How does a car engine turn fuel into motion?
Tiny explosions, thousands of them a minute. Here’s one cylinder, slowed way down so you can watch it work.
Slowed down 74×
At this speed, each cylinder fires 20.8 times a second.
Press Hear it, then drag the slider into the red. The sound plays at real speed.
Stroke 1 of 4 · Intake
Suck
The intake valve opens and the piston slides down. A mist of air and fuel gets pulled into the cylinder.
Wait, what?
Only 1 of the 4 strokes makes any power.
The other three just coast. The heavy crankshaft keeps spinning, and the other cylinders take turns firing. That’s why most car engines have 4, 6 or 8 cylinders: someone is always on a Bang.
Up-and-down becomes round-and-round.
The piston only ever pushes in a straight line. But wheels need to spin. The crankshaft turns one into the other.
If you’ve ever ridden a bike, you already know this trick. Your knees go up and down, and the pedals go around. Swap legs for pistons and you have an engine.
Four cylinders, one crankshaft.
Every Bang’s push over the last 4 turns now
Firing order
- 1
- 3
- 4
- 2
Cylinder 4 is on its Bang.
A car engine is usually four of these cylinders in a row, all shoving the same crankshaft. They take turns: while one is on its Bang, the others suck, squeeze and blow. A Bang lands every half turn, so the push never stops.
Press Only cylinder 1 and watch the crankshaft lurch: a shove, then three strokes of coasting.
Pressed Hear it up top? That buzz is four cylinders’ Bangs blurred together. At 2,500 rpm, that’s 83 a second.
It’s mostly air.
You’d think the gas pedal squirts in more fuel. Not directly. It tilts a flap called the throttle and lets in more air. The engine’s computer measures that air and sprays in just enough fuel to match: about 1 gram of fuel for every 15 grams of air.
More air and fuel in each cylinder means a bigger Bang and a harder shove on the crankshaft. That’s all “step on it” really means.
Burning one liter of gasoline takes about 9,000 liters of air. That’s roughly 45 bathtubs’ worth.
Wait, what?
Most of the fuel’s energy never becomes motion.
Only about a quarter to a third of the energy in the fuel ends up turning the crankshaft. The rest turns into heat.
Where the rest goes.
Every Bang makes heat. Much of it goes out the tailpipe with the burnt gas. Much of the rest soaks into the metal, then gets carried to the radiator at the front of the car and blown away by the air rushing past.
So a car engine is really a heater that happens to make a little motion on the side. Why it doesn’t melt is its own question →
Talk like a gearhead
RPM
Revolutions per minute. How many times the crankshaft spins in a minute.
“She idles at 800.”Redline
The fastest the engine can safely spin. Go past it and parts can get damaged.
“Don’t bounce it off the redline.”Displacement
The total space all the pistons sweep through. It’s the “2.0” in a 2.0-liter engine.
“Big displacement, lazy power.”Torque
The twisting force on the crankshaft. It’s the shove you feel in your seat when a car speeds up hard.
“Diesels are all torque.”Combustion
Burning. Fuel and air combine, give off heat, and the hot gas expands.
“Internal combustion: the fire is inside the engine.”Throttle
The flap that lets air into the engine. The gas pedal moves it.
“Full throttle!”Firing order
The order the cylinders take their Bangs. Most 4-cylinder engines go 1-3-4-2.
“One, three, four, two.”Misfire
When a cylinder doesn’t fire. One push goes missing and the engine shudders.
“She’s running rough, probably a misfire.”Go down the rabbit hole
WatchA see-through engine, filmed in slow motionSmarter Every Day films real combustion through a glass cylinder head and talks you through every stroke.ReadHow do car engines work?Explain that Stuff walks through the four strokes and the crankshaft in plain words.
TryNext time you’re in a car, peek at the rev counterIt’s usually the dial next to the speedometer. When the needle points at 2, the engine is doing 2,000 RPM. That means each cylinder fires about 17 times every second. Try counting that fast.
WatchFour cylinders taking turns, animatedAnimagraffs shows the 1-3-4-2 dance with the whole engine moving. The link starts right at the firing-order bit.Talkr/explainlikeimfiveAsk the questions that feel too basic. Start yours with “ELI5:” and people explain it simply.