Fault detection of a motor vehicle oxygen sensor

Abstract: This invention relates to the fault detection of an oxygen gas sensor (14) in a motor vehicle, where the oxygen gas sensor (14) is used to measure the oxygen content of the combusted air fuel mixture of a motor vehicle exhaust. The method involves measuring the fall time for the sensed oxygen level to fall to a pre-determined lower threshold after the fuel supply to the engine (12) has been cut off, and if the measured fall time exceeds a pre-set time, producing an oxygen sensor fault signal (22).

Description: This invention relates to the fault detection of an oxygen gas sensor in a motor vehicle, in particular where the oxygen gas sensor is used to measure the oxygen content of the combusted air fuel mixture of a motor vehicle exhaust. In order to improve the efficiency of an internal combustion engine in a motor vehicle, an oxygen sensor is often used to sense the oxygen content of the exhaust gas, and the air-fuel mixture admitted to the engine is adjusted by the engine management system according to the sensed oxygen level of the exhaust gas. As the oxygen sensor deteriorates with age, the response time of the oxygen sensor can increase, leading to a maladjusted air-fuel mixture and a reduced engine efficiency. A known method of monitoring the efficacy of the oxygen sensor involves measuring the response of the oxygen sensor when the amount of fuel admitted to the engine is forcibly changed during feedback control, as disclosed in US 5,685,284. However, this method is complicated and requires a degree of accuracy in the control of the fuel supply. Document US 5 672 817 discloses a further method of failure determination for an oxygen sensor. In this method the time which the sensor signal needs to drop below a threshold after the fuel supply reinstatement is measured. According to a first aspect of the present invention, there is provided a failure determination method for an oxygen sensor that detects oxygen concentration in the exhaust gas from an internal combustion engine, characterised in that the method comprises the steps of: cutting off the fuel supply to the internal combustion engine and allowing the oxygen content of the exhaust gas to rise; reinstating the fuel supply after the oxygen level has risen above a pre-determined upper threshold; from the moment fuel is reinstated, measuring the fall time for the sensed oxygen level to fall to a pre-determined lower threshold; and if the measured fall time exceeds a pre-set time, producing an oxygen sensor fault signal. Since the failure determination method does not involve feedback control of the fuel supply, the accuracy in the control of the fuel supply is not important in determining whether the oxygen sensor has failed. The internal combustion engine may be in a motor vehicle, and the oxygen sensor may be placed in the motor vehicle exhaust system, in order to monitor the exhaust gas emitted from the motor vehicle. The fuel supply may conveniently be cut off by an engine management system when an accelerator pedal controlling the fuel supply is released, to ensure that the failure determination method does not affect the fuelling of the engine. Cutting off the fuel supply will normally result in a rapid rise in the oxygen content of the exhaust gas since the angular momentum of the engine or the momentum of a vehicle driven by the engine will keep the engine turning and drawing in air after the fuel has been cut off. The upper threshold in the oxygen content after which fuel is reinstated need not be sensed, and may therefore be assumed to have been reached after a pre determined time interval after fuel cut off has occurred, but preferably the oxygen sensor is used to determined when the upper threshold has been reached. The upper threshold may be the oxygen concentration at which the oxygen sensor saturates, and the lower threshold will typically be fixed at a value between 70% and 85% of the upper threshold oxygen concentration. However, the lower threshold may be varied as a function of the reinstated fuel level in order to take into account any effect of the reinstated fuel level on the actual oxygen content in the exhaust. After the upper threshold has been reached, the fuel may be reinstated by the engine management system when the accelerator pedal is depressed, or alternatively the fuel may be reinstated just before the engine speed has dropped to a low enough value for the engine to stall, so that in either case the failure determination method does not interfere with the fuelling of the engine. To provide reproducible starting conditions, the fall time for the sensed oxygen content to reach the lower threshold may be measured from the moment the engine management system issues a command signal for fuel reinstatement. The engine