5
$\begingroup$

The temperatures of internal combustion engines are well above the melting points of the component materials.

For example, temperatures can reach 2200° C whereas the melting point of aluminium is 659°, of iron is 1530°, of cast iron 1260°,of high carbon steel 1353° etc.

I have read that the reason for this is the phenomenon called "microboiling". The cooling water boils locally at temperatures below the boiling temperature, and the created steam has higher thermal conductivity and cools the engine.

What are the reasons behind microboiling?

And why steam has higher thermal conductivity than water?

Improve this question
$\endgroup$
14
  • 1
    $\begingroup$ Peak temperatures inside the engine's working fluid (the air-gas mixture that gets combusted) have little to do with the temperature of the cooling liquid for the engine block or even the temperature of the cylinder walls, which must be below the point where the lubricants stop working (and the metals weld together). Once bubbles form in the coolant, things are going downhill fast because the bubbles greatly decrease thermal conduction between the metal and the liquid. That's typically where engine death begins. $\endgroup$
    – CuriousOne
    Apr 25, 2016 at 22:20
  • 2
    $\begingroup$ @CuriousOne It is not bubbles, it is a layer between the metal and the liquid. Thermal conduction due to this layer is contrary to what think, increased. The average temperature of a liner don't exceed 220 degrees, the limit of lubricant failure. If you know the reason why, we will become both smarter after you write an answer $\endgroup$
    – veronika
    Apr 25, 2016 at 22:30
  • $\begingroup$ Are you talking about pool boiling? That's a totally different regime than forced convection. Maybe you want to add a citation for the claim that a gas film increases conduction. $\endgroup$
    – CuriousOne
    Apr 25, 2016 at 22:55
  • 1
    $\begingroup$ In typical coolant loops, boiling is a bad thing, reducing thermal transfer to the working fluid and leading to damage (e.g. pitting) of the heat exchanger. Relying on boiling to take that extra bit of heat away runs in to the problem of how to keep supplying liquid everywhere evenly. The coolant in your car's engine should not be boiling. $\endgroup$
    – Jon Custer
    Apr 25, 2016 at 23:37
  • 1
    $\begingroup$ @CuriousOne you say the temperature of the working fluid has little to do with the temperature of the engine block. Why? That's very counter-intuitive to pretty much anyone reading these comments. Also, it would be nice if you could provide an answer, ignoring the details brought out in your second comment, which are not really needed for the main question. $\endgroup$
    – DanielSank
    Apr 26, 2016 at 4:50

6 Answers 6

Reset to default
7
$\begingroup$

It is all a matter of engineering balance, between the water circulating in the radiator circuit of the car, which enfolds the engine and with water-metal contact which takes heat away at a certain rate.

In automobiles and motorcycles with a liquid-cooled internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which a liquid (coolant) is pumped. This liquid may be water (in climates where water is unlikely to freeze), but is more commonly a mixture of water and antifreeze in proportions appropriate to the climate

Microboiling are those small bubbles one sees at the bottom of the saucepan before water starts boiling uniformly at 100C. They are removed with the water circulation raising the radiator water's temperature.

If the radiator loses its water the engine seizes up because of loss of lubricating oil and deformations due to heat, long before the melting point is reached, ( as observed by tfb in comments) and is destroyed. If the water boils it would remain at the same temperature so cannot work as a coolant. Therefore the water circulates to remove the micro boiling points from the surface of the metal to the rest of the reservoir and cool it at the radiator.

The engineering design takes all this into account, to keep the metal surfaces well below melting and at a good temperature for the lubricating oil by the rate of circulation of water around the engine fast enough. With red lights coming up to stop immediately if the water circulation fails ( has happened to my old car and not only once).

Now as far as containers and temperatures, take the temperature of a propane heated oven, it is at 2800C, but we cook food in the oven at 180C. Do the oven walls melt? Or even the inlet grid? It is all about rates of heat transfer and and it depends on the engineering parameters.

Improve this answer
$\endgroup$
5
  • $\begingroup$ Thank you very much for your answer. Anna, the temperature that you read on your car is always 90. Why then a layer of water evaporates? Your car stopped because the water pump is centrifugal and pressure of these pumps depend on density, so it cannot handle air and steam unlike a positive displacement pump. Steam can cool. The question is what is behind this microboiling. Thank you again $\endgroup$
    – veronika
    Apr 26, 2016 at 6:27
  • $\begingroup$ Micro boiling is the beginning of the steaming process next to the source of heat, the bottom of the pan in the kitchen, or the walls of the engine in the car. The water circulates fast enough so that it removes the 100C + tiny bubbles towards the refrigirator grid where air cools the water. people.smu.edu/ahumason/technotes/microboiling%20point.htm . all this is relevant to engineering designs. $\endgroup$
    – anna v
    Apr 26, 2016 at 7:17
  • $\begingroup$ In fact after coolant loss the engine fails long before anything melts: temperatures rise leading to a combination of differential expansion and loss of the oil film on the cylinder walls, and the whole thing seizes up. $\endgroup$
    – user107153
    Apr 26, 2016 at 7:39
  • $\begingroup$ @tfb correct. melts is a wrong term I guess, though it is the effect of too much heat. $\endgroup$
    – anna v
    Apr 26, 2016 at 8:14
  • $\begingroup$ Cast iron will develop cracks due to allotropic expansion. Melt was a dramatic word used to draw attention to the question $\endgroup$
    – veronika
    Apr 26, 2016 at 12:06
2
$\begingroup$

