How Cars Work
The Spark
CHAPTER THREE
We left the cylinder full of mist. The piston has just finished the SQUEEZE stroke — the gasoline-and-air mixture is compressed into a tiny space at the top of the cylinder, hot from the squeezing, ready to burn.
It still needs a kick. Without that kick, the mixture will just sit there, hot but not burning, and the engine won’t run. From last chapter you already know what the kick is: a spark. A small electric spark, jumping across a gap inside the cylinder, at exactly the right millisecond.
In a fast-running engine, this happens in each cylinder somewhere between thirty and a hundred times per second. Every spark has to land in the right cylinder at the right phase of the four-stroke cycle. If a spark fires too early, the mixture explodes before the piston has finished squeezing. If it fires too late, the piston has already started moving back down. Either way, the engine runs badly or stops.
This whole chapter is about how an old car pulls off this trick.
What a spark actually is
If you have ever shuffled across a carpet in socks and then touched a metal doorknob, you have made a spark. You felt the zap. You may even have seen the flash, in a dark hallway. That is a spark. Electricity, briefly jumping through air.
This is supposed to be impossible. Normally, air does not conduct electricity. Engineers call air an insulator — a substance that doesn’t let current flow through it. That is why bare power lines in the sky don’t continuously zap each other. The air between them is in the way.
But under enough pressure — the right kind of pressure — air gives up. If you push electricity hard enough across a small gap of air, the air will briefly become a conductor and let the current through. That moment of giving up is the spark you see and feel.
The “pressure” in this case is voltage. Voltage is how hard the electricity is being pushed. A typical AA battery in a TV remote pushes at one and a half volts. A wall outlet pushes at one hundred and twenty volts. A lightning bolt is somewhere around a hundred million volts.
The amount of voltage it takes to make a spark depends on the size of the gap. For the tiny gap inside a spark plug — about the width of a paperclip wire — it takes around twenty thousand volts.
Twenty thousand volts. The car has a battery that makes twelve. We are going to need to do some work to get from twelve to twenty thousand.