How does one drop of petrol push a whole car?

Gasoline has a temper: it loves to burn. Atomise one tiny droplet, mix it with air, add a single spark — the mixture catches at once, turning into hot, high-pressure gas that shoves outward. A car’s galloping strength hides inside this controlled burn.

The room that receives that push is the cylinder: a tube with a piston sliding inside, like the plunger of a syringe. The tube’s top has two doors — the intake valve and the exhaust valve — and between them stands the spark plug, ready to strike.

The piston dances a four-step dance: ① intake — the intake door opens and the fuel-air mix is breathed in; ② compression — both doors shut and the piston squeezes the mix good and tight; ③ power — the spark snaps and sets the mix alight; the hot gas expands and presses the piston firmly down — this is the only step that truly pushes; ④ exhaust — the exhaust door opens and the burnt gas leaves. Four steps, one round; round after round after round.

Pumping up and down alone can’t turn a wheel. A connecting rod joins the piston to the crankshaft, rather like a bicycle pedal: the piston shoves down, the crank swings half a circle, and a heavy flywheel stores just enough momentum to carry the piston back up. The piston’s up-and-down becomes the wheel’s spin.

A cylinder receives a combustion push once per four strokes. It keeps moving through the other strokes, carried by the flywheel or other cylinders. Multiple cylinders stagger their power strokes to smooth the rotation. This common inline-four example fires 1-3-4-2; other engines use different cylinder counts and orders.

Detailed controls

1. Find every part. Chapter two labels the valves, spark plug, piston and crank; scrub the timeline and watch the piston rise and the doors open and close — the rod stays pinned to the piston and the crank pin throughout.
2. Count the four steps. The phase table in chapter three highlights the current stroke: intake → compression → power → exhaust. Only “power” lights the orange combustion-heating glow in the cylinder.
3. Turn a line into a spin. Watch the crank in chapter four: each down-push swings it half a circle while the flywheel’s marker keeps turning — it remembers, for the piston, when to come home.
4. The relay of four brothers. In chapter five, whoever glows is pushing: 1-3-4-2. This sketch places the power strokes in sequence; real torque still fluctuates, and the flywheel helps smooth it.
5. Spin it wild. In the exploration, pull the speed to 6000 rpm, then switch to one cylinder and watch the push rhythm — a single cylinder pushes once every two turns; four in relay keep the pushes coming.

Guess first: do four cylinders firing together run smoother, or taking turns? Choose, then check — firing together leaves a long gap between pushes; taking turns is the relay.

Compression lets combustion add energy while the gas occupies a smaller volume. The following expansion can then convert more of that heat into mechanical work. Compression raises temperature and pressure too, but more is not always better: petrol engines must avoid knock. A diesel engine usually compresses air until hot, then injects fuel that ignites without a spark plug.

The piston position here follows the exact slider-crank geometry, p = r·cosθ + √(L² − r²sin²θ): the rod’s two ends stay pinned to the piston and the crank pin at a constant length all the way round. Combustion, however, is drawn as if it finished in an instant — real burning takes an amount of time that varies with rpm and operating conditions, with a travelling flame front, and the best spark moment advances with rpm. The exhaust route follows the common layout — engine → catalytic converter → muffler; the catalyst reduces specific pollutants rather than making exhaust harmless, and both devices are sketched, not drawn to their real internals.

A petrol engine turns only about a third of the fuel’s energy into motion at the wheels; the rest leaves with hot exhaust, the cooling water and friction. An electric motor burns nothing and pushes its rotor with magnetic fields, so it usually runs more quietly — though where its electricity comes from belongs in the same ledger. See electric cars.