Abstract:
Method and apparatus to monitor secondary air injection and catalyst conversion efficiency. The method includes operating an engine in a rich condition after detecting an engine steady state condition. The secondary air injector injects air into an exhaust stream to simulate a lean engine condition. The injection of the air into the exhaust stream is ceased after both inlet and outlet sensors detect the lean condition. After ceasing air injection, a lag time is determined between the inlet sensor detecting the rich condition and the outlet sensor detecting the rich operating condition. An oxygen storage capacity of the catalytic converter is calculated based on the lag time. An efficiency of the catalytic converter is determined as a function of the storage capacity. Additionally, performance of the secondary air injector is monitored. If the inlet sensor fails to detect the lean condition after the secondary air injector is active, a fault is signaled.

Description:
TECHNICAL FIELD 
     The present invention relates to diagnostic systems for vehicles, and more particularly to a method and apparatus for monitoring catalytic converter efficiency and secondary air injection. 
     BACKGROUND OF THE INVENTION 
     During the combustion process, gasoline is oxidized, and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO 2 ), water (H 2 O), carbon monoxide (CO), nitrogen oxides (NO x ), unburned hydrocarbons (HC), sulfur oxides (SO x ), and other compounds. 
     Automobile exhaust systems include a three-way catalytic converter that reduces CO, HC and NO x  in the exhaust gas. The efficiency of the catalytic converter is periodically monitored to prevent excess CO, HC and NO x  in the exhaust gas. Typically, the catalytic converter is monitored during engine steady state conditions. At idle, for example, the engine controller adjusts the air to fuel (A/F) ratio to achieve consistent emissions output. Traditional monitoring methods force the A/F ratio to a lean or rich condition for a predetermined period. Afterwards, the controller switches to the rich or lean condition. The controller estimates an oxygen storage capacity (OSC) of the catalytic converter based on a lag time between an inlet oxygen sensor and an outlet oxygen sensor detecting the lean/rich condition. The OSC is indicative of the efficiency of the catalytic converter. 
     The intrusive catalytic converter monitoring test adversely impacts emissions and driveability. For example, operation in a lean A/F ratio may cause engine instability. Compensation involving more intrusive control of other engine parameters is typically required to prevent engine instability. 
     A secondary air injector may also be provided to inject air into the exhaust stream. The secondary air injector normally operates during a short start-up period of the engine. During the startup period, the engine is still “cold” and combustion of the gasoline is incomplete, which generates dense emissions, especially CO and HC. Additional air injected by the secondary air injector is used to quickly heat the catalyst by oxidizing the CO and HC. The warmed catalytic converter further oxidizes CO and HC, and reduces NO x , to lower emissions levels. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides method and apparatus for monitoring both a secondary air injector and a catalytic converter. The method includes operating an engine in a rich condition after detecting an engine steady state condition. The secondary air injector injects air into an exhaust stream to create a lean condition. The injection of the air into the exhaust stream is ceased after both inlet and outlet sensors detect the lean condition. After ceasing air injection, a lag time is measured between the inlet sensor detecting the rich condition and the outlet sensor detecting the rich condition. An oxygen storage capacity of the catalytic converter is calculated based on the lag time. An efficiency of the catalytic converter is determined as a function of the storage capacity. Additionally, a secondary air injector fault is signaled if the inlet sensor fails to detect the lean condition. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a functional block diagram of a vehicle exhaust system; 
     FIG. 2 is a flowchart illustrating a method for monitoring catalyst efficiency and secondary air injection according to the present invention; 
     FIG. 3 is a graph showing inlet and outlet oxygen sensor voltage as a function of time according to the present invention; 
     FIG. 4 is a graph showing a corresponding commanded equivalence ratio as a function of time for the data in FIG. 3; 
     FIG. 5 is a graph showing inlet and outlet oxygen sensor voltage as a function of time according to the prior art; and 
     FIG. 6 is a graph showing a corresponding commanded equivalence ratio as a function of time for the data of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. 
     With reference to FIG. 1, a vehicle  10  includes a controller  12 , an engine  14 , a secondary air injector  16  and an exhaust system  18 . The controller  12  communicates with various sensors, actuators and valves. The engine  14  includes a throttle  20  that communicates with the controller  12 . The throttle  20  controls the amount of air drawn into the engine  14  during an intake stroke of the pistons (not shown). The amount of power produced by the engine  14  is proportional to a mass air flow rate of air into the engine  14 . The engine  14  operates in a lean condition (i.e., reduced fuel) when the A/F ratio is higher than a stoichiometric A/F ratio. The engine  14  operates in a rich condition when the A/F ratio is less than the stoichiometric A/F ratio. Internal combustion within the engine  14  produces exhaust gas that flows from the engine  14  to the exhaust system  18 , which treats the exhaust gas and releases the exhaust gas to the atmosphere. 