All very interesting. Of course if you pull down an engine that has had a piston seize from overheating you will see the aluminium has started to melt as the piston expands and is dragged up and down the cylinder bore. Pistons do melt but by the time they start to melt they have expanded to a point where they are so tight in the cylinder bore the loss of energy through friction stalls the engine. More fuel ,more heat, more expansion, loss of cooling , piston heats up, piston expands a few thousands of an inch and seizes. This is very obvious when one piston seizes due to poor combustion (faulty diesel injector) and the other pistons are working overtime to maintain engine speed at full power. Piston starts to melt and then seizes. The engine can tear the piston in half. I have had it happen and repaired a few. John

Improve this answer
$\endgroup$
1
$\begingroup$

I'll venture the guess, that the explanation you read was wrong. What I'd find plausible is:

liquid mixed with gas has a higher effective thermal capacity - you don't need conductivity since the stuff is pumped through the cooling cycle, right?
What I mean by this: the liquid can receive a lot of heat by boiling. It carries the heat away not only in the form of higher temperature, but in the form of beeing a gas. In the other part of the cycle it can give away a lot of heat by condensing.

At least this is how heating works in dairy factories where I have been: when pasteurizing the big steel tanks, you pump vapour (with pressure greater than atmospheric, of course) in them, which condenses (therefore pressure drops, and more vapour is sucked in), which heats them very effectively.

The engineering problem in the cooling system is probably, that it's more difficult to pump such a mixture (like the blood in you veins, when delving upwards from a great depth too fast, might form bubbles and stop flowing, very dangerous). That's why it might be advanatageous, if all the liquid touching the engine gets evaporized, so you have no mixture, only arriving liquid and departing gas.

That are "educated guesses" which hopefully explain part of the surrounding physics. For a practical explanation of the occuring issues probably engineers are the better audience.

Improve this answer
$\endgroup$
6
  • $\begingroup$ Thanks for your answer. I think your distinction is useful; it is not conductivity but capacity that the presence of steam increases. My question is what is the reason behind microboiling; a similar question to what could be the reason behind osmosis or capillary rise or normal boiling. Maybe you know a good link or book about this $\endgroup$
    – veronika
    Apr 26, 2016 at 6:19
  • $\begingroup$ well... I'm not sure that microboiling is important here (to understand why engines dont melt). Why shouldn't the water boil, if the engine is hot ;) the reason behind normal boiling? maybe physics.stackexchange.com/questions/129512/… helps? The reason behind osmosis and capillary rise are again quite different. And I would say, that if microboiling means what happens in the kitchen, then this is due to gases that were dissolved in the water and get free; it's not water vapour; and it doesnt consume much energy $\endgroup$
    – Ilja
    Apr 26, 2016 at 7:11
  • $\begingroup$ PS: the answers in the link above are not very good, but "vapour pressure" is the keyword to understand boiling $\endgroup$
    – Ilja
    Apr 26, 2016 at 7:12
  • $\begingroup$ I have seen the microboiling argument on MAN company tutorial (a reliable decent source) but i can't find it now. Thanks for the link $\endgroup$
    – veronika
    Apr 26, 2016 at 7:47
  • $\begingroup$ there was just another new question about boiling :) so I tried to explain it myself too: physics.stackexchange.com/a/252122/111915 $\endgroup$
    – Ilja
    Apr 26, 2016 at 7:52
1
$\begingroup$

Engine will not be running at 2200 deg for all the time.It only happens at the power stroke at negligible time i.e. it is in such a way that heat is dissipated in fraction of second through Fins and oil.As you said engine is made up of Aluminium, Aluminium has the highest thermal Conductivity.Hence Engine is not melted.

Improve this answer
$\endgroup$
0
$\begingroup$

Combustion gas temperature can be as high as you quoted. But metal temperature is much lower than its melting temperature. At high temperature, metal will lose its strength. Engine design will not allow this to happen. During the cycle, combustion gas tries to heat up the metal. But there is coolant flow carrying away the heat. The net is that the metal keeps the temperature low. When the coolant system failed somehow, metal will melt.

Improve this answer
$\endgroup$
2
  • $\begingroup$ yeah, she explained it in the question; the problem was rather how it works in detail $\endgroup$
    – Ilja
    Apr 26, 2016 at 5:48
  • 1
    $\begingroup$ Sorry, I didn't answer the second part. In general, a coolant system doesn't like vapor because the bubble can block and slow down the coolant flow and therefore reduce the heat transfer. The concept of micro boiling tries to avoid this situation and makes the maximum heat transfer especially for today's downsized heavy duty engine. However, it is not because of the heat capacity of anything but because of the latent heat when boiling occurs. $\endgroup$
    – user115350
    Apr 26, 2016 at 13:02
0
$\begingroup$

Air fuel ratio also plays a major role in controlling the peak temperatures attained in an engine.The temperature will be the highest at the stoichiometric ratio because of the complete combustion of higher quantity of fuel.Since the engine operates at very high speeds, complete combustion occurs at a slightly leaner than stoichiometric ratio.But usually this leaner mixture is not used .The engine management system with the help of temperature sensors will optimise the air fuel ratio.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.