Method to adapt the O2 signal of an O2 sensor during overrun

A system for compensating for changes in behavior of an oxygen sensor. In one embodiment, the system includes an oxygen sensor configured to produce an output indicative of an oxygen level in an exhaust stream produced by an internal combustion engine. An electronic control unit receives the output of the oxygen sensor and is configured to cause the internal combustion engine to operate in an overrun mode. The electronic control unit is programmed or otherwise configured to determine whether a change in oxygen level over time is approximately zero. When the change in oxygen level is approximately zero or near zero, the electronic control unit determines a compensation factor for the oxygen sensor.

BACKGROUND

The present invention relates to a method of calibrating O2 sensors used in the exhaust systems of internal combustion engines. More particularly, the invention relates to Zirconia-based O2 sensors, such as those used in diesel exhaust systems.

In general, internal combustion engines need a specific air-to-fuel ratio (or ratio range) to operate correctly. For gasoline engines, the ideal ratio is 14.7 parts of air to one part of fuel. When the ratio is less than 14.7, not all fuel in the air-fuel mixture is burned or combusted. This situation is referred to as a rich mixture or rich condition and has a negative impact on exhaust emissions because the leftover fuel becomes pollution in the form of hydrocarbons (“HCs”) and carbon monoxide (“CO”). When the air-fuel ratio is less than 14.7, excess oxygen is present in the air-fuel mixture. This situation is referred to as a lean mixture or lean condition. When an engine burns lean, it produces nitrogen-oxide pollutants and, in some cases, engine performance decreases, engine damage occurs, or both events occur.

In modern engines, the air-fuel mixture is controlled, in part, through use of an O2 sensor. The O2 sensor communicates with an engine control unit in a feedback loop which typically either controls a fuel quantity or an Exhaust Gas Recirculation (“EGR”) rate. In some engines, the engine control unit uses the O2 sensor's input to adjust the fuel mixture. The O2 usually sensor measures the oxygen level inside the exhaust manifold.

The performance of an O2 sensor degrades when its exhaust gas inlets ports become fouled or blocked. This blockage could occur due to being coated with oil or by being covered with an exhaust by-product such as soot. The performance of an O2 sensor can also degrade due to age. Typically, when a sensor ages it produces an incorrect signal or no signal at all. A properly operating O2 sensor (e.g., one used in a gasoline engine, located upstream of the catalytic converter, and not aged or contaminated or blocked by soot or other combustion by-products) should fluctuate between a rich and lean mixture at least once a second to keep the amount of harmful emissions low. A properly working O2 sensor in a diesel application should provide an output or reading that changes with changes to engine loading (within a reasonable amount of time).

The result of aging in a Zirconia-based O2 sensor is typically manifested in signal drift and incorrect sensor readings. To compensate for signal drift, an overrun adaptation is used. “Overrun” refers to a situation in which a vehicle's exhaust pipe is purged with air from outside the engine. As is known, atmospheric air has an oxygen content of about 21%. If an O2 sensor reading (after signal pressure compensation) in overrun differs from 21%, the engine control unit assumes that the deviation is due to aging of the sensor. The common procedure is to compensate for this signal deviation by determining a correction factor which is then applied to all following readings. In the next overrun cycle, the correction factor can be trimmed or adjusted to account for sensor aging between the current and past overrun cycles.

SUMMARY

One challenge associated with compensating for O2 sensor signal errors relates to the time needed to completely purge the exhaust pipe from combustion gases to make sure that the sensor reading in overrun is not initiated too early. Exhaust gas residue could lead to an incorrect O2 reading (e.g., an O2 level below 21%) and, as a consequence, the correction factor could be based on an incorrect calibration point. A common technique used to determine the timing of reading an O2 signal involves determining the amount of the gas or air that is required to purge the exhaust pipe from exhaust gas residue. To determine the amount of purge air, an O2 measurement is used during the calibration phase. The required purge air mass is the mass that has been pumped through the pipe until the O2 reading of an oxygen sensor results in a stable signal. This amount of purge gas is calibrated and is fixed thereafter. As a consequence, the calibration is static and can not adapt dynamically based on changes that may occur in the operation of the O2 sensor or another engine component (e.g., a stuck or jammed EGR valve).

