What an engine actually does

Strip away a century of refinement and a car engine has exactly one job: turn the energy locked in petrol into a spinning shaft. Everything else — the cylinders, the valves, the turbo, the redline — is machinery in service of that single transformation. Understand the transformation and the rest of this course is details.

Energy changes costume

Petrol is astonishing stuff. One kilogram holds about 44 megajoules of chemical energy — roughly forty times more than the best lithium battery per kilogram. That is the entire reason cars burn fuel: it is nature's densest convenient energy store.

The engine unlocks it in a chain of costume changes: chemical → heat → pressure → motion. Burn the fuel and its chemical bonds release energy as heat. Heat a trapped gas and its molecules hammer the walls harder — pressure. Let one wall of the container move (the piston) and pressure becomes force, force becomes motion, and a crank turns straight-line motion into spin. Four arrows, and every one of them obeys physics you learn at school.

The explosion, tamed

A cylinder full of petrol-air mixture doesn't detonate like a bomb — it burns, a flame racing across the chamber in a few milliseconds, controlled and repeatable. That control is everything: the pressure must peak just after the piston starts moving down, pushing it like a perfectly timed hand on a swing. In a modern engine this happens up to a hundred times per second per cylinder, each event identical to the last, for years.

How hard does the push have to be? Pressure in the cylinder peaks at 50–100 bar — fifty to a hundred times atmospheric pressure, several tonnes of force on a piston the size of your fist.

The price of the trick

There is a catch, and it is fundamental: most of the fuel's energy never reaches the wheels. A typical petrol engine is about 30% efficient — the rest leaves as heat in the exhaust and the cooling system. This is not sloppiness; it is thermodynamics. Any engine that works by heating a gas must dump some of that heat to a colder place to keep the cycle going, and physics sets a hard ceiling on the fraction you may keep:

Hotter combustion ($T_{\text{hot}}$ up) means more of the energy is keepable — which is why engineers chase high compression and lean burning, and why the most efficient piston engine in the world (an F1 power unit, over 50%) is a story about temperature management. Keep that 30% number in mind for the whole course: almost every clever device you will meet exists to claw back a little of the lost 70.