Cylinder-selective control of the air-fuel ratio

Cylinder-selective control of the air-fuel ratio in a multi-cylinder internal-combustion engine is provided, wherein a lambda probe arranged in the exhaust pipe system generates a voltage signal corresponding to an air-fuel ratio. The voltage signal is supplied to a computing unit which determines the air-fuel ratio for each individual cylinder. A fuel metering unit determines a fuel injection quantity at least as a function of a basic fuel injection value and the determined air-fuel ratios of the individual cylinders, and a fuel supply unit supplies the fuel injection quantity determined by the fuel metering unit to the cylinders of the internal-combustion engine. The computing unit crank-angle-synchronously detects the voltage signal and assigns it to a certain cylinder. A voltage deviation and corresponding injection quantity correction is determined for each cylinder in relation to the voltage signals of the adjacent cylinders.

BACKGROUND AND SUMMARY OF THE INVENTION
 This application claims the priority of German Application No. 198 46
 393.6, filed Oct. 8, 1998, the disclosure of which is expressly
 incorporated by reference herein.
 The invention relates to a cylinder-selective control of the air-fuel ratio
 in the case of a multi-cylinder internal-combustion engine as well as to a
 system for implementing-such a cylinder-selective control.
 It is known that a controlled catalyst operation is required for a high
 conversion rate of the pollutants present in exhaust gases. In this
 operation, the exhaust gas composition is monitored by a lambda probe and,
 in the event of a deviation from an air ratio .lambda.=1, the air-fuel
 composition is corrected.
 The lambda probe is normally installed as a sensor in the exhaust gas flow
 in front of the catalyst, specifically behind a junction of the exhaust
 pipes from the individual cylinders. As a result, the lambda probe
 supplies an averaged value concerning the individual cylinders. However,
 as a rule, mixture fluctuations between the individual cylinders are not
 compensated and cause a deterioration of the emissions for two reasons. On
 the one hand, the control frequency of the lambda control is shortened by
 mixture differences. This falsifies the average lambda value set by way of
 control parameters. On the other hand, the flows from the individual
 cylinders, as a rule, flow against different areas of the catalyst. As the
 result of the mixture differences, these areas do not operate in the
 optimal lambda range.
 European Patent documents EP 0 670 419 A1 and EP 0 670 420 A1 describe
 systems for estimating the air-fuel ratio in the individual cylinders of a
 multi-cylinder internal-combustion engine. By means of these systems, the
 mixture fluctuations between the individual cylinders are taken into
 account. In this case, a mathematical model is developed in order to
 describe the system performance as a function of an output signal of a
 broad-band air-fuel sensor. An observation of the development of the
 condition of the mathematical model supplies information on the air-fuel
 ratio in the individual cylinders, whereupon a corresponding adjustment of
 the fuel-air ratio can be performed for each cylinder.
 However, the above-described process requires relatively high computing
 expenditures and is based on the signals of broad-band lambda probes.
 It is an object of the invention to provide a simple, cylinder-selective
 control of the air-fuel ratio in the case of multi-cylinder
 internal-combustion engines of the above-mentioned type, which operates
 reliably for a long operating period and whose development and securing
 expenditures are lower. In addition, a reasonable system is to be
 obtained.
 This object is achieved by a cylinder-selective control process of the
 air-fuel ratio in the case of a multi-cylinder internal-combustion engine,
 wherein a lambda probe arranged in the exhaust pipe system generates a
 voltage signal corresponding to an air-fuel ratio. The voltage signal is
 supplied to a computing unit which determines the air-fuel ratio for each
 individual cylinder. A fuel metering unit determines a fuel injection
 quantity at least as a function of a basic fuel injection value and the
 determined air-fuel ratios of the individual cylinders. A fuel supply unit
 supplies the fuel injection quantity determined by the fuel metering unit
 to the cylinders of the internal-combustion engine. The computing unit
 crank-angle-synchronously detects the voltage signal and assigns it to a
 certain cylinder. A voltage deviation is determined for each cylinder in
 relation to the voltage signals of the adjacent cylinders. A correction of
 the injection quantity is carried out as a function of the voltage
 deviation.
 The object is further achieved by a system for implementing the
 cylinder-selective control of the air-fuel ratio in the case of a
 multi-cylinder internal-combustion engine, wherein a lambda probe is
 provided in the exhaust pipe system for generating a voltage signal
 corresponding to an air-fuel ratio, a determination unit is provided to
 which the voltage signal is fed in order to determine the air-fuel ratio
 for each individual cylinder, a fuel metering unit is provided which
 determines a fuel injection quantity at least as a function of a basic
 fuel injection value and the determined air-fuel ratios of the individual
 cylinders, and a fuel supply unit is provided which supplies the fuel
 injection quantity determined by the fuel metering unit to the cylinders
 of the internal-combustion engine. The determination unit is constructed
 for (1) detecting the voltage signal in a crank-angle-synchronous manner
 and assigning it to a certain cylinder, (2) determining the voltage
 deviation for each cylinder in relation to the voltage signals of adjacent
 cylinders, and for (3) carrying out a correction of the injection quantity
 as a function of the voltage deviation.
