Contents: Spark plug heat rating ↳ Spark plug gap ↳
Fig. 9.4. Spark plug: 1 and 2 – threaded tips for connecting the high-voltage wire; 3 – ceramic insulator with barrier to eliminate surface leakage current; 4 – rod; 5 – spark plug housing; 6 – compression and heat shrinkage zone; 7 – central electrode; 8 – side electrode; 9 – insulator support; 10 – sealing ring
Spark plugs (Fig. 9.4) are designed to ignite a small cloud of finely dispersed fuel-air mixture with a spark, sufficient to initiate the entire ignition process in the combustion chamber. This creates a temperature of about 2500°C and a pressure of up to 60 bar. To ensure a stable spark between the spark plug electrodes, the spark plug rod is surrounded by a ceramic insulator. The central electrode and rod are inserted into a glass mass that conducts electric current, which securely fastens these parts and serves as a sealing insulation in the combustion chamber. For a spark to occur, the voltage between the spark plug electrodes must increase sharply from zero to the voltage required to form an arc. After the spark discharge occurs, the voltage drops to the level required for the spark to spread, and the fuel-air mixture ignites.
Spark plug heat rating
The most important parameter characterizing a spark plug is the temperature regime of its elements, primarily the central electrode and the surface of the insulator skirt. In order for the spark plug to work reliably, after starting the engine, it must quickly reach a temperature that ensures its self-cleaning. This temperature is approximately 400°C. Otherwise, the remains of combustion products will settle on the insulator support. At full engine load, the temperature should not exceed 800°C. In addition, the operating conditions for spark plugs are not the same in all engines. Only the heat rating determines whether the spark plug and the engine are suitable for each other. For example, if you use a spark plug with a very low heat rating, the insulator support may heat up significantly, which will lead to uncontrolled pre-ignition, which can even destroy the engine. If, on the contrary, you use a spark plug with too high a heat rating, the spark plug will not reach the self-cleaning temperature and the insulator support will become dirty.
The most convenient designation of the glow number, once adopted by most European companies, is the time (in seconds) after which glow ignition begins during testing on a special installation. In recent years, most companies have switched to the conventional designation of the glow number of spark plugs.
Spark plug gap
Fig. 9.5. The location of measurement of the interelectrode gap (X) of the spark plug between the central (2) and side (1) electrodes
In addition to the correct heat rating, it is important to know the spark plug gap, since too large or too small a gap changes the spark size, which reduces engine efficiency. The gap should always meet technical requirements and be 1.3 mm (Fig. 9.5).
When the fuel-air mixture burns, the spark plug electrodes burn slightly. This is facilitated by a spark discharge, which knocks out metal particles from the electrodes, as a result of which the interelectrode gap increases with the duration of operation of the spark plugs.
A higher voltage is required to break through an excessively large interelectrode gap. This can also lead to misfires, or the engine may not start at all. Therefore, it is necessary to check the gap in a timely manner and, if necessary, adjust it. The gap is checked with a wire template or feeler gauge, and its adjustment is made by bending the side electrode.
When replacing spark plugs, be sure to pay attention to the prescribed heat range and spark plug threads.
Spark plugs recommended for use on Fiesta vehicles are listed in Table 9.1.
PRACTICAL ADVICE
Precautions when working with the ignition system
Ignition systems are a source of increased danger. Therefore, when working with ignition systems, it is necessary to observe safety regulations. It is prohibited for people with a pacemaker to work on the ignition system. In order not to expose yourself to unnecessary danger, entrust work on the ignition system to a service center. Extreme caution must also be observed when performing maintenance work.
Never touch live elements of the primary and secondary circuits of the ignition system when the ignition is on - this is life-threatening.
Before performing any work on the ignition system, turn off the ignition first. This rule must also be followed when disconnecting and connecting electrical wires or connecting measuring instruments.
When the ignition is on, a slight rocking of the car is enough to cause a high voltage pulse. Therefore, when working in the engine compartment, you put your life at risk, in addition, the main components of the ignition system can be damaged.
When performing welding work on a vehicle, disconnect the wires from both terminals of the battery.
TECHNICAL DICTIONARY
Diagnosis of engine condition by the appearance of spark plugs
The appearance of spark plugs can be used to judge the condition of the engine, deviations in its operation from optimal parameters. The check should be carried out after warming up the car engine on a high-speed highway. Checking after a short trip can lead to erroneous results. During the check, it is necessary to carefully inspect the upper part of the thermal cone of the insulator with the central electrode and the side electrode.
Light grey or brown colour of the insulator cone -
indicates that the fuel injection and ignition systems are operating normally, ensuring economical engine operation.
White color of the insulator cone -
indicates that the engine is running on a lean fuel-air mixture or with too much ignition advance angle (early ignition), which is possible due to faulty ignition advance angle adjustment and control unit.
Large soot deposits -
are caused by the fact that the spark plug does not reach the self-cleaning temperature, i.e. the car is often used for short-distance trips, so the engine does not warm up to normal operating temperature; too rich a fuel-air mixture, which increases the carbon monoxide content.
Oil layer on the insulator cone and electrodes -
appears when piston rings, valve guides or valve stem seals are worn. You may have used motor oil or fuel with additives. Replace spark plugs, motor oil and fuel, recheck the condition of spark plugs.
If the appearance of the spark plugs does not indicate any problems with the engine, but its operation is disrupted, this may be due to defective spark plugs.
(This article is a copy from the website: FordBook.ru)