1. Field of the Invention
The apparatus relates to the field of fuel-air mixture preparation for internal combustion engines. More particularly, it relates to fuel mixture preparation in which a .lambda. or oxygen sensor is used in the exhaust system of the engine to provide a signal related to the presence of oxygen in the exhaust gas and permitting deductions as to the relative richness or leanness of the fuel mixture supplied to the engine. Typically, the .lambda.-sensor signal is fed to a comparator where a comparison is made between the magnitude of the .lambda.-signal and a local, possibly adjustable, set-point voltage. The output of the comparator is then fed to an integrator which engages a final control element in the mixture preparation system to adjust the fuel-air ratio.
2. State-of-the-Art
The use of so-called .lambda.-sensors or oxygen sensors in the exhaust system of an internal combustion engine for providing an actual control value to control the fuel-air mixture is known. When such a sensor is used, the overall system may be identified in the following manner. The carburetor or fuel injection system together with the engine is the overall control system. In that system, the engine itself is the controlled variable and the mixture preparation system is the controller which receives an output signal from the .lambda.-sensor that acts as the actual, operational value for the loop. The nominal or desired fuel-air ratios are determined on the basis of the rpm and the air flow rate aspirated by the engine. For example, fuel injection systems are known which inject fuel intermittently or continuously to the combustion chambers or the induction tube of the engine. One of the problems encountered in such control processes has been the fact that the time constant of the .lambda. control is adjusted for optimum exhaust gas conditions, i.e., the time constant of the controller is held relatively small so as to permit a rapid response to changing operational conditions. However, and especially if the controller is capable of substantial adjustments, the engine often see-saws at idling, i.e., there are periodic rpm changes due to the fact that the time constant of the engine itself is not constant but depends on the engine speed. This means that when the engine runs relatively slowly, for example at idling, the engine time constant is increased due to the slower passage of gases through the engine. This type of increase in the engine response time or engine dead time leads to pronounced control oscillations unless the time constant of the control process is adapted to the changed conditions which occur when the engine idles.