Since the time to stabilize on O2 sensor signal typically also depends on clogging residues in the sensor's protection tube, current calibration techniques employ a safety factor that reflects a relatively slow sensor dynamic (which might result, e.g., from a soot-clogged sensor-protection tube or poisoning that affects the sensor electrodes pumping capabilities or diffusion barriers). These side effects can significantly change the required amount of purge gas. As a consequence, it is hard to decide how big the safety factor must be to cover all possible aging effects. If the purge air mass is estimated too low by the calibrator, the incorrect estimate can lead to significant errors in the O2 reading and, therefore, negatively impacts emissions and component aging. The situation may also interfere with on-board diagnostics. If the purge air mass is estimated too high the system may never be able to compensate for signal drift. This depends on the driver's driving behavior in a case where the engine does not stay in “overrun” long enough to calculate a new O2 compensation factor. Also, this would affect on-board diagnostics and could also affect emissions

Another problem with many compensation techniques is that they actually operate less optimally when new sensors are monitored. Generally, the adaptation trigger (purge air mass) for the compensation process is based on the signal response of aged parts. As a consequence, when a new sensor is used, the system has to wait relatively long periods of time to determine the new correction factor. This can be critical for the very first adaptation at or after what is know as the “end-of-line” (“EOL”) stage of vehicle production, since an initial adaptation is typically required to release the O2 sensor signal for system usage. In other words, engine control systems in the vehicle will either ignore or not receive the signal from the O2 sensor unless an adaptation has occurred. Thus, there is a risk that after EOL, cars could be driven without any active O2-sensor signal because the driver doesn't operate the vehicle in situations that meet the purge gas threshold during overrun phases or because turnover time in vehicle production doesn't allow a long enough roller-dyne-testing to have the sensor signal initially calibrated.

Instead of calculating or guessing an amount of required purge gas to release the O2 signal adaptation, embodiments of the invention monitor the stability of the O2 sensor during overrun directly. If the signal slope (ΔO2/Δt) of the pressure compensated O2 sensor signal becomes (or is close to) zero, the O2 sensor signal is considered to be stable and signal adaptation is initiated. Slope monitoring increases the reliability the sensor adaptation, since such monitoring inherently compensates or accounts for fouling and aging of the sensor (soot clogging, electrode poisoning, diffusion barrier plugging) as well as effects that might lead to long purge gas poisoning (engine blow-by, clogged exhaust gas recirculation valves, etc.).

In one embodiment, the invention provides a system for compensating for changes in behavior of an oxygen sensor. The system includes an oxygen sensor configured to produce an output indicative of an oxygen level in an exhaust stream produced by an internal combustion engine. An electronic control unit receives the output of the oxygen sensor and is configured to cause the internal combustion engine to operate in an overrun mode. The electronic control unit is programmed or otherwise configured to determine whether a change in oxygen level over time is approximately zero. When the change in oxygen level is approximately zero or near zero, the ECU determines a compensation factor for the oxygen sensor.

Embodiments of the invention may be implemented to provide various benefits, including improving automatic transmission calibration (“ATC”) control. As noted, past methods for purging exhaust systems relied on fixed amounts of purge gas and fixed amounts of time. Both of these techniques require safety margins that are accounted for during calibration. The safety margins change for different vehicles, and unless they are calculated correctly, the risk for calibrating the wrong values during overrun is relatively high when using static purge techniques. If the calibration is incorrect, a complete purge may not be achieved. In a vehicle with an automatic transmission, when a driver releases the gas pedal (or accelerator), the automatic transmission opens up (or disengages) the clutch and fuel is injected so that the engine idles and overrun is not entered. If no overrun occurs or the length of overrun is short, the exhaust system is not completely purged and the oxygen sensor is not properly calibrated.

Embodiments of the invention can also be used to address challenges in exhaust gas recirculation (“EGR”) control strategies. In some vehicles, the EGR valve is kept open all or most of the time to help ensure complete combustion of fuel. However, if the EGR valve is kept open during overrun more air is needed to properly purge the exhaust system. This is so because when the EGR valve is open the volume of the exhaust system increases, so more air is needed to purge the system.

Some vehicles have large exhaust pipes which cause less gas to pass by the oxygen sensor in a given amount of time. While purging an exhaust system, with a large exhaust pipe or one that has lower exhaust speeds, the process can be lengthened because of the flow of gas past the sensor. By implementing an embodiment of the invention, it is possible to release O2 signal adaptation in a more reliable manner than with at least some of the currently used techniques. This is because the embodiment automatically compensates for non-linear or unexpected signal response behavior of the sensor resulting from varying air flow speeds. Slow or unpredictable O2 sensor response time can impact tailpipe emission levels. Thus, monitoring the dynamic behavior of an O2 sensor can be made part of the on-board diagnostic (“OBD”) system.

