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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.
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Without going into actual numbers: |
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Air is much less stiff than oil, so the areas would need to be much larger. This means a heavier engine, not lighter. |
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Tighter tolerances means higher manufacturing costs and greater susceptibility to thermal expansion. |
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The specific heat capacity of air is much lower than that of oil, so cooling of the bearing surfaces would be less effecient. |
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The addition of an air pump would would drain power and add weight and complexity to the engine. |
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These ar the problems to look at, the sums will tell you how big these problems are. |
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But the main reason is that air gets out of the way when you squish it, oil does not. |
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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. |
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Some reciprocating aero engines have been built with roller bearings rather than journal bearings, but the engineering challenges are formidable. |
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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.) |
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Air bearings are currently used in some jet engines. This might work if the IC engine always ran at a sufficiently high RPM. |
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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. |
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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. |
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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. |
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