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Air Bearings in IC Engines

Oil-free engine uses air for lubrication
  [vote for,

With extremely tight tolerances and wide bearing journals, air would be able to support the bearing load in an IC engine instead of the oil that is currently used. The engine may be lighter and more fuel efficient, and cooling could be efficient with a constant supply of fresh air over large surfaces.
kevinthenerd, Aug 31 2009


       Do some sums.
egbert, Sep 01 2009

       Without going into actual numbers:   

       Air is much less stiff than oil, so the areas would need to be much larger. This means a heavier engine, not lighter.   

       Tighter tolerances means higher manufacturing costs and greater susceptibility to thermal expansion.   

       The specific heat capacity of air is much lower than that of oil, so cooling of the bearing surfaces would be less effecient.   

       The addition of an air pump would would drain power and add weight and complexity to the engine.   

       These ar the problems to look at, the sums will tell you how big these problems are.
Twizz, Sep 01 2009

       But the main reason is that air gets out of the way when you squish it, oil does not.
egbert, Sep 01 2009

       In alloy journal bearings, the oil (and its non-hydrocarbon components, of which there are many) actually react with the bearing surface, so as to reduce friction. You wouldn't get that with air.   

       Some reciprocating aero engines have been built with roller bearings rather than journal bearings, but the engineering challenges are formidable.
8th of 7, Sep 01 2009

       I guess you're not understanding the tolerances involved. Air sucks as a lubricant, a heat transfer fluid, and as a journal bearing support; all of that is true. Getting the necessary reaction force would only be a matter of reducing the tolerances down to the microscopic scale, but you'll never recover the pumping losses on the air. Think of a large stationary industrial engine. In applications such as these, a great deal of time and money is spent shutting down the machine to change the oil, and production is lost in the mean time. I will retract the assumption that it could be made lighter, cheaper, or more fuel efficient, but operating at a constant speed, I would imagine that thermal expansion can be designed into it, and there may be a way to get it started on oil (externally supporting the crank). Thinking about this idea some more, however, I guess the best option would be to introduce new preheated oil via bypass channels. (This would assume using two switchable reservoirs on a dry sump system.)
kevinthenerd, Sep 27 2009

       Air bearings are currently used in some jet engines. This might work if the IC engine always ran at a sufficiently high RPM.
sninctown, Sep 27 2009

       Air bearings work in turbines and other balanced rotary devices due to the lack of side loads. In an automotive engine all the loads are side loads and the total "load" (stress applied to the crankshaft through the bearings) is greater than the HP produced at the flywheel. An air bearing of any design is simply going to be inadequate in a piston engine. If it were possible to "float" the main, small, journal, and cam bearings on air you would still need oil to lubricate the cam lobes and piston rings where an air lubrication system would be impossible.
WcW, Sep 28 2009

       Gas turbines are awesome, but they produce a lot of NOx emissions. The trouble is... NOx emissions are formed at high temperatures, and high temperatures are necessary for good Carnot efficiency. I'll bet designing a crappy turbine that runs cool enough to decrease NOx formation would lead you right back to the drawing board of the piston engine.
kevinthenerd, Oct 01 2009

       Air bearings have one major advantage, they are essentially frictionless. To counter that, they are ridiculously temperamental, and extremely expensive to manufacture. The machining tolerances are high, and thermal expansion can !NOT! be "designed into it" unless you are controlling the temperature to plus or minus a few 10s of degrees, not the plus or minus a few hundred of an IC engine's operating regime (Or build the entire system out of Invar, but good luck on that).
Not to mention that, unlike most other sorts, air bearings are prone to catastrophic and accelerating failure. If a journal or roller bearing starts to fail, it gives warnings as the damage builds up over time. If the air is cut off to an air bearing, the first instant produces shaft/bearing contact, which turns into one or both surfaces becoming slightly scratched. Even if the air is restored within milliseconds, those scratches are enough to stop the bearing from working properly, thus producing a catastrophic failure in extremely short order.
MechE, Oct 01 2009


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