DETAILED DESCRIPTION

FIG. 1illustrates a diesel engine10that is controlled by an electronic control unit (“ECU”)12. For the sake of simplicity, only one cylinder of the engine10is shown inFIG. 1. However, a typical engine includes multiple cylinders and, as a consequence, also includes duplicates of other components illustrated in the drawing such as fuel injectors, valves, and the like. The ECU12receives information from a mass air flow (“MAF”) sensor14and an O2 sensor16. The O2 sensor produces an electric signal or output that is correlated to the amount of oxygen measured by the sensor. The engine includes an exhaust gas recirculation (“EGR”) valve19, an exhaust manifold21, a fuel injector23, and a throttle or throttle valve25. The ECU12sends data and instructions to the EGR valve19and fuel injector(s)23. Air from outside the engine10enters intake manifold30and, in the embodiment shown, is forcefully drawn into the engine by a turbo31. Air flows past the throttle valve25(when the throttle valve is open), past an intake valve27(when it is open), and into a cylinder32. A piston33moves up and down within the cylinder32. The fuel injector controls delivery of fuel to the cylinder32and fuel from the fuel injector23is mixed with the air in the cylinder32and combusted or burned. Exhaust or exhaust gases from combustion flow out of the cylinder32past an exhaust valve29(when it is open) into exhaust manifold21.

While a vehicle is in operation, exhaust or exhaust gases (or, in other words, an exhaust stream) pass (or passes) the O2 sensor16. The ECU12continuously tests for lean and rich mixture conditions based on information from the O2 sensor16. Based on the measured oxygen content, the ECU12adjusts the EGR valve position. The ECU12also controls operation of the fuel injector23and may send a command signal to reduce or increase the amount of fuel injected into the cylinder32depending on whether too rich or too lean a condition exists in comparison to the desired state.

During overrun, no fuel is delivered to the engine10. In other words, to operate the engine in an overrun mode, the ECU sends a command signal to the fuel injectors to turn off or otherwise operate so that the fuel injectors deliver no fuel to the engine. In the overrun mode, piston33continues to move. As a consequence, the engine acts like an air pump. Intake valve27and exhaust valve29are operated in overrun and outside air passes through cylinder32and into exhaust manifold21. Once outside air reaches exhaust manifold21, the oxygen level is sensed by the O2 sensor16. The overrun process continues until the level of oxygen sensed by the O2 sensor is approximately 21% (or, more precisely, 20.95%). When this O2 level is reached, it is assumed that the exhaust manifold has been purged of exhaust gases and residues.

The turbo diesel engine10inFIG. 1is just one type of engine in which adapting the O2 signal of an O2 sensor during overrun can occur.FIG. 2illustrates a gasoline engine40in which embodiments of the invention may be implemented or utilized. The gasoline engine40includes an air injection system41. The air injection system41includes an air pump42, a diverter valve44, and a check valve46. The engine40includes an exhaust valve48, an inlet valve50, a cylinder51, a throttle valve52, an exhaust gas recirculation valve54, an exhaust manifold58, and an inlet manifold62. The engine also includes an ECU64and an O2 sensor66.

In normal operation, air flows from an air cleaner68, past the throttle valve52, into the inlet manifold62. Fuel is mixed with the air and the resulting air-fuel mixture is combusted in the cylinder51. Combustion is triggered by a spark from a spark plug69. Exhaust gases generated as a result of combustion flow pass the exhaust valve48into the exhaust manifold58. The O2 sensor66is located in the exhaust manifold58and senses the level of oxygen in the exhaust.

As with the diesel engine10, the gasoline engine40purges its exhaust manifold during overrun. Outside air is delivered to the exhaust manifold58and the level of oxygen is sensed by the O2 sensor66. Once the O2 sensor66has detected an oxygen level of about 21%, the ECU64assumes that the exhaust manifold has been purged of exhaust gases.

FIG. 3graphically illustrates some of the deficiencies that have been observed in current adaptation methods. The graph shows the signal behavior of a new O2 sensor and an aged O2 sensor versus time. The behavior of the new O2 sensor is shown with a solid line (or, more appropriately, curve)74. The behavior of the old O2 sensor is shown with a dashed line (or curve)76. Behavior line78represents an O2 level of approximately 21%, which (as noted) is the level of oxygen that indicates a successful overrun purge of the exhaust manifold. Point79represents the time at which the new sensor reaches the desired O2 level (i.e., the level indicated by line78). Point81represents the time at which the aged sensor reaches the desired O2 level (again, the level demarcated by line78).

Vertical line83indicates the start of adaptation for an aged sensor. Line84, which has a constant slope (and is shown as a dot and dash pattern), represents the purge gas mass (i.e., the mass of the gas that flows through the exhaust manifold during overrun until a reading of 21% is achieved). Point89represents the calibrated air mass limit or threshold at which adaptation starts. The distance from point79to point83represents a delay or “wasted adaptation release time” due to the negative effects of aging in a sensor.

FIG. 4graphically illustrates the calibration error that may be caused by signal response time delay. Signal response delay can be caused by a clogged exhaust gas recirculation valve or a clogged or fouled O2 sensor among other things. The graph shows signal behavior for a new O2 sensor, an aged O2 sensor, and an O2 sensor displaying unexpected behavior. The behavior of the new O2 sensor is shown by curve95, the behavior of the aged O2 sensor is shown by curve97, and the behavior of the O2 sensor operating unexpectedly is shown by curve99. Line100represents the purge gas mass (calibrated in the engine controller) and line101represents the O2 level at the end of an overrun.

Line103indicates the O2 level at which the sensor displaying unexpected behavior begins calibration. Vertical line107indicates the start of adaptation (at a time t1) for an aged sensor (or a sensor that is not displaying unexpected behavior). The distance between lines101and103is an indication of an O2 adaptation error due to unexpected signal behavior. Since the adaptation begins at time t1, the sensor does not communicate to the control unit (e.g., ECU12or64) that it is misreading the amount of oxygen in the gases present in the exhaust manifold since the start of adaptation relies solely on the fixed, calibrated purge air mass.

FIG. 5illustrates an improved O2 sensor calibration or adaptation strategy. In the illustrated technique or strategy, adaptation is delayed until ECU detects that the change in oxygen level (as measured by the O2 sensor) over time is zero or near zero. The signal behavior of a new sensor is represented by lines110and112. Line112represents the change in the measured oxygen level over time (ΔO2/Δt) for a new sensor. Line114represents the start time of adaptation for a new sensor. The signal behavior for an aged sensor is represented by lines116and118. Line118represents the change in oxygen level over time (ΔO2/Δt) for the aged sensor. The adaptation start time the aged sensor is represented by line120. The difference or gap between the adaptation start times (114and120) represents an amount of extra time that an aged sensor takes to achieve a correct O2 reading. ThusFIG. 5illustrates (unlikeFIG. 3and FIG.4) that the adaptation strategy adjusts and starts the signal compensation process based on the sensor behavior instead of a predetermined amount of purge gas. In other words, when the ECU determines that the O2 sensor is reading a constant level of oxygen (or when the slope of the oxygen sensor signal is approximately or substantially zero) it starts the compensation process. This provides a time advantage for new sensors over prior compensation strategies and has a benefit of automatically self-adapting to changes in signal behavior.

The compensation process or calculation of a compensation or correction factor may be accomplished in a variety of ways. One compensation technique includes performing the calculation as shown in Equation 1, below, to determine a correction factor.
Correction Factor=20.95%/measured oxygen level (at the time that ΔO2/Δt=0)  Eqn. 1.
Once the calculation factor is determined, it is applied to subsequent O2 sensor readings by multiplying the readings by the factor. In many implementations, the correction factor is also analyzed to assess its quality. A quality assessment is often required since compensation depends on the environmental conditions where the overrun event happens. For example, if an overrun occurs when a vehicle is in a tunnel, the correction factor is different when the overrun occurs in an unrestricted location (which has cleaner air as compared to the air in the tunnel). Depending on the exact circumstances, the differences can be large. In some embodiments, the ECU recognizes such changes (e.g., by executing appropriate software) in the correction factor and filters the correction factor to avoid a situation where an incorrect change in the correction factor occurs or an incorrect correction factor is used.

As can be seen from the above, certain embodiments of the invention provide a more reliable and self-adapting methodology to initiate O2-sensor compensation. Additionally, embodiments of the invention help address challenges associated with automatic transmission control states and help compensate for unpredicted changes of the sensor response behavior due to low exhaust flow speeds, as was discussed above. The time between the start of exhaust gas purging and the start of the O2 adaptation process can also be used to determine the dynamic response time of an O2 sensor. The dynamic response time provides an indication of the level of sensor clogging (e.g., by soot).