As Simon mentions, there are differing theories about what goes on inside
an engine. My instructor worked on aircraft engines in WWII and he was
very sceptical of anything written after that. I do seem to remember a
film (or a series of stills?) made of a clear engine that showed the
detonation process although it was obviously done at fairly low engine
speed and was only as good as the camera equipment available. Since flame
fronts don't really have any mass and since many engines (the aircraft
industry is full of them) use twin spark plugs and, by nature, have flame
fronts colliding with every combustion cycle, I am skeptical that you could
hear the collision of flame fronts.
Concerning the use of two spark plugs: Reducing flame travel and allowing
higher compressions is one theoretical reason for using them. A second
reason is that faster burn time means that an engine will reach peak
combustion pressure faster and, consequently, the peak pressure will act on
the cylinder longer, providing more power. If you are ever in a small
airplane, you will notice that for preflight, the pilot cuts each magneto
in turn to test that both are working. The engine runs slower (less power)
when it runs on one plug, not two.
As far as the resurgance of twin plugs on passenger cars in the seventies
and eighties, I would guess that that had more to do with lame attempts to
meet emissions standards than a quest for horsepower.
And finally, as to whether any of this is right, I don't know. It fits the
facts as we know them today. But the person who designed the MGA head
thought he knew what was going on, also. A classic example of this type of
change is the hemi head. The thought at one time was that a hemispherical
combustion chamber was the answer since it provided a small surface area to
the charge while providing huge area for valves. However, in order to get
any compression, you had to use domed pistons which pretty much shot the
surface area advantage and definately increased flame travel. Today we use
very flat combustion chamber to minimize flame travel. Also, engineers try
very hard to maintain a central spark plug for the same reason.
If I have confuse anyone, I am sorry. If I am boring anyone, I am sorrier
still.
Regards
Bill Eastman.
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