How Cars Work · The Spark

Where the spark actually happens

At the end of the chain is the spark plug. We met it briefly in Chapter 1; here is what is actually inside it.

A spark plug is a small ceramic-and-metal device, about the size and shape of a thick crayon, that screws into the top of each cylinder. The bottom end pokes down into the cylinder. The top end sticks out where the wire from the distributor can connect to it. Inside the spark plug, the wire runs down through a ceramic insulator and ends in a small metal post called the electrode. Right next to the electrode, hooked to the body of the plug (which is in turn screwed into the metal of the engine), is a second metal post. There is a tiny gap between them, about the width of a paperclip wire.

When the twenty-thousand-volt pulse arrives, it jumps across that gap, through the gasoline-and-air mist, and ignites it.

That is the spark.

Try It: The Balloon and the Doorknob

You will need:

• A latex balloon, blown up and tied

• A clean head of hair (yours)

• A metal doorknob

• A dark or dim room

Rub the balloon vigorously back and forth on top of your head for ten or fifteen seconds. Your hair will probably try to follow the balloon when you pull it away. That is a clue. You have moved electrons from your hair onto the balloon. The balloon is now slightly negatively charged. Your hair is slightly positively charged.

In a dark room, slowly bring the charged balloon close to a metal doorknob — but not quite touching. Watch the gap.

If you get close enough, you will see a tiny spark and hear a faint snap. The electrons crowded onto the balloon, pushed by the voltage of being concentrated all in one place, found a path through the air to the metal of the doorknob. Air briefly stopped insulating and started conducting.

That is the same kind of event that happens between the two electrodes of a spark plug, scaled differently. The spark plug uses an ignition coil to build the voltage. You used your hair. The physics is identical: enough voltage, narrow enough gap, air gives up, lightning happens.

Did You Know? A lightning bolt is the same kind of thing that happens between the two electrodes of a spark plug. Charge builds up — in lightning’s case, between a cloud and the ground, or between two clouds — until the voltage is so high that air briefly conducts electricity. The spark from a spark plug is about a quarter of an inch long. A lightning bolt is several miles long. The voltage scales accordingly: about twenty thousand volts for the spark plug, around a hundred million volts for the lightning. Same physics, very different size. Both look about as bright if you stare directly at them, but please don’t stare directly at lightning.

The whole chain, in one breath

The battery sends twelve volts through the points and into the primary coil. A magnetic field builds up around the iron rod in the middle. At the right instant in each cylinder’s cycle, the engine pushes the points apart, the current stops, the magnetic field collapses, and twenty thousand volts springs out the other end of the secondary. That pulse travels down the center wire of the distributor, jumps to the rotor, and is passed to whichever spark plug the rotor happens to be pointing at. The pulse runs down the spark plug wire, into the spark plug, jumps the tiny gap inside the cylinder, and ignites the mist.

BANG.

Two strokes left in the cycle. The piston is now flying down toward the bottom of the cylinder, and somehow this turns into the wheels rolling under your seat. How? That is Chapter 4.

All 13 chapters of How Cars Work · Library · All courses