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Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression ignition engines may use glow plugs to improve cold start characteristics.Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.The spark plug is connected to thousands of volts generated by the ignition coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. An ionized gas becomes a conductor and an ionized gas can pass electrons.
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