Engines · Fundamentals · Demo
One number decides power, economy and emissions — and they all want a different one.
14.7 : 1 is chemically perfect for petrol (λ = 1). Slide rich for power, lean for economy — and watch the exhaust temperature spike between them, which is why “a bit leaner” has melted a lot of pistons.
Standard SI fuelling shapes: power peaks ~12.8 : 1 where all the air burns, efficiency ~15.8 : 1 where all the fuel does, exhaust temperature in between, and everything falls off the lean-misfire cliff past ~16.5. The green band is the catalyst’s legal residence.
In plain terms
Petrol burns completely at one precise mixture: 14.7 grams of air per gram of fuel — stoichiometric, lambda 1. But complete isn’t the same as best. A little extra fuel (rich, ~12.8:1) makes sure every last molecule of air finds a partner: maximum power. A little extra air (lean, ~15.5–16:1) makes sure every drop of fuel burns thoroughly: best economy. And the three-way catalyst that cleans the exhaust only works in a razor-thin window around exactly 14.7 — which is why your engine cruises there, whatever it might prefer.
At full throttle the limiting resource is air — the cylinder holds only so much. Extra fuel guarantees all of that air burns, and the surplus fuel’s evaporation even cools the charge: peak power lives around 12.5–13:1. At cruise the limiting resource is fuel you’re paying for — surplus air guarantees all of it burns, so best efficiency lives lean of stoichiometric.
Exhaust temperature peaks close to stoichiometric — no surplus fuel to evaporate and cool the charge, no surplus air to soak up heat. Tuners fear the “lean spike”: leaning out from rich toward perfect, temperatures climb before they fall, which is how lean-of-stoich mistakes melt pistons under load.
A three-way catalyst needs the exhaust to oscillate tightly around lambda 1 — richer and it can’t oxidise the CO and hydrocarbons, leaner and it can’t reduce the NOx. So the oxygen sensor holds cruise at exactly 14.7 by law and by chemistry, and the ECU only slides rich when you demand full power. The pedal is quietly choosing between these mixtures for you.