Outline Of Diagnosis

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To detect malfunctions, the diagnostic uses a predictive model, individual cylinder fuel control, and the primary oxygen sensor measurement. The diagnostic does not directly attempt to separate the primary oxygen sensor measurement into individual cylinder contributions, but uses a predictive model to do so. Individual cylinders STFT are independently adjusted to obtain the moving 720 degree average lambda of all four cylinders using feedback from primary oxygen sensor. If any individual cylinder's STFT is different than the average of the other cylinder's STFT by a calibratable amount, that cylinder will be identified as failing.

Monitor Method

This diagnostic monitor performs a functional check of the fuel system to determine an air-fuel ratio cylinder imbalance, as required by the regulations in Section (e) (6.2.1) (c). This method uses a parameter called "imbalance value" which is calculated for each cylinder. The "imbalance value" is the difference between the individual STFT of the specific cylinder and the average value of the individual STFT of the other three cylinders. These imbalance values are compared to each other in order to determine the "imbalanced cylinder", which is the cylinder with the maximum "imbalance value". A diagnostic value is calculated for the "imbalanced cylinder". When this diagnostic value is more than the predetermined threshold (lean malfunction) or less than the predetermined threshold (rich malfunction), a DTC for the most imbalanced cylinder is determined.

DTC for "Imbalanced cylinder" is following.

P219C: Cylinder 1 Air-Fuel Ratio Imbalance

P219D: Cylinder 2 Air-Fuel Ratio Imbalance

P219E: Cylinder 3 Air-Fuel Ratio Imbalance

P219F: Cylinder 4 Air-Fuel Ratio Imbalance

Malfunction Criteria.

Rich malfunction (negative) Diagnostic value < Threshold

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: STFT Cylinder Waveform

Courtesy of SUBARU OF AMERICA, INC.

*individual cylinder λ deviation: u(k)

= Individual cylinderλ - averageλ during 720°CA

Fig 2: Cylinder Diagnosis Flowchart

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: Cylinder Diagnosis Estimation Model 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: Cylinder Diagnosis Estimation Model Block 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.)