeze and heat transfer.

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hey,
I just got out of Chemestry class and this was on my mind. The teacher was explaining how when a solute is added to a solvent to form a solution, as long as the solute is not ionized(which basicly means it woun't evaporate, I think) then the freezing point of that solution will go down. He used antifreeze ina car as an example. However, he went on to say that this also causes the bioling point to go up, so in the summer higher coolant temperatures can be reached without boiling over. I always thought that pure water provided better heat transfer and did a better job of keeping the engine cool. Wouldn't I want to lower the water temp. as much as possible, thereby increasinf performance and efficience rather that make the water harder to boil and equally harder to get rid of the engines heat?
Just wondering
Paul
 
You want your coolant to transfer as much heat as possible (reach a higher temperature) without ionizing (boiling over).
 
You are correct, in that water has better heat transfer properties than the antifreeze mixture. However, as with everything there is a compromise. Water does not have the freeze protection that we desire. It also will corrode the system fairly rapidly without any corrosion inhibitor packages. These are the principal factors as to why pure water is not used.

The higher boiling point of antifreeze coolants comes at the cost of less heat capacity. However, this higher boiling point and the vapor pressure properties of the glycol coolant is useful in some applications where cavitation is of concern. Cavitation is usually a localized phenomenon where the coolant vaporizes. In a simplied world, the gas bubbles then implode on themselves when they hit localized areas of higher pressure. This causes localized erosion/corrosion. It can occur in the water pumps or on some engines or on wet cylinder liners of larger engines.
 
pacman":256b0ais said:
Wouldn't I want to lower the water temp. as much as possible, thereby increasinf performance and efficience rather that make the water harder to boil and equally harder to get rid of the engines heat?
Nope. You want to keep the engine's internal temperatures as high as possible. The higher the operating temperature of the engine, the more efficient the combustion process. In other words, the more heat you can prevent from being transmitted to the coolant, the more it stays in the cylinder creating higher pressures (HP at the crank). In an effort to increase these operating temperatures, anti-boiling agents are added to the water, and the system is pressurized, raising the boiling point even further.

On the other hand, we don't want temperatures so hot as melt parts and burn up the oil...
 
Hey SuperMag,
Im a little confused. I am pretty sure that in a perfect world if all the heat could be harnessed in the combustion and contained in the chamber them the engine would be 100% or so effecient.This is the reasoning behind thermal coatings, right? So if this is true I am led to believe that the bigger the difference in temperature between the chamber and surrounding metal, the higher the efficiency. Is this not correct. I kinda thought it was like a hott air balloon in a cold morning compared to a hot summer day.
Somebody set me straight
Paul
 
A higher temperature difference between to surfaces will promote heat loss. In heat transfer, the temperature difference is like the pressure in a fluid system in that it is the driving force for the heat exchange.
Besides retaining some heat in the head, the coatings on headers etc help to keep the exhaust gases from cooling. As they cool, they will try to occupy more space, and tend to slow the gases down and create more back pressure. Exhaust velocity and cylinder scavenging will be enhanced with a higher temp, faster velocity exhaust.
Doug
 
Most of the energy in the gasoline (perhaps 70%) is converted into heat, and it is the job of the cooling system to take care of that heat. The primary job of the cooling system is to keep the engine from overheating by transferring this heat to the air, but the cooling system also has several other important jobs.
The engine in our cars run best at a fairly high temperature. When the engine is cold, components wear out faster, and the engine is less efficient and emits more pollution. So another important job of the cooling system is to allow the engine to heat up as quickly as possible, and then to keep the engine at a constant temperature.
Inside a car's engine, fuel is constantly burning. A lot of the heat from this combustion goes right out the exhaust system, but some of it soaks into the engine, heating it up. The engine runs best when its coolant is about 200 degrees Fahrenheit (93 degrees Celsius). At this temperature the combustion chamber is hot enough to completely vaporize the fuel, providing better combustion and reducing emissions. Also the oil used to lubricate the engine has a lower viscosity (it is thinner), so the engine parts move more freely and the engine wastes less power moving its own components around, and metal parts wear less.
As this liquid passes through the hot engine it absorbs heat, cooling the engine. After the fluid leaves the engine, it passes through a heat exchanger, (radiator) which transfers the heat from the fluid to the air blowing through the exchanger. Cars operate in a wide variety of temperatures, from well below freezing to well over 100 F (38 C). So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.
Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.
The temperature of the coolant can sometimes reach 250 to 275 F (121 to 135 C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.
The cooling system uses pressure to further raise the boiling point of the coolant. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45 F (25 C) so the coolant can withstand the high temperatures.
One interesting way to reduce the demands on the cooling system is to reduce the amount of heat that is transferred from the combustion chamber to the metal parts of the engine. Some engines do this by coating the inside of the top of the cylinder head with a thin layer of ceramic. Ceramic is a poor conductor of heat, so less heat is conducted through to the metal and more passes out of the exhaust.
 
