Method for detecting combustion misfires and cylinder equalization in internal combustion engines with knock control

Methods for determining engine rough running by evaluating the engine rpm are used for spark-ignition engines as well as in diesel engines in order to detect a non-uniform running of the engine and to minimize this non-uniformity via suitable control arrangements. The non-uniform running of the engine can be caused, for example, by valve coking or, in direct injection engines, by scattering of the characteristic values of the injection valves.

Methods of this kind utilize the realization that a non-occurring or incomplete combustion within a cylinder of an engine is associated with characteristic changes of the torque trace of the engine compared to the normal operation.

From the comparison of the torque traces, one can distinguish between normal operation of the engine without misfires and so-called operation associated with misfires or non-optimal combustion. An incomplete or poor combustion of one or several cylinders contributes to a total torque trace of the engine with a reduced contribution. This contribution can be determined from a detection of the actual torques of the cylinders via an evaluation of the time-dependent trace of the crankshaft rotation or the camshaft rotation.

According to the method of the invention, a crankshaft angular region characterized as a segment is assigned to a specific region of the piston movement of each cylinder. The segments belonging to each cylinder are realized by markings on a transducer wheel coupled to the crankshaft. The segment time is the time in which the crankshaft passes through the corresponding angular region of the segment and is dependent essentially on the energy converted in the combustion stroke. Misfires or a poor combustion lead to an increase of the ignition synchronously detected segment times as a consequence of the deficient torque contribution. These segment times are determined for each cylinder by scanning the markings on the transducer wheel with a suitable sensor. The more uniform the engine runs, the smaller will be the differences between the segment times of the individual cylinders.

In accordance with methods already known from the state of the art, such as disclosed in German patent publication 4,138,765 (corresponding to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned), an index for the rough running of the engine is computed from differences of the segment times. Additional conditions such as the increase of the engine rpm for a vehicle acceleration are compensated by computation. The rough-running value, which is computed for each ignition, is compared in a subsequent method step in synchronism with the ignition in a desired value comparison to a threshold value. If the determined rough-running value exceeds the threshold value, which is dependent upon operating parameters such as load and rpm, then this is evaluated, for example, as a misfire of the particular cylinder.

In a further method step, equalization or correction factors are formed for individual cylinders in an evaluation unit and, with the aid of these factors, injection times, ignition time point times, or the charge of the individual cylinders, which are affected by the torque changes, can be influenced. For example, a change of the ignition time point can change the torque component of a cylinder. Furthermore, by influencing injection times and injection duration, the differences in the injection performance of injection valves can be compensated. Furthermore, for a system with cylinder-individual adjustment of the cylinder charge (for example, via individual throttle flaps or fully variable inlet and/or outlet valves), the cylinder individual charge can be adapted.

The above-described method, which is known from the state of the art, has been proven in the context of engine management systems for bringing about a cylinder equalization. The cylinder-individual interventions undertaken here can, however, lead to the situation that a knocking combustion in one or several cylinders, which occurs under specific conditions, is additionally amplified. For this reason, in known engine management systems, which have a knock control in addition to the cylinder equalization, a use of both systems at the same time is precluded. This can be attributed to the situation that the changes of the ignition angle, which are undertaken because of the knock control, can lead to a change of the rough-running value. These changes of the ignition angle are caused, for example, by an ignition angle retardation. Should the cylinder equalization by means of injection time correction be active in this case simultaneously with the knock control, then the cylinder equalization for reducing the rough-running values would effect an enrichment of the gas mixture because of changed injection times in the cylinder subjected to knocking combustion.

However, an enrichment of the above kind perforce leads to a further increase of the knocking combustion within the cylinder during the combustion so that neither the objective of a quiet engine running nor the elimination of the knocking combustion can be achieved by the cylinder equalization and the knock control of the engine management system.

From the state of the art, possibilities are known for acting on the operating state of internal combustion engines. In contrast thereto, the method of the invention affords the advantage that, for the first time, a knock control and a cylinder equalization can be combined without the concern that there will be a larger misidentification in the detection of misfires in the context of the cylinder equalization and, on the other hand, without the knock tendency of the particular engine being increased because of the measures of cylinder equalization. This takes place in accordance with the invention in that the equalization factors (that is, correction factors) for the change of the injection times or the ignition time points serve as the basis for the determination of a rough-running increase value effected by the change and in that, with the determined rough-running increase value, a computation of a corrected rough-running value takes place. The above-mentioned equalization factors and/or correction factors are computed in the context of the rough-running computation for the cylinder equalization. The corrected rough-running value is used for the computation of the final equalization factor and/or correction factor for influencing the injection times or the ignition time points.

