A glow plug engine, or glow engine, is a type of small internal combustion engine [1] typically used in model aircraft, model cars and similar applications. The ignition is accomplished by a combination of heating from compression, heating from a glow plug and the catalytic effect of the platinum within the glow plug on the methanol within the fuel.
German inventor Ray Arden invented the first glow plug for model engines in 1947. [2]
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The glow plugs used in model engines are significantly different from those used in full-size diesel engines. In full-size engines, the glow plug is used only for starting. In model engines, the glow plug is an integral part of the ignition system because of the catalytic effect of the platinum wire. 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.
There are three types/shapes (at least) of glow plugs. The standard glow plug, which comes in long/standard and short (for smaller engines), in both open and idle-bar configurations, has a threaded tube that penetrates the combustion chamber to varying degrees. Due to the small size of the combustion chamber changing brands or styles of standard glow plug can affect the compression ratio. Turbo style (European/metric) and Nelson style (North American/English) glow plugs do not penetrate the combustion chamber. Instead they have an angled shoulder that seals against a matching surface at the bottom of the glow plug hole. As a Turbo or Nelson plug is installed and seals the combustion chamber, they create a smooth surface inside the head. This smooth surface is very desirable for high-performance application such as Control Line Speed events and also high-revving RC Cars. The design of Turbo/Nelson plugs allow switching between brands without the possibility of affecting compression. Turbo and Nelson plugs are not interchangeable as they have different threads and dimensions.
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. 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, which requires such fuel in some competitions.
To start a glow engine, a direct current of around 3 amps and 1.5 volts is applied to the plug from a "glow plug igniter" or "glow driver", powered by a high current single cell rechargeable battery, or a purpose-built "power panel" running on a 12VDC source. [3] The current heats the platinum filament, causing it to glow red hot, hence the name. 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 hot, which along with the catalysis of methanol oxidation by the platinum, allows the ignition of the next charge in a self-sustaining power cycle. [4] [3]
The rechargeable battery may be of NiMH, NiCD, Li-ion, or lead-acid type. The higher fully-charged voltages of lead-acid (2.0) and Li-ion (4.2) cells, if applied directly to a regular 1.5 volt glow plug, will cause it to burn out instantaneously, so 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 be used to limit the current through the plug to an appropriate level. Even with an appropriate power input, glow plugs can burn out at any time, and hobbyists are encouraged to carry spares. [5]
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. A leaner mixture produces more power, but the engine is less well lubricated, which can cause overheating and detonation. 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 most resembles 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). [4] 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. [5]
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A glow plug engine must be operated with the correct glow plug temperature. Large engines can operate with lower temperatures, while smaller engines radiate heat to the air more quickly and require a hotter glow plug to maintain the correct temperature for ignition. The ambient temperature also dictates the best glow plug temperature; in cold weather, hotter plugs are needed. Since glow plug engines are air-cooled, an engine that "runs hot" can sometimes benefit from a lower plug temperature, although this may cause rougher idling and difficulty in tuning. The operating speed of the engine must also be considered; if the engine is to run at consistently high RPM, such as with an airplane or a car on a mostly straight track, a lower plug temperature is more efficient. If the engine is to operate at lower RPM, combustion will not heat the engine as much, and a hotter plug is required.
The fuel type and the fuel/air mixture must also be considered. The greater the nitromethane content in the fuel, the hotter the fuel will burn; high "nitro" fuels require cooler glow plugs. Lean mixtures (low fuel-to-air ratio) burn hotter than rich mixtures (higher fuel-to-air ratio) and operating temperatures can be raised to levels that can prematurely destroy the glow plug if too lean a mixture is used ("over-leaning").
If the engine slows down ("sags") when the battery power is removed, the plug temperature or the nitromethane content of the fuel should be increased, as the engine is not sufficiently hot. If the engine backfires when it is hand-cranked, it is operating too hot and the glow plug temperature or "nitro" content should be lowered.
Glow plugs have a limited lifetime and users are advised to have several replacement plugs on hand. Replacement plugs must be the correct type; plugs for turbo engines are not compatible with plugs for standard engines. The plugs should be tightened a quarter-turn past a snug fit to avoid over-tightening. Glow plugs, like all incandescent objects, are extremely hot, and glow plugs should never be removed when hot. Likewise, care must be taken when fueling because a hot glow plug can ignite fuel. Overheating of the battery can also be dangerous and only well-made connectors should be used.
