How does a car work?

A petrol car is a machine for turning fuel into motion. Follow the chain: petrol from the fuel tank burns inside the engine. The hot gas pushes pistons down, and they turn a crankshaft.

The spinning crankshaft goes into the gearbox, which picks how many engine turns make one wheel turn. In this car a long driveshaft carries the spin to the back, where the differential turns it sideways and splits it between the two axles. The tyres push backwards on the road, and the road pushes the car forwards.

Around that chain sit the helpers. The radiator gets rid of spare heat. The exhaust carries burnt gas away, through a catalytic converter that cleans it. A 12-volt battery starts the engine and runs the lights. Brakes, steering and springs let you stop, turn and ride over bumps.

Inside each cylinder a piston slides up and down. A connecting rod joins it to the crankshaft, which turns the up-and-down into round-and-round. Each cylinder repeats four strokes, taking two turns of the crank:

1. Intake. The piston slides down and the intake valve opens. Air rushes in and an injector sprays in a mist of petrol: about 1 g of fuel for every 14.7 g of air.
2. Compression. Both valves close and the piston squeezes the mixture into about a tenth of the space. That is the compression ratio, 10.5 : 1 here.
3. Power. The spark plug fires just before the top. The mixture burns, the pressure leaps, and the gas shoves the piston down. This is the only stroke that makes work.
4. Exhaust. The exhaust valve opens and the piston pushes the burnt gas out.

Two camshafts, turning at half the crank's speed, open the valves at the right moments. With four cylinders firing in the order 1-3-4-2, one of them is always on its power stroke, so the crank gets a push every half turn. Water flows through passages in the block and takes spare heat to the radiator.

Torque is twisting force. Push on a wrench and the twist on the bolt is your push times the length of the handle: torque = force × lever arm, measured in newton metres (N·m). Push 200 N on a 0.5 m handle and you make 100 N·m. A longer handle gives more twist for the same push.

Inside the engine, the burning gas pushes the piston, the rod pushes the crank pin, and the crank throw is the wrench. At the very top the rod points straight at the centre, so there is no lever arm and no twist. A little later the arm is longest and the twist is huge.

Power is how much work you do each second: power = torque × speed. In numbers, kW = N·m × rpm ÷ 9,549. One horsepower is 745.7 W, a unit James Watt picked in the 1780s to compare his steam engines with horses.

An engine's torque rises, peaks and falls as it spins faster: at high rpm there is less time to fill each cylinder with air. But power keeps climbing past the torque peak, because the rpm is still going up faster than the torque is falling. That's why peak power comes at a higher rpm than peak torque.

A petrol engine only works between about 800 rpm (idle) and 6,500 rpm (the redline), and it's strongest in the middle. The wheels need to turn anywhere from 0 to about 2,000 rpm. Gears bridge the gap.

A small gear driving a big one turns it slower but with more twist. First gear here has 10 teeth driving 36: the output turns 3.6 times slower and with 3.6 times the torque, enough to pull away up a hill. Fifth gear has 26 teeth driving 20, so the engine can loaf along at motorway speed. A last pair, the final drive (41 teeth to 10), slows everything another 4.1 times. So:

wheel torque = engine torque × gear ratio × final drive × efficiency, and the push on the road is that torque ÷ the tyre's radius.

The clutch is two plates squeezed together by springs. Press the pedal and they part, so the engine can keep turning while the car stops or you change gear. An automatic uses a fluid-filled torque converter instead.

In a corner the outside wheel travels further than the inside one, so it must turn faster. The differential lets it: little spider gears inside start to spin, speeding one side up exactly as much as they slow the other.

All that engine torque ends up as one thing: a push where the tyres meet the road. Three things push back.

Rolling resistance. Tyres squash a little as they roll, which wastes energy. It is about 1% of the car's weight, roughly 150 N, at any speed.
Air drag. The car has to shove air out of the way: drag = ½ × air density × Cd × frontal area × speed². Double the speed and the drag is four times bigger.
Hills. Going up a 10% slope, about a tenth of the car's weight pulls it back.

Whatever push is left over makes the car speed up: F = m × a, so acceleration = spare force ÷ mass. A heavier car speeds up more slowly with the same engine. At top speed there is no spare force left: the engine's whole power goes into pushing air.

Drag is also why fuel use climbs so fast on the motorway. The power needed is force × speed, so it grows with speed cubed. Stopping costs energy too: brakes turn all the car's motion energy, ½mv², into heat.

An electric car swaps the whole engine, fuel tank and gearbox for three parts. A battery under the floor stores the energy: 60 kWh here, the same as about 7 litres of petrol. An inverter turns the battery's direct current into the alternating current the motor needs, and controls how much flows. The electric motor drives the wheels through a single reduction gear.

A petrol engine needs to be spinning before it makes any twist. An electric motor gives its full torque from 0 rpm, then its torque falls once it reaches full power. It happily spins to 16,000 rpm, so one gear covers every speed and there's no clutch.

Lift off or brake and the motor runs backwards as a generator. That's regenerative braking: instead of all the car's motion turning into heat, much of it goes back into the battery.

The big win is efficiency. An EV turns about 85–90% of its battery energy into motion. A petrol engine turns only about a quarter of its fuel's energy into motion; the rest leaves as heat. At 100 km/h this EV uses about 15 kWh per 100 km, while the petrol car burns about 6.5 litres, which holds 58 kWh.