This thread to explain in a few lines the basic principles of spark ignited (SI) engine (= petrol or gasoline engine) fuel calibration, in order for you to better understand what's going on in your engine / PCM and the data I will share with you on the CAN OBD thread. And I'll be glad to answer any question (ook in het Nederlands wanneer het mogelijk is).
---------- A few definitions first :
- AFR, Air Fuel Ratio : it's the ratio between the mass of air and the mass of fuel in the air-fuel mixture. AFR = M(Air)/M(Fuel). Mainly used in the USA.
- Stoichiometric (in short : stoich) operation : operating mode where ther is complete combustion of the fuel, without excess or lack of air. Stoich AFR for 95 RON lead free fuel is around 14.7:1. Therefore 14.7 kg of air are theoretically needed to obtain a complete combustion of 1 kg of 95 RON fuel.
- Lambda (greek letter) : it's defined by the ratio between the actual measured AFR and the stoich AFR. Therefore, if an AFR of 14.7 is measured in the exhaust gas, Lambda = 1. Lambda is often used in Europe.
- Lambda > 1 , AFR > 14.7 => lean mixture, with an excess of oxygen (air)
- Lambda < 1 , AFR < 14.7 => rich mixture, with an excess of fuel.
----------Usual operating modes in SI engines :
- Lambda = 1 (stoich) : this operating mode is needed for maximum exhaust gas cleansing by a 3-way catalyst. Indeed, whern the mixture gently remains around stoich (Lambda = 1), the catalyst can simultaneously reduce (as in chemical reduction - taking off oxygen ions) Natrium Oxydes (NOx) and oxydise Carbone Monoxyde (CO) and unburnt Hydrocarbons (HC) for an optimal aftertreatment. This operating mode is the default mode for idle and when at part loads, when one cruises or drives calmly.
- Slightly lean mode : gives a combustion optimised for fuel economy. Back then when our cars had carburettors, they operated at this AFR in cruise mode and at part throttle. Today, because of the 3-way catalyst gas aftertreatment and emission regulations, this mode is not used any more. Only DI (direct injection) petrol engines run in lean burn mode. It's good for fuel economy but bad for NOx emissions. They need a NOx trap that momentarily absorbs the NOx and reject them during stoich or rich mode for aftertreatment in the 3-way catalyst. It is indeed very difficult to reduce (= take off oxygen ions) NOx when there is an excess of oxygen.
- Slightly rich mode : give maximum torque and therefore max power too. Good NOx reduction, however high levels of unburnt HC and CO in the exhaust gas. This operating mode is only used under heavy load, when the driver requests a high engine utput, e.g. under WOT (Wide Open Throttle) operations.
All for now, your questions and remarks are welcome.