Glow plug...What is it? Let's check out Webster's Dictionary:
Main Entry: glow plug
Date: circa 1941
: a heating element in a diesel-engine cylinder to preheat the air and facilitate starting; also : a similar element for ignition in other internal combustion engines
Basically, the glow plug is used to provide ignition for the fuel/air mixture in an engine...in a similar manner to a spark plug in a gasoline engine.
So how does it work? To start the motor we must first heat up the element (the small coil of wire inside the plug) by connecting it to a battery. Most plugs use 1.5 volts to pre-heat the coil. Once the motor is running the battery can be disconnected and the plug will continue to glow with an orange heat to provide ignition and keep the engine running.
How does it do this without a battery? It all hinges on the fuel we use (or part of the fuel) and what the element is made from. The working part of the fuel is methanol, a type of alcohol (but not the drinking kind) which is quite poisonous. The element is made from several metals alloyed together to make it strong enough to handle the heat and vibration induced from engine operation. The specific metal in this alloy that we are interested in is platinum. When platinum comes in contact with alcohol there is a catalytic reaction between the two which heats the platinum while causing the alcohol to ignite.
But if the element is glowing the entire time the motor is running, how does the fuel know when to start burning (in other words, what determines the ignition point)?
Well this goes back to the catalytic reaction again. This reaction depends on two things, one is the temperature of the element (the hotter it is the easier it will react) and the pressure of the fuel/air mixture inside the cylinder (the higher the pressure the easier it will react).
So is this where "hot" and "cold" plugs come into play? Exactly. Glow plug temperature is controlled by using different heat range plugs. Just like automobile spark plugs, glow plugs come in different heat ranges from hot to cold with maybe half a dozen steps in between. If in doubt, use the plug specified by the engine manufacturer. Using a hotter plug than normal will advance the ignition point and a colder plug will retard the ignition.
You talked about pressure of the fuel/air mixture...what the heck was that all about? The pressure of the air/fuel mixture is determined by the compression ratio of the engine and normally is fixed by the manufacturer. You can vary this compression ratio slightly by adding or removing shims under the cylinder head. This is not something to play with unless you know what you are doing.
Ok, so can I run any glow plug with any fuel? Not really. There are a couple of plugs that are said to be for general use (O.S. #8 and #0), but the more nitro you run, the more you advance the ignition point. What's that mean? In an ideal situation the glow plug will ignite the air fuel mixture when the piston is at top dead center (TDC), which will force the piston down and back up for another compression stroke. When you run a higher nitro percentage (and don't go to a colder plug) you'll advance your ignition point (the point at which the glow plug ignites the air/fuel mixture), which will result is less than optimum performance since the piston is still on it's compression stroke when the air/fuel is ignited and not at TDC.
The general rule regarding glow plugs and nitro fuel is:
The higher the nitro percentage, the colder the plug should be.
The lower the nitro percentage, the hotter the plug should be.
OK, cool enough. Where can I find info on these plugs?
Glad you asked. Check out this list of info on McCoy's Glow plugs, with heat ranges and applications.
MC-4C: 4-cycle engines
MC-55: The MC-55 is considered a hot plug and is designed to run on fuel with a low nitro content. Applications for this plug would be planes, tether cars, etc.
MC-59: Like the MC-55, the MC-59 is considered a hot plug. The difference between the 55 and 59 is that the MC-59 is designed to run on fuel with a higher nitro content (low to medium nitro content) than the MC-55. Applications: Boats, planes, heilcopters, and cars.
MC-8: The MC-8 is a medium to cold plug, deisgned to run on fuels with a medium to high nitro content. Applications: Boats, cars, and planes.
MC-9: The McCoy MC-9 is considered a cold plug and is designed to run on fuels with a medium to high nitro content. Applications: Cars, boats, ducted fan and Giant Scale planes.
MC-14: The MC-14 is a hot plug, designed for helicopters and 4-stroke engines.
From reading just these bits of info on McCoy's plugs, you can easily see that general rule...if you're running fuel with a high nitro percentage, you should be using a glow plug that's considered to be a "cold" plug. Conversely, if you're running a low nitro content in your fuel, you should be using a "hot" plug.
The following are lists of OS and Novarossi plugs. These plugs follow the same nitro/heat range rules as demonstrated with the McCoy's plugs.
OSMG6300- #0 Economy standard plug similar to the O.S. #8.
OSMG6304- #1 Hot plug for low nitro (0-5%).
OSMG6308- #3 Medium plug for medium nitro (5-25%).
OSMG6312- #5 Cold plug for high nitro (25% +).
OSMG6316- #7 2 stroke engines requiring an idle bar.
OSMG2691- #8 "The" standard. For use in all 2 strokes, any nitro.
OSMG6320- #9 For general use in 2 stroke engines.
OSMG2692- #F For O.S and other four strokes.
10001 R1 Extra hot from .8 TO 2CC
10002 R2 Hot from2 TO 3.5CC
10003 R3 Medium from 3.5 TO 6CC
10004 R4 Cold from 6 TO 10CC
10005 R5 Extra cold for nitro fuel and RC
10006 R6 Cold for nitro from 10 - 13CC
10007 R7 Cold for nitro from 13 - 15CC
10008 R8 Super Cold for nitro from 15-30CC
10010 RC Hot for RC from 2.5 TO 6CC
10020 RC Cold for RC from 6 TO 15CC
10011 G1 Hot for R15 Speed
10022 G2 Medium for R15 Speed
10033 G3 Cold for nitro fuel from 18%-30% for R15 Speed.
10044 G4 Extra cold for nitro fuel from 30%-15% for R15 Speed
10055 G5 Extra cold for nitro fuel from 50%-70% for R15 Speed.
To quote Cartman from South Park:
"So you see, Simon and Simon were not brothers in real life, only on television."
Actually, I hope that this helps you...there's more to glow plugs than telling the guy at the hobby shop "I need a plug for a .21". Don't just buy a plug, buy the right plug.