Brian Nystrom wrote:
> hyundaitech wrote:
>> "Why would it be shielded? The spark is between the tip and electrode
>> and
>> a portion will be behind it, another portion ahead, another portion
>> right across. While one good spark will get the job done, I don't see
>> that
>> four
>>
>> would be a hinderence. No necesarly better though, as you point out."
>>
>>
>> Four electrodes don't make four sparks. They only make one. The spark
>> jumps whatever gap offers the least resistance.
>
> Exactly. All the other electrodes do is block the path of the flame
> front as it moves outward from the ignition point. They are unnecessary,
> detrimental and therefore pointless.
>
> About the only thing one can do to an ignition system that has any
> potential to improve it is to go with a multiple-spark system, where
> more than one spark (typically 4 at low revs and 2 at higher revs) is
> generated within milliseconds of each other at each plug. This is common
> practice in racing engines, though the benefits in the less harsh
> environment of normally-aspirated street engines would probably be
> marginal at best.

I would not call it a pointless. All aircraft engines are using four
electrodes spark plugs. It makes sure that one will fire for sure. It is
a redundancy feature, not better performance feature. Four point spark
plug will fire more reliably than one point, perhaps requiring more
maintenance, but in the aircraft when the fan stops, you go down...........

 

happy wrote:
> Brian Nystrom wrote:
>> hyundaitech wrote:
>>> "Why would it be shielded? The spark is between the tip and
>>> electrode and
>>> a portion will be behind it, another portion ahead, another portion
>>> right across. While one good spark will get the job done, I don't
>>> see that
>>> four
>>>
>>> would be a hinderence. No necesarly better though, as you point out."
>>>
>>>
>>> Four electrodes don't make four sparks. They only make one. The spark
>>> jumps whatever gap offers the least resistance.
>>
>> Exactly. All the other electrodes do is block the path of the flame
>> front as it moves outward from the ignition point. They are
>> unnecessary, detrimental and therefore pointless.
>>
>> About the only thing one can do to an ignition system that has any
>> potential to improve it is to go with a multiple-spark system, where
>> more than one spark (typically 4 at low revs and 2 at higher revs) is
>> generated within milliseconds of each other at each plug. This is
>> common practice in racing engines, though the benefits in the less
>> harsh environment of normally-aspirated street engines would probably
>> be marginal at best.
>
> I would not call it a pointless. All aircraft engines are using four
> electrodes spark plugs. It makes sure that one will fire for sure.

Actually no, it doesn't guarantee that at all. If one electrode shorts
from carbon bridging, there will be no spark at all, as the current will
follow the path of least resistance. OTOH, if one electrode somehow wore
away or broke off, there would be redundant electrodes to take it's place.

> It is a redundancy feature, not better performance feature.

True, but it's limited in what it can do. I suspect that it's really
just a holdover from the days when spark plugs were less reliable than
they are now. I'm curious;, have you seen or experienced a plug failure
in an aircraft engine in the last 20 years or so?

> Four point spark
> plug will fire more reliably than one point, perhaps requiring more
> maintenance,

That's simply not true. It depends entirely on the reason for the misfire.

but in the aircraft when the fan stops, you go down...........

That's painfully true!