Because of the fact that the rough-running value, which is decisive for the final computation of the equalization factors and/or correction factors, is subjected to a correction step in advance of the computation, the rough-running value increase, which takes place normally without correction, is eliminated so that an unwanted interaction between knock control and cylinder equalization is avoided.

The rough-running increase value, which is to be determined, can, according to the invention, be determined in different ways. Thus, it is conceivable to determine this value from the computed equalization factor and/or correction factor via a characteristic field computation. The relationship between the rough-running increase, which is to be determined, and an ignition angle shift, which is to be carried out, is stored in a characteristic line within the engine management system so that the rough-running increase can be determined in a simple manner. Furthermore, the possibility is present to determine the rough-running increase value from the torque as a function of the ignition angle efficiency. This affords the advantage that the relationship between torque and ignition angle efficiency is already stored in the engine system as a function. Furthermore, it is conceivable to determine the rough-running increase value as a function of the injection time span in the expansion phase of the particular cylinder (so-called double injection). This applies also for the individual charge of a cylinder. Here too, the charge difference to the other cylinders can be considered via the rough-running increase value.

The invention will now be described with reference to the single FIGURE (FIG. 1) of the drawing which shows the sequence of steps of the method of the invention in the context of a block diagram.

FIG. 1 shows a method for a cylinder of a multi-cylinder internal combustion engine by way of example. The sequence of the method steps for the corresponding remaining cylinders takes place in correspondence to the illustrated block diagram in the components of the engine management system which are provided for the method.

In FIG. 1, the internal combustion engine M is shown as a block. A cylinder-individual segment time t s is determined in the engine M by means of a transducer wheel on which the individual segments are provided. The transducer wheel is connected to the crankshaft or to the camshaft. This segment time t s can be extended compared to the normal engine operation in the event that an incomplete combustion or a combustion misfire takes place in the corresponding cylinder. The segment time t s , which is determined via suitable sensors by means of the transducer wheel, is next supplied to a computation block 1 which computes cylinder-individual rough-running values LUT from the segment times t s . The block 1 includes a filter element wherein the determined rough-running values LUT are subjected to filtering. The values LUT filtered in this manner are identified as FLUT. The LUT/FLUT-values are next subjected to a desired value comparison with a threshold value SW in a comparison block 2 . If the desired value comparison yields that the cylinder-individual rough-running values LUT/FLUT are greater than the threshold value SW, then the amount of the LUT value is transmitted further to a PI-controller 3 . 1 . The LUT value is a direct index for the control deviation from the normal state within the particular cylinder which is intended to be eliminated in the context of the cylinder equalization. The PI-controller 3 . 1 determines equalization values or correction values GL from the control deviation for the particular cylinder. These values GL are decisive for an adaptation of the injection time and/or of the ignition time point and/or of the charge of the particular cylinder in order to effect a return into the normal state. To this extent, the method steps explained above are from methods for cylinder equalization known from the state of the art. As output quantities of the PI-controller 3 . 1 , the rough-running value LUT as well as the equalization factor GL 1 are shown in FIG. 1 . The computed equalization factor GL 1 is supplied to a computation module in the next method step. This computation model determines a rough-running increase value dLUT in dependence upon the input quantity GL 1 . This can take place, for example, via a characteristic field contained in the computation module wherein the relationship is stored between an ignition angle shift, which is to be undertaken, and/or an injection time correction and/or the charge correction or charge change of the rough-running increase, which results therefrom.

The characteristic field values can, however, also be determined via application. Furthermore, it is conceivable to determine the rough-running increase value, for example, from the torque as a function of the ignition angle efficiency or as a function of the injection time or as a function of cylinder-individual charge differences or as a function of the injection time span in the expansion phase (with a double injection) of the particular cylinder. The decision, in which way the rough-running increase value is computed, results from the other conditions of the engine management system. The output quantity of the computation block 4 . 2 , the rough-running increase value dLUT, serves subsequently as the input quantity of the block 4 . 3 wherein a corrected rough-running value LUT corrected is determined. This rough-running value LUT corrected is determined from the original uncorrected rough-running value LUT while considering the rough-running increase value via subtraction. This corrected rough-running value is subsequently supplied to a computation block 3 . 2 wherein the final equalization factors for the individual cylinders are determined analog to the computation block 3 . 1 . The final equalization factors are needed for the cylinder equalization.

Because of the fact that the rough-running value is subjected to a correction method, it is precluded that the function of a knock control is negatively affected.

Furthermore, by applying the corrected rough-running values, the recognition quality of a misfire identification is improved which takes place usually by means of a comparison of the rough-running value with a threshold value.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.