A vehicle will run without an alternator or a bad alternator for around 5 to 10 minutes although sometimes it may run for 30 minutes depending on the battery and the charge in the battery.
Once the battery goes dead the the vehicle will stop running.
The alternator does help fire the spark plugs by providing the spark plugs with current.
The alternator also helps to keep the battery charged and keep power running to the vehicle to continue to provide power to the engine and vehicle.
If the alternator goes out then the car will run but only for so long until the battery drains and then the vehicle will die out.
The alternator keeps voltage producing to the battery as well as the ignition system and the vehicles electrical system.
That electricity is then used to supply the vehicle with power and also send voltage to the ignition coil which then steps up the voltage from the 12 volts to the thousands of volts needed to fire the spark plugs and ignite the fuel.
The ignition coil along with the spark plugs, ignition control module and sometimes a distributor if your car has one is responsible for firing the spark plugs and igniting the fuel.
12 volt ignition coils work by increasing the voltage from the cars battery and alternator and the coil sends the high voltage to the spark plugs to ignite the fuel.
In order to produce the required high voltages the 12 volt ignition coils make use of the relationships that exist between electricity and magnetism.
When an electric current flows through an electrical conductor such as a coil of wire, it creates a magnetic field around the coil.