Fuel Injection

Complete combustion of petrol needs about 14.7 kg of air per kilogram of fuel (the stoichiometric ratio). Engineers describe mixture with λ = actual AFR ÷ stoichiometric AFR: λ < 1 is rich, λ > 1 is lean. A three-way catalytic converter can only oxidise CO and HC while reducing NOx when λ is held in a narrow window around 1, so closed-loop control is fundamental to modern emissions compliance.

The ECU determines the air mass per cylinder per cycle either directly from a hot-film mass-air-flow (MAF) sensor or indirectly from manifold absolute pressure, intake air temperature and engine speed (speed-density). Dividing by the target AFR gives the fuel mass required. An injector at a fixed pressure difference flows at a nearly constant rate, so fuel delivered is roughly proportional to the pulse width minus a dead time (the opening delay, which grows as battery voltage falls and is compensated in software).

Port fuel injection (PFI) sprays at roughly 3–5 bar onto the back of the intake valve, giving time for the fuel to evaporate and mix. Gasoline direct injection (GDI) sprays straight into the cylinder at roughly 100–350 bar. Evaporating fuel inside the cylinder cools the charge, allowing a higher compression ratio before knock, and gives precise control of injection timing; the trade-offs are higher particulate emissions (hence gasoline particulate filters) and carbon build-up on intake valves that are no longer washed by fuel. Some engines combine both. The ECU runs open loop when the oxygen sensor cannot be trusted or a different target is needed: cold-start enrichment, full-load enrichment to protect the catalyst and exhaust valves, and fuel cut-off on the overrun.