Method for recognizing erratic combustion

A method is provided for recognizing erratic combustion in a multicylinder internal combustion engine by using a time measuring device to measure successive time periods which a crankshaft needs to rotate through predetermined angles during operating strokes of successive cylinders. The method includes determining a static component by using a calculating device to subtract a time period of the next cylinder in an ignition order from the time period of a cylinder to be examined. The static component is multiplied with a standardizing factor by using the calculating device. A dynamic component is determined by using the calculating device to subtract a time period of a next cylinder in the ignition order from a time period of a preceding cylinder in the ignition order, and a dynamic component is negated. A change component for a lack of smoothness value is formed, the lack of smoothness value is multiplied with a weighting factor to form a weighted value, and the weighted value is set equal to zero if the weighted value is negative. A sliding average is formed from the weighted value in a sliding averaging process. A lack of smoothness value is determined from addition of the static component, the dynamic component, and the sliding average. A combustion misfire is recognized if the smoothness value falls below a given limit value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for recognizing erratic combustion in 
multicylinder internal combustion engines. In engines equipped with a 
catalytic converter, erratic combustion can cause damage to the catalytic 
converter, since after-reactions of the uncombusted mixture of fuel and 
air can cause high temperatures in the catalytic converter. Regardless of 
whether or not a catalytic converter is used, erratic combustion causes 
poorer-quality exhaust. 
Methods are already known that measure the instantaneous angular speed of 
the crankshaft in order to detect erratic combustion. The period of time 
during which the crankshaft rotates about a defined angle is measured. The 
time measurement is typically carried out with the aid of markings on a 
wheel mounted on a crankshaft. The difference between successive periods 
of time that are measured is compared with the limit value. Erratic 
combustion leads to a temporary slowing of the angular speed of the 
crankshaft, since the energy that would be contributed by the misfiring 
cylinder is missing in the drive of the crankshaft. If the angular speed 
slows down, the difference between successive measured time periods 
increases. If a predeterminable limit value is exceeded, an ignition 
misfire is recognized and indicated or displayed. Control provisions can 
then optionally be made, with one example being to turn off the applicable 
injection nozzle. 
A primary disadvantage of the known methods is that they are suitable only 
for steady-state operation at a constant speed, without braking and 
acceleration. In braking, for instance, the angular speed of the 
crankshaft decreases. The measured time period and the difference between 
successive time periods become greater, and as soon as that increase in 
the time period exceeds a limit value, a combustion misfire is indicated. 
Yet no misfire has in fact yet occurred, rather simply entirely normal 
braking took place. German Published, Non-Prosecuted Application DE 40 09 
895 A1 describes a method that overcomes that disadvantage. 
In the method described in German Published, Non-Prosecuted Application DE 
40 09 895 A1, not only a static component but also a dynamic component is 
calculated, which takes into account the mean linear increase in speed 
(acceleration) or the mean linear decrease in speed (deceleration). The 
dynamic component is calculated so that periods of time of a plurality of 
cylinders that are successive, but spaced farther apart timewise, are 
compared with one another. If the dynamic component is then subtracted 
from the static component, that compensates for the influence of changes 
in speed on the measured time periods. The remaining changes in the time 
periods are then in fact predominantly due to erratic combustion. 
A disadvantage of the method described above is that once again it is 
unsuitable for a markedly unsteady-state operation, because it can 
compensate only for the influences of constant acceleration or constant 
deceleration. Yet those driving states tend to be the exception in 
everyday operation of a motor vehicle. Under everyday conditions, usually 
highly unsteady conditions prevail instead, with examples being uneven 
deceleration, uneven acceleration, an often abrupt change back and forth 
between acceleration and deceleration, or even very hard changes in speed, 
such as in fast clutch engagement and disengagement. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a method for 
recognizing erratic combustion, which overcomes the hereinafore-mentioned 
disadvantages of the heretofore-known methods of this general type and in 
which reliable erratic combustion recognition is possible even in such 
highly unsteady operating states. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a method for recognizing erratic combustion 
in a multicylinder internal combustion engine by using a time measuring 
device to measure successive time periods which a crankshaft needs to 
rotate through predetermined angles during operating strokes of successive 
cylinders, which comprises determining a static component by using a 
calculating device to subtract a time period of the next cylinder in an 
ignition order from the time period of a cylinder to be examined; 
multiplying the static component with a standardizing factor by using the 
calculating device; determining a dynamic component by using the 
calculating device to subtract a time period of a next cylinder in the 
ignition order from a time period of a preceding cylinder in the ignition 
order, and negating a dynamic component; forming a change component for a 
lack of smoothness value, multiplying the lack of smoothness value with a 
weighting factor to form a weighted value, and setting the weighted value 
equal to zero if the weighted value is negative; forming a sliding average 
from the weighted value in a sliding averaging process; determining a lack 
of smoothness value from addition of the static component, the dynamic 
component, and the sliding average; and recognizing a combustion misfire 
if the smoothness value falls below a given limit value. 