pacman":3odzo2ju said:
....I am led to believe that the bigger the difference in temperature between the chamber and surrounding metal, the higher the efficiency. Is this not correct....

Umm, no... When gasoline is burned in an engine, energy is released in two forms. The first is pressure, whose force is brought to bear directly on the pistons, thus creating HP at the flywheel. The other type of energy formed is heat, obviously, half of which goes straight out the tail pipe, and the other half of which makes its way up to the radiator. The ratio of HP to tailpipe heat exhaust to radiator heat exhaust is about 1/3-1/3-1/3.

Now let's forget the chemistry for a moment and concentrate on the physics. Simply stated, heat goes from where it is to where it ain't. This temperature differential is referred to as ∆T. (IIRC, the rate of heat transfer is increased not in a linear relationship, but rather as a function of the square of ∆T.) But without getting into the thermodynamics of it all, suffice it to say that if you can bottle up the heat so that it doesn't escape, your pressures will be much higher, and thus so will your HP output at the flywheel.

So to get as much pressure as possible in the cylinders, you try to retain as much heat inside the cylinder as possible, and yes, through the use of ceramic and/or other reflective/insulative materials. You also try to run the colant temperatures as high as practicable so as to prevent the heat from wanting to get out in the first place by minimizing ∆T.
 
Efficiency in a motor always increases when the temperature well difference is maximized.
 
Sorry for the double post. Not to confuse the systems, but temperature difference between wells meaning that of the combutions process and that of the working fluid - in this case air.

The coolant only serves as a transport medium, so I question of the specific heat really matters under steady state conditions.

Heat Xfer from the block to the coolant via conduction and convection is linear.

The aid in extending the operating temperatures is really the kicker although I find the anti-cavitation concept interesting...I'll have to look into that.
 
The coolant only serves as a transport medium, so I question of the specific heat really matters under steady state conditions

The specific heat capacity of the coolant with a properly designed and operating system is not likely to matter much if it does not vary significantly. However, on an old cooling system that has scale and rust buildup, the difference in fluid heat capacity may be the difference in staying cool. As an example 100% ethylene glycol has heat capacity that is 30 to 34% less than the a coolant composed of 100% water. Assuming that all other things are equal, that means that each ounce of 100% glycol circulating through the engine can only carry 66% as much heat as each ounce of water going through the engine.

Although I agree, that the coolant basically transports the heat to the radiator, the heat capacity of the coolant comes into play on the initial design of the system. The system design is constrained by radiator size, air flow, pump size etc. You could fill your radiator with thermal oil and not have to worry about boiling the fluid out at 300 deg F or higher. However, the engine would still over heat, because thermal oil does not carry as much heat. In order for it to work, you would have to increase the flow rate, and/or the size of the radiator in order to transport the same amount of heat to the radiator over a given period of time.
 
