The composition and working principle of the ignition coil

The ignition coil is actually a transformer made by the principle of electromagnetic induction, which turns the 6 volt low-voltage current output by the battery or magneto into a high-voltage current of 11,000~15,000 volts. The ignition coil is mainly composed of low-voltage coil (also known as primary coil), high-voltage coil (also known as secondary coil), iron core, etc. The low-voltage coil is wound with thicker copper enameled wire with a small number of turns, and one end of the coil is connected to the power supply and the other end is connected to the circuit switch (relay). The high-voltage coil is wound with a thin copper enameled wire with a large number of turns, and one end of the coil is connected to the spark plug and the other end is connected to the body iron.

The working principle of the ignition coil is that when the relay contact is closed, the low-voltage circuit is turned on, and the current magnetizes the core through the low-voltage coil, and a magnetic field is generated around the low-voltage coil. When the relay contact is jacked open by the cam, the low-voltage circuit is cut off and the current in the low-voltage coil disappears. At this time, the iron core is demagnetized, and the magnetic field lines immediately shrink and cut the high-voltage coil, so that the high-voltage coil generates an extremely high voltage induced current. This high-voltage current is delivered by the high-voltage line to the spark plug to generate a jump, which ignites the combustible mixture in the combustion chamber to burn. On the other hand, when the magnetic field wire cuts the high-voltage coil, it also cuts the low-voltage coil, so that the low-voltage coil also produces a self-inductive voltage, with a voltage of 200~300 volts. The direction of this self-inductive voltage is the same as the direction of the original current, which will hinder the rapid disappearance of the original current, make the magnetic field contract slowly, reduce the induced voltage of the high-voltage coil, so that the spark plug can not jump fire, and make a strong spark form between the relay contacts, causing the contacts to burn out quickly.

Therefore, in order to eliminate the harmful effect of the self-inductance voltage, a capacitor is connected in parallel at the low-voltage coil connection. Because the capacitor has the function of capacitance and discharge, when the relay point is pushed open, the self-inductive voltage of the low-voltage coil charges the capacitor, and this part of the electrical energy is stored by the capacitor without forming a strong spark at the relay contact, protecting the relay contact. After the current in the low-voltage coil disappears, the charged capacitor is discharged in the opposite direction through the low-voltage coil, which also causes the original magnetism to shrink rapidly, thereby increasing the induced voltage of the high-voltage coil.

The low-voltage coil and the high-voltage coil are wound on the core, and in order to enhance the magnetic conductivity, a shingled magnetic permeability sheet is installed on the periphery of the two coils, and then loaded into an iron shell. The iron shell has a plastic insulating cover equipped with a stud that serves as a guide to the low- and high-voltage coils. The terminal posts of the high voltage coil are trapped in the center, and this terminal post is surrounded by raised insulating plastic, which prevents spark discharge between the high voltage and low voltage coil terminal posts. In order to strengthen the insulation and avoid moisture in the ignition coil, in addition to the coil itself impregnated with rosin paraffin or oil, the shell of the coil is also filled with asphalt with a melting point of 145~160 °C, filling all the space and pores.

The capacitor is mounted on the housing of the ignition coil, which is separated by two layers of tin foil and two layers of wax paper, rolled into a cylinder, and packed in a metal case. One layer of tin foil of the capacitor is connected to the power supply by a wire from the cover of the metal case.