The powertrain control module (PCM) monitors an air/fuel ratio imbalance of any cylinder compared with the other cylinders of the engine. A cylinder individual air-fuel ratio imbalance can be caused by a drifting injection valve, an intake air delivery variation, or by uneven internal or external exhaust gas recirculation (EGR) distribution. The air/fuel imbalance monitor utilizes the principle of engine roughness (ER) change during cylinder individual mixture step to lean. Depending on initial cylinder specific air/fuel ratio, the ER change will be unique for each cylinder. If the air/fuel ratio for a single cylinder increases (becomes leaner), the ER will increase. The ER is averaged over a calibrated number of combustion cycles. The difference between the reference ER and the average ER after the lean step is the Delta ER.
In order to decrease monitoring time, all cylinders are evaluated in pairs in a defined order by the enleanment and pre-/post-enrichment of the individual cylinders. To provide this, the fuel quantities for each pair of cylinders are modified by trimming the injection times to specific values. The resulting ER change for each cylinder is evaluated. The change of ER depends on the initial air/fuel ratio of the tested cylinder. Depending on whether the cylinder contains excessive fuel or lacks fuel, the change of ER will be either low or high.
As shown in the figure, after the release of the air/fuel imbalance monitor, the first pair of test cylinders enters the fuel modulation phase. At this step, the fueling of the cylinder A is shifted towards “lean” and the fueling of the cylinder B is shifted towards “rich” simultaneously and the cylinder individual ER signals are stored. The stored value of ER of cylinder A is a cylinder A test value and the stored value of ER of cylinder B is a cylinder B reference value. In the next step, cylinder A enters the "reference" phase and cylinder B enters the "enleanment" phase and ER values are stored again. The second stored ER value of cylinder A is a cylinder A reference value and the second stored ER value of cylinder B is a cylinder B test value. The cylinder specific change of engine roughness is calculated based on its captured reference and test values and then stored. To keep an exhaust bank specific air/fuel ratio of 1.0, the cylinders that are not being tested are additionally enriched/leaned off contrary to the enrichment/enleanment of each pair of cylinders.
If no misfires are detected, the test will be repeated for each pair of cylinders. If the enable conditions are at any time no longer satisfied, the test will be halted. It starts again as soon as the enable conditions are met again for any pair of cylinders for which the test has not yet been completed. When all cylinders have been checked, the cylinder specific changes of ER are converted into cylinder specific air/fuel deviation values by means of defined correlation maps and the bank average air/fuel ratio is calculated. Then the difference between the bank average air/fuel ratio and the cylinder specific air/fuel deviation value of each cylinder is calculated. If the resulting value of cylinder individual air/fuel ratio is outside of a calibrated range, the corresponding cylinder air/fuel ratio imbalance fault will be set.