Malfunction Criteria

Rich malfunction (negative)  Diagnostic value < Threshold

or

Lean malfunction (positive)  Diagnostic value > Threshold

Diagnostic value = "Imbalance value n" of the "Imbalanced cylinder"

(n: Cylinder number (#1 or #2 or #3 or #4))

• Imbalance value n = iSTFTn - Average (other 3 iSTFTs)

  For example: Imbalance value #1 = iSTFT # 1 - (iSTFT # 2+ iSTFT # 3 + iSTFT # 4)/3.

• Imbalanced cylinder: The cylinder with maximum (ABS (Imbalance value #1, #2, #3, #4)) is selected
• iSTFT: Individual cylinder STFT (Refer to the following about the detail)

  Individual Cylinder STFT (iSTFT n (k)) 

  Individual cylinder STFT (= iSTFT n (k)) are calculated by integral control and individual cylinder λ deviation u(k) converges to target (no deviation).

  iSTFT n (k) = iSTFT n (k-1) + C(tgt - u(k)) 

  iSTFT n (k) = Individual cylinder STFT

  u(k): Estimated individual cylinder λ deviation*

  tgt: Target (No deviation = zero)

  C: Control gain (negative value)

Fig 1: Individual Cylinder Amplitude Waveform

Courtesy of SUBARU OF AMERICA, INC.

* individual cylinder λ deviation: u(k)

= Individual cylinderλ - averageλ during 720°CA

Fig 2: Individual Cylinder Amplitude Deviation Waveform

Courtesy of SUBARU OF AMERICA, INC.

Estimated individual cylinder λ deviation "u(k)" is calculated by model (as described below).

Estimation Model for individual cylinder λ deviation (u(k)) 

Subaru developed a model for estimating individual cylinder λ deviation* by the output signal of the primary oxygen sensor (mounted on the collector section of the exhaust manifold). This estimation model makes it possible to control cylinder A/F individually. The estimation model is designed as follows.

Fig 3: Individual Cylinder Amplitude Deviation Block Diagram

Courtesy of SUBARU OF AMERICA, INC.

Model equations for individual cylinder λ deviation (u(k)) 

"u(k-1)": Estimated individual cylinder λ deviation is calculated by the following model.

Fig 4: Individual Cylinder Amplitude Deviation Model Diagram

Courtesy of SUBARU OF AMERICA, INC.

a1 - 4, b1 - 4: Model parameters calibrated by primary oxygen sensor output λ and 'measured' individual cylinder λ.

(These parameters are calibrated and fixed prior to production. However these models are calculating the predicted λ of each cylinder during real world driving.)