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All about GLOW PLUGS
Written by Brian Gardiner, and Central Coast Model Aero Club Inc.
Submitted by Wayne Beasley
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How Does A Glow Plug Work?
Contrary to what many have previously been lead to believe the following is an
explanation of how a
glow plugfunctions in a motor. The plug is initially heated by applying a
voltage (typically 1.5 volts) to it.
This is to cause it to glow so as to ignite the fuel at compression and start
the internal combustion cycle.
Once the cycle has started, the power source can be disconnected, as with the
heat generated at
combustion the CATALYTIC Reaction generated between the methanol and platinum in
the plug coils
becomes sufficient to keep the process going. The catalytic reaction is a
reaction whereby platinum will
glow in the presence of methyl alcohol vapour. This will happen without any
external power source
being applied.
How do you select the correct PLUG for your application, and why ?
To do this you need to understand a little more of the theory behind the
process. In glow fuel the catalytic
reaction is generated between the methanol and platinum only. Castor oil,
synthetic oil, nitro methane,
etc do not generate a catalytic reaction with the platinum.
Next you need to understand that a certain surface area of platinum is required
to generate a sufficient
catalytic reaction to keep the internal combustion process going. Also it is
necessary to allow extra
surface area for the reaction to be great enough when it diminishes with the
available methanol dropping
as in the case at motor idle. Simply put, cold plugs are manufactured using a
thicker wire to give greater
surface area to facilitate a greater reaction and thus the required catalytic
reaction where less methanol
is present in the fuel mixture.
So! More nitro means less methanol which in turn means a greater surface area to
platinum will be
required to generate a sufficient catalytic reaction. Suddenly it all makes
sense! To work out which
temperature plug to use, you need to know how much methanol is in your fuel, not
how much nitro or oil.
As a rough rule of thumb;
80% methanol or above, use a hot plug.
70%-75% use a medium plug.
60%-75% use a cold plug.
65% or less use a very cold plug.
Idle Bars and Other Stuff
Again, contrary to what many believe, the idle bar on a glow plug is not
necessarily what its name would
suggest. It is in fact to stop any fuel not vapourised from dousing the platinum
coil of the glow plug by
dispersing it away from the coil.
Why are plated coils not as good as platinum alloy coils?
Plated coils suffer from very quick degeneration as the plating breaks down
under operating conditions.
As bits of plating come off, the plug is effectively becoming a hotter and
hotter unit until in a
comparatively short time it is
no longer able to perform its function. Conversely, a platinum alloy coil will
still degenerate, but as it is
platinum alloy throughout, the surface remains as platinum alloy and the plug
continues giving much the
same characteristics for quite a long time.
Plated coils are very poor value when compared to platinum alloy coiled glow
plugs.
courtesy, Warringah Radio Control Society Incorporated
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