     The exhaust system  18  includes an exhaust manifold  22 , a catalytic converter  24 , an inlet oxygen (O 2 ) sensor  26  located upstream from the catalytic converter  24 , and an outlet (O 2 ) sensor  28  located downstream from the catalytic converter  24 . The catalytic converter  24  controls the engine-out emissions by increasing the rate of oxidization of hydrocarbons (HC) and carbon monoxide (CO), and the rate of reduction of nitrogen oxides (NO x ) to decrease tail-pipe emissions. To enable oxidization, the catalytic converter  24  requires air or O 2 . When the exhaust is in rich condition, the converter can release the O 2  stored in lean condition or from excess O 2  generated by the reduction reaction. The O 2  storage and release capacity of the catalytic converter  24  is indicative of the catalytic converter&#39;s efficiency in oxidizing the HC and CO, and reducing NO x . The inlet O 2  sensor  26  communicates with the controller  12  and measures the O 2  content of the exhaust stream entering the catalytic converter  24 . The outlet O 2  sensor  28  communicates with the controller  12  and measures the O 2  content of the exhaust stream exiting the catalytic converter  24 . 
     The secondary air injector  16  includes an air pump  30  and a valve  32 . The secondary air injector  16  is operated during a short start-up period (approximately 30 to 40 seconds) after the engine is started. If the engine  14  is “cold”, the fuel within the cylinders (not shown) is not sufficiently burned, which increases HC and CO levels in the exhaust gas. The secondary air injector  16  injects secondary air into the exhaust stream to increase HC and CO oxidization. Additionally, the oxidization quickly heats the catalytic converter  24 , significantly benefiting the conversion of HC, CO, and NO x . In this manner, emissions during the cold start-up period are adequately controlled. Both the air pump  30  and valve  32  communicate with the controller  12 . The controller  12  initiates operation of the pump  30  and opening of the valve  32  to enable injection of air into the exhaust. 
     With reference to FIG. 2, a method of measuring the O 2  storage capacity of the catalytic converter  24  is shown. Control begins with step  100 . In step  102 , the controller  12  determines whether the engine  14  is operating at idle. If the engine  14  is not operating at idle, control loops back to step  100 . If the engine  14  is operating at idle, the controller  12  causes the engine  14  to run in a rich condition in step  104 . In step  106 , the controller  12  initiates operation of the pump  30  and opening of the valve  32  to supply air into the exhaust manifold  22 . In this manner, O 2  is injected into the rich exhaust stream to create a lean exhaust stream. 
     In step  108 , the inlet O 2  sensor  26  is checked by the controller  12  to determine whether the inlet O 2  sensor  26  has detected the created lean condition. If the inlet O 2  sensor  26  does not detect the lean condition, a fault is signaled in step  110  to indicate that the secondary air injector  16  is not functioning properly. If the inlet O 2  sensor  26  does detect the lean condition, a signal indicates that the secondary air injector  16  is functioning properly, in step  111 , and control continues with step  112 . In step  112 , the controller  12  determines whether the outlet O 2  sensor  28  has yet detected the lean condition. If the outlet O 2  sensor  28  has not detected the lean condition, control loops until the outlet O 2  sensor  28  detects the lean condition. 
     Once the outlet O 2  sensor  28  detects the lean condition, the controller  12  continues to operate the pump  30  for a predetermined period of time, to make the catalytic converter  24  saturated. Once the predetermined period of time expires, the controller  12  turns off the pump  30  and closes the valve  32  in step  114 . With the secondary air injector  16  turned off and the engine  14  still running rich, the O 2  level of the exhaust stream decreases. The inlet O 2  sensor  26  eventually detects the rich condition. As the rich exhaust stream is treated in the catalytic converter  24 , the outlet O 2  sensor  28  eventually detects the rich condition. Control continues with step  115 , where the controller  12  tracks the time it takes the inlet O 2  sensor  26  and the outlet O 2  sensor  28  to achieve a reference voltage. It should be noted that prior to executing step  115 , the controller may optionally command the engine to operate the same as, more rich, or less rich than commanded in step  104 . 
     In step  116 , the controller  12  measures the lag time between the inlet O 2  sensor  26  achieving the stoichiometric, or rich reference voltage, and the outlet O 2  sensor  28  achieving the same (see FIG.  3 ). After determining the lag time, control continues with step  118  where the controller  12  determines the oxygen storage capacity (OSC) of the catalytic converter based upon the lag time. In step  120 , the controller  12  determines whether the OSC of the catalytic converter  24  is above a pre-calibrated level. If the OSC is not above the calibration level, then a fault is signaled in step  122 . If the OSC is above the calibration level, then the algorithm signals that the catalytic converter  24  is functioning properly in step  123 . The method ends at step  124 . 