In accordance with another mode of the invention, there is provided a 
method which comprises taking the limit values from performance curves 
determined as a function of load, rpm, and temperature of the engine. 
In accordance with a further mode of the invention, there is provided a 
method which comprises reading out performance values from the performance 
curves, buffer storing the performance values in a buffer store, and 
comparing lack of smoothness values and limit values for lack of 
smoothness for the same time periods. 
In accordance with an added mode of the invention, there is provided a 
method which comprises performing a sliding averaging process on the 
smoothness value GLUK.sub.n according to the equation GLUK.sub.n 
=GLUK.sub.n-1 *(1-MITKO)+LUK.sub.n *MITKO, wherein MITKO is an averaging 
constant having a range of values between 0 and 1, and LUK.sub.n is the 
lack of smoothness value. 
In accordance with an additional mode of the invention, there is provided a 
method which comprises calculating the standardizing factor NO of the 
static component as a function of a measuring window n-x through n+y of 
the dynamic component by the formula NO=x+y. 
In accordance with yet another mode of the invention, there is provided a 
method which comprises selecting the measuring window for computing the 
dynamic component and the change component such that for given cylinders, 
the time period of the same mechanical segment of the crankshaft is 
measured. 
In accordance with a concomitant mode of the invention, there is provided a 
method which comprises selecting the angular range of the crankshaft over 
which the time periods are measured, such that with respect to the top 
dead center of the motion of a respective piston, at an unfavorable 
operating point, it furnishes a maximum signal rise in the event of 
erratic combustion. 
Besides the above-described static component and the dynamic component that 
takes into account the general rpm trend, the method of the invention 
additionally includes a so-called change component, with which changes in 
acceleration and deceleration can be taken into account. In order to 
calculate this change component, instead of comparing the difference 
between the time periods of directly successive cylinders with one 
another, the difference in the time periods of cylinders that are farther 
apart is compared instead. The value obtained thereby is additionally 
weighted and enters the method of the invention only in the form of a 
sliding averaging process. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. Although the invention is illustrated 
and described herein as embodied in a method for recognizing erratic 
combustion, it is nevertheless not intended to be limited to the details 
shown, since various modifications and structural changes may be made 
therein without departing from the spirit of the invention and within the 
scope and range of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures of the drawing in detail and first, 
particularly, to FIG. 1 thereof, there is seen an illustration of a method 
according to the invention, taking a six-cylinder engine as an example. 
In a method step S1, using markings on a crankshaft, time periods T.sub.n 
that the crankshaft needs to rotate about a certain crankshaft angle, in 
this case 120.degree., for instance, during the working stroke of a 
cylinder, are measured with a time measuring device. 
The continuously measured values are buffer-stored in a method step S2. 
In a method step S3, a static component LUS.sub.n is calculated with a 
calculating device, in each case from two successive time periods T.sub.n, 
T.sub.n+1. 
A standardizing factor NO used in the method step S3 is dependent on a 
measuring window (n-x through n+y), which is used in a method step S4 for 
calculating a dynamic component. This is calculated from an equation 
NO=x+y, or in other words in this exemplary embodiment NO=3+3=6. 
In the method step S4, the dynamic component LUD.sub.n is calculated by 
forming a difference between the farther-apart time periods. In this 
exemplary embodiment, the third-from-last time period T.sub.n-3 or the 
third time period T.sub.n+3 from then, with respect to the current time 
period T.sub.n, is used. It would also be possible in this case to use an 
asymmetrically placed measuring window with respect to the current time 
period T.sub.n. The dynamic component LUD.sub.n is then negated. 
In a method step S5, a change component LUK.sub.n is calculated. Two 
differential values are formed symmetrically to the current time period 
T.sub.n. In the exemplary embodiment, these are T.sub.n-3 -T.sub.n and 
T.sub.n -T.sub.n+3. From these two values, a difference is again formed, 
and that difference, if it is greater than 0, is weighted by a weighting 
factor BF. If this difference is less than 0, it set to zero. 
The thus-calculated change component LUK.sub.n is stored in a buffer store 
in a method step S6, and then in a method step S7 is used to determine a 
sliding average, for instance by the following formula: 
EQU GLUK.sub.n =GLUK.sub.n-1 *(1-MITKO)+LUK.sub.n *MITKO, 
where MITKO is an averaging constant with a range of values between 0 and 
1. 
Measuring windows (n, n-x, n+y, n-z, n+z) in the calculation of the dynamic 
component and the change component are advantageously selected in such a 
way that at the corresponding cylinders n, n-x, n+y, n-z, n+z, the time 
period of the same mechanical segment of the crankshaft is measured. Any 
inequality in the size of the individual crankshaft segments that may be 
present will therefore not cause any measurement error. 
