What Causes Spark Plugs to Spark?

[fourthindiana]

What makes the spark plugs in an automobile engine or any other type of gasoline engine spark?

 

[phyzguy]

What makes the spark plugs in an automobile engine or any other type of gasoline engine spark?

The coil generates a large voltage (over 10,000 volts), which is applied across the spark plug gap. This voltage exceeds the breakdown voltage of the air in the gap, so the air breaks down. The breakdown separates some of the neutral gas atoms in the air in the gap into electrons and ions. A large current is carried by the electrons and ions in the gap, which is what you see (and hear) as the spark.

 

[fourthindiana]

The coil generates a large voltage (over 10,000 volts), which is applied across the spark plug gap. This voltage exceeds the breakdown voltage of the air in the gap, so the air breaks down. The breakdown separates some of the neutral gas atoms in the air in the gap into electrons and ions. A large current is carried by the electrons and ions in the gap, which is what you see (and hear) as the spark.

What does "the air breaks down" mean? Do you mean that the molecules in the air such as nitrogen and oxygen split up into individual atoms?

 

[AZFIREBALL]

It is exactly like lightning but, inside the engine.

 

[phyzguy]

What does "the air breaks down" mean? Do you mean that the molecules in the air such as nitrogen and oxygen split up into individual atoms?

More than this. Some of the atoms of oxygen and nitrogen get dissociated into electrons and ions, so the air which is normally an insulator becomes conductive. Try reading this Wikipedia description.

 

[Ranger Mike]

i n the good old days of carburators and ignition points, the 12 volt input was transformed to 25,000 volts required to jump the 0.025" spark plug gap. This is the ionization voltage. At idle you only need about 10,000 volts to bridge the gap but more volts as RPM climbs. So 25,000 volt dc at 3000 to 5500 rpm is typical.
You have voltage seeking a ground. Once enough voltage is produced to over come the resistance of fuel / air mixture in the spark plug gap and the arc is made we have ignition of the fuel air mixture. When the air gap grows due to eroding of material, higher voltage is required.

You could only get about 30,000 miles out to the spark plugs before the gap grows so wide the spark has a tuff time jumping it. Later on the CD ignition came along so you had 60,000 volts jumping a .060" spark plug gap and could get 60,000 out of the plugs. Now with new materials like platinum spark plugs and computer ignitions the system is even more fine tuned to where you have an ignition coil at each cylinder and you can go 100,000 miles on a set of spark plugs. The chemistry thing in the combustion chamber is a whole other big thing so won't dwell ( pun) on it here.