On 05/04/2011 09:57, Michael Rigby-Jones wrote:
>>
>> If you had two spark gaps in one plug they would be separated only
>> by a very short distance, so any advantage of dual flame fronts
>> would be lost.

Oli Glaser <oli.glaser@talktalk.net> wrote:
>
> Out of interest, how far apart are the spark gaps in a twin spark=20
> system?

Only engine of which I have first hand knowledge is my Lycoming
O-360 (360 cubic inch, 4 cylinder, horizontally opposed) engine.
Bore is 5.125" (130mm).  Sparkplugs come in diagonally from the
top & bottom edges of cylinder heads.  Given the angle, I estimate
the tips of the 2 sparkplugs are 3" to 3-1/2" (75-90mm) apart.

Aircraft sparkplugs come in massive and fine wire versions. The
massive electrode version has 2 arc-shaped ground pieces around
a cylindrical center electrode. Spark can arc from center to one
or the other or both ground pieces.  I've never paid the premium
for fine wire aircraft sparkplugs so I can't comment on them.


Geo <buggiesmith@gmail.com> wrote:

> The double spark makes little difference to the RPM but it is only=20
> checked on the ground - you do not switch off one mag in flight (ok
> you do if the engine starts misfiring as one mag could have lost
> sync) so I am not sure if a /single/ mag failure would be noticed.

Depends on the instrumentation you have installed. If you are doing
it "by ear" then you may quite likely not notice early on.

If you have a multi-probe EGT gauge, a misfiring cylinder shows up
clearly as a change in exhaust gas temperature; particularly during
the run-up check when you only have 1 sparkplug firing per cylinder.
I prefer a bar graph style EGT gauge. I feel exact temperatures are
not as important as relative values and trends (easier to watch the
bar heights). A magneto failure shows up as a more subtle change in
all cylinders.

Note: exhaust gas temperatures are NOT the same for all cylinders.
Each one has a characteristic temperature that you learn to expect
in various flight regimes. Variation is due to differences in length
& shape of each intake manifold runner, mixture distribution, mixing
of air-fuel, cylinder cooling (cooling air flow varies by cylinder
location), etc.  That's why I like a bar graph.  Humans easily learn
a "picture" to expect when at cruise -- if the picture changes, you
perk right up.  Hopefully before you become a glider pilot. :-)

						Lee Jones
--=20
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