It may be desirable to accurately determine exhaust temperatures of an engine. By determining engine exhaust temperatures, it may be possible to provide mitigating actions when exhaust temperatures are higher than is desired. Further, determining exhaust temperatures may be useful for assessing operation of exhaust after-treatment devices. One way to determine exhaust temperatures is to install thermocouples, thermistors, or other temperature sensors in an exhaust passage that directs engine combustion by-products to exhaust after treatment devices. However, the thermocouples or thermistors may degrade if they are exposed to higher exhaust temperatures. Further, performance of exhaust temperature sensors may degrade if acidic combustion byproducts accumulate on the temperature sensors. This can result in the need for frequent replacement of temperature sensors, and related warranty issues.
Another example approach for determining exhaust temperatures involves inferring the exhaust temperature based on a heating element. For example, as shown by Ma et al. in U.S. Pat. No. 8,152,369, the resistance of a heating element coupled to an exhaust gas sensor may be used for exhaust temperature estimation. Therein, a change in the resistance of a heater that is used for maintaining the temperature of a sensing element of the exhaust gas sensor (e.g., a universal exhaust gas oxygen sensor or UEGO) is leveraged for estimating the exhaust temperature.
However, the inventors have recognized potential issues with such an approach. As one example, exhaust temperature estimation may be delayed due to the slow time response of the heater circuit. As another example, a heat shield may be required for the sensor, adding hardware requirements. As yet another example, using the heating element for sensing element temperature control and for exhaust temperature estimation may require complex control algorithms that are computationally intensive. Even if the heating element were used, accurate exhaust temperature estimation requires steady-state engine operation for an extended period of time. In particular, transients can cause sudden temperature changes at the exhaust pipe and exhaust sensor, which affect the heating element current. However, these temperature may not correlate with changes in the exhaust temperature. For example, transients can result in catalyst and sensor cooling which result in additional catalyst heater operation for sensor temperature maintenance. This would incorrectly suggest a drop in the exhaust temperature. Even if there are small inaccuracies in exhaust temperature estimation, they can result in large errors in engine operation. For example, if engine operation were adjusted based on the underestimated exhaust temperature, exhaust over-heating could result. As such, this narrows the window of operating conditions where the exhaust temperature can be accurately determined. Therefore, it may be desirable to determine engine exhaust temperatures in a way that reduces the possibility of sensor degradation. Further, it may be desirable to determine exhaust temperatures in a way that is accurate and dynamic such that rapid changes in exhaust temperatures may be accurately observable.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for an engine, comprising: inferring a composite transient exhaust temperature based on a duty cycle of an exhaust gas sensor heating element and further based on vehicle conditions during transient vehicle operation, the vehicle conditions including engine load, vehicle speed, and modeled exhaust flange temperature; and adjusting engine operation based on the transient composite exhaust temperature. In this way, exhaust temperature can be estimated reliably over a larger range of engine operating conditions using existing engine hardware.
As one example, the duty cycle of a heater coupled to an exhaust gas oxygen sensor (such as a UEGO sensor coupled upstream of an exhaust catalyst or a CMS sensor coupled downstream of the exhaust catalyst) may be captured during vehicle operation. This includes data captured during steady-state vehicle operation as well as transient vehicle operation. As such, the heater is operated to maintain the temperature of the exhaust gas sensor at an operating temperature. Thus, during conditions when the exhaust temperature is low, the duty cycle of the heater may increase to provide sufficient heat to warm the sensor. In contrast, during conditions when the exhaust temperature is high, the duty cycle of the heater may decrease since the exhaust provides sufficient heat to warm the sensor. An engine controller may convert an inverse of the captured duty cycle into a steady-state exhaust temperature. Then, a composite transient exhaust temperature may be determined that compensates for exhaust temperature changes arising from vehicle conditions during transient vehicle operation, such as the transient changes in vehicle speed, engine load, the occurrence of tip-in and tip-out events, and changes in a modeled exhaust flange temperature. For example, the controller may use a transfer function (e.g., a multiplier) to convert the inverse of the duty cycle into the steady-state temperature, and then ramp in a transient adjustment, wherein both an amount of the transient adjustment as well as a ramping rate is based on the engine load, vehicle speed, and modeled exhaust flange temperature. As an example, the rate of ramping may be decreased as the vehicle speed increases to compensate for a drop in exhaust pipe temperature (which requires heater operation) which does not correlate with a corresponding drop in exhaust temperature. As another example, the ramping rate may be decreased during a tip-in and increased during a tip-out to compensate for the different effects of the transient events on the exhaust temperature. Likewise, the ramping rate may be adjusted to compensate for load changes. The composite transient exhaust temperature may then be used for adjusting engine operations, such as to mitigate exhaust overheating. As one example, if the composite transient exhaust temperature is elevated, an engine load may be limited to reduce peak exhaust temperatures.
In this way, by estimating exhaust temperature via a heater of an oxygen sensor, it may be possible to provide the technical result of measuring exhaust temperature using existing hardware. Further, by compensating for changes incurred during transient vehicle operation to the heater operation that are distinct from changes to the exhaust temperature, the accuracy of exhaust temperature measurements may be improved. Consequently, it may be possible to provide over-temperature mitigating actions in a timely manner, and reduce engine warranty issues.
The present description may provide several advantages. Specifically, the approach may improve exhaust gas temperature estimates. Additionally, the approach may reduce exhaust gas temperature sensor degradation. Further, the approach may compensate for exhaust temperature sensor changes that occur over time, instead of one time sensor compensation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.