Where the energy goes

Drive to work and back and you might burn four litres of petrol. Roughly one of those four actually moved you down the road. The other three left the car as heat — out of the exhaust pipe, off the radiator, and into the brake discs at every set of lights.

That is not a complaint about your car. It is the shape of the entire subject, and once you can see it, everything else in the academy is detail hung off it.

Nothing appears and nothing disappears

Energy is not created and it is not destroyed. It changes form. That is the first law of thermodynamics, and it is the only rule you need to make sense of a vehicle — because a vehicle is a machine for moving energy out of the form you bought it in and into the form you wanted.

You bought chemical energy: bonds between carbon and hydrogen atoms in a tank, or lithium ions sitting on one side of a battery cell. You wanted kinetic energy: two tonnes of metal at a hundred kilometres an hour. Everything between those two states is the machine.

The forms, in order

For a petrol car, the chain runs:

1. Chemical, in the tank. About 32 megajoules in a litre of petrol.
2. Thermal, in the cylinder. The fuel burns and the gas above the piston gets very hot and very high-pressure.
3. Mechanical, at the crankshaft. Pressure pushes a piston, the piston swings a rod, the rod turns a crank.
4. Kinetic, in the car. Through the gearbox, the driveshafts, the tyres, into motion.
5. Thermal again, at the end. Every joule that moved you is eventually handed to the brakes, the air, and the road.

For an electric car it runs: electrochemical, electrical, mechanical, kinetic — and then, at the end, thermal again. Different middle. Same start and same finish.

Everything the machine cannot use becomes heat

This is the second law, and it is the reason engineering has ceilings rather than aspirations. Heat is the form energy falls into when it has nowhere better to go. You can turn work into heat completely and effortlessly — rub your hands together. You cannot turn heat back into work completely, no matter how well you build the machine.

So an engine is not inefficient because the people who designed it were lazy. There is a mathematical ceiling on how much of a hot gas's energy can be turned into shaft work, it depends on how much you squeezed the gas before you lit it, and it is calculable.

For an idealised Otto cycle: η = 1 − r^(1−γ)

where r is the compression ratio and γ is the ratio of specific heats of the working gas.

Take r = 10.5 and the air-standard γ = 1.4:

η = 1 − 10.5^(−0.4) = 1 − 0.390 = 0.61

So even a perfect engine at that compression ratio hands you 61% and throws the rest away as heat. Real engines reach under 40% at their single best operating point, because the working gas is combustion products rather than air, heat leaks into the cylinder walls, the pistons rub, and the engine has to pump air past a throttle plate.

The number matters less than what it does to the question. "Why is my car only 25% efficient" is a complaint. "The ideal ceiling is 61% and we get 38% at best — where did the other 23 points go" is a diagnosis, and every one of those points has a lesson behind it.

One stop at the lights, accounted for

Here is the accounting made concrete, because a worked number stays with you where a principle does not.

Kinetic energy: E = ½mv²

Car mass m = 1500 kg

Speed v = 60 km/h = 60 ÷ 3.6 = 16.67 m/s

E = 0.5 × 1500 × 16.67² = 0.5 × 1500 × 277.8 = 208,000 J, or 208 kJ

Two hundred and eight kilojoules, delivered into two lumps of cast iron over about four seconds. Nothing was destroyed and nothing was lost — you converted 208 kJ of motion into 208 kJ of warm brake disc, and then the disc gave it to the air over the next few minutes.

Do that a hundred times on the way home and you converted 20 megajoules. Every one of those joules was bought at the servo.

Predict, then run

A car rolls to a stop on a flat road without the brakes being touched once. Where did its kinetic energy end up?

Friction destroyed it

As heat — in the tyres, the bearings, the driveline, and the air it pushed aside

Stored in the springs and the suspension

Returned to the fuel tank

Commit

You learn more from being wrong on purpose than right by accident.

Why this is the first lesson

Because it makes the rest of the academy answerable rather than memorisable.

Why do brakes fade? Because they are converting motion into heat, and heat has to leave as fast as it arrives. Why is a turbocharger worth having? Because the exhaust is carrying away energy you already paid for and a turbine can take some of it back. Why does an electric car go further in traffic than on the highway, the exact opposite of a petrol one? Because braking is where a petrol car throws energy away and an electric one collects it.

Four different questions from four different corners of a car. One answer underneath all of them.

Follow the energy. It went somewhere, it went there for a reason, and the reason is always a mechanism you can name.