Regarding boxer engines and air cooling

Discussion in 'Technical' started by maserati, Aug 3, 2009.

  1. I have heard that the boxer engines lends itself to utilize air cooling more effectively than other designs. This definitely is backed up when you look at airplane motors and porsche pre-996.

    I was just wondering why exactly this is, and since modern boxer engines seemed to have gone the liquid cooled route anyways, how effective is an air cooled boxer compared to a liquid cooled one?
     
  2. compare an inline 4 to a boxer 4. First off, the banks of cylinders on an opposed engine are completely opposite each other, and then there is more surface area (marginally) to cool them. If it's kept inline, the heat transfer is throughout the bank, and the heat will rise much more rapidly.

    And lastly, liquid cool will always be more efficient compared with air cooled, because air cooled just dont have the same cooling effect, unless you are up in the sky. Although liquid cooling requires more machining and components, it still does a better job and doesn't necessarily require material that will expand with heat as to prevent damage and cracking, like making a cylinder out of pure cast iron.

    main reason for having the engines is just to save on space, or in the realm of vehicles, to have a low COG.
     
  3. liquid cooling is NOT always more efficient. It weighs more, costs more, adds complexity, etc. the main downside is that its not very consistent at cooling. water cooling cools even when the car is at a standstill where the air doesnt. but at speed air cooling is great, mainly because the free stream air temp is much lower than the water temp that cools the motor and the weight savings.
     
  4. yeah at speed its great, lots of air and good cooling, but like the harley sitting in traffic, if it sits too long its not going to be that great for the engine overall.

     
  5. *highfive*
     
  6. Love the off beat sound of boxer engines :D
     
  7. The boxer isn't much better suited to aircooling. In a car you need a fan to cool an aircooled engine anyways so it doesn't really matter what configuration you use. In an airplane a boxer is somewhat better than an inline engine since the boxer is shorter for a given displacement/number of cylinders and the trouble with air cooling is that the cooling becomes worse the more cylinders you have in a line. So from a cooling perspecitive for an airplane, if we talk about air cooled engines, the radial engine is the best suited configuration.

    A liquid cooled engine isn't heavier than an air cooled engine. To begin with, a liquid cooled engine can handle higher brake mean effective pressures due to the better cooling provided by the liquid. In other words, with a given displacement the watercooled engine is able to produce a higher power output. The reason a liquid cooled engine can handle higher brake mean effective pressures is because in an engine we need to transfer a lot of heat away from a small area, we have what is called a high 'thermal flux', and liquid cooling performs better in such situations. So it doesn't matter that the cooling air is cooler than the cooling liquid, liquid cooling outperforms air cooling.

    The aircooled engine is typically also limited to two valves per cylinder and the engine needs large cooling fins on the block and head. These fins are bad from a NVH perpective aswell as in terms of packaging where you usually must increase the bore spacing compared to a liquid cooled engine.

    As for costs I would expect a liquid cooled car engine to be cheaper to manufacture than an aircooled engine. The head och block casting is simpler with liquid cooling and the radiator and cooling pump are cheap to produce. Infact, some early liquid cooled engines didn't have a cooling pump at all. With an additional small radiator and an electric fan we have also solved the heat and ventilation of the coupé in a simple way.

    For an airplane though, there is more that can go wrong with liquid cooling and that means aircooling is safer, even if we have to accept an engine that is larger and probably a bit heavier too compared to a liquid cooled engine.
     
  8. Air cooling is less effective and its more difficult to get a constant engine temperature.

    To cope with uneven cooling, air engines must have larger tolerances that is larger clearances between engine parts to allow for expansion when heated and contraction when cooled.
    Larger tolerances allow more combustion gasses to escape thus hurting emissions and fuel economy.

    That's why you don't see aircooled cars anymore.
     

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