I have an engine temperature gauge on the dashboard of my car (as most people do), but if for some reason the engine temperature is caused to increase (due to a lack of cooling or what have you), the car will break down and the engine needs time to cool down again.

Cars have overheated, on Top Gear for example, many times which has always caused them to break down (and often smoke is produced).

What actually happens inside the engine to cause it to function incorrectly at higher temperatures?

One idea that I had was that the engine coolant will reduce in viscosity. How this makes the engine break down I still have no idea.

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You asked

What actually happens inside the engine to cause it to function incorrectly at higher temperatures?

Response

One of the many ways that engines fail due to high temperature is the increased diameter of a piston due to the heat. As you have noted heat will make things get bigger. In a situation where an engine seizes due to heat the OD (outside diameter) of the piston increases in size so much that is larger than the ID (inside diameter) of the bore it is traveling.

Upon seizing because of heat, due to very high friction, the piston stops the crankshaft from turning as the motor cannot create enough power to overcome the friction of the seizing piston in the cylinder bore.

Upon cool down of the ICE the piston will shrink and allow the engine to turn over if there is no physical impedance from the actual seizure.

In some cases an engine can be started after a heat related seizure. This does not mean that it is good operational order or that lifespan and durability have not effected it. More than likely rings, piston and cylinder bore have irrevocably damaged and require replacement and machining in order for the ICE to continue to operate normally.

Other Affects

  • Plain bearings for the crankshaft and connecting rods

  • Radiator/Coolant hoses can burst due to the increased pressure in the cooling system.

  • Radiators that are old or poorly constructed can fail due to increased internal pressure of the cooling system

  • Head gaskets can fail due to the increased temperature of the coolant OR due to warping during cool down or an overzealous operator putting cold water into the radiator before the ICE can cool slowly and evenly

  • Overhead cam cam holders

  • Various other components

Another issue is that also as the temperature increases, the metal components also slightly increase in size. How much this effects the ability to function I'm not sure, as the increase is still small while the temperature increase is small.

My understanding is that the temperature warning doesn't so much warn you that things are too hot, but rather that the coolant is not acting sufficiently at taking the heat away from the engine. If the temperature reaches 100 degrees centigrade, then there is actually minimal water in the system, but rather steam. This steam is substantially less effective at heat transfer than liquid water. You therefore get a positive feedback cycle, and this easily runs away on you and the engine gets to extreme temperatures, and this is when the damage is caused.

Your question more asks what does damage the engine. I suspect it is a combination of the reduction in viscosity resulting in less lubrication, and also the slight swelling - which at 200 degrees is more of a noticeable increase. Furthermore at that temperature there could be some alterations of the chemical properties of your coolants and fuels, which cause differing effects than what the engine was designed for. Also at high temperatures your rubber/plastic seals can begin to deform and denature, reducing the effectiveness of them.

  • You mentioned that there will be minimal water in the system, but the engine is cooled using automotive oil, which I thought evaporated at around 300 deg C. Other than that, your other points do sound plausible. – Max Goodridge Jan 11 '16 at 22:49
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    My understanding is that the automotive oil is for lubrication within the engine, and possibly some heat transfer from the middle of the engine to the outside of the engine, where the heat is transferred to the engine coolant. The engine coolant is pumped in an enclosed circuit and is cooled at the radiator. The coolant is normally some water-based substance, as oils are too viscous to travel around the circuit in the small pipes. – Lui Jan 11 '16 at 22:53
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    Since the engine coolant is pressurised, the water will not turn to steam at 100C. – HandyHowie Jan 11 '16 at 23:14
  • Good point, didn't think about that. "Factory radiator caps typically increase the cooling-system pressure by 14 or 15 psi and raise the boiling point about 43 degrees F", source. So not a massive increase (~ 6 degrees C) – Lui Jan 11 '16 at 23:16
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    It's not 6 degrees extra, it's 23 decrees C extra. Water boils at 212F, if you add the 43F that you said, that comes to 255F, which is 123C. – HandyHowie Jan 11 '16 at 23:28

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