AML EOBD System Operation Summary
Rory O’Curry Aston Martin Lagonda CONFIDENTIAL 1 May 2009
[email protected] AML EOBD Monitors 07 ROC.doc Page 4 of 43
Catalyst Efficiency Monitor
The Catalyst Efficiency Monitor uses an oxygen sensor before and after the catalyst to infer Hydrocarbon
conversion efficiency, based on oxygen storage capac ity. Under normal closed-loop fuel conditions, high
efficiency catalysts have significant oxygen storage, which makes the switching frequency of the rear
HO2S quite slow compared with the switching freque ncy of the front HO2S. As catalyst efficiency
deteriorates, its ability to store oxygen declines and the post-catalyst HO2S signal begins to switch more
rapidly, approaching the switching frequency of the pre-catalyst HO2S.
In order to assess catalyst oxygen storage, the monitor compares front and rear HO2S signals during
closed-loop fuel conditions after the engine is warm ed-up and inferred catalyst temperature is within
limits. Front H02S signals are accumulated in up to ni ne different air mass regions or cells although 3 air
mass regions is typical. Rear H02S signals are counted in a single cell for all air mass regions. Currently
there are two algorithms that can be used to compare the front and rear HO2S signals:
1. Switch Ratio method;
The Switch Ratio method compares the 'switch frequencies' of the front and rear HO2S sensors. A
'switch' is counted every time the HO2S voltage output passes through a defined threshold (0.45 V).
The catalyst condition is diagnosed by dividing the number of rear H02S switches by the number of
front HO2S switches.
2. Index Ratio method.
The Index Ratio method calculates and compares the length of the front and rear HO2S signals. The
catalyst condition is diagnosed by dividing the length of the rear HO2S signal by the length of the front
HO2S signal.
A Switch / Index Ratio near 0.0 indicates high oxygen storage capacity, hence high HC efficiency. A
Switch / Index Ratio near 1.0 indicates low oxygen storage capacity, hence low efficiency. To improve the
robustness of the monitor, the Switch / Index Ratio is calculated using an Exponentially Weighted Moving
Average (EWMA) algorithm. If the Switch / Index Ratio exceeds the threshold, the catalyst is considered
failed.
AML EOBD System Operation Summary
Rory O’Curry Aston Martin Lagonda CONFIDENTIAL 1 May 2009
[email protected] AML EOBD Monitors 07 ROC.doc Page 25 of 43
VCT Monitor
Variable Cam Timing System Monitor
VCT Hardware
Variable Cam Timing (VCT) enables rotation of the camshaft(s) relative to the crankshaft (phase-shifting)
as a function of engine operating conditions. Intake Only (phase-shifting only the intake cam) is used in
the AML application.
VCT is used primarily to increase internal residua l dilution at part throttle to reduce NOx, and to
improve fuel economy. With Intake Only VCT, the in take camshaft is advanced at part throttle and WOT
(at low to mid-range engine speeds) to open the in take valve earlier for increased residual dilution and
close the intake valve earlier in the compression stroke for increased power. When the engine is cold,
opening the intake valve earlier warms the charge which improves fuel vaporization for less HC
emissions; when the engine is warm, the residua l burned gasses limit peak combustion temperature to
reduce NOx formation.
The VCT system hardware consists of a contro l solenoid and a pulse ring on the camshaft. The PCM
calculates relative cam position using the CMP input to process variable reluctance sensor pulses coming
from the pulse ring mounted on the camshaft. Each pul se wheel has N + 1 teeth where N = the number of
cylinders per bank. The N equally spaced teeth are used for cam phasing; the remaining tooth is used to
determine cylinder # 1 position. Relative cam position is calculated by measuring the time between the
rising edge of profile ignition pickup (PIP ) and the falling edges of the VCT pulses.
VCT Diagnostic
The PCM continually calculates a cam position error value based on the difference between the desired
and actual position and uses this information to cal culate a commanded duty cycle for the VCT solenoid
valve. When energized, engine oil is allowed to flow to the VCT unit thereby advancing and retarding cam
timing. The VCT logic calculates the instantaneous va riance in actual cam position (the squared difference
between actual cam position and commanded cam position), then calculates the long term variance using a
rolling average filter (Exponentially Weighted Moving Average).
If the VCT system is stuck or operates with an consta nt error relative to the target position, the monitor
will detect a variance which will quickly accumulate. There are three variance indices that monitor cam
variance in the retard direction, the advance directi on, and for V engines, the difference between banks. If
any variance index is greater than the malfunction threshold, a VCT target error malfunction will be
indicated (P0011, P0012 Bank 1, P0021, P0022 Bank 2).
The VCT solenoid output driver in the PCM is check ed electrically for open circuit and shorts (P0010
Bank 1, P0020 Bank 2).
VCT Monitor Operation:
