Diagnosis of Oxygen Sensing System Mixture Adaptation

Diagnosis of Oxygen Sensing System Mixture Adaptation







General
The adaptive oxygen sensing system corrects longer lasting deviations of the fuel/air mixture of lambda = 1 by changing the anticipatory control calculated in the DME control module and the injection timing altered as a result.

Through mixture adaptation, the anticipatory control is multiplied and augmented in such a way that corrections to the oxygen sensor (FR) itself can be kept to a minimum, and the mixture remains within the range
lambda = 1 even during open-loop (as opposed to closed- loop) control.

This process is based on the following assumptions:
Over the life span of the engine and through different tolerances, two basic faults may occur:
The multiplicative fault and the additive fault per time unit (false air). At idle speed the false air fault dominates (low air flow rate, low rpm); with a high air flow rate the multiplicative fault dominates.

A combination of multiplicative and additive faults is compensated correctly when a corrective value is adapted in its dominant range. Therefore, there are three major adaptation ranges:

System description
The adaptation values can be read out with the Porsche System Tester 2.
The key to the adaptation values is as follows:
RKAT = Adaptation in range close to idle Cylinders 1 - 3
RKAT2 = Adaptation in range close to idle Cylinders 4 - 6
FRAU = Adaptation in lower load range Cylinders 1 - 3
FRAU2 = Adaptation in lower load range Cylinders 4 - 6
FRAO = Adaptation in upper load range Cylinders 1 - 3
FRAO2 = Adaptation in upper load range Cylinders 4 - 6
FR = Oxygen sensor for cylinders 1 - 3
FR2 = Oxygen sensor for cylinders 4 - 6

Function
After the DME control module has been disconnected from constant B+, the numerical values are at 1.00 (FRA) or 0 (RKAT). After the engine is started, first the programmed map is displayed.

Example:
Range 1:
RKAT: 0 %
RKAT2: 0 %

After oxygen sensing has been enabled (depending on engine temperature), the oxygen sensor (FR) moves around its mean value (e.g. 0.98-1.03).

Example:
Oxygen sensor:
FR: 1.03
FR2 : 1.02

If the engine now runs too lean (e.g. through a 10 % addition of false air) in this operating condition, the FR value (oxygen sensor) will deviate from its mean value by this value (by 1.10), to compensate for the false air. Despite the 10 % false air, the mixture is now once more in the lambda = 1 range and the oxygen sensor is working at around 1.11.

Example:
Oxygen Sensor:
FR: 1.09
FR2: 1.11

Since the range (adjustment) of the oxygen sensor is limited, however, the adaptation must relieve the oxygen sensor after a few minutes so that the full adjustment range is available again.

During this process, the RKAT value in range 1 shifts up (when the engine is idling), i.e. time is added to the injection time until the FR once more works at around its mean value. With the RKAT value the Porsche System Tester 2 shows the added time in percent.

Example:
Range 1:
RKAT: 1.5%
RKAT2: 1.0 %

Should the adaptation phase take place under load, the FRA value would change. The value is displayed by the Porsche System Tester 2 as a factor.

Example:
Range 2
FRA. 1.06
FRA2: 1.07

Since, however, the false air of 10 % has a greater effect at idle than under load, the FRA value only changes until the oxygen sensor (FR) fluctuates around its mean value once more.

The changes due to the adaptation in RKAT or in FRA also affect the other adaptation range.

The maximum adjustment of the FR is 25 % towards rich and 25 % towards lean. Therefore, an incorrect fuel/air mixture towards either rich or lean can be briefly corrected by the oxygen sensor. For corrections of longer duration, the adaptation takes care of the correction. This correction factor remains stored in the DME control module until the latter is disconnected from constant B+.

Changes in the fuel/air ratio caused by fuel tank ventilation are corrected by an additional adaptation factor (FTEAD).

Therefore, the adaptation value for fuel tank ventilation (FTEAD) with high absorption of the EVAP canister could look like this:

Example:
Fuel tank ventilation:
FTEAD: 0.93

Adaptation thresholds:
Rich threshold: RKAT 4.5 %
FRAU 1.32
FRAO 1.32

Lean threshold: RKAT = 4.5 %
FRAU = 0.70
FRAO = 0.70


NOTE: Map changes in the load range have a multiplicative effect map changes in the idle range have an additive effect.