Advantageously, the angular span of the crankshaft over which the time 
periods are measured is selected, with respect to top dead center of the 
motion of the associated piston, in such a way that they span the range of 
maximum signal rise. A combustion misfire thus exhibits more-pronounced 
effects. Since these angle ranges are load and rpm-dependent, this 
optimization is performed at the least favorable operating point (high rpm 
and low load), so that even in this range, an adequately great signal rise 
will still be present. 
From these individual components, in a method step S8 a value for the lack 
of smoothness is formed for the period of time T.sub.n observed, by means 
of adding together the individual components LUS.sub.n and GLUK.sub.n, and 
the previously negativated LUD.sub.n. 
In a method step S9, simultaneously with the time period T.sub.n, the load, 
rpm and temperature of the engine were measured. In accordance with these 
characteristic variables, in a method step S10, a limit value for lack of 
smoothness LUG.sub.n was taken from associated performance graphs and 
buffer-stored in a method step S11. 
By buffer-storing the values, it is then possible in a method step S12 to 
compare the limit value for lack of smoothness LUG.sub.n in the time 
period T.sub.n with a value LU.sub.n for lack of smoothness calculated for 
the same period of time. 
If the lack of smoothness value LU.sub.n is less than the limit value of 
lack of smoothness LUG.sub.n, then in a method step S13 a combustion 
misfire is recorded. If LU.sub.n is greater than or equal to LUG.sub.n, no 
misfire is recorded (in a method step S14). Both cases are delivered for 
statistical evaluation in a method step S15, since when individual 
combustion misfires are recognized it is not yet possible to take 
controlling actions, such as shutting off injection nozzles. Such actions 
cannot be taken until the statistical frequency of such combustion 
misfires exceeds a certain limit. 
FIG. 2 shows curves plotted over time for a highly unsteady-state driving 
mode, of the following measured variables: air mass LM, time period 
T.sub.n of crankshaft revolution, and the lack of smoothness value 
LU.sub.n calculated for it (with and without a change component 
LUK.sub.n). Additionally shown is the limit value for lack of smoothness 
LUG.sub.n, which is taken as a function of operating variables from the 
performance graphs. 
A distinction can be made between chronologically different operating 
ranges. In a range I, an overrunning shutoff prevails. The fuel injection 
has been shutoff, and no erratic combustion recognition takes place. 
In a range II, the driver has depressed the gas pedal at high speed, 
causing an expulsion of air, which is apparent from a sharp rise and an 
overswing at the top in the LM curve shown in dashed lines. The rpm 
increases with corresponding fluctuations and the measured time periods 
T.sub.n of the crankshaft revolutions become shorter, which can be seen 
from a drop in the T.sub.n curve shown in solid lines. 
Two shaded curve bands represent the calculated lack of smoothness value 
LU.sub.n. This value fluctuates within the boundaries of the scattering 
bands that are shown. 
In order to provide comparison, the lack of smoothness value without the 
change component LUK.sub.n is shown (the lower curve with shading 
diagonally downward toward the left), as is the lack of smoothness value 
with the change component (the curve nearer the top with shading 
diagonally downward toward the right). The areas of overlap are apparent 
from the cross-hatching. The two curves for the lack of smoothness value 
deviate from one another in the range II. However, both curves still 
remain above the limit value for the lack of smoothness LUG.sub.n (lower 
doted line), which is taken from performance graphs. In other words, no 
mistakes in recognition are tripped by this unsteady operating range. The 
upper curve, that is the curve of the lack of smoothness value with the 
change component, maintains a greater signal-to-noise interval from the 
curve of the limit value of the lack of smoothness LUG.sub.n than the 
lower curve, that is the curve for the lack of smoothness value without 
the change component. This greater signal-to-noise interval aids in 
increasing the certainty with which mistakes in recognition can be 
precluded. 
In the next range, a range III, five combustion misfires are produced. Both 
curves of the lack of smoothness value assume such major fluctuations that 
they drop below the curve of the limit value LUG.sub.n, whereupon these 
five combustion misfires are recognized. This occurs as highly 
unsteady-state operation continues, which can be seen from the dropping 
curve of the time periods T.sub.n. This curve also has small zig-zags 
upward, which are a clear sign that the combustion misfires are causing a 
brief slowing down of the angular speed of the crankshaft and therefore a 
prolongation of the time periods T.sub.n. 
A following range IV again exhibits a highly unsteady operating range. It 
is characterized by an abrupt letup on the gas and a following, equally 
sudden, depression of the gas pedal again. This can be seen from the sharp 
break in the air mass curve LM. The curve for the time periods T.sub.n 
also decreases in the presence of major fluctuations. The two lack of 
smoothness curves LU again deviate from one another. The lower curve, that 
is the curve of the lack of smoothness value without the change component 
LUK.sub.n, drops repeatedly below the curve of the limit value LUG.sub.n. 
Each time