Depending on the purpose, candles have a number of design features. So, for cars and motorcycles operating in difficult road conditions, when water can get on the engine, and with increased requirements for the level of radio interference, shielded candles with a hermetically sealed high-voltage wire are used. In order to increase the stability of the candles, several electrodes are sometimes made on the body or there are no side electrodes at all (with floating electrode).
Depending on the features of the working process, candles are made with an insulator recessed inside the body or with a protruding insulator (for combustion chambers with intensive turbolization of the mixture in the combustion chamber).
The most important parameter characterizing the candle is the temperature regime of its elements, and first of all, the central electrode and the surface of the insulator skirt. The main problem in creating a spark plug for a high-speed engine is the fulfillment of conflicting requirements. On the one hand, it is necessary to ensure a sufficiently high temperature of the insulator skirt in order to prevent the formation of soot on it during long-term operation at low loads and idling, which leads to shunting, especially at low temperatures. When switching to high loads at high temperatures, the spark plug insulator self-cleansing due to the burnout of deposits on it. On the other hand, at high speeds and high load, the temperature of the central electrode and the lower part of the insulator should not exceed the upper limit at which glow ignition occurs (ignition of the mixture even before a spark from the ignition system occurs). From cycle to cycle, ignition occurs earlier and earlier, the temperature of the parts rises until something burns out - at best a candle, at worst a piston. So because of a small candle there can be big troubles.
How to prevent the possibility of the occurrence of incandescent ignition? First of all, you need to follow the correct (according to instructions) setting the ignition timing, preventing too early ignition. With a contact system, the ignition timing may suddenly increase, for example, if one of the soldered breaker contacts bounces.
The second condition is to always fill in gasoline corresponding to the level of engine boost (compression ratio, rated speed, filling, etc.). The lower the octane number, the greater the tendency to glow ignition. Well, if you still fill in low-octane gasoline, then you can’t go to high loads at high speeds.
The third condition, sometimes the most important thing, is to install candles with the required glow number in accordance with the recommendation of the engine manufacturer.
The most convenient designation of the glow number, at one time adopted by most European firms in time (in seconds), after which, during tests on a special installation, glow ignition begins. In recent years, most firms have switched to the symbol for the glow number of candles.
A few practical tips for choosing candles and their maintenance
Installing new spark plugs after a short run (about 1000 km), it is advisable to unscrew them, marking the cylinder number, and carefully examine the condition of the insulators. With the right glow number and the optimal composition of the mixture and the ignition timing, the insulator will be light brown, brown or light gray. Multi-colored deposits on the insulator and electrodes indicate the presence of a large amount of additives in the fuel or engine oil. Black, matte soot on the insulator, electrodes and body is a sign of operation in most modes on an over-enriched mixture or too high heat number. Carburetor or injection system adjustments need to be checked (e.g. gas analyzer) or install a hotter spark plug. Shiny black soot indicates that lubricant has entered the cylinder through the piston rings and valve guides. Engine repair required.
If the insulator "snow white" coloring - it means that the candle worked at the maximum allowable thermal regime. Causes: too early ignition, too lean mixture or too "hot" spark plug for this engine. It is necessary to check the carburetor settings, the characteristics of the ignition timing device and, if they are normal, put in a colder spark plug. Well, the next degree of overheating of the candle is the melted electrodes of the candle (it's good that the piston bottom did not burn out).
Sometimes it may turn out that all candles of the same brand in different cylinders have insulators of different colors. There may be several reasons. As a rule, in carbureted engines, the mixture (composition) and anti-knock additives are unevenly distributed over the cylinders.
There are engines with different compression ratios for each of the cylinders. Well, between engines of the same model, the compression ratio sometimes differs by a whole unit. Due to the deviation from technological processes, materials, etc., among candles of the same brand, there is also a large spread in actual glow numbers. With a large difference in the condition of the candles, it is sometimes necessary to install candles with different glow numbers even in the same engine. In order to always have a reserve for the glow number in various operating conditions, it is advisable to have two sets of candles and install colder ones in summer, and in winter "hot".
Recently, a number of companies have been producing candles with an extended thermal field of the candle. To do this, the central electrode is made of copper, a coating of heat-resistant materials is applied on top (nickel, chrome). Sometimes the side electrodes are made of metal with good thermal conductivity and a heat-resistant coating. Another original solution is to make a miniature platinum central electrode that does not protrude from the insulator. Extra cold spark plugs with a glow number of 300 and higher for uprated engines (turbocharged, two-stroke, etc.) usually made with a small diameter silver electrode and a short insulator skirt. Before installing such candles, it is sometimes recommended to first warm up the engine on hot candles and only then install cold ones.
It must be borne in mind that when cleaning the candle in a sandblaster, the surface of the insulator is destroyed. It is better to turn the candle over and pour solvent, gasoline into the body or use a special aerosol can. Then clean the insulator with a wooden stick and blow it with compressed air.
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