Fuel and spark

Every power stroke is a small, precisely staged event: the right amount of fuel, mixed into the right amount of air, lit at exactly the right moment. Get all three right and the engine hums for two hundred thousand miles. Get any one wrong and it coughs, overheats — or starts quietly destroying itself. This lesson is about those three decisions.

The mixture

Petrol burns completely at one ratio: 14.7 : 1 air to fuel by mass — the stoichiometric ratio. Richer (more fuel) burns cooler and makes a touch more power, at the cost of economy and unburnt fuel out the exhaust; leaner saves fuel but burns hotter and edges toward misfire. Modern engines hold the ratio within a percent or two, hundreds of times a second, using an oxygen sensor in the exhaust as the feedback — a closed loop your right foot never notices.

Getting the fuel in has its own history: carburettors (clever plumbing, no electronics) gave way to port injection (a squirt behind the intake valve), which is giving way to direct injection — fuel fired straight into the cylinder at over 200 bar, timed to the crank degree, enabling the precise, lean, high-compression burns modern efficiency demands.

The spark

The flame takes time to cross the chamber — a few milliseconds — but peak pressure must land just after the piston passes the top. So the spark fires before the piston gets there, typically 10–40 crank degrees early, and the faster the engine spins, the earlier (in crank angle) it must fire. This is ignition timing: advance it and pressure builds against the still-rising piston; retard it and the burn wastes itself chasing a piston already running away. The engine computer re-decides this number every single firing.

Knock — the engine killer

Compress and heat the mixture too far and the last pocket of unburnt gas stops waiting for the flame — it detonates spontaneously, all at once. That is knock: a pressure spike like a hammer blow, audible as a metallic pinging, capable of eroding pistons in minutes at full load. Knock is THE limit on compression ratio and boost — nearly every performance ceiling in this course traces back to it.

Which is what octane actually means: a fuel's resistance to detonating spontaneously. 98-octane contains no more energy than 95 — it simply keeps its nerve under higher compression, letting the engine run more aggressive timing and boost. Modern engines carry knock sensors — accelerometers listening for the ping — and retard the spark within one engine cycle of hearing it, riding the knock threshold as closely as they dare. That closed loop is why today's engines survive compression ratios that would have wrecked engines of the 1970s.