January 2003 77
Joe Wagner
T h e E n g i n e S h o p
212 S. Pine Ave., Ozark AL 36360
IN the prevIouS column I discussed catalysis: how some
substances (such as zinc) promote reactions between other chemicals
without being chemically affected themselves. I mentioned an
experience with a brass fuel-tank clunk weight that catalyzed glow
fuel into acetic acid (a potent rust inducer!) and told about how a diecast
zinc alloy model engine part once converted a jarful of “cleaning
solvent methanol” into acid, which then completely ate away the
casting.
However, much useful work in model engines is performed by
catalysis. Glow plugs wouldn’t work without it! Glow plugs provide
fuel-air ignition for practically all the engines—two- and fourstroke—
that power American model airplanes. They also provide
many of the difficulties we have to overcome to get our model
engines to perform the way we want. Much of that trouble is caused
by the fact that few modelers understand exactly how glow plugs
work.
The usual explanation of glow-plug operation is: “The plug
contains a coiled wire heating element which glows orange-hot when
connected to a battery. This glowing filament ignites the fuel-air
mixture in the combustion chamber when you flip the propeller. Then
after the engine starts, the battery can be disconnected because the
burning fuel keeps the plug element hot, and the engine continues
firing.”
That sounds logical. But if that was really all there was to it, glow
plugs could be made with nichrome alloy heating coils (such as the
ones toasters use) instead of expensive platinum—and glow plugs
would retail for approximately 50 cents apiece.
What actually makes a glow plug ignite a fuel-air mixture is the
catalytic action of the platinum alloy in its coiled-wire element. The
ability of platinum to initiate combustion of alcohol fuels was
discovered (by famous British chemist Sir Humphrey Davy) way
back in 1816; “automatic lighters” using platinum as an ignitor were
on sale in the 1830s—long before practical matches had been
developed.
Platinum can “start fires” without becoming a participant in the
combustion process—and even without being hot. To explain how, I
have a grossly oversimplified analogy.
Think of platinum atoms as being like tiny golf balls, with deep,
tapered dimples all over them. These strongly attract atoms of
hydrogen and oxygen. But in nestling snugly down into the platinum
atom’s “dimples,” the oxygen and hydrogen atoms wedge tightly
The first model engine glow plugs: 1949 Champion, 1947 Arden.
Both work as well in modern glow engines as most newer plugs do.
GloBee plug’s flat spiral coil gave competition engines great
performance. Its reputation made it popular with sport fliers too.
Veco’s unique “super-shielded” glow plug from 1960s seems to
offer no way for fuel-air mixture to get in and out, but it worked!
Modern Fireball plugs come in various heat classifications.
(“Hot” plugs are for use in cool-running engines and vice versa.)
against each other—so tightly that they combine. That produces heat
and initiates combustion.
As hydrogen and oxygen merge catalytically on a platinum atom
(thus becoming a molecule of H2O), the energy released by the
combustion expels the H2O from the “dimple.” That empties it to allow
new, uncombined oxygen and hydrogen atoms to enter and undergo the
same process. And none of this activity requires heating of the
platinum itself! The platinum will probably get hot from the
combustion occurring on it, but the catalytic action can initiate at
“room temperature.”
Because of that, it’s possible to start a glow engine without a
battery. True, that’s not easy, but I’ve done it more than once—not on
purpose, though!
Out to fly Control Line one day, I had a frustrating time getting my
engine going. Ordinarily it was a two-flip starter-upper, but that day it
turned balky. However, I wanted to fly and refused to give up. After
lots of fast and furious flipping—nobody used starters in those days!—
my engine eventually burst to life.
I repeated that routine two or three more times that day. Later, as I
was putting my equipment back in the car to head home, I noticed that
one of the wires to my big “doorbell-cell” glow-plug battery had
broken off at its solder lug. I’d been starting my engine with a cold
glow plug all afternoon …
Heating a glow-plug filament definitely eases engine starting! It has
other beneficial effects too. Heat drives carbon monoxide (CO) out of
platinum’s “dimples.” Molecules of CO adhere firmly to the surface of
room-temperature platinum whenever they get a chance, thus
“poisoning” its catalytic activity. But heating the material red-hot
drives off such “impurities” and restores the metal’s ability to initiate
hydrogen-oxygen combustion.
