It is generally known to convert toxic components of the exhaust gas of an internal combustion engine such as HC, NO.sub.x and CO by means of a catalyzer which is mounted in the exhaust gas system of the engine. The toxic components are converted into non-poisonous gases to the greatest extent possible.
What is however decisive for the so-called degree of conversion is that the oxygen content of the exhaust gas lies within optimal values. For a so-called three-way catalyzer, these optimal values lie in a narrow range about the value which corresponds to an air/fuel mixture of lambda equals 1.
In order to maintain this tight range, it is conventional to control the air/fuel ratio for an engine by means of oxygen probes which are disposed in the exhaust gas system of the engine.
The control operation can be accelerated especially in transition regions. For this purpose, and in addition to the control based on the signal of the oxygen probe, the determination of a so-called precontrol value takes place based upon the operating characteristic variables of the engine such as the air quantity Q supplied thereto and the engine speed n. The determination of the air quantity Q can take place in different ways such as by determining the opening angle of a throttle flap or based on the signal of an air flow sensor.
The precontrol value determined on the basis of Q and n is corrected in accordance with the signal of the oxygen probe in such a manner that the optimal air/fuel mixture is determined. This corrected signal then controls a fuel metering arrangement which meters the optimal quantity of fuel to the engine.
If a fuel injection unit is utilized as the fuel metering arrangement, then the drive signal supplied thereto constitutes a so-called injection time ti which, for the required conditions such as constant fuel pressure ahead of the injection valves and the like, is a direct measure for the fuel quantity supplied per work stroke.
The drive signal for other fuel metering arrangements is determined in a corresponding manner. This is known to persons in the field and the description which follows will be made with reference to a fuel injection unit but the invention should not be construed as to be limited thereto.
Published international application WO90/05240 discloses a system wherein two lambda probes are used to control the air/fuel mixture. A first one of the probes is disposed ahead of the catalyzer and the second one downstream of the catalyzer.
The signal of the second lambda probe is compared to a desired value and the difference of the two values is integrated and the value obtained in this way functions as the desired value for the signal of the first lambda probe.
It has also been shown that modern three-way catalyzers exhibit a gas storage capability and especially an oxygen storage capability of approximately 1.5 liters.
This means that when the engine emits an exhaust gas composition having an increased oxygen content, which corresponds to a lean air/fuel mixture, this is partially stored in the catalyzer.
For a rich air/fuel mixture, the exhaust gas of the engine is deficient in oxygen. In this case, the oxygen stored in the catalyzer is again emitted. As indicated above, the degree of conversion in a region about lambda=1 is optimal. If the engine is now supplied with a rich air/fuel mixture and the catalyzer supplies a portion of its stored oxygen, then this leads temporarily to an increase in the degree of conversion compared to that degree of conversion which corresponds to the air/fuel mixture which is supplied.
The evaluation of the gas storage capacity of a catalyzer is disclosed in U.S. Pat. No. 4,231,334. A system is disclosed here for determining the proportions of the air/fuel mixture supplied to an engine which utilizes the gas storage effect of a catalyzer.
The system described in U.S. Pat. No. 4,231,334 is applied in internal combustion engines which have at least two oxygen probes in their exhaust gas system and wherein the output signals are integrated and are utilized in a supplementary manner for precontrol for the constituent determination of the air/fuel mixture.
The special feature of the system disclosed in U.S. Pat. No. 4,231,334 is that the value computed by the mixture preparation unit for the composition of the mixture is wobbled about a pregiven value such as .lambda.=1. It has been further shown that exhaust gas catalyzers have, in a specific manner, a gas storage capacity which can be described as a first approximation by a delay of the first order. Accordingly, if the composition of the mixture to be combusted is wobbled at a relatively high frequency for example with a wobble frequency of f.sub.min &gt;2 Hz about a pregiven lambda value, approximately .lambda.=1, then it can be expected that the catalyzer acts on the exhaust gas composition so as to form a mean value.
The system disclosed in U.S. Pat. No. 4,231,334 does not however permit a targeted enrichment or leaning of the air/fuel ratio about a pregiven desired value whereby the gas storage effect of the catalyzer can be utilized in a still better manner and the toxic components of the exhaust gas can be considerably reduced.