“COLD” etc.) and test the plugs to really
get an accurate understanding of the
each manufacturer’s scale.
A glow plug’s heat rating is all about
engine timing. The colder the plug, the
closer to top dead center an engine will
fire. Conversely a hotter plug will cause
the engine to combust sooner thereby
advancing the timing of the engine. Here
is an interesting example of the complex
interplay of the plug, fuel, engine and
pipe: by putting a colder plug in the
engine you actually retard the timing.
The result is the combustion will happen
later, causing the exhaust gases to exit
hotter, maybe while still burning. The
result is the hotter gases will exit the
engine at a faster rate, meaning the sonic
wave from the pipe is going to come
back faster, and the net result is that we
have to lengthen the pipe to make sure
the intake port is closed before we
compress the fuel slug. All if this is
important because the fuel does not burn
all at once. It actually burns over a very
fast duration and the trick is the get the
piston at the top of the stroke at the
optimal timing of the burn.
A plug’s heat rating is particularly
important when running fuels with
different nitro levels. The more nitro that
is present in the fuel, the colder the plug
you want to use. Without going into a lot
of detail about the specific burn
characteristics of nitro and methanol for
our purposes we will use the following
rules of thumb:
More Methanol and less Nitro:
Use a Hotter Plug
More Nitro and less Methanol:
Use a colder Plug
A “hot” plug means that the element
remains hotter through the ignition
cycle. If a plug is too hot then an engine
can suffer from pre-detonation. The heat
range of a plug is mainly determined by
three things: the thickness of the coil, the
sidewalls of the body and the number of
turns in the coil. A hotter plug will have
a thinner coil and sidewalls and more
turns than a colder plug.
Remember in the previous paragraphs
where we talked about how the glow
plug works by igniting the methanol
vapors? The more methanol in the
mixture, the easier it is to ignite the fuel,
and conversely the more nitro, the more
difficult it is to ignite the fuel.
PROPWASH
16
November 2012
Know Your Plug, Know Your Engine
By David Santistevan
District 20 Member
I am always amazed when running my model how little is understood about the
glow plug and how critical it is to a good running engine. Further, few people
understand the black art of reading a plug and the big information it can give you
about the way your engine is running. Having a working knowledge of plugs can both
make you more competitive at the pond and save heartache and big bucks in engine
repair. This article will go over the basic anatomy and basic theory of the model glow
plug, but most importantly, it will address how reading a plug can aid you to tune your
engine for maximum performance and longevity.
Theory of the Glow Plug
The primary purpose of the glow plug of course is to ignite the fuel. Using a heated
coil, usually made of platinum, the glow plug ignites the methanol vapors in the fuel
mixture. This is a very important point, as later we will see how different fuel mixtures
will affect the plug and the combustion process. During the compression stroke of a
two cycle engine, a catalytic process takes place where the heated coil of the plug
ignites the methanol fuel vapors to create the power stroke. The superheated coil then
remains hot until the next compression cycle and subsequent ignition again.
When starting the engine we have to apply an external power source (the igniter) to
the plug, thereby heating the coil and starting the process. Once the engine begins to
combust on its own we can remove the external power source and the engine will
continue to run on its own through the catalytic process of combustion and heating the
plug again for the next cycle.
Anatomy of a Glow Plug
The glow plug consists of several basic parts: the body, the electrode and the coil.
There are a couple other smaller parts used in the assembly but for the purposes of this
discussion, they are not relevant. The electrode is the post that sticks out of the top of
the plug and connects to the starting battery to provide electric current to the coil. The
body of the plug is a machine body that serves to hold the coil and electrode and
screws into the engine head to provide a positive compression seal.
The coil is the business end of the plug. It glows red when it is heated either by the
catalytic reaction of combustion in the presence of methanol vapors in the chamber or
through an external battery source during starting. There are a couple of really
important things to consider about plugs when it comes to the coil. A plug containing
a coil that is solid platinum will be much more expensive than a plug with a coil that is
made with a plated element. A good example is that you can pay around $8 for an OS
#8 plug, where you can buy some other brands of plugs for around $3 or $4. The less
expensive plugs may seem like a better value, but in reality you will get less life and
less performance from a less expensive, plated plug. During the normal running of a
well-tuned engine you will actually burn away some amount of the coil. With a plated
coil you can burn through the plating fairly quickly and then a plug loses its ability to
properly ignite the fuel. Once the plating is gone, the plug is subject to failure and you
could go dead in the water.
Selecting the Right Plug
Simple, right? Well not so fast, this is where we begin to separate the guys who
know what they are doing from the rest. First of all, I am sure that most of you know
that plugs come in a variety of heat ranges. A lot of guys will pick a medium heat
range plug and go with it. But there is a lot more to it than that, as you will see when
we get into discussing, tuning and reading the plug. Most plugs are numbered
according to their heat ranges by the manufacturers.
They are usually numbered progressively hotter, with a number three plug being
colder than a number six. It is important to understand that there is no standard for
measuring a plug’s temperature between manufacturers. So an Enya #6 is not the same
as an OS #6. You really have to look at their descriptions (“HOT”, “MEDIUM HOT”,