Ever stricter legal requirements with respect to permissible emissions of pollutants of motor vehicles with internal combustion engines require emissions of pollutants to be kept as low as possible during the operation of the internal combustion engine. This can be done, on the one hand, by reducing the emissions of pollutants which occur during the combustion of the air/fuel mixture in the respective cylinders. On the other hand, exhaust gas post-treatment systems are used in internal combustion engines, said systems converting the emissions of pollutants which are produced during the combustion process of the air/fuel mixture in the respective cylinder into non-damaging substances.
For this purpose, catalytic converters are used which convert carbon monoxide, hydrocarbons, and nitrogen oxides into non-damaging substances.
Both the reduction of emission of pollutants during the combustion and the conversion of the pollutant components with a high level of efficiency by means of a catalytic converter require a very precisely set air/fuel ratio in the respective cylinder.
The manual “Handbuch Verbrennungsmotor [internal combustion engine manual]”, published by Richard von Basshuysen, Fred Schafer, 2nd Edition, Vieweg & Sohn Verlagsgesellschaft mbH, June 2002, pages 559 to 561, discloses a binary lambda control system with a binary lambda probe which is arranged upstream of the exhaust gas catalytic converter. The binary lambda control system comprises a PI controller, wherein the P and I components are stored in characteristic diagrams of the engine speed and the load. In the binary lambda control system, the excitation of the catalytic converter, also referred to as lambda fluctuation, occurs implicitly as a result of the two-point control. The amplitude of the lambda fluctuation is set to approximately 3%.
In order to satisfy legal requirements relating to the emissions of pollutants, catalytic converters are moved increasingly close to the engine. Said catalytic converters may operate well with very low tolerance in the air/fuel ratio in the individual cylinders of an exhaust gas bank, specifically a significantly lower tolerance than is the case with an arrangement of the catalytic converters remote from the engine, owing to the short mixing section from the outlet valves to the catalytic converter. In this context, a cylinder-specific lambda control can be used.
DE 19846393 A1 discloses cylinder-selective control of the air/fuel ratio in a multi-cylinder internal combustion engine having a lambda probe which is embodied as a discrete-level probe. Within the scope of the cylinder-selective control, the voltage difference of the lambda probe voltage signal of one cylinder is formed in relation to the voltage signals of the adjacent cylinders. The injection is then corrected with the difference value. This takes into account the fact that it is actually the strong change in the probe voltage in the region of the precisely stoichiometric air/fuel ratio which permits even small differences from an optimum air/fuel ratio to be detected.
DE 10 2007 015 362 A1 discloses a lambda control method in an internal combustion engine having at least one discrete-level lambda probe which is arranged in an exhaust-gas-conducting exhaust gas system of the internal combustion engine upstream of an exhaust gas purification device and which is configured to generate a first measurement signal which is dependent on the oxygen content of the exhaust gas. The first measurement signal is used and corrected at at least two points.
DE 10 2007 016 276 A1 discloses a binary lambda control system. A change of the mixture of the internal combustion engine is pilot-controlled by means of the engine control system. A stored characteristic curve of the lambda probe is adapted using the pilot-controlled changes of the mixture.
DE 10 2007 029 029 A1 describes a lambda control method in an internal combustion engine with an engine control system for mixture formation and a front lambda probe, arranged in an exhaust gas system of the internal combustion engine, for detecting a front probe signal which characterizes a front oxygen content of an exhaust gas which is conducted in the exhaust gas system, upstream of a catalytic converter arranged in the exhaust gas system. In addition, arranged upstream of the catalytic converter in the exhaust gas system is a rear lambda probe for generating a rear probe signal which characterizes a rear oxygen content of the exhaust gas which is conducted downstream of the catalytic converter in the exhaust gas system. In order to make available an improved lambda control system provision is made to determine a lambda value by means of the front probe signal and a conversion rule. In addition there is provision to determine a lambda difference from the rear probe signal and to adapt the conversion rule by means of the determined lambda difference.
WO 2013/045526 A1 discloses determining a correction signal for a fuel mass which is intended to be metered, by means of a lambda controller, in particular as a function of a measurement signal of an exhaust gas sensor. If at least one predefined condition is satisfied, within a predefined operating range of the internal combustion engine, a cylinder-specific diagnosis is carried out which relates to the emissions of pollutants on the basis of a consideration of unsmoothed running, wherein the cylinder-specific diagnosis is carried out actively only in a corresponding detection phase in which a modified integral parameter of the lambda controller is used, which integral parameter is reduced in terms of its absolute value compared to the integral parameter in a normal operating mode of the lambda controller.
DE 10 2005 009 101 B3 describes a method for determining a correction value for influencing an air/fuel ratio in a respective cylinder of an internal combustion engine having a plurality of cylinders, injection valves assigned to the cylinders and an exhaust gas probe which is arranged in an exhaust gas section and whose measurement signal is characteristic of the air/fuel ratio in the respective cylinder. At a predefined sampling crankshaft angle related to a reference position of the piston of the respective cylinder, the measurement signal is detected and assigned to the respective cylinder. A controller value for influencing the air/fuel ratio in the respective cylinder is determined by means of one controller in each case, as a function of the measurement signal detected for the respective cylinder. A first adaptation value is determined as a function of the controller value when predefined first conditions are met, said conditions including a predefined first temperature range of a temperature which is representative of a temperature of the respective injection valve and including the fact that a quasi-steady-state operating state is present. When predefined second conditions which include a predefined second temperature range of the temperature which is representative of the temperature of the respective injection valve and which include the fact that a quasi-steady-state operating state is present are satisfied, a second adaptation value is determined as a function of the controller value.
The correction value for influencing the air/fuel ratio in the respective cylinder is determined as a function of the first or second adaptation value as a function of the temperature which is representative of the temperature of the respective injection valve.
DE 10 2008 058 008 B3 describes a device for operating an internal combustion engine with a plurality of cylinders, which are each assigned an injection valve, an exhaust gas section which comprises an exhaust gas catalytic converter, and a lambda probe which is arranged upstream or in the exhaust gas catalytic converter.
An assignment unit is provided which is designed to determine cylinder-specific lambda signals as a function of the measurement signal of the lambda probe and to determine lambda difference signals for the respective cylinders as a function of the cylinder-specific lambda signals, with respect to a lambda signal which is averaged over the cylinder-specific lambda signals.
In addition, an observer is provided which comprises a sensor model of the lambda probe, which sensor model is arranged in a feedback branch of the observer. The observer is embodied such that the cylinder-specific lambda difference signals are fed to it on the input side, and observer output variables related to the respective cylinder are representative of differences of the injection characteristic of the injection valve of the respective cylinder from a predefined injection characteristic.
In addition, a parameter detection unit is provided which is designed to impress an interference pattern composed of cylinder-specific mixture differences, in reaction to the respectively predefined interference pattern, to change at least one parameter of the sensor model as a detection parameter, until at least one of the observer output variables represents, in a predefined fashion, the portion of the interference pattern which is assigned to its cylinder, and the at least one detection parameter is output.