Ignition coils are fundamental components in gasoline engines, responsible for generating the high-voltage spark needed to ignite the air-fuel mixture within the cylinders. While modern vehicles incorporate sophisticated electronic control systems, the core principle of ignition remains rooted in technology developed over a century ago. Understanding how an ignition coil works is crucial for anyone seeking to grasp the basics of automotive engine operation and maintenance. This article will delve into the workings of ignition coils, exploring their history, underlying principles, and role in creating the spark that powers your car.
The story of the ignition coil begins with Charles Kettering’s groundbreaking invention around 1910-1911. Recognizing the limitations of earlier ignition systems, Kettering developed a coil-based system that revolutionized automotive technology. His innovation, designed for a major vehicle manufacturer, was the first electrical system to simultaneously power both the starter motor and the ignition system. This system included a battery, generator, and a more complete electrical circuit, providing a stable power source for the ignition coil.
The Kettering ignition system, illustrated in Figure 1, employed a single ignition coil to generate a high voltage. This voltage was then directed to a rotor arm within the distributor assembly. The rotor arm, acting as a rotating switch, pointed the voltage to a series of electrical contacts – one for each engine cylinder. These contacts, in turn, were connected to the spark plugs via spark plug wires, ensuring the high voltage was delivered to the correct cylinder in the engine’s firing order.
Figure 1: The main components of a Kettering ignition system
The Kettering system became the dominant ignition technology for mass-produced gasoline vehicles, holding its position until the advent of electronically switched and controlled ignition systems in the 1970s and 1980s. Even today, the fundamental principles pioneered by Kettering remain at the heart of modern ignition coil design.
To comprehend how an ignition coil produces the necessary high voltages, we must explore the relationship between electricity and magnetism. A foundational principle is that when an electric current flows through a conductor, such as a coil of wire, it generates a magnetic field around that coil. This magnetic field, more accurately termed magnetic flux, essentially stores energy that can be converted back into electrical energy.
Figure 2 visually represents this phenomenon: as electric current is switched on and flows through the coil, the current rapidly increases to its maximum level. Simultaneously, the magnetic field expands and strengthens until it reaches its maximum capacity and stabilizes along with the current. Conversely, when the electric current is switched off, the magnetic field collapses back towards the coil of wire.
The strength of this magnetic field is influenced by two key factors:
- Current Strength: Increasing the electric current flowing through the coil directly strengthens the magnetic field.
- Number of Windings: A coil with a higher number of wire windings will produce a stronger magnetic field for the same current.
The principle of electromagnetic induction is central to the operation of an ignition coil. If a coil of wire is placed within a magnetic field, and that magnetic field changes or moves relative to the coil, an electric current is induced in the wire. This is the essence of inductance.
This effect can be readily demonstrated by moving a permanent magnet near a coil of wire. The motion of the magnet, causing a change in the magnetic field around the coil, induces an electric current within the coil wire, as depicted in Figure 3.
Figure 3: A changing or moving magnetic field induces an electric current in a coil
The magnitude of the induced voltage is governed by two primary factors:
- Rate and Magnitude of Magnetic Field Change: The faster the magnetic field changes (or the speed of movement) and the greater the change in the magnetic field’s strength, the higher the induced voltage.
- Number of Windings in the Coil: A coil with more windings will experience a greater induced voltage for the same change in magnetic field.
In an ignition coil, the high voltage needed for spark plugs is generated by utilizing a collapsing magnetic field. When an electric current is used to create a magnetic field around a coil, any fluctuation in the current (increase or decrease) causes a corresponding change in the magnetic field. Critically, when the electric current is abruptly switched off, the magnetic field collapses rapidly. This collapsing magnetic field then induces an electric current back into the coil, as shown in Figure 4.
Figure 4: If an electric current used to create a magnetic field is switched off, the magnetic field collapses, which induces another electric current into the coil
Similar to how increasing the speed of a moving magnetic field increases induced voltage, a faster collapsing magnetic field will induce a higher voltage. Furthermore, increasing the number of windings in the coil will also result in a higher induced voltage.
Ignition coils utilize the principle of mutual inductance, often seen in transformers, to achieve substantial voltage amplification. Imagine two coils of wire placed in close proximity. When an electric current flows through the first coil (the primary winding), it creates a magnetic field that also surrounds the second coil (the secondary winding). When the current to the primary winding is switched off, and the magnetic field collapses, it induces a voltage in both the primary and secondary windings – this is mutual inductance, as illustrated in Figure 5.
Figure 5: The magnetic field in the primary winding also surrounds the secondary winding. Collapsing the field induces electric currents in both windings
For ignition coils, and transformers in general, the secondary winding is designed with significantly more windings than the primary winding. Consequently, when the magnetic field collapses, a much higher voltage is induced in the secondary winding compared to the primary winding (Figure 6).
Figure 6: Here, the secondary winding has more coils than the primary winding. When the magnetic field collapses, the voltage in the secondary coil will be greater than the voltage induced in the primary winding
A typical ignition coil’s primary winding might have 150 to 300 turns of wire, while the secondary winding can have 15,000 to 30,000 turns – approximately 100 times more.
The process begins when the vehicle’s 12-volt electrical system is applied to the ignition coil’s primary winding, building the magnetic field. When a spark is needed at a spark plug, the ignition system interrupts the current flow to the primary winding. This causes the magnetic field to collapse rapidly. The collapsing magnetic field induces a voltage of around 200 volts in the primary winding. However, due to the 100-fold increase in windings in the secondary coil, the voltage induced in the secondary winding is amplified to approximately 20,000 volts.
This voltage transformation, achieved through mutual inductance and the winding ratio between primary and secondary coils, allows the ignition coil to convert the vehicle’s low voltage supply into the very high voltage required to create a spark across the spark plug gap. This process of stepping up voltage is known as “transformer action.”
To further enhance efficiency, ignition coils incorporate an iron core around which both the primary and secondary windings are wrapped. The iron core concentrates and strengthens the magnetic field and flux, making the ignition coil more effective at voltage transformation.
In conclusion, ignition coils are sophisticated yet elegantly simple devices that leverage electromagnetic principles to generate the high voltage sparks essential for internal combustion engines. From the historical Kettering system to modern designs, the fundamental physics remains the same: manipulating magnetic fields to induce high voltages and initiate combustion, ultimately powering your vehicle.
