Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
1999-09-13
2001-08-28
Denion, Thomas (Department: 3748)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
C073S023210, C073S023320, C073S118040, C204S401000, C204S424000, C205S784500, C060S274000
Reexamination Certificate
active
06279372
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the automotive technology field. More specifically, the invention relates to a method of correcting the characteristic of a linear lambda probe arranged upstream of a catalytic converter in an emission control system of an internal combustion engine.
In internal combustion engines which operate according to the spark-ignition principle (Otto engines), the quality of the combustion air is usually set via a throttle valve arranged in the intake pipe. However, it is also possible to control the inlet valves such that these, for example, electromechanical valves take over this function.
In order to clean the exhaust gas of an internal combustion engine, a three-way catalytic converter is usually arranged in the exhaust gas tract of the internal combustion engine. Provided upstream of this catalytic converter is a linear lambda probe whose output signal is dependent on the residual oxygen component contained in the exhaust gas. The residual oxygen component, in turn, is dependent on the mixture which has been fed to the internal combustion engine. The residual oxygen component in the raw exhaust gas is lower for fuel excess (rich mixture), and higher for air excess (lean mixture).
A linear lambda probe supplies a unique, monotonically rising signal in a wide lambda range (0.7 to 4). The signal is converted into a lambda value by means of a characteristic curve in a control unit. The internal combustion engine is controlled such that the lambda probe displays a value assigned to lambda=1. Since a three-way catalytic converter shows optimum catalytic properties for a raw exhaust gas with lambda=1, the predetermined mean value or the value assigned to lambda=1 should then also actually correspond to lambda=1. In other words, the characteristic must contain the correct assignment of signal and lambda value.
The dynamic and static properties of the lambda probe upstream of the three-way catalytic converter undergo change through aging and poisoning, however. As a result, the probe signal corresponding to lambda=1 shifts its position. In order to be able to correct this, it is known in accordance with the prior art to arrange downstream of the three-way catalytic converter a further lambda probe which is used as monitor probe to monitor the catalytic conversion and permits fine control of the mixture. For this purpose, the assignment of the signal of the lambda probe to the lambda value is corrected so that the lambda value most favorable for the conversion can always be observed. That method is referred to as guiding control or trimming control.
The correction therefore corresponds to shifting the characteristic curve. However, because of aging or a certain component tolerance, the gradient of the characteristic of the linear lambda probe can also deviate from the characteristic stored in the control unit. The consequence of such a deviation is that in the case of lambda values not equal to 1 the control unit converts the signal of the lambda probe into a defective lambda value. This error caused by an erroneous characteristic gradient becomes larger the more the lambda value of the exhaust gas deviates from lambda=1. This error can assume intolerable magnitudes, particularly in the case of lean-burn operation of the internal combustion engine.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of correcting the characteristic of a linear lambda probe, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows a proper correction of the gradient of the characteristic.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of correcting a characteristic of a linear lambda probe arranged upstream of a catalytic converter in an emission control system of an internal combustion engine, which comprises:
in an overrun fuel cut-off phase of the internal combustion engine:
opening a valve to set a defined minimum pressure in a cylinder of the internal combustion engine;
assigning a signal of the lambda probe to a lambda value corresponding to an oxygen concentration of ambient air; and
correcting a gradient of the characteristic with the signal.
In other words, an overrun fuel cut-off phase of the internal combustion engine is used to correct the characteristic of the linear lambda probe. In such an overrun fuel cut-off phase, the exhaust gas of the internal combustion engine theoretically always has the oxygen concentration of the ambient air, since no combustion is taking place. The linear lambda probe then would indicate consistently the value corresponding to lambda=1. Experimental investigations have shown, however, that the signal of the linear lambda probe also fluctuates in the overrun fuel cut-off phases. In this case, the fluctuation is so large that a sufficiently precise correction of the characteristic is not possible. The fluctuation determined was ±5%.
Further investigations led to the finding that the fluctuation of the signal of the lambda probe in overrun fuel cut-off phases is caused by hydrocarbon and carbon monoxide emissions of the oil of the internal combustion engine, since the lambda probe exhibits a cross sensitivity to the substances, and therefore the signal of the lambda probe then fluctuates impermissibly even given a constant oxygen content in the exhaust gas.
It has emerged that the hydrocarbon/carbon monoxide emissions of the oil no longer occur to an extent incapacitating the correction when the pressure in the internal combustion engine exceeds a certain threshold, because the emissions from the oil are suppressed by reduction of the underpressure, normally prevailing in overrun phases, in the cylinder/cylinders. The signal of the linear lambda probe then has a constancy sufficient for correcting purposes.
Thus, according to the invention, the throttle valve and/or at least one gas exchange valve is opened to carry out the correction of the characteristic gradient.
In accordance with an added feature of the invention, the emission control system of the internal combustion engine comprises a measuring sensor detecting a substance concentration in an exhaust gas disposed downstream of the catalytic converter, and the correcting step comprises:
using the signal of the measuring sensor to correct an assignment of a signal to the lambda probe to lambda=1; and
determining a corrected gradient of the characteristic from the signal of the lambda probe assigned to lambda=1 and from the signal assigned to the lambda value corresponding to the oxygen concentration of the ambient air.
In accordance with an additional mode of the invention, the above-noted step of correcting the gradient comprises:
calculating a correction factor from the signal assigned to the lambda value corresponding to the oxygen concentration, and a reference value for the signal; and
calculating the corrected characteristic from a characteristic having the reference gradient, and the correction factor.
In accordance with another mode of the invention, when the calculated correction factor lies outside a prescribed range, the correction factor is set to a neutral value and the lambda probe is diagnosed as defective.
In accordance with a further mode of the invention, the signal of the lambda probe is temporarily smoothed prior to the step of assigning the lambda value corresponding to the oxygen concentration of the ambient air.
In accordance with yet an added feature of the invention, the assigning step and the step of correcting the characteristic gradient are performed only when a variation in the signal of the lambda probe to be assigned to the lambda value corresponding to the oxygen concentration of the ambient air lies below a threshold value.
In accordance with yet an additional feature of the invention, the method processing is held in abeyance in overrun fuel cut-off phases during which t
Denion Thomas
Greenberg Laurence A.
Lerner Herbert L.
Nguyen Tu M.
Siemens Aktiengesellschaft
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