     With reference to FIG. 3, O 2  sensor voltage (measured in mV) is shown as a function of time (measured in seconds). More specifically, the graph of FIG. 3 is divided into three sections, section A (pre-test), section B (test), and section C (post-test). The continuous line represents the inlet O 2  sensor voltage and the dashed line represents the outlet O 2  sensor voltage. The sinusoidal form of the inlet O 2  sensor voltage through sections A and C indicates the cycling between lean and rich engine conditions that enables consistent exhaust emissions content during idle. 
     To initiate measurement of the catalytic converter&#39;s OSC, the controller  12  causes the engine  14  to operate rich upon detecting engine idle. The controller  12  initiates operation of the pump  30  and opens the valve  32  to inject air into the exhaust stream to create a lean exhaust stream. As the lean exhaust stream flows through to the catalytic converter  24 , the voltage of the inlet O 2  sensor  26  drops as the O 2  content of the exhaust stream increases. This is indicated at the beginning of section B. There is a lag between the inlet O 2  sensor  26  and the outlet O 2  sensor  28  detecting the lean condition, while the exhaust stream is treated within the catalytic converter  24 . Once the outlet O 2  sensor  28  detects the lean condition, the controller  12  continues operation of the pump  30  for a predetermined period of time, to achieve saturation of the catalytic converter  24 . Once the predetermined period of time expires, the controller  12  ceases air injection into the exhaust stream and the exhaust stream reverts to a rich condition. 
     As the engine  14  operates rich, thereby decreasing the O 2  content of the exhaust stream, the voltage of the inlet O 2  sensor increases and stabilizes through section B. As the exhaust stream is gradually treated in the catalytic converter  24 , the O 2  content decreases, and the outlet O 2  sensor  28  detects the decreased O 2  content of the exhaust stream. Eventually, a sharp increase in the outlet O 2  sensor voltage occurs, and the O 2  content of the catalytic converter  24  is at a minimum, as indicated at the end of section B. Once the outlet O 2  sensor  28  detects the rich condition of the catalytic converter, the controller  12  initiates normal idle operation of the engine  14 , as indicated by the sinusoidal form of the inlet O 2  sensor voltage in section C. 
     A lag time X in section B identifies the lag between the inlet O 2  sensor  26  and the outlet O 2  sensor  28  detecting a low O 2  content condition of the catalytic converter  24 . This value is determined at a reference voltage, preferably indicative of a stoichiometric condition. The controller  12  measures the lag time X, from which the OSC is determined. The efficiency of the catalytic converter  24  can also be determined by the controller  12  based upon the OSC. 
     With reference to FIG. 4, a commanded engine equivalence (CEE) ratio is graphically shown as a function of time. The CEE ratio is defined as the stoichiometric A/F ratio (A/F stoich ) divided by the actual A/F ratio (A/F actual ), as determined by the controller  12 . The periods Y 1 , and Y 2  represent the commanded periods of the two-stage A/F transitions. The values Z 1  and Z 2  separately represent the commanded magnitudes, offset to stoichiometric, of the two-stage A/F transitions. If the commanded CEE ratio is more than 1.0, the magnitude offset Z is positive; otherwise, it is negative. The value Z 1  may be calibrated to match the engine rich condition to the secondary air flow, in order to obtain reasonable lean conditions of the catalytic converter  24 . Although the A/F transitions are shown as step transitions, it is anticipated that ramp transitions may be substituted therefore. A ramp transition would be preferable in a situation where the CEE ratio is large enough to effect engine stability. 
     Comparing FIGS. 3 and 4 to FIGS. 5 and 6 highlights advantages of the present invention. In particular, the section B of the traditional monitoring method is greater than that of the present monitoring method. This is due to a shorter lean period, and increased exhaust air flow with the present method. Thus, the present method is less intrusive than the traditional method. FIGS. 5 and 6 show the O 2  sensor  26 ,  28  responses and the CEE ratio for the conventional catalytic converter monitoring method. In FIG. 5, lean operation of the engine  14  results in a longer overall, intrusive monitoring period, whereby the engine  14  operates in the lean condition for an extended period of time until the outlet O 2  sensor  28  detects the lean condition. In FIG. 6, the extended lean operation of the engine  14  is caused by the dip in the CEE ratio. The lean to rich or rich to lean transition magnitude (|Z 1 |+|Z 2 |) of the traditional catalyst monitoring method is much larger than that (|Z 2 −Z 1 |) of the present catalyst monitoring method. Larger transitions are detrimental to engine performance, therefore, the reduced transition achievable by the present invention provides a significant improvement. 
     The method of the present invention monitors the secondary air injector and the catalytic converter to reduce intrusive operation of the engine. By reducing the magnitude of engine A/F transitions during the testing period, engine stability and vehicle drivability are improved as compared with conventional monitoring methods. Additionally, the catalytic converter monitoring time is reduced due to increased exhaust air flow and faster response, smaller A/F ratio transitions and decreased transition delay. Further, emissions are reduced as a result of the shorter engine rich condition and secondary air injection in period Y 1 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.