find the anti-cavitation concept interesting
My experience with cavitation in engines is on wet liners on stationary engines. But I understand that truck diesel engines may have the same problem. The replaceable liners are not as stiff as bored cylinders, and they oscillate at some frequency. This ringing creates minute pressure changes near the skin of the liner. Coupled with high liner temps, the coolant can vaporize into very small bubbles. The pressure variation from the ringing can cause the bubble to implode or collapse violently. (these are small bubbles) Anyway, it strips the corrosion inhibitors off of the liner, and makes it susceptible to corrosion and further erosion from the cavitation. It forms a honeycomb pattern that can eat through a 1/2" liner wall fairly rapidly. Then you get water in the combustion chamber and then it won't take long for you to find out that water in the cylinder is not conducive to long engine life or meeting your maintenance budget. Anyway, propylene glycol has more resistance to cavitation than ethylene glycol. But as it turns out, our problem was eventually traced back to a leaking $10 radiator cap on the surge tank. Surge tank was high up in the air, an no one had ever bothered to inspect the cap that was common for all three engines.
 
Yeah, propylene glycol has more cavitation resistance than ethylene glycol(and therefore, possibly more anti-foaming action, not that foaming is a major problem here), but it has even less ability to transfer heat than does ethylene glycol(the most common anti-freeze additive).

Personally, I run a reduced amount of ethylene glycol (temps in SoCal seldom run below -10 F.) and toss in a bottle of Redline's Water Wetter. It supposedly reduces surface tension in the coolant mix to assist the heat transfer process, both from engine-to-coolant and coolant-to-radiator. It sure has helped out in vehicles that were operating a little too warm for comfort. It has also allowed return to a factory temperature rated thermostat, for better combustion & lower emissions.

The thermal barrier coatings as applied to combustion chamber surfaces have some other helpful properties. They can be polished like a mirror to further reduce 'hot spots' and pre-ignition. Applied to valve faces they can shield the valve heads from some of the intense heat. When applied to polished exhaust ports & even internally-polished exhaust manifolds, more heat will be delivered to a turbocharger for faster spool-up, ie. less turbo lag.

On the subject of a rapidly-cooled exhaust, the gases will take up LESS, NOT more room, and that is why the velocity slows down, reducing the scavenging/"extractor" effect. OTOH the exhaust out of a turbocharger needs to expand in size to get rid of backpressure(according to MacInnes) & keep the turbo spinning.

J.R.
 
On the subject of a rapidly-cooled exhaust, the gases will take up LESS, NOT more room, and that is why the velocity slows down, reducing the scavenging/"extractor" effect

I did say that didn't I? OOPS my bad. I should have read that post again. Sorry for the mistake.:oops:
Doug
 
I apologize for the error above when I said the gas cools exiting the heads taking up more space. We all know gas expands when heated. Apparently I had a brain fart and did not write what I was thinking about, which was that gas expands as it leaves the pressurized cylinder. The expansion of the gas into the low pressure exhaust pipes slows down the gas velocity. The expansion rapidly cools down the exhaust gases. And as the gases cool, the density increases. But the pressure reduction has a greater influence on the gas velocity than the cooling.

In order to maintain a mass balance, the denser cooler gas at the tailpipe must be traveling slower than the hotter less dense gas exiting the heads. Headers have much more surface area than a single exhaust pipe. The extra surface area cools the gas down quicker. Heat wrap or coatings help minimize some of the heat loss. Once the gas has expanded into the headers, if the temperature is maintained over a longer distance, then the gas velocity will be more constant over that distance. The higher velocity will enhance scavenging. Also the higher temps can help keep the moisture in the gas stream. Once liquids start to condense or drop out, the friction increases rapidly.
Doug
 
Let's not forget that antifreeze also contains lubricants essential to the waterpumps bearing and seal surviving. If you have never experinced a siezed waterpump, trust me, you don't want to.
 
Right-O on that! Seized one up and shot a new fan & the front of a (fairly new) pump right into a brand-new custom 4-layer cored radiator a few years ago. A disappointing experience... damned near exhausted all my vocabulary on that one! Coolant was always maintained properly but that bearing froze up anyway. Just a freak occurence AFAIK, but very costly.

BTW RedLine Water Wetter now does have lubricant & anti-corrosive substances added to it (it originally did not), so I don't see any problem substituting it in for part of the ethylene glycol's expected anti-corrosion properties, if the projected outside temperature ranges are OK.

J.R.
 
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