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Monday, June 15, 2015

Platinum Spark Plugs

Wright Brothers engine: The spark that ignited the gasoline was supplied by a set of electrical "points" inside each cylinder. The steel arms of the points tipped with tiny amounts of platinum to forestall corrosion. As the engine runs, the points momentarily close, making an electrical connection, and then open again breaking the connection and creating a spark. This was called a "make-and-break" ignition system.
Spark plugs didn't exist when the Wright Brothers built their flying machine, so they had to come up with their own mechanism for igniting the fuel-air mixture. They used a pair of contacts in the combustion chamber that were opened and closed by a camshaft. When they were closed, current flowed through the contacts, and when they were opened there was a spark.

Write Brothers engine, camshafts, valves and ignition.
The intake valve is red, the exhaust is blue, and the ignition is green. The piston can be seen moving through the hole in the center.
   It is an unusual mechanism, but not surprising since people were still trying to figure out how to make engines. The surprising part is that the contacts were made of platinum. Nowadays we use platinum spark plugs because they last longer. Presumably the Wright brothers used it for the same reason. But why does platinum last longer? Are the flames afraid of damaging the precious metal? Somehow I don't think so. Turns out the big reason is because of platinum's very high melting point - over 3,000 degrees Fahrenheit. Those little sparks of electricity don't have much mass, but they are very hot, and each time you get a spark a tiny bit of metal is vaporized. With platinum, you lose less metal with each spark and so your plugs, or in this case, your contacts, last longer.
    The Wright brothers used a magneto to drive their ignition system. I'm thinking it must have been producing a substantial amount of current (relative to a modern ignition system) which would have aggravated the erosion problem. Hard to believe that erosion would have been a problem for the short amount of time this engine was run, but maybe by using platinum on their points they were attempting to placate the gods of obstinate machinery. After all, they are always lurking in the shadows, waiting for any little flaw to appear, a flaw they can exploit to bring the whole thing to a halt. If it creates a catastrophe, so much the better. The gods of machinery are vicious animals.

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