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Platinum plugs according to Bosch...



I spent some quality time with the spark plug and ignition chapters in the
Bosch Automotive Electrical/Electronic Systems handbook, which is a highly
valuable engineering text, whether you're big on Bosch or not. Anyway, I've
gained some insight about Platinum plugs which I will share:

First, why do Pt plugs have a cooler spark?  Well, they don't, spark
temperature, per se, is not a function of the electrode materials, but is a
function of the voltage and current during the spark duration phase. The plug
"temperature" or heat range, is affected by a whole host of factors that
include insulator design, electrode composition and shape, plug body design and
its interface to the cylinder head, etc., but these have no influence on the
arc temperature.

Spark occurs as three distinct phases: The first is the rising voltage across
the spark gap, which occurs as the energy storage device ( the coil ) is
discharged. This phase usually lasts about 0.2 milliseconds. When the gap
flashover, or ignition voltage is reached, typically at about 15 kilovolts,
significant current begins to flow in the gap and the spark progresses to phase
two, called the spark duration phase. During spark duration, which can last
several milliseconds, the gap voltage drops to about 1 kilovolt. It is during
spark duration that the air/fuel mixture is ignited. Phase 3 is called spark
decay, which occurs when there is insufficient energy in the coil to maintain
gap current, and the gap voltage decays to zero.

Secondly, to answer Dennis Sly's query, ignition voltage follows a sort of
parabolic function wrt/air/fuel mixture. Stoichiometric mixtures require the
lowest ignition voltages, rich mixtures need slightly higher voltages to
ignite, but lean mixtures require the highest ignition voltages. Optimum
ignition occurs for Lambda values of 0.9 to 1.0.

Other factors that affect ignition voltage:

1. Electrode gap - ignition voltage increases as gap increases.

2. Electrode geometry - small electrode dimensions increase the field strength,
allowing lower ignition voltages.

3. Electrode material - materials which more readily emit electrons when hot
(electron emissivity, or thermonic emission for us vacuum tube types...), will
lower the ignition voltage.

4. Engine compression - higher compression requires higher ignition voltage.

5. Mixture composition - explained above.

6. Mixture turbulence - quiescent mixtures ignite more easily then turbulent,
but turbulence is necessary to create a homogeneous mixture so maximum power
can be extracted from the burn. Over-turbulent mixtures can adversely affect
ignition, the spark can actually be extinguished in extreme cases.

Now, electrode materials can affect the ignition voltage, but once the spark
duration phase is reached, electrode materials become irrelevant. Platinum has
lower electron emissivity then the Nickel/Chromium/Manganese/Silicon alloys
used in compound electrode plugs, so, under boosted conditions, which is the
same as raising compression, Pt plugs will require a higher ignition voltage
than compound electrode plugs. I can see that Pt plugs might be difficult to
ignite in a boosted engine, especially if boost pressures were high and/or the
ignition system was marginal, which often happens at high RPM.

The Bosch Pt plug appears to have advantages and disadvantages in the boosted
engine/Pt plug debate. A plus would be its small electrode geometry, making
ignition easier at lower voltages. Another plus with this design is that the
center electrode and its insulator run at nearly the same temperature,
something that Bosch says equalizes the heat balance, which is the temperature
differential between the center electrode and the insulator. The benefits of
this (according to Bosch), are that the thermal working range is wider, meaning
the heat range is more generic, combustion residues don't accumulate on the
electrode as readily, and the self-cleaning temperature is reached more
quickly. The problem I see with Bosch Pt's is this: As the center electrode
erodes and recesses inside the insulator, which it will*, this plug is going to
require increasingly higher ignition voltages.

*Bosch actually sells a pin gauge to measure the center electrode recession on
their Pt plugs.

OK, my take on all this is that the Bosch Pt's may not be appropriate for the
SVO, especially long term, but a more conventional, compound electrode plug
with a Pt tip should work no better or worse than a non-Pt plug if the ignition
system is in good shape. Owing to Pt's erosion resistance, I would expect a
double Pt plug to maintain its gap spec longer than one without Pt.

Its 6PM Friday and I'm going home. See you all Monday.

Gary Morrell
gmorrell@ford.com


On Sep 10, 11:41, Sly, Dennis wrote:
> Subject: RE: Installed new parts, car runs like shit
> Do you know if Ford used the same plugs every year in the SVO's. My
> questions revolves around your point about The platinum plugs do not
> have as hot of a spark, thus do not perform well under high boost
> conditions. In the later years of the SVO the injectors were upgrades to
> 36#. Would they not need a hotter spark to burn more gas?
>
> Dennis Sly.
> 84 SVO, Silver
>
> >-----Original Message-----
> >From:	Leiding,Steve [Wpg] [SMTP:Steve.Leiding@EC.GC.CA]
> >Sent:	Friday, September 05, 1997 4:57 PM
> >To:	Mustang SVO Mailing List
> >Subject:	RE: Installed new parts, car runs like shit
> >
> >>> Until someone provides me with scientific evidence that Platinum Coated
> >>> Spark Plugs are bad for my Turbo 2.3, I'm just going to ignore this
thread
> >>> about "platinums being bad."
> >>>
> >
> >Because of the high cylinder pressure in a turbo engine, it is a bit
> >harder to light off the mixture than in a comparable normally aspired
> >engine. The problem is that with high cylinder pressures increased
> >voltage is
> >necessary to make the spark jump the gap. This is why we run a small
> >plug gap
> >of .035" or less.  The higher the pressure, the higher the voltage
> >needed.
> >The platinum plugs do not have as hot of a spark, thus do not perform
> >well under
> >high boost conditions.  But, they last a really long time !  I figure,
> >as our plugs
> >are fairly easy to change - go with a cheap conventional plug, and
> >replace them
> >more frequently.
> >
> >>STeve Leiding
> >>"According to my calculations the problem doesn't exist."
>-- End of excerpt from Sly, Dennis