The ignition system provides a spark between the spark plug electrodes. The spark must occur at precisely the right time in the engine cycle, and it must have sufficient energy to bridge the gap and ignite the air-fuel mixture under all operating conditions.
The energy required can be obtained from the vehicle's electrical system, but the nominal battery voltage of 12 volts must be increased or "stepped up" to provide a firing voltage of many thousands of volts. This high firing voltage causes the spark gap to become electrically conductive, enabling an ignition spark to occur.
It must have sufficient heat energy to ignite the mixture so that it can continue to burn by itself. Exactly how much energy is required varies according to the condition of the mixture, and the pressure in the cylinder at the end of the compression stroke.
With an engine at normal operating temperature, and under a light load, a mixture with a ratio close to the ideal of 14.7:1 ignites readily. However, suddenly depressing the accelerator to increase speed, or to maintain speed when hill-climbing, causes cylinder pressures to rise. This increases the firing voltage needed.
The ignition system is designed to have reserve energy available in excess of its normal requirements so that it is able to produce ignition, even when conditions are unfavorable.
Most light vehicle ignition systems are of the inductive type: they use an induction coil with primary and secondary windings to "step-up" the nominal 12 volts at the battery to the required firing voltage.