Catalyst Efficiency Monitor

Catalyst Efficiency Monitor

The Catalyst Efficiency Monitor uses an oxygen sensor before and afier the catalyst to infer the hydrocarbon (HC) efficiency based on the oxygen storage capacity of the catalyst. Under normal closed-loop fuel conditions high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear heated oxygen sensor (HO2S) very slow and reduces the amplitude of those switches as compared to the switching frequency and amplitude of the front HO2S. As the catalyst efficiency deteriorates due to thermal and/or chemical deterioration its ability to store oxygen declines. The post-catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude approaching the switching frequency and amplitude of the pre-catalyst or upstream H02S.

Note: The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst) not thermal deterioration.

All vehicles use a Federal Test Procedure based catalyst monitor. This simply means that the catalyst monitor must run during a standard Federal Test Procedure emission test. This differs from the 20-second steady state catalyst monitor used in 1994 through some 1996 vehicles. Currently the 2 slightly different versions of the catalyst monitor that are used are the Switch Ratio method and the Index Ratio method. Beginning with the 2001 model year and beyond both versions will continue to be used in subsequent model years.

Switch Ratio Method

1. In order to assess catalyst oxygen storage the monitor counts front and rear H025 switches during the part-throttle closed-loop fuel condition afier the engine is warmed-up and the inferred catalyst temperature is within limits. Front switches are accumulated in up to 9 different air mass regions or cells although 3 air mass regions is typical. Rear switches are counted in a single cell for all air mass regions. When the required number of front switches has accumulated in each cell the total number of rear switches is divided by the total number of front switches to compute a switch ratio. A switch ratio near 0.0 indicates high oxygen storage capacity hence high HC efficiency. A switch ratio near 1.0 indicates low oxygen storage capacity hence low HC efficiency. If the actual switch ratio exceeds a calibrated threshold switch ratio~ the catalyst is considered failed.

Inputs from the engine coolant temperature (ECT) or cylinder head temperature (CHT) intake air temperature (IAT) mass air flow (MAF) crankshafi position (CKP) vehicle speed and throttle position (TP) are required to enable the Catalyst Efficiency Monitor.

Typical Switch Ratio Monitor Entry Conditions:

^ Part throttle with no rapid throttle transients.
^ Minimum 330 seconds since start-up at 21°C (70°F).
^ Engine coolant temperature is between 76.6°C - 1 10°C (170°F - 230°F).
^ Intake air temperature is between -6°C - 82°C (20°F - 180°F).
^ Engine load is greater than 10%.
^ Time since entering closed-loop is 30 seconds.
^ Vehicle speed is between 8 and 112 km/h (5 and 70 mph).
^ Inferred catalyst mid-bed temperature of 482°C (900°F).
^ Mass air flow is between 1 and 5 lbs/mm.
^ Fuel level greater than 15%.
^ EGR is between 1 and 12%.

2. The DTCs associated with this test are DTC P0420 (Bank 1 or Y-pipe system) and P0430 (Bank 2). Because an exponentially weighted moving average algorithm is used for malfunction determination up to 6 driving cycles may be required to illuminate the MW during normal customer driving. If the KAM is reset or the battery is disconnected a malfunction illuminates the MIL in 2 drive cycles.

Index Ratio Method

1. In order to assess catalyst oxygen storage the catalyst monitor counts front HO2S switches during part-throffle closed-loop fuel conditions afier the engine is warmed-up and the inferred catalyst temperature is within limits. Front switches are accumulated in up to 3 different air mass regions or cells. While catalyst monitoring entry conditions are being met the front and rear HO2S signal lengths are continually being calculated. When the required number of front switches has accumulated in each cell the total signal length of the rear HO2S is divided by the total signal length of the front HO2S to compute a catalyst index ratio. An index ratio near 0.0 indicates high oxygen storage capacity hence high HC efficiency. A switch ratio near 1.0 indicates low oxygen storage capacity hence low HC efficiency. If the actual index ratio exceeds the threshold index ratio~ the catalyst is considered failed.

Inputs from ECT or CHT IAT MAF CKP TP and vehicle speed are required to enable the Catalyst Efficiency Monitor.

Typical Index Ratio Monitor Entry Conditions:

^ Minimum 330 seconds since start-up at 21°C (70°F).
^ Engine coolant temperature is between 76.6°C - 1 10°C (170°F - 230°F).
^ Intake air temperature is between -6°C - 82°C (20°F - 180°F).
^ Time since entering closed-loop is 30 seconds. Inferred rear HO2S sensor temperature of 482°C (900°F).
^ EGR is between 1% and 12%.
^ Part throttle maximum rate of change is 0.2 volts/0.050 sec.
^ Vehicle speed is between 8 and 112 km/h (5 and 70 mph).
^ Fuel level is greater than 15%.
^ First Air Flow Cell
^ Engine RPM 1000 to 1300 RPM.
^ Engine load 15 to 35%.
^ Inferred catalyst temperature 454°C - 649°C (850°F - 1200°F).
^ Number of front HO2S switches is 50.
^ Second Air Flow Cell
^ Engine RPM 1200 to 1500 RPM.
^ Engine load 20 to 35%.
^ Inferred catalyst temperature 482°C - 677°C (900°F - 1250°F).
^ Number of front HO2S switches: 70.
^ Third Air Flow Cell
^ Engine RPM 1300 to 1600 RPM.
^ Engine load 20 to 40%.
^ Inferred catalyst temperature 510°C - 704°C (950°F - 1300°F).
^ Number of front HO2S switches is 30.

2. The DTCs associated with this test are DTC P0420 (Bank 1 or Y-pipe system) and P0430 (Bank 2). Because an exponentially weighted moving average algorithm is used for malfunction determination up to 6 driving cycles may be required to illuminate the MW during normal customer driving. If the KAM is reset or the battery is disconnected a malfunction illuminates the MIL in 2 drive cycles.

General Catalyst Monitor Operation

Monitor execution is once per drive cycle. The typical monitor duration is 700 seconds. In order for the catalyst monitor to run the HO2S monitor must be complete and the Secondary AIR and EVAP system functional with no stored DTCs. If the catalyst monitor does not complete during a particular driving cycle the already accumulated switch/signal data is retained in the KAM and is used during the next driving cycle to allow the catalyst monitor a better opportunity to complete.

Rear HO2S sensors can be located in various configurations to monitor different kinds of exhaust systems. In-line engines and many V-engines are monitored by their individual bank. A rear HO2S is used along with the front fuel control HO2S for each bank. Two sensors are used on an in-line engine and 4 sensors are used on a V-engine. Some V- engines have exhaust banks that combine into a single underbody catalyst. These systems are referred to as Y-pipe systems. They use only 1 rear H025 along with the 2 front fuel- control H02 S. The Y-pipe system uses 3 sensors in all. For Y-piped systems the 2 front LI02S signals are combined by the PCM sofiware to infer what the H025 signal would have been in front of the monitored catalyst. The inferred front H025 signal and the actual single rear H025 signal is then used to calculate the switch ratio. The rational for this strategy is that the catalyst nearest the engine will deteriorate first allowing the catalyst monitor to be more sensitive and illuminate the MIL properly at lower emission standards.

Note: Exhaust systems that use an underbody catalyst without a downstream/rear HO2S are not monitored by the catalyst efficiency monitor.

Most vehicles that are part of the Low Emission Vehicle (LEV) catalyst monitor phase-in will monitor less than 100% of the catalyst volume. Ofien this is the first catalyst brick of the catalyst system. Partial volume monitoring is done on LEV and Ultra Low Emission Vehicle (ULEV) vehicles in order to meet the 1.75 emission standard. The rationale for this strategy is that the catalyst nearest the engine deteriorate first allowing the catalyst monitor to be more sensitive and illuminate the MW properly at lower emission standards.

Many applications that use partial-volume monitoring place the rear HO2S afier the first light-off catalyst can or afier the second catalyst can in a 3-can per bank system. (A few applications placed the HO2S in the middle of the catalyst can between the first and second bricks).

Some Partial Zero Emission Vehicles (PZEV) use 3 sets of HO2S per engine bank. The front sensors or stream 1 (HO2S1 1/H02S21) are the primary fuel control sensors. The next sensors downstream or stream 2 in the exhaust are used to monitor the light-off catalyst (H02S12/H02S22). The last sensors downstream or stream 3 in the exhaust (H02S13/H02S23) are used for very long term fuel trim in order to optimize catalyst efficiency (fore afi oxygen sensor control).

Index ratios for ethanol (flex fuel) vehicles vary based on the changing concentration of alcohol in the fuel. The malfunction threshold typically increases as the percent of alcohol increases. For example a malfunction threshold of 0.5 may be used at E1O (10% ethanol) and 0.9 may be used at E85 (85% ethanol). The malfunction thresholds are therefore adjusted based on the percentage of alcohol in the fuel.