Patent Application
20110073086
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.
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.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
While a vehicle is in operation, exhaust or exhaust gases (or, in other words, an exhaust stream) pass (or passes) the O2 sensor 16. The ECU 12 continuously tests for lean and rich mixture conditions based on information from the O2 sensor 16. Based on the measured oxygen content, the ECU 12 adjusts the EGR valve position. The ECU 12 also controls operation of the fuel injector 23 and may send a command signal to reduce or increase the amount of fuel injected into the cylinder 32 depending on whether too rich or too lean a condition exists in comparison to the desired state.
During overrun, no fuel is delivered to the engine 10. 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, piston 33 continues to move. As a consequence, the engine acts like an air pump. Intake valve 27 and exhaust valve 29 are operated in overrun and outside air passes through cylinder 32 and into exhaust manifold 21. Once outside air reaches exhaust manifold 21, the oxygen level is sensed by the O2 sensor 16. 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 engine 10 in
In normal operation, air flows from an air cleaner 68, past the throttle valve 52, into the inlet manifold 62. Fuel is mixed with the air and the resulting air-fuel mixture is combusted in the cylinder 51. Combustion is triggered by a spark from a spark plug 69. Exhaust gases generated as a result of combustion flow pass the exhaust valve 48 into the exhaust manifold 58. The O2 sensor 66 is located in the exhaust manifold 58 and senses the level of oxygen in the exhaust.
As with the diesel engine 10, the gasoline engine 40 purges its exhaust manifold during overrun. Outside air is delivered to the exhaust manifold 58 and the level of oxygen is sensed by the O2 sensor 66. Once the O2 sensor 66 has detected an oxygen level of about 21%, the ECU 64 assumes that the exhaust manifold has been purged of exhaust gases.
Vertical line 83 indicates the start of adaptation for an aged sensor. Line 84, 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). Point 89 represents the calibrated air mass limit or threshold at which adaptation starts. The distance from point 79 to point 83 represents a delay or “wasted adaptation release time” due to the negative effects of aging in a sensor.
Line 103 indicates the O2 level at which the sensor displaying unexpected behavior begins calibration. Vertical line 107 indicates the start of adaptation (at a time t1) for an aged sensor (or a sensor that is not displaying unexpected behavior). The distance between lines 101 and 103 is 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., ECU 12 or 64) 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.
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).
Various features and advantages of the invention are set forth in the following claims.