 In order to minimize mixture fluctuations between the cylinders,
 working-cycle-synchronous voltage fluctuations of the lambda probes
 selected in the form of surge probes are analyzed and are assigned to the
 individual cylinders. In particular, the voltage deviation of the lambda
 probe voltage signal of a cylinder is formed in relation to the voltage
 signals of the--relative to the ignition sequence--adjacent cylinders. By
 means of the differential value, a correction of the injection will then
 be made.
 According to a preferred embodiment of the invention, a correction value
 for the injection quantity is obtained from a characteristic curve or a
 characteristic diagram.
 In order to reduce the computing expenditures, the cylinder-individual
 mixture adaptation may be switched off above a defined threshold.
 Preferably, two correction values per cylinder are computed for the
 injection quantity, for example, one term for long-period deviations and
 one term for short-period deviations (such as tank ventilation).
 If defined conditions are met for lambda adaptation, the long-period term
 can form an adaptation value for the cylinder mixture adaptation and,
 after the engine is switched off, can be stored in the holding phase of
 the control unit in a non-volatile manner.
 On the whole, the present invention has the advantage that a long operating
 period with high control precision can be used as the basis. In addition,
 surge probes are clearly lower in cost than broad-band lambda probes, so
 that generally lower development and manufacturing costs can be expected.
 Other objects, advantages and novel features of the present invention will
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS
 FIG. 1 illustrates a system for implementing the cylinder-selective control
 according to the invention. In this case, an engine 10 has a plurality of
 cylinders. In the present case, the engine 10 has four cylinders.
 The engine 10 is supplied with air by way of an intake pipe system 12, the
 air flow being determined by an air flow sensor 16. A corresponding signal
 is emitted to a control unit 24.
 The exhaust gases of the engine are removed into the environment by way of
 an exhaust pipe system 14.
 In the exhaust pipe system, a catalyst 18 is provided for converting the
 pollutants into non-toxic substances. Between the engine 10 and the
 catalyst 18, a lambda probe 30 is arranged. The lambda probe 30 is
 constructed as a surge probe. The lambda probe 30 emits a voltage signal,
 which corresponds to the exhaust gas composition, to the control unit 24.
 In the case of a lean mixture (.lambda.&gt;1), the probe voltage amounts, for
 example, to about 100 mV. In the range .lambda.=1, the probe voltage
 changes almost in a surge-type manner, and, in the case of a rich mixture
 (.lambda.&lt;1), reaches values of 800 mV or more. Specifically the intense
 change of the probe voltage in the range .lambda.=1 permits the detection
 of even minimal deviations from the optimal air-fuel ratio. The present
 invention is based on the fact that, although the surge is manifested by a
 fast voltage rise, it is not manifested by a purely rectangular surge
 characteristic. In addition, it is known that surge probes are very
 reliable and reasonable in cost.
 In the present case, the control unit 24 also receives temperature values T
 of the coolant, rotational speed values n concerning the rotational speed
 of the engine, as well as an operating voltage U.sub.B.
 Since, in the present invention, the voltage fluctuations of the lambda
 probes are analyzed and assigned to the individual cylinders, it is
 necessary that the momentarily existing working cycle of each cylinder be
 known. For this purpose, a crankshaft sensor 32 is used in the existing
 signal. The signals of the crankshaft sensor 32 are also supplied to the
 control unit 24.
 On the basis of the existing information, the control unit 24 computes an
 injection time t.sub.i for each cylinder and transmits it to the injection
 valves 20. The injection valves 20 supply the fuel obtained from the fuel
 supply 22 by way of lines 26 corresponding to the injection time t.sub.i
 to the cylinders operating in the engine 10.
 The control unit 24 first computes an injection time for each cylinder on
 the basis of the data available to the control unit, such as the
 temperature T, the rotational speed n and the air flow signals, and
 generates a basic injection time ti_zyl_z, wherein the letter z is a
 defined cylinder. For this basic injection time, a cylinder-specific
 mixture adaptation is then computed, specifically from the difference of
 two--relative to the ignition sequence--adjacent cylinders.
 This will be explained in the following by way of FIG. 2. FIG. 2
 illustrates a probe voltage signal ULS_1_z over time s. In the course of
 the voltage, the probe voltage is indicated for different cylinders z.
 The voltage deviation of a cylinder z is now calculated on the basis of the
 voltage values of the cylinders, which are adjacent relative to the
 ignition sequence. The voltage difference for the first cylinder (z=1)
 ULS_1_diff_1 is calculated as follows:
EQU ULS_1_diff_1=((ULS_1_3+ULS_1_2)/2)-ULS_1_1.
 In this case, ULS_1_z is the probe voltage at the z-th cylinder. The
 differences ULS_1_diff_z at the other cylinders are calculated
 correspondingly.
 Corresponding to the determined voltage deviation, an injection correction
 KF_ti_zyl_z is obtained from a characteristic curve. The basic injection
 time ti_zyl_z is corrected by means of this correction injection time.
 If the conditions for lambda adaptation have been met, an adaptation value
 of the cylinder mixture adaptation is formed and is stored in a
 non-volatile manner.
 On the whole, the present invention provides simple cylinder-selective
 control at reasonable cost.
 The foregoing disclosure has been set forth merely to illustrate the
 invention and is not intended to be limiting. Since modifications of the
 disclosed embodiments incorporating the spirit and substance of the
 invention may occur to persons skilled in the art, the invention should be
 construed to include everything within the scope of the appended claims
 and equivalents thereof.