Academy · Car Engines · Lesson 10/11
To the wheels
Clutch, gearbox, differential: combustion becomes motion. ~5 min
The engine's story ends with a spinning crankshaft — but the crank turns at thousands of rpm and a wheel at motorway speed turns at barely a thousand, and the engine only makes real power in a narrow band. Between combustion and tarmac stand three translators: the clutch, the gearbox and the differential. This is the drivetrain — the last metre of the journey from fuel to motion.
The clutch
An engine cannot spin below ~700 rpm without stalling; a stationary car's wheels turn at zero. The clutch bridges the impossible gap: two plates — one driven by the engine, one connected to the gearbox — pressed together by springs. Fully clamped, they turn as one. Pedal down, they separate. And in between lives the art of slipping: letting the plates rub with controlled friction so the engine can keep running while the car creeps from rest. Every smooth getaway you have ever felt was friction, precisely managed and paid for as a little heat. (Automatics do the same job with a fluid torque converter or robotised clutches — different hardware, same problem.)
The gearbox
A gear pair is a torque lever: mesh a small gear driving a large one and the output spins slower but twists harder, in exact proportion. First gear trades a lot of speed for a lot of torque — that is how 200 Nm at the crank becomes thousands at the wheels for pulling away. Top gear does the reverse, letting the engine loaf at low rpm while the car cruises fast. Five to eight ratios keep the engine inside its happy band (lesson 5's power curve) at every road speed — the gearbox is the adapter between the engine physics wants and the journey you want.
The differential
One last problem, hiding in every corner: the outside wheel travels a longer arc than the inside one, so driven wheels on a shared solid axle would fight each other, scrubbing and hopping through every turn. The differential — a small planetary dance of bevel gears — lets the two wheels spin at different speeds while still delivering torque to both. Its one weakness is famous: torque follows the path of least resistance, so one wheel on ice can spin uselessly while its partner with grip gets nothing. Limited-slip differentials (as met in the F1 course) exist precisely to close that loophole.
And with that, the chain is complete: air and fuel to fire, fire to pressure, pressure to spin, spin traded and split until rubber pushes road. Everything else in engine engineering — every lesson in this course — is refinement of that one unbroken line. The exam awaits.