Glow plug (model engine)

"For the glow plugs used in automotive diesel engines, see Glowplug"A glow plug (alternatively spelled glowplug or glow-plug) is a device, similar to a spark plug, used to ignite the fuel in the very small internal combustion engines typically used in model aircraft, model cars and similar applications. The ignition is accomplished by a durable, mostly platinum, helical wire filament recessed into the plug's tip. When an electric current runs through the plug, or when exposed to the heat of the combustion chamber, the filament glows, enabling it to ignite the special fuel used by these engines. Power can be applied using a special connector attaching to the outside of the engine, and may use a rechargeable battery or DC power source.

Glow fuel (usually methanol, sometimes with nitromethane for greater power, and oil for lubrication and heat-control) burns when ignited by the hot wire of the plug. Between strokes of the engine, the wire remains hot, continuing to glow partly due to thermal inertia, but largely due to the catalytic combustion reaction of methanol remaining on the platinum filament. This keeps the filament hot, allowing it to ignite the next charge, thus sustaining the power cycle.

To start a glow plug engine, a direct current (around 3 amps and 1.25 to 2 volts, sometimes provided by an AA battery or special adapter.) is applied to the glow plug, initially heating the filament. (The name 'glow plug' comes from the fact that the plug's filament glows red hot.) The engine is then spun from the outside using a manual crank, spring-loaded motor, or hand to introduce fuel to the chamber. Once the fuel has ignited and the engine is running, the electrical connection is no longer needed and can be removed. Each combustion keeps the glow plug filament glowing red hot, allowing it to ignite the next charge, thus sustaining the power cycle.

Technically a glow plug engine is fairly similar to a diesel engine in that it uses internal heat to ignite the fuel, but since the ignition timing is not controlled by fuel injection (as in an ordinary diesel engine), or electrically (as in a spark ignition engine), it must be adjusted by changing fuel/air mixture and plug/coil design (usually through adjusting various inlets and controls on the engine itself.) A richer mixture will tend to cool the filament and so retard ignition, slowing the engine. This "configuration" can also be adjusted by using varying plug designs for a more exact thermal control.

Glow plug engines can be designed for two-cycle operation (ignition every rotation) or four-cycle operation (ignition every two rotations). The two-cycle (or two-stroke) version produces more power, but the four-cycle engines have more low-end torque, are less noisy and have a lower-pitched, more realistic sound. [ [http://www.rc-airplane-world.com/model-airplane-engines.html Model airplane engines - how they work ] ]

Considerations when using glow plugs

* High-nitro fuels produce more power than low-nitro fuels, but also produce more heat. The higher the nitro content, the colder the plug, also due to the lower methanol content.
* Depending on engine type, usage of a turbo plug may be required. For turbo engines use turbo plugs. Never install a turbo plug in a standard engine or vice versa.
* Big engines have more mass and retain heat better. Smaller, lighter engines don't, and need the help a hotter plug can offer.
* The "right" plug for an engine can change with the temperature. The hotter the day, the colder the plug.
* Hot plugs promote better idling and acceleration. If your engine runs rough or accelerates sluggishly, a hotter plug will help.
* Cold plugs produce more power and may improve performance if your engine runs hot. The downside is rougher idling and more difficulty in tuning.
* For cars: If the track/course has a lot of twists and turns, a hot plug is fine. If the track/course has long straights where you'll reach maximum rpm, a colder plug is best.
* Over-leaning an engine can harm it, by raising operating temperatures; "burn up" a plug inside it's product lifetime.
* Higher nitro means hotter fuel: needs colder plugs, and vice versa.
* If the engine sags when the battery is disconnected, the plug is too cold or more nitro is needed (or the plug is at the end of its life), and if the engine bites back or backfires when hand cranking, the plug is too hot or less nitro is needed.
* Glow plugs get very hot, enough to glow the filament red or white hot, and removing a glow plug while power is applied can cause burning if appropriate care is not taken. Special caution must be taken while near fuel sources.
* Some connectors for glow plugs can short circuit and damage batteries, or cause them to explode. Batteries may get hot during the use of a glow plug. This especially applies to home-made or nonstandard connectors.

Sources: [ [http://www.fubarhill.com/GLOW.htm All about glow plugs ] ] [ [http://www.osengines.com/accys/choosing-glowplugs.html How to Choose the Right Glow Plug ] ]

References