"-b-" wrote in message
...
One question beginning pilots frequently ask, and rightly so, is given the
obvious performance effect of two functioning spark plugs per cylinder,
why do
automobile engines not adopt this? The answer, in recent years, is of
course
that many do, but not so many years ago this was not the case.

The effect of two or more spark plugs per cylinder and the development of
the
flame front in a combustion chamber has been the object of a great deal of
research, and experimental engines have been built with up to four plugs
per
cylinder. The results have shown a strong performance improvement by using
two
plugs, and diminishing returns thereafter. Part of the performance
increase
must be attributed to redundancy - plugs simply do not fire every time,
and
doubling the number of plugs greatly enhances the probability of a fire
each
stroke. Redundancy is not alone though, there is an ample body of evidence
for
enhanced uniformity of the flame front in a combustion chamber with two
sparking points instead of one. Automobile manufacturers have known this
since
the 1940's, so we can only assume that economy is the driving factor,
overcome
in aircraft engines by safety concerns related to engine failures.

Aircraft engines are unique, in that the driving force for dual ignition
really is redundancy and the same performance--and that, with the edxception
of redundancy, similar performance could be achieved by a very slight change
in timing.

OTOH, Wankel rotaries are simply unable to achieve the required flame
propagation at high RPM without a second starting point; and a similar
problem exists in some engines with dradically peaked pistons--which can be
resolved by a second ignition system or, in some cases, by a channel bored
across the crown of each piston.

And, yes, a lot has been known since the 1940s that was not practical to
implement at that time. Some of it still is not.

Peter