Where the energy goes

Drive to work and back and you might burn four litres of petrol. Over a typical drive cycle, roughly one litre's worth of that fuel energy may reach the wheels. Most of the rest leaves through the exhaust and cooling system or is consumed by engine and driveline losses. The energy that did reach the wheels also ends up in the surroundings when the car slows — through the brakes, tyres, road and air.

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 stored energy: chemical energy in fuel, or electrochemical potential in a battery. You wanted motion: perhaps two tonnes of metal at a hundred kilometres an hour. Everything between those 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

This 61% is an air-standard model result, not a performance promise for a real engine. It assumes an ideal gas, constant heat capacities, instantaneous constant-volume heat addition and no friction, pumping or wall heat transfer. Real petrol engines peak below 40% brake thermal efficiency because those assumptions do not hold and because combustion, gas exchange, friction and accessories all carry losses.

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 is the car's kinetic energy at that speed. In a conventional friction-brake stop, most of it becomes heat in the brake system, with smaller shares going to tyre deformation, air resistance and the driveline. The front-to-rear split and stopping time depend on the vehicle and the stop; it is not necessarily two discs or four seconds. An EV may also return part of it to the battery through regenerative braking.

Repeat that same stop a hundred times and the initial kinetic-energy total is about 20 megajoules. In a combustion car that energy ultimately came from fuel, although the amount of fuel needed was larger because the powertrain was not perfectly efficient.

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 can brakes fade? Heat can arrive faster than the system sheds it. Why can a turbocharger help? A turbine can recover some exhaust-gas energy. Why can stop-start driving favour an electric powertrain? Regenerative braking can recover part of the car's kinetic energy. Each outcome still depends on the vehicle and conditions.

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.