Glow plug engine

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A display of vintage and modern model aircraft engines, of both spark ignition and glow plug ignition types

A glow plug (alternatively spelled glowplug or glow-plug) is a device, similar to a spark plug, used to help ignite the fuel in the very small internal combustion engines typically used in, model cars and similar applications. The ignition is accomplished by a combination of heating from compression and heating from the glow plug. The glow plug is 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 help 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 generally consists of methanol with varying degrees of nitromethane content as an oxidizer for greater power, generally between 5% and 30% of the total blend. These volatiles are suspended in a base oil of castor oil, synthetic oil or a blend of both for lubrication and heat control, again, in varying degrees of overall content. The lubrication system is a "total loss" type, meaning that the oil is expelled from the exhaust after circulating through the engine. The fuel ignites when it comes in contact with the heating element of the glow 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.

Some aircraft engines are designed to run on fuel with no nitromethane content whatsoever. Glow fuel of this type is referred to as "FAI fuel" after the aeronautical governing body of the same name.

To start a glow engine, a direct current (around 3 amps and 1.25 to 2 volts, often provided by a single, high current capacity rechargeable NiCd, NiMH or lead-acid battery cell, or a purpose-built "power panel" running on a 12VDC source) 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, built-in rope-based recoil starter, spring-loaded motor or purpose-built electric motor, or by 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.

Lead-acid battery cells that are used to ignite a model engine glow plug, due to their two volt output when freshly charged, usually cause a regular 1.5 volt glow plug to burn out instantaneously, and either a resistor of the proper value and wattage, or a high-power germanium transistor's base/emitter junction (in a series connection with one of the plug's terminals) can reduce the lead-acid cell's voltage to a suitable 1.5 volt level for engine starting.

Technically a glow plug engine is fairly similar to a diesel engine and hot bulb 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. Of all internal combustion engine types, the glow plug engine resembles most the hot bulb engine, since on both types the ignition occurs due to a "hot spot" within the engine combustion chamber.

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.[1]

Considerations when using glow plugs

  • 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 an engine runs rough or accelerates sluggishly, a hotter plug will help.
  • Cold plugs produce more power and may improve performance if an 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; "burning up" a plug inside its 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.
  • Glow plugs have a limited lifespan. Always keep three or four of them on hand.
  • Glow Plugs do not need to be very tight. Just seat them, then another 1/4 turn.

Technical specs

Turbo Glow Plug
  • Overall Length: 17mm (.67")
  • Diameter: .35" (9mm)
  • Thread size: M8x.75mm[2]
Normal Glow Plug
  • Length: .8"
  • Diameter: .35"
  • Threads: 1/4-32 UNEF[3]