So, under acceleration, mixtures get richer. Why? Maximum power is made between 0.85 to 0.95 Lambda (12.5 to 14.0 A/F with iso-octane). However, when we need acceleration, the mixture gets richer. At cruise and idle, mixture is held tightly to 1.0 to keep the catalytic convertor at optimal efficiency, so the emissions are minimized. So, why don't we always run at 1.0 all the time? Well, we do MOST of the time. It should be noted that the ratios are mass-based, not volume-based. If Lambda is less than 1.0, then there is a surplus of fuel and the engine is running rich. With the oxygenated gasoline that most of us use, actual A/F ratio of 15:1 is closer to stoichiometric.If Lambda is greater than 1.0, then there is a surplus of air and the engine is running lean. This is known as 'Stoichiometric', a condition where there is a perfect balance between oxygen molecules and the various hydrogen and carbon based molecules in petroleum. With iso-octane ('ideal' gasoline), Lambda of 1.0 is equal to 14.7:1 A/F.
A/F is expressed as either a ratio (14.7:1 for example) or as a Lambda value. The ECU controls Air/Fuel mixture in order to maintain power, efficiency, and emissions. Some of the most common Fault Codes (DTC's) pertain to fuel trim (rich mixture, lean mixture, etc.) Here is an explanation of fuel trim and what it does for us.