The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected. This is in contrast to igniting the fuel-air mixture with a spark plug as in the Otto cycle (four-stroke/petrol) engine. Diesel engines are used in aircraft, automobiles, power generation, diesel–electric locomotives, and both surface ships and submarines.
A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a ceramic insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. The central electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the side, earth, or ground electrode(s).
A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.
A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
In spark-ignition internal combustion engines, knocking occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel–air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
Nitromethane, sometimes shortened to simply "nitro", is an organic compound with the chemical formula CH
3NO
2. It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and as a cleaning solvent. As an intermediate in organic synthesis, it is used widely in the manufacture of pesticides, explosives, fibers, and coatings. Nitromethane is used as a fuel additive in various motorsports and hobbies, e.g. Top Fuel drag racing and miniature internal combustion engines in radio control, control line and free flight model aircraft.
Top Fuel is a type of drag racing whose dragsters are the quickest accelerating racing cars in the world and the fastest sanctioned category of drag racing, with the fastest competitors reaching speeds of 338 miles per hour (544.0 km/h) and finishing the 1,000 foot (304.8 m) runs in 3.62 seconds.
Pre-ignition in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.
Methanol fuel is an alternative biofuel for internal combustion and other engines, either in combination with gasoline or independently. Methanol (CH3OH) is less expensive to sustainably produce than ethanol fuel, although it is more toxic than ethanol and has a lower energy density than gasoline. Methanol is safer for the environment than gasoline, is an anti-freeze agent, prevents dirt and grime buildup within the engine, has a higher ignition temperature, and produces horsepower equivalent to that of super high-octane gasoline. It can readily be used in most modern engines. To prevent vapor lock due to being a simple, pure fuel, a small percentage of other fuel or certain additives can be included. Methanol may be made from fossil fuels or renewable resources, in particular natural gas and coal, or biomass respectively. In the case of the latter, it can be synthesized from CO2 (carbon dioxide) and hydrogen. The vast majority of methanol produced globally is currently made with gas and coal. However, projects, investments, and the production of green-methanol has risen steadily into 2023. Methanol fuel is currently used by racing cars in many countries and has seen increasing adoption by the maritime industry.
Dieseling or engine run-on is a condition that can occur in spark-plug-ignited, gasoline-powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, drawing fuel through the carburetor, into the engine and igniting it without a spark.
Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction produces heat that can be transformed into work in a heat engine.
A nitro engine generally refers to an engine powered with a fuel that contains some portion of nitromethane mixed with methanol. Nitromethane is a highly combustible substance that is generally only used in very specifically designed engines found in Top Fuel drag racing and in miniature internal combustion engines in radio control, control line and free flight model aircraft.
A spark-ignition engine is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engines, typically diesel engines, where the heat generated from compression together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.
The hot-bulb engine, also known as a semi-diesel, is a type of internal combustion engine in which fuel ignites by coming in contact with a red-hot metal surface inside a bulb, followed by the introduction of air (oxygen) compressed into the hot-bulb chamber by the rising piston. There is some ignition when the fuel is introduced, but it quickly uses up the available oxygen in the bulb. Vigorous ignition takes place only when sufficient oxygen is supplied to the hot-bulb chamber on the compression stroke of the engine.
In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
A model engine is a small internal combustion engine typically used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight, control line aircraft, or ground-running tether car model.
Glow fuel is a fuel source used in model engines – generally the same or similar fuels can be used in model airplanes, helicopters, cars and boats. Glow fuel can be burned by very simple two-stroke engines or by more complicated four-stroke engines, and these engines can provide impressive amounts of power for their very small size. Glow fuel is primarily for two-stroke engines with the need for oil mixed in the fuel and limited exhaust and fuel/air between cycles. Top Fuel race cars with 4-stroke engines may also use glow fuel, but in this case it does not contain appreciable oil.
In a diesel engine, a glow plug is a heating device used to aid starting of the engine in cold weather. This device is a pencil-shaped piece of metal with an electric heating element at the tip.
A carbureted compression ignition model engine, popularly known as a model diesel engine, is a simple compression ignition engine made for model propulsion, usually model aircraft but also model boats. These are quite similar to the typical glow-plug engine that runs on a mixture of methanol-based fuels with a hot wire filament to provide ignition. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engines.
A cold start is an attempt to start a vehicle's engine when ambient temperatures are much lower than its normal operating temperature. A cold start can be difficult for an engine due to higher viscosity of oil and fuel in cold temperatures.
