4.8: Design Example: Automobile Ignition System
The automobile's ignition system plays a vital role by ensuring the timely ignition of the fuel-air mixture in each cylinder. This ignition is facilitated by a spark plug, which is composed of two electrodes separated by an air gap. A spark forms across this air gap when a substantial voltage is generated between the electrodes, leading to the ignition of the fuel.
One can generate a large voltage using a car battery of 12 volts with the help of inductors. Inductors are known for opposing rapid changes in current, which make them the perfect tool for generating sparks.
In the ignition system of an automobile, a specific type of inductor, referred to as the spark coil, is employed. By creating a significant change in current over a brief period, the voltage across the inductor can be amplified. When the ignition switch is engaged, the current passing through the inductor gradually increases until it reaches a stable state. At this juncture, the rate of current change and the voltage of the inductor are both zero.
However, when the switch is suddenly disengaged, a high voltage is created across the inductor due to the rapidly collapsing magnetic field. This results in a spark or arc in the air gap. The spark persists until all the energy stored in the inductor is exhausted in the spark discharge.
This can be illustrated with an example of designing an automobile ignition system. Assuming that the system's spark coil has an inductance of 20-mH and a resistance, and is supplied with a voltage of 12 V. The task at hand is to calculate the time required for the coil to charge completely, the energy stored in the coil, and the voltage generated at the spark gap if the switch opens in 2 microseconds.
After conducting the necessary calculations, the conclusion drawn is that the coil will be fully charged in 20 milliseconds, will store an energy of 57.6 millijoules, and will generate a voltage of 24 kilovolts at the spark gap when the switch is opened. This high voltage is responsible for creating the spark that ignites the fuel-air mixture in the cylinder, thereby powering the engine of a car.
