Exhaust temperature sensor(s) may be provided to monitor the temperature of an emission control device in an engine exhaust system. These sensors may be placed near or in the emission control device to provide accurate temperature readings. In some examples, additional exhaust temperature sensors have been used to evaluate whether the exhaust temperature sensors placed inside an emission control device, for example, are providing accurate exhaust temperature readings. However, having additional exhaust temperature sensors may be costly, and may create further diagnostic requirements.
As an alternative, exhaust gas sensors, such as air-fuel ratio (AFR) sensors, have been used. For example, JP 2000-227364 provides a method that uses an AFR sensor. The AFR sensor heater is temporarily deactivated while it is used for air-fuel ratio measurement. The exhaust temperature is then estimated based on the AFR sensor heater resistance, which is measured using a small current. The above-mentioned technique, however, may degrade engine operation since it may require placing an active AFR sensor temporarily out of service when it is needed for a feedback control of air-fuel ratio control during engine operation.
As a further alternative, as provided by US patent application 2006/0117737, the AFR sensor is kept active and the exhaust temperature is estimated from the amount of heat released from the AFR sensor heater. However, to accurately estimate the exhaust temperature from the heat released from the heater, one must accurately determine the correlation coefficient(s) used to correlate the heat released to the exhaust temperature. The correlation coefficient(s) may be affected by various factors that may or may not be accurately determined and/or controlled. For example, the correlation coefficient(s) may be affected by the exhaust gas mix (e.g., the relative contents of H2O, CO, CO2, CH, and NOx), the exhaust flow rate, and the exhaust flow pattern. Failure to accurately determine the correlation coefficient(s) may render the exhaust temperature estimation inaccurate and unreliable.
The applicants herein recognize that the above mentioned interrelated issues may at least be partially addressed by providing a system for diagnosing an exhaust temperature sensor in an engine exhaust; the exhaust further including an emission control device, the system comprising an exhaust temperature sensor disposed in the exhaust pathway for indicating temperature; an air-fuel ratio sensor having a heater, the air-fuel ratio sensor disposed in the exhaust pathway; a control system, the control system enabling and disabling the air-fuel ratio sensor heater based on operating conditions, the controller further adjusting fuel injection to provide a desired air-fuel ratio based on feedback from the air-fuel ratio sensor when the heater is enabled, and determining degradation of the exhaust temperature sensor based on the air-fuel ratio sensor when the heater is disabled.
In this way, it is possible to take advantage of the opportunities associated with using the air-fuel ratio sensor to accurately diagnose temperature sensor operation under conditions where the air-fuel ratio sensor is either unneeded or unable to provide air-fuel ratio feedback information, since the temperature reading from the air-fuel ratio sensor is needed only under select diagnostic conditions. Thus, during certain engine operating conditions, such as cold start and engine lean burn combustion, a heater for one or more AFR sensors may be temporarily turned off, (as when measuring air/fuel ratio is either not possible or unnecessary) and the sensor may then be used to determine exhaust temperature (e.g., on the basis of exhaust temperature indicative signal(s) received from the AFR sensor(s), such as an AFR sensor heater impedance or a time function of an sensor heater impedance). The estimated exhaust temperature may in turn be used for a number of different purposes, including as an indication of the actual exhaust temperature, as a reference for diagnosing the functionality of an adjacent exhaust temperature sensor, and/or as a basis for recalibrating the exhaust temperature sensor, in some instances, the estimated exhaust temperature may be adjusted for thermal contribution of one or more emission control devices to the exhaust.