Power plants – Internal combustion engine with treatment or handling of... – Having sensor or indicator of malfunction – unsafeness – or...
Reexamination Certificate
2002-02-01
2003-03-18
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Having sensor or indicator of malfunction, unsafeness, or...
C060S274000, C060S276000, C060S285000
Reexamination Certificate
active
06532734
ABSTRACT:
BACKGROUND
The present invention relates to a method and system for determining the efficiency of a catalytic converter based on signals generated by pre-catalyst and post-catalyst exhaust gas oxygen sensors.
As is known in the art, increasingly stringent federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy. Catalytic converters, positioned in the engine exhaust path, are often used to reduce emission levels of regulated exhaust gases. The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst O
2
sensor positioned upstream from the catalytic converter and a post-catalyst O
2
sensor positioned downstream from the catalytic converter.
One method known for indicating conversion efficiency of the catalyst is to calculate a ratio of downstream sensor transitions or switches to upstream sensor transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst. When the switch ratio exceeds a threshold value, a malfunction indicator light (MIL) is illuminated so the vehicle operator will seek service. This method of catalyst monitoring is disclosed in Orzel U.S. Pat. No. 5,357,751, assigned to the assignee of the present invention, and is referred to as the Switch Ratio (SR) method. Another method for indicating conversion efficiency of the catalyst is based on the ratio of the arc lengths of the downstream sensor signal to the arc lengths of the upstream sensor signals identified as an Index Ratio (IR) method in contrast to the SR method. This method is disclosed in U.S. Pat. No. 5,899,062, assigned to the assignee of the present invention, and incorporated herein by reference.
The test cycle for catalyst monitoring requires collection of data from each of the sensors while the engine is operating in each of a plurality of inducted airflow ranges or air mass (AM) cells. In each method a predetermined number of transitions or switches of the upstream sensor in each AM cell is required to complete the test cycle. These methods rely on AM cell calibration and assume that sensor signal transitions occurring in a defined AM cell are valid for ratio computation regardless of engine speed and load conditions. The determination of SR and IR based on data taken while the driver is operating the vehicle at a high load, low rpm or low load, high rpm condition results in increased SR and IR variability even though operation is within one of the plurality of inducted airflow ranges. The determination of catalyst conversion efficiency based solely on AM conditions may result in error, and may reduce the ability to discriminate between a good and a failed catalyst.
A method of detecting catalytic converter deterioration based on the ratio of the arc length or the number of transitions of signals from sensors upstream and downstream of the converter where ratio determination is restricted to predefined air mass ranges within corresponding predefined engine speed/load ranges in order to avoid areas of engine speed and load instability that might impair test to test repeatability of the deterioration detection is described in U.S. Pat. No. 6,195,986 assigned to the assignee of the present invention, and incorporated herein by reference.
The inventors have recognized that all presently known methods are designed to monitor total oxygen storage degradation using different upstream to downstream O
2
sensor signal calculations. Oxygen storage can be found in two different catalyst wash coat components: Ceria (cerium oxides) and precious metal. Total oxygen storage availability is a function of the Ceria and precious metal content in wash coat and as well their dispersions and mutual locations (within the wash coat). The high O
2
storage (Ceria) catalyst has been the standard for monitoring starting in circa 1994 model years. Ceria is the weaker link in the wash coat when compared to the precious metal (PM). Ceria degrades sooner than does the PM when exposed to thermal or chemical (phosphorus) degradation. The current production\Index Ratio (IR) catalyst monitor measures the change in the O
2
sensor signal amplitude, as the catalyst ages. The rear O
2
sensor signal increases in activity as the catalyst loses ability to store oxygen. The Index monitor measures the catalyst O
2
storage (ceria) only and infers the emissions. The ratio of the rear O
2
sensor is compared to the front O
2
sensor, as the ratio approaches 1.0 the catalyst failed.
During the catalyst monitor calibration process, the emission and catalyst index relationship are established testing differently aged catalyst. The index vs. Tail Pipe FTP emission function typically referred as a “hockey stick curve”. Monitoring the index in the field allows the catalyst “health” or tailpipe emission from catalyst index. (
FIG. 1
) to be inferred. For the
FIG. 1
hockey stick curve, the “slope” is attributed to the loss of O
2
storage that is measured as an increase in the amplitude of the rear, or CMS O
2
sensor signal compared to the amplitude of front O
2
sensor signal. At the knee of the curve, basically all of cerium oxides (oxygen storage) are gone. After the knee the emissions are still increasing but the Index ratio is constant. This flat portion of the “hockey stick curve” is insensitive to the existing catalyst monitor. The real world failure illustrated in
FIG. 1
demonstrates a catalyst that has lost it's ability to storage oxygen (high Index ratio) yet has good emissions. The cerium oxides based oxygen storage is gone (very susceptible phosphorus contamination) while the precious metal based oxygen storage still stay untracked. This catalyst was phosphorous poisoned in the field and turned on the malfunction indicator light (MIL). The concern is that while the catalyst has lost it's ability to storage oxygen it may still be a very good emission catalyst. No longer is catalyst monitoring limited to a single measurement of O
2
storage. At the knee of the “hockey stick curve” the A/F signal amplitude entering and exiting the catalyst is almost the same. From this time on, the catalyst has no Ceria based O
2
storage. The precious metal(s) (PM) alone continues to degrade but still carry some O
2
storage due to affinity of oxygen and PM. The PM crystals grow larger (degrade) due to thermal aging thus reducing the active surface area and increase the emissions. The exhaust gas residency time or presence next to the active PM sites with some oxygen storage is the detectable (measured) metric, here time delay, &tgr; (tau) or phase shift through the catalyst. On the flat portion of the hockey stick curve &tgr; (tau) is still changing. &tgr; (tau) is the transport delay (time) between the front and rear O
2
signals which measures the change of catalyst activity. As the (no Ce O
2
storage) catalyst degrades the value of &tgr; (tau) decreases (FIG.
2
). The measurable O
2
sensor signal change for a catalyst as it ages to low or no cerium oxides based oxygen storage is the time constant tau. Tau is the time, or transport, delay between the upstream and downstream ) O
2
sensor signals. Tau or time delay varies at different rpm, loads, air mass and monitor volume for a given aged catalyst. However, &tgr; decreases over time as the catalyst ages.
To put it another way, the inventors have recognized that the are two different types of material in the converter: one highly oxidizable (e.g., Ceria); and the other relatively less oxidizable. Thus, while increases in the amplitude of the oxygen sensed by the downstream converter indicates deterioration in the oxidizable material, and therefore its loss of effectiveness, there may still be effectiveness in the less oxidizable material performing the requisite emission reductions. Applicants further recognized that the effectiveness of the less oxidizable material may be measured by measuring the time delay, or phase shift, between the signals produced by the upstream
Jerger Robert Joseph
Kluzner Michael Igor
Nader David Robert
Shane Michael Daniel
Uhrich Michael James
Buckert John E.
Denion Thomas
Ford Global Technologies Inc.
Lippq Allan J.
Tran Binh
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