Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Car engine block/head metal temp limits?

  1. Nov 26, 2011 #1
    Good Morning,

    I am working on a project that is designing a water pump for a car engine without any mechanical parts (works on waste heat building pressure to circulate the water). I know that most of the car engine heads are aluminum and that blocks are part cast steel and part aluminum.
    In order for my design to work i need to reach boiling temps of pressurized water, which might get high (depending on the pressure).

    What are the highest temperatures that can be reached in a car coolant system without harming the mechanical properties of the metals surrounding?
    Also to be considered is the oil temps, what temps to oils break down and stop being efficient?

    *note: What i am designing isn't based on the same principles as Thermosyphons.
    Last edited: Nov 26, 2011
  2. jcsd
  3. Nov 26, 2011 #2
    Found the answer in a Auto Engine Engineering book: Internal Combustion Engines Fundamentals, J Heywood.
    Cast Iron limited to about 400 Deg. C
    Aluminum limited to about 300 Deg. C
    Gas side surface of cylinder wall must be kept lower then 180 Deg C in order to prevent deterioration of the lubricant film.
  4. Nov 26, 2011 #3
    Take a look at the temperature gauge on a vehicle. The red zone generally begins when the coolant temperature exceeds 120 C or so. The cooling system in all vehicles has a pressure relief valve that is incorporated in the radiator cap. It is no more than 16 psig. Therefore the pressure in the system will not exceed 16 psig. The coolant system is pressurized to limit the boiling of the coolant by raising the boiling point. Once the relief valve opens, boiling can increase to the point where the cylinder walls are no longer fully wetted. In other words there is so much boiling that the bubbles cannot get away fast enough from the cylinder walls so the walls are blanketed to a great extent by vapor. When this occurs, there is a reduction of heat transfer away from the cylinder walls and temperatures greatly increase. This destroys the oil film on the inside of the walls and can warp engine blocks/heads. The engine is then doomed to failure.
  5. Nov 26, 2011 #4
    Quick noobue question don't mean to thread jack. If oil breaks down and becomes deteriorated by 180 c why do they sell thermostats that keep the car engines at 170-190 c? Atleast for my Honda it they do.
  6. Nov 26, 2011 #5
    That would be 170-190 Fahrenheit, not Celsius.

    180 C = 356 F
  7. Nov 26, 2011 #6
    170-190 F. Whoops, did not read previous post before adding my
  8. Nov 27, 2011 #7

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    i been here too many times..you can race an engine at 220 -230F degrees F for miles as long as the coolant temp at the point it returns to the radiator is 200 -210 max and you run an oil cooler/ Oil temps are 250 to 30 degrees higher than the radiator temps. We even raced at 230F and as long as we did not pop the radiator cap and boiled, we were making good horsepower. This gets real hairy if you run over 13:1 compression ration. Also if you have an aluminum cylinder head and cast iron block..you are subject to blown head gaskets. We never run anti freeze as this hurts heat transfer. Watter Wetter is a chemical we typically add to distilled water to increase heat transfer. Today's synthetic oil really steps up and can handle the temps. BUT..hot is literally hell on valve springs and this drive train area must be properly oiled to survive and you MUST run an oil cooler sized for the engine.
  9. Nov 27, 2011 #8
    "We never run anti freeze as this hurts heat transfer. Watter Wetter is a chemical we typically add to distilled water to increase heat transfer."

    Antifreeze is not run in races because it is very slippery. When it gets on the track it is almost as slippery as oil.

    Water Wetter decreases the surface tension of the coolant. This reduces the size of the bubbles during nucleate boiling, thereby increasing heat transfer due to reduction of vapor. The overall effect is to lessen the temperature difference between the surface and moving coolant.

    Commercial engines are designed to last well over 100K miles. Keeping block temperatures lower is a means to that end. Head gasket replacement for a typical car owner is a major expense.
  10. Nov 27, 2011 #9

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    well said LawrenceC...
    but for passenger cars antifreeze is a must at least in Ohio..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook