Diagnostic method for recognizing combustion misfiring in an internal-combustion engine

In a diagnostic method for recognizing combustion misfiring in an internal-combustion engine, a map of characteristics, in which characteristic numbers dependent on load and on rotational speed are filed, is provided. One or more different diagnostic methods are selected by a diagnostic device depending on the respective characteristic number and allow as reliable information as possible of combustion misfiring for the respective load and rotational-speed range.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a diagnostic method for recognizing combustion 
misfiring in an internal-combustion engine. 
2. Description of the Related Art 
The requirement to recognize misfiring under all the operating conditions 
of an internal-combustion engine, to make it possible to counteract 
increased emissions of harmful materials associated therewith or to 
perform protective functions with a catalytic converter, have increasingly 
becoming part of the corresponding regulations. Various methods have 
already become known for recognizing such combustion misfiring, such as, 
for example, the evaluation of erratic running or a lambda-probe signal or 
monitoring of the ignition current, etc. 
It has been shown, however, that none of these methods can provide 
generally valid evidence of combustion misfiring. On the one hand, there 
is the problem that one method does not work with sufficient accuracy in 
all load and rotational-speed ranges, as, for example, in the 
consideration of erratic running. On the other hand, in some other 
methods, there is the problem that no reliable information about a proper 
combustion can be obtained, since only sub-functions necessary for the 
combustion method are diagnosed, as, for example, in ignition-current 
diagnosis. 
SUMMARY OF THE INVENTION 
In contrast to this, the object of the present invention is to provide a 
diagnostic method for detecting combustion misfiring, in such a way that 
information about a proper combustion in all the load and rotational-speed 
ranges of the internal-combustion engine can be obtained with greater 
reliability. 
The solution according to the invention is provided by a diagnostic method 
for recognizing combustion misfiring in an internal-combustion engine, 
characterized in that a memory with a map of characteristics, in which 
characteristic numbers dependent on load and on rotational speed are 
filed, is provided, in that a selection device selects at least one 
diagnostic method from at least two diagnostic methods for detecting 
combustion misfiring depending on the read-out characteristic number, and 
in that the diagnostic device recognizes a combustion misfire when at 
least one of the diagnostic methods detects a combustion misfire. 
Advantageous developments of the invention are provided by the selection 
method selecting the diagnostic method from a group of diagnostic methods 
which evaluate method output quantities of the combustion method, such as 
erratic running, noise, vibrations, lambda-probe signal. A further 
improvement provides that the selection method selects the diagnostic 
method from a group of diagnostic methods which evaluate the method input 
quantities of the combustion method, such as ignition current or output 
signals from the output stages for the injection valves. The solution 
according to the invention proceeds from the consideration that diagnostic 
methods for detecting combustion misfiring can be divided into three 
groups. 
The first group considers method output quantities, such as erratic 
running, noise, vibrations or the lambda-probe signal. The reaction of the 
internal-combustion engine to the combustion operation is therefore 
evaluated. 
These methods are highly cost-effective since the sensors necessary for 
them are usually already present in engine control systems. The 
corresponding diagnosis therefore necessitates only additional software 
routines. However, the high dependence of many of these methods on load 
and on rotational speed is a problem, no reliable information being 
obtained in specific operating ranges, for example as a result of the 
influences of vibration and resonance. 
The second group of diagnostic methods considers method input quantities, 
such as, for example, the ignition current or the output signals from the 
output stages for the injection valves. One of the preconditions for 
proper combustion is therefore respectively evaluated. 
These methods are likewise highly cost-effective since only the 
corresponding electrical signals have to be evaluated. A problem of these 
methods is that in a negative diagnosis, admittedly a combustion misfire 
is reliably recognized, since no combustion can take place if the ignition 
current is absent or if output signals for the injection valves are 
absent. Nevertheless, in a positive diagnosis, no information is obtained, 
since, despite the presence of an ignition current or the presence of 
output signals, a combustion misfire can occur as a result of other 
influencing factors. 
Finally, the third group considers the method itself, such as, for example, 
ionic current, pressure or light measurements in the combustion space. The 
progress of the combustion method is therefore considered directly here. 
These methods are cost-intensive, since a corresponding sensor has to be 
installed in the combustion space in each case. The advantage of these 
methods is that they provide generally valid information for the 
combustion method. 
In the solution according to the invention, a diagnostic method composed of 
the first two groups which can be executed in an especially cost-effective 
way is used. To avoid the disadvantage of the dependence on load and on 
rotational speed and the lack of general validity for a positive 
diagnosis, a diagnostic method is selected from a plurality of existing 
ones depending on load and on rotational speed. 
For this purpose, characteristic numbers are filed in a map of 
characteristics depending on load and on rotational speed. A selection 
device selects the diagnostic method depending on the read-out 
characteristic number. All combinations are possible here. For example, in 
a particular operating range, a diagnostic method, such as that relating 
to erratic running, can be sufficient. In contrast, in another operating 
range, two or more diagnostic methods can be employed simultaneously. 
Any combinations are possible also for recognizing a combustion misfire. A 
combustion misfire can be recognized, for example, when only a single 
diagnostic method or one diagnostic method of a plurality detect a 
combustion misfire. It is also possible to recognize a combustion misfire 
only when a plurality or all of the diagnostic methods used detect this 
case. 
The selection of a specific combination is determined by test-bench or 
driving tests. By means of the characteristic numbers stored in the map of 
characteristics, different combinations can also be used in different load 
and rotational-speed ranges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present diagnostic method makes use of a diagnostic device which is 
used within the framework of an engine control system with a 
microcomputer. The diagnostic device is part of the microcomputer system, 
essential functional parts being a memory with a map of characteristics 
stored in it and a further memory, in which corresponding diagnostic 
program routines are filed. 
The flow diagram, shown in FIG. 1, of the recognition of a combustion 
misfire is called up cyclically by the engine control. The values for the 
rotational speed n and the intake air mass LM are obtained from other 
program routines of the engine control. Such program routines are, for 
example, those for the continuous injection-time and ignition-timing 
calculation which work respectively with the current load and 
rotational-speed values. 
In step S1, a characteristic number Z is determined depending on the 
rotational speed n and on the intake air mass LM. This characteristic 
number Z is filed in a map of characteristics which is shown in more 
detail in FIG. 2. 
In the example, this contains three characteristic numbers Z which each 
represent a range of the map of characteristics. These ranges of the map 
of characteristics are fixed on the engine test bench and differ in that, 
in each range, a different combination of diagnostic methods for detecting 
combustion misfiring gives the best results in terms of evaluation 
reliability. The example shown applies to a specific 6-cylinder engine. In 
other engines, other combinations or also a larger or smaller number of 
ranges of the map of characteristics can be more suitable. 
In steps S2 and S3, the various diagnostic methods for detecting combustion 
misfiring are then called up according to the characteristic number Z. 
In the range of the map of characteristics with the characteristic number 
1, this is a diagnostic method relating to erratic running. In this, the 
rotational speed for each cylinder is evaluated by means of two crank 
shaft positions and a combustion misfire is detected when the gradient of 
the rotational-speed trend falls below a limit value. Such a method is 
described in the applicant's European Patent Application 90123131.6. 
Step S7 enquires whether a combustion misfire has been detected by means of 
the method. If so, there follows step S10, in which the diagnostic device 
recognizes a combustion misfire. If not, the program routine is 
terminated. 
When the range of the map of characteristics with the characteristic number 
2 is-recognized in step S3, a plurality of diagnostic methods are 
activated in step S5. These are the methods relating to erratic running, 
to the ignition current and to the output signals from the output stages 
for the injection valve. With regard to the diagnostic method relating to 
the ignition current, a check is made as to whether a corresponding 
ignition current is flowing in the primary or in the secondary circuit of 
the ignition. In the diagnostic method relating to the injection valves, 
the output signals from the output stages assigned to the injection valves 
are checked. 
In step S8, a combustion misfire is detected when the diagnostic method 
relating to erratic running or to the ignition current or to the output 
signals from the output stages for the injection valves respond. The 
reason for this measure is that, in this range of the map of 
characteristics identified by the characteristic number 2, the diagnostic 
method relating to erratic running provides less reliable information 
particularly as a result of the influences of vibration. A combustion 
misfire is therefore additionally recognized in step S10 when the 
diagnostic method relating to the ignition current or to the output 
signals from the output stages for the injection valves have responded. 
When the answer in step S3 is no, the characteristic number 3 must be 
present. Then, in step S6, the diagnostic methods relating to the ignition 
current and to the output signals from the output stages for the injection 
valves are activated. If there is an answer in the affirmative in step S9, 
a combustion misfire is recognized in step S10 when one of the two 
diagnostic methods responds. In this range of the map of characteristics, 
the diagnostic method relating to erratic running no longer provides 
virtually any useful information and is therefore omitted. 
Although other modifications and changes may be suggested by those skilled 
in the art, it is the intention of the inventors to embody within the 
patent warranted hereon all changes and modifications as reasonably and 
properly come within the scope of their contribution to the art.