The ignition system of a spark-ignition engine produces the high voltage needed to ignite the fuel charges in the cylinders. The system must create an electric arc across the gaps at the spark plugs. These arcs must be timed so they happen exactly as each piston nears the top of its compression stroke. The heat of the arcs starts combustion and produces the engine's power stroke.
In recent years, different types of ignition systems have been developed to reduce emissions and improve engine performance，fuel economy, and dependability.
An ignition system performs the following functions：
．Provides a method of turning a spark ignition engine on and off
．Operates on various supply voltages (battery or alternator voltage)
．Produces high-voltage arcs at the spark plug electrodes to start combustion
．Distributes high-voltage pulses to each spark plug in the correct sequence
．Times the spark so that it occurs as the piston nears TDC on the compression stroke
．Varies spark timing as engine speed, load，and other conditions change
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