Energy and combustion: where the power comes from
Everything an engine does starts with one chemical reaction: hydrocarbons meeting oxygen, fast. Understand what fuel actually is, what burning actually does, and one surprise — that every engine is starved of air, not fuel — and half of engine design suddenly makes sense.
What fuel actually is
Petrol and diesel are hydrocarbons: molecules made of carbon and hydrogen, refined from crude oil. What makes them precious is not anything exotic — it is the sheer amount of energy packed into every kilogram, and how easy that kilogram is to carry. A kilogram of petrol holds about 44 MJ. A state-of-the-art lithium-ion battery holds about fifty times less. That one ratio explains why aircraft still burn kerosene, why electric cars need half-tonne batteries, and why the piston engine has been so hard to replace.
The reaction in the cylinder
Burning is just oxidation in a hurry: fuel molecules combine with oxygen from the air, the carbon leaves as CO₂, the hydrogen leaves as water vapour, and the difference in chemical bond energy comes out as heat. In the cylinder that heat has nowhere to go but into the trapped gas, whose pressure leaps — and pressure on a piston is force, force on a crank is torque. That is the entire chain: chemistry → heat → pressure → force. Everything else on an engine is plumbing in service of it.
The stoichiometry behind 14.7 : 1for engineers
Take octane, a stand-in for petrol. Balanced combustion is:
Work through the molar masses and each kilogram of octane needs ~3.5 kg of pure oxygen. But air is only 23% oxygen by mass, so it takes ~15.1 kg of air — and averaging over the real hydrocarbon blend in pump petrol gives the famous stoichiometric ratio of about by mass. The mixture demo's magic number is nothing but this arithmetic. It also shows how lopsided the recipe is: the fuel you pay for is 6% of the mass entering the cylinder. The other 94% is free — which is exactly why engines are built to breathe.
Air is the limit, not fuel
Here is the surprise hiding in that recipe. Squirting more fuel into an engine is trivial — a bigger injector pulse, done. But every gram of fuel needs almost fifteen grams of air to burn, and air is bulky: a 2-litre engine at full throttle swallows about 2 litres of it per revolution and physically cannot take more. So the power of every naturally-aspirated engine is set by how much air it can capture, not how much fuel you can afford. Once you see that, a century of engine tricks collapses into one sentence: almost everything — bigger displacements, more revs, four valves per cylinder, tuned intakes, turbochargers — is a scheme to get more air.
Burning vs exploding
One more distinction carries a lot of weight: a healthy engine does not contain explosions. The spark lights a flame front that walks across the cylinder in about a millisecond — fast, but orderly, and the pressure rises as a controlled shove. An explosion is what happens when the mixture stops waiting for the flame: squeezed and heated past its patience, the last unburned gas detonates all at once. That is knock, it hammers pistons like a mallet, and the fear of it dictates the compression ratio, the octane rating on the pump, and the timing of every spark. The ignition demo lets you cross that line and watch the pressure trace grow teeth.
Flame speed, revs, and why timing existsfor engineers
A laminar petrol flame walks at under 1 m/s, far too slow for an engine at speed — but in-cylinder turbulence folds and stretches the flame front, multiplying its effective area, and turbulence scales with piston speed. The upshot: burn anglestays roughly constant (~25–55° of crank) across the rev range, which is the only reason high-rpm engines work at all. The spark still has to fire earlier as revs rise or load falls (the burn takes a near-fixed slice of crank angle, and the pressure peak must land ~10–20° after TDC), which is exactly the MBT-chasing game in the ignition & knock demo. Octane rating, meanwhile, measures nothing but resistance to detonation: higher octane burns no hotter and carries no more energy — it simply tolerates more squeeze before letting go.