management system may provide a command signal for fuel reinstatement that comprises a single step, so that fuel reinstatement is as abrupt as possible. The fall time may conveniently be measured by a counter-timer that is set to run by a microprocessor when the microprocessor senses the negative edge of the command signal for fuel reinstatement issued by the engine management system. The counter-timer may be re-set to zero by the microprocessor after the lower threshold has been reached, but preferably the counter-timer will be re-set before fuel reinstatement. The pre-set time at which the oxygen level fall time is deemed excessive and a fault signal is produced may be set as a function of the reinstatement fuel level and the value for the lower threshold, but typically, the pre-set time will be fixed at about 2 s ±20%. The oxygen sensor fault signal produced if an excessive fall time is measured may cause a light or other warning device to turn on in order to alert the person operating the engine that the oxygen sensor needs replacement. The failure determination method may be carried out on board a vehicle as it is travelling, rather than in a garage, for example. According to a second aspect of the present invention, there is provided a fault detector for detecting a fault in an oxygen sensor that senses the oxygen concentration of the exhaust gas of an internal combustion engine with an engine management system, characterised in that the fault detector comprises: a microprocessor having a counter-timer and being adapted to receive signals from the engine management system and the oxygen sensor, wherein if the microprocessor receives a command signal from the engine management system indicating that fuel to the engine has been cut off, followed by a signal from the oxygen sensor indicating that the sensed oxygen level has reached an upper threshold, the microprocessor is adapted to measure the elapsed time from the moment the engine management system issues a command for fuel reinstatement until the sensed oxygen has fallen to a lower threshold value, and if the elapsed time is greater than a pre-set time, to issue an oxygen sensor fault signal. The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a block diagram for a fuel controller according to the invention; Figure 2 is a graph illustrating how a normal oxygen sensor and a faulty oxygen sensor can be distinguished according to the invention, and; Figure 3 is a flow diagram showing the steps of the failure determination method. In Figure 1, an engine management system (EMS) 10 controls the air and fuel input to an internal combustion engine 12 of a motor vehicle by issuing a command signal 15 to the engine 12. In order to optimise the ratio of the air-fuel mixture admitted to the engine 12, the engine management system 10 takes into account the oxygen content of the exhaust gas as sensed by an oxygen sensor 14. When an accelerator pedal controlling the engine throttle 16 is depressed or released, the command signal 15 passed to the engine 12 causes fuel to the engine to be respectively reinstated or cut off. The command signal 15 from the engine management system 10 is also passed to a microprocessor (µP) 18 connected to the oxygen sensor 14 and a counter-timer (T) 20. (The microprocessor 18 could be integrated into the engine management system 10, but is shown here as a separate component). If the oxygen level sensed by the sensor 14 is above an upper threshold immediately before a command signal 15 for fuel reinstatement is issued by the engine management system 10, the microprocessor 18 is able to reset and cyclically increment the counter-timer 20 until the sensed oxygen level reaches a lower threshold. The microprocessor 18 is able to read the counter-timer 20 so that if the time for the sensed oxygen level to reach the lower threshold exceeds a pre-set time, the microprocessor 18 can send an oxygen sensor fault signal 22 to a vehicle instrument panel (IP) 24 where for example a warning light will light up. Although in Figure 1 the oxygen sensor 14 is connected directly to the microprocessor 18, the oxygen sensor could alternatively be connected indirectly to the microprocessor via the engine management system 10. Figure 2 shows experimental traces in arbitrary units for the sensed oxygen content of the exhaust gas of the internal combustion engine 12, in this example a V8 4 litre engine, as a function of time when the oxygen sensor 14 is in the normal state and in the failed state. Here time is measured in units of seconds. The command signal 15 from the engine management system 10 governing the fuel supply varies with time so as to produce a trace in the form of a top hat as shown in Figure 2. A high command signal 15 indicates that fuel to the engine 12 is cut off, whereas a low command signal indicates that fuel is being supplied to the engine 12. Initially, at T = 175 s, the command signal 15 is low, so fuel is being supplied to the engine, and the oxygen content of the exhaust gas is low. This represents the steady state fuelling of the engine 12, when the accelerator pedal controlling the engine throttle 16 is depressed. When the accelerator pedal is released, the command signal 15 changes from low to high as indicated in Figure 2 at about T = 178 s, and fuel cut-off takes place. The oxygen levels sensed by the failed sensor and the normal sensor rise quickly to a common saturation value since only air is being drawn into the engine. When the accelerator pedal is depressed again, the command signal 15 changes from high to low abruptly, in a stepwise fashion, and the oxygen levels sensed by the failed sensor and the normal sensor both drop, but at a different rate, the oxygen level sensed by the failed sensor taking longer to drop that that sensed by the normal sensor. The response time of the normal sensor and the failed sensor can be compared from the time at which the sensed oxygen level drops below a lower threshold value, here about 80 % of the maximum sensed oxygen concentration as indicated by the dotted line in Figure 2. The oxygen level sensed by the normal sensor reaches the lower threshold about 0.95 s after fuel reinstatement has been initiated, as measured from the negative edge of the command signal step. In contrast, with the failed sensor the sensed oxygen level reaches the lower threshold about 2.65 s after fuel has been reinstated. The time taken for the sensed oxygen level to fall to the lower threshold is due to the fall time of the actual oxygen concentration and the response time of the oxygen sensor. Since the actual oxygen fall time in the traces for the normal sensor and the failed sensor is expected to be similar, the difference in the sensed fall times, here about 1.7 s, is due to the increased response time of the failed sensor. The response time of a normal sensor, here a Universal Heated Exhaust Gas Oxygen sensor, is typically about 10 ms, a very short time on the time scale of Figure 2, and about two orders of magnitude lower than the increase in response time of the failed sensor. Although the difference in the response times of the two sensors could in principle be measured when the oxygen level is rising, just after fuel cut off, the difference is relatively small as can be seen from Figure 2, making the measurement more difficult. The failure determination procedure can be more clearly described with reference to Figure 3, which is a flow diagram of the steps involved, carried out by the microprocessor 18. First, the microprocessor 18 waits until it receive a command signal 15 indicating that that the engine 12 is in fuel cut mode. The engine will be in fuel cut off mode after the accelerator pedal controlling the engine throttle 16 has been released (this is the situation at T = 178 s in Figure 2 when the command signal has risen to a high value). Before the failure determination procedure can continue, sufficient time must have elapsed for the fuel to be flushed out of the engine so that the oxygen sensor reaches saturation and produces a lean response (in Figure 2 this occurs at approximately T = 180 s). So when the microprocessor 18 has received a signal 17 from the oxygen sensor 14 indicating that the sensed oxygen level has saturated, the microprocessor 18 sets to zero the response counter-timer 20 in preparation for the next step in the procedure. The next step occurs when the engine management system 10 issues a command signal 15 that changes to low, indicating the onset of fuel reinstatement, which in Figure 2 occurs at T = 186 s. The negative edge of the step when the command signal 15 changes from high to low triggers the microprocessor 18 which increments the counter-timer 20 in a cyclic fashion until the microprocessor 18 receives a signal 17 from the oxygen sensor 14 indicating that the sensed oxygen level is below the pre-set lower threshold shown in Figure 2. The microprocessor 18 then ceases to increment the counter-timer 20 and if the counter-timer reading is above a pre-determined value, a failure condition is set, and the microprocessor 18 sends a fault signal 22 to the vehicle instrument panel 24 that consequently displays a warning to show that the oxygen sensor is faulty. (The pre-set time reading above which the fault signal 22 is sent is 2 s for the V8 4 litre engine 12 used in producing the graph of Figure 2, but the pre-set time may be different with a different engine or if the sensor 14 is placed in a different position in the engine exhaust system). The driver of the vehicle is thereby informed that the oxygen sensor 14 requires attention, and can take the vehicle to a garage for corrective action.

Claims: A failure determination method for an oxygen sensor (14) that detects oxygen concentration in the exhaust gas from an internal combustion engine (12), characterised in that the method comprises the steps of: cutting off the fuel supply to the internal combustion engine (12) and allowing the oxygen content of the exhaust gas to rise; reinstating the fuel supply after the oxygen level has risen above a pre-determined upper threshold; from the moment fuel is reinstated, measuring the fall time for the sensed oxygen level to fall to a pre-determined lower threshold; and if the measured fall time exceeds a pre-set time, producing an oxygen sensor fault signal (22). A failure determination method as claimed in Claim 1, wherein the internal combustion engine (12) is in a motor vehicle, and the oxygen sensor (14) is placed in the motor vehicle exhaust system, in order to monitor the exhaust gas emitted from the motor vehicle. A failure determination method as claimed in any preceding claim, wherein the fuel supply is cut off by an engine management system (10) when an accelerator pedal controlling the fuel supply is released. A failure determination method as claimed in any preceding claim, wherein the fuel is reinstated by the engine management system (10) when the accelerator pedal is depressed. A failure determination method as claimed in any preceding claim, wherein the oxygen sensor (14) is used to determined when the upper threshold has been reached. A failure determination method as claimed in any preceding claim, wherein the upper threshold is the oxygen concentration at which the oxygen sensor saturates. A failure determination method as claimed in any preceding claim, wherein the lower threshold is fixed at between 70% and 85% of the upper threshold oxygen concentration. A failure determination method as claimed in any of claims 1 to 6, wherein, the lower threshold is varied as a function of the reinstated fuel level. A failure determination method as claimed in any preceding claim, wherein the fall time for the sensed oxygen content to reach the lower threshold is measured from the moment the engine management system (10) issues a command signal (15)for fuel reinstatement. A failure determination method as claimed in any preceding claim, wherein the engine management system provides a command signal (15) for fuel reinstatement that comprises a single step. A failure determination method as claimed in Claim 10, wherein the fall time for the sensed oxygen content to reach the lower threshold is measured by a counter-timer (20) that is set to run by a microprocessor (18) when the microprocessor (18) senses the negative edge of the command signal for fuel reinstatement issued by the engine management system. A failure determination method as claimed in Claim 11, wherein the counter-timer(20) is re-set to zero by the microprocessor (18) after the lower threshold has been reached. A failure determination method as claimed in any preceding claim, wherein the pre-set time is fixed at 2 s ±20%. A failure determination method as claimed in any one of claims 1 to 12, wherein the pre-set time is set as a function of the reinstatement fuel level and the value for the lower threshold. A failure determination method as claimed in any preceding claim, wherein the oxygen sensor fault signal causes a warning device to turn on. A failure determination method as claimed in any preceding claim, wherein the failure determination method is carried out on board a vehicle as it is travelling. A fault detector for detecting a fault in an oxygen sensor that senses the oxygen concentration of the exhaust gas of an internal combustion engine (12) with an engine management system (10), characterised in that the fault detector comprises: a microprocessor (18) having a counter-timer (20) and being adapted to receive signals from the engine management system (10) and the oxygen sensor (14), wherein if the microprocessor (18) receives a command signal (15) from the engine management system (10) indicating that fuel to the engine has been cut off, followed by a signal from the oxygen sensor (14) indicating that the sensed oxygen level has reached an upper threshold, the microprocessor (18) is adapted to measure the elapsed time from the moment the engine management system (10) issues a command (15) for fuel reinstatement until the sensed oxygen has fallen to a lower threshold value, and if the elapsed time is greater than a pre-set time, to issue an oxygen sensor fault signal (22)

Inventors: KENNEDY TIMOTHY JOHN, KING PAUL JOHN, KENNEDY, TIMOTHY JOHN, KING, PAUL JOHN
Applicants: JAGUAR CARS, JAGUAR CARS LIMITED
Application year: 2000
Main classification: F02D41
IPC codes: F02D41