Spark Plug Construction.
All internal combustion gasoline engines have one thing in common; they use spark plug. The spark plug is made up of three major parts: the electrodes, insulator, and shell.
The typical modern spark plug may be expected to last from 15,000-30,000 miles (24 000-48 000 Km) Spark plug can last as long as 100,000 miles (160 000 Km). To accomplish this, the spark electrodes must be constructed of material that will be resistant to heat, oxidization, and burning. Typical materials used to make spark plug center electrodes include copper and nickel alloy . Platinum, although expensive, is sometimes used. The center electrode is insulated from the rest of the spark by a ceramic insulator. The top terminates in a push-on terminal to which the plug wire may be attached.
In a modern spark plug there are two electrodes, the space between the two is called the plug gap. On older vehicles, the spark plug gap may be as small as .025`` (0.63 mm). The spark plugs in newer vehicles may require gaps of as much as .045`` to .065`` (1.1 to 1.65 mm). Always check the service manual for the exact plug gap.
Electrons will flow more easily from a hot surface to a cooler one. As the center electrode of a spark plug is always Hotter than its side electrode, the plug`s center electrode has negative polarity. This ensures a better flow of current from the center electrode. Air is more easily ionized (broken down from a nonconductor to a conductor) near the hotter center electrode. As a result, less voltage is needed to create a spark which lightens the load on the coil.
In distributorless ignition systems, the waste spark travels from the side electrode to the center electrode, since the cylinder is not under compression, an extra load is not placed on the coil. However, reverse firing can cause electrode damage to certain types of spark plug. The technician must always make sure that the replacement plugs are designed for use in a distributorless system.
Spark Plug insulators.
Spark plug insulators must have special properties. They must heat, cold, chemical corrosion, and sudden voltage changes. Insulators must also be resistant to vibration and physical shock. A common materials used for making spark plug insulators is aluminum oxide. The aluminum oxide is fired at high temperature to produce a glassy smooth, dense, and very hard insulator. The length, diameter, and location of the insulator ha a direct bearing on the heat range of the particular plug. The top end of the insulator is often ribbed, or grooved, to prevent flash-over.
Spark Plug Shells.
The center electrode, surrounded by the insulator, is placed in a steel shell. The steel shell is generally crimped insulator tightly and also forms a pressure seal at both the top and bottom of the insulator. This is prevents combustion leaks and arcing.
The side electrode is welded to the steel shell. The shell is threaded so it will screw into a threaded hole in the cylinder head. The shell`s threaded area will vary in length and thread pattern to conform to the cylinder head in which it is installed. The shell forms a seal with the head by means of a cooper or aluminum gasket, or by a beveled edge that wedges against a similar bevel in the cylinder head. The thread seal is important since this is an area through which a great deal of the plug`s heat is transferred to the head metal. The insulator is subjected to tremendous temperatures. In order to prevent burning, it must get rid of surplus heat.
Spark plug thread seal. Not all spark plug us a gasket for
sealing purpose.
Spark Plug Heat Range.
Spark plug are designed to operate at a certain temperature, neither too hot nor cold. If a plug runs too cold, it will contact deposits and become fouler. If it runs too hot (much above 1,700 degree F or 968 degree C), it will cause preignition from hot carbon deposits ignition the fuel charge before the plug fires. It is important that the plug extend into the combustion chamber just the right amount. The heat range is determined by the diameter and length of the insulator as measured from the sealing ring down to the tip.
Notice in the next figure that the heat generated in the insulator must travel up until it can escape through the seal to the shell and then to the head. The longer and thinner the insulator tip, the less efficiently it can transfer heat. As a result, it will run hotter (hot plug). The short, heavy insulator carries heat well and will operate much
cooler (cold plug). All engine manufacturers specify heat engines for the plugs to be used in their engines.
A--Cold-to-hot spark plug heat range.
B--A cold type, short heat path and a hot type,
long heat path.
Projected Nose Plugs.
One spark plug design extends the electrode tip and insulator further down into the combustion chamber. This projected nose places the insulator more directly in the path of the incoming fuel charge, Which cools the tip. The projected nose plug, due to its long tip, tends to run hot at low engines speeds because the incoming fuel charge is moving slowly and provides little cooling effect. This prevents low speed fouling and assists in producing a park without voltage from the coil.
Projected nose core plug.
At high engine speeds, the insulator temperature is kept from exceeding acceptable limits by the rapid flow of the incoming fuel mixture. The washing effect of the ho escaping exhaust trends to keep the plug clean. The projected nose plug will perform satisfactory within acceptable temperature limits over a wider vehicle speed and load range.
Speed/temperature range performance.
Resistor Spark Plugs.
The spark plug electrodes is delivered in two stages. The voltage at the plug`s center electrode will rise rapidly until the voltage is sufficient to ionize the gab and cause the plug to fire. This is the first stage. The second stage is longer and follows the first. It is produced by the remaining residual voltage in the coil.
The combustion process takes place during the first stage. The second stage causes radio interference and can cause the plug electrodes to wear. To shorten the second stage, a resistor of around 100,000 ohms is placed in the ceramic insulator. The resistor shortens both the stages. It does not require any higher voltage and will lengthen electrode life as well as suppress ratio interference. A resistor plug is shown in figure illustrates the difference between the spark produced by a standard plug and a resistor type plug. No additional voltage is required for the resistor plug. You have seen how the necessary voltage is produced, how this voltage is delivered to each plug at the proper time will now be covered.
A cutaway view of one type of a resistor spark plug.
Spark plug discharge graph. Notice how the resistor plug reduces
undesirable inductive portion of spark .
Distributor Cap.
High voltage from the coil is carried by an insulated wire to the center terminal of a distributor cap. On some vehicles, the coil is installed in the distributor cap. Addition terminals, one per cylinder, will be arranged in a circle around the center terminal. Each one of these will have a heavily insulated wire connected it wirh a spark plug. The distributor cap is made of a plastic material, sometimes mica-filled to reduce flash-over tendency. The materials used must provide excellent insulation.
A distributor cap for A four-cylinder engine.
On the inside of the cap-like cap, the plug terminals have metal (aluminum, brass, etc.) lugs extended down past the cap material. Note the fixed carbon rod used in the distributor cap contains the coil terminal and a small metal connector or brush which forms the connection between the coil and rotor.
A cross-section view of distribution cap. Note the carbon rod.
Rotor.
To carry the secondary voltage from the center cab terminal to any one side terminal requires what is known as a rotor. In cases where the coil is installed in the distributor cap, voltage is discharged directly to the rotor through a metal conductor in the cap. The rotor body is made of an insulating material, usually plastic, it is attached to the top of the distributor shaft. Its outer age has a brass terminal that passes, as it turns, very close to side terminals. This outer edge terminal is connected to a spring that rubs against the center coil terminal carbon post.
When current from the coil arrives at the center terminal, it travels down the carbon rod, though the spring, and out to the rotor`s outer age terminal. From this point, it jumps the small gap between the rotor and side terminal and goes on to the plug. Jumping the gap between the rotor and distributor cap terminal requires about 3000 extra volts, well within the coil`s capacity.
Cutaway view of distributor cap and rotor. The rotor transfers current
from the center terminal to the cuter terminal.
Spark plug wires.
The spark plug wires carry the high voltage current from the distributor cap to the spark plugs. Spark plugs wires are converted with a thick layer of insulation to protect the wire and reduce the possibility of arcing. Original equipment plus wires contain a built-in resistance to reduce ratio interference. The plug wires are arranged in the distributor cap in that order. Direction of rotor rotation dictates whether a clockwise or counter clockwise order is required.
The cap, rotor and wires distribute the spark to each plug at the proper time. It is essential that the plug wires be arranged in the proper sequence. The firing sequence is never used. An example of a typical firing order (order in which pistons reach TDC on the compression stroke in a six-cylinder engine is 1, 6, 5, 2, 4, 3
A simplified firing order for a six-cylinder engine 1, 2, 3, 4, 5, 6.
Notice how the plug wires are arranged in the distributor cap to
produce this firing order.