Unfortunately for us, glow-plug filaments can be “poisoned” in
other ways, where heat is of no help. One of those turned up in
connection with the all-aluminum Sportsman engines K&B made some
years ago. During break-in, wear particles from the aluminum piston
would contact the hot glow-plug element and “alloy into” the platinum.
Stock glow head for Cox .15, and another altered to accept
standard plug. Deep counterbore is needed for proper
performance.
78 M ODEL AVIATION
That reduced its melting temperature, then the plug burned out.
But even an intact and brightly glowing plug element can be
poisoned, and it isn’t such a rare occurrence. The major cause appears
to be some form of silicone compound in the glow fuel. That may be
from a “surfactant” or foam-inhibiting chemical intentionally added to
the fuel by its manufacturer, or it may have dissolved from the interior
of a modeler’s rubber squeeze-bulb fueler.
Either way: as the engine runs, a glassy coating slowly but
permanently deposits onto the platinum wire coil. That coil will still
glow as brightly as it ever did, but its catalytic ability is expiring with
every piston stroke. The only cure is a new plug, followed by
silicone-free fuel. (Some model fliers add a “dash” of Armor All to
a gallon of glow fuel to reduce fuel foaming. That’s not a lot, but I
advise against doing it anyway.)
A great variety of glow-plug designs have
been marketed since the first ones appeared
in 1947. Those were the Arden plugs, easily
identified by their ball-shaped brass tops.
(They featured replaceable elements too.)
Various coil shapes have been used
successfully; K&B once made a plug with a
W-shaped element, and for years GloBee’s
flat-spiral glow plug was a favorite among
competition fliers.
When throttled Radio Control engines
came along, so did another problem with
glow plugs. Low-throttle cruising often
allowed liquid fuel to collect in an engine’s
crankcase. After that, opening the throttle
could suddenly splash raw fuel up through
the bypass onto the plug element and quench
it.
But a fix for that problem already existed!
The Champion spark-plug people had been
making model-engine glow plugs since 1949,
and they originated the “idle bar” plug
design. Its purpose was to protect the plug
element from raw fuel squirted into the
exhaust port while starting the engine. (We
all did that back in the dear, old premuffler
days.)
Later-era glow-plug makers were quick to
adopt Champion’s “idle bar” configuration,
and different forms of that soon developed.
Fox introduced a still-popular line of glow
plugs featuring a massive machined-as-partof-
the-plug-body idle bar. (The more
common welded-on bar was known to come
off occasionally. That happened to me in
flight once, and it ruined one of my favorite
engines.)
However, the all-time ultimate “raw fuel
shield” used on a model glow plug was in
plugs Veco made in the 1960s. Those had the
entire plug inner end covered with a steel cap.
A tiny central hole in the cap provided the
only entrance for the combustion chamber’s
fuel-air mixture. It hardly seems possible that
this plug worked—but it did.
Then we have glow heads. Used primarily
in small-size model engines, their main
reason for existence (it seems to me) was to
compel users of those engines to buy
“genuine factory replacement parts” when
their glow elements failed.
Plenty of glow-head-equipped engines
survive today—but not the replacement heads
for them. To keep their faithful glow-headed
power plants perking, some of these engines’
owners have drilled and tapped their engine
heads to accept a standard (1⁄4-32) glow plug.
That’s easy enough to do, but many of the
results have been disappointing because glow
heads are rather thick. For good running
performance it’s necessary to drill, tap, and
counterbore the new hole so that the glow
element of the installed plug will end up in
exactly the same position as the original
“built-in” element.
I’ve reworked quite a few model engines
that way: Coxes, Foxes, Gilberts, even GMark
.031s. Sometimes I had to try glow
plugs of various heat ranges to optimize
performance, but each of my reworked
“glow-headers” ran at least as well with a
standard removable glow plug as it ever did
in its original configuration.
Glancing through recent model-airplane
magazines and catalogs can give someone a
strong impression that electric power has
overwhelmed piston engines as a choice for
Radio Control flying. “Quiet power” does
have its advantages and “E-technology”
keeps improving with no end yet in sight.
However, internal-combustion engines
still have significant advantages of their own
when it comes to powering model aircraft. As
for the “noise factor,” consider this quotation:
“People who make no noise are dangerous.”
(Jean de la Fontaine, 1621-1695). MA
January 2003 79