1.6.2.3 Diagnostic Principle

The cylinder imbalance monitoring is an intrusive diagnosis which applies a lean shift to the air fuel ratio of one single cylinder and evaluates the measured reaction in the engine roughness.

The diagnosis is attached to the cylinder balancing function, which stores cylinder individual adaptation values to be used as correction factors to the injected fuel mass. Hence, in the following the cylinder balancing function and the cylinder imbalance monitor are described together.

Fig 1: A/F Ratio Versus ER-Value Graph

Courtesy of BMW OF NORTH AMERICA, INC.

• Start at unknown A/F-Ratio with known (absolute) ER-Value.
• Shift towards lean until ΔER_sp is reached.
• Control variable t_inj and change Δ_tinj contain information about Δλ.
• Unknown λ_Start can be calculated with λ of all cylinders (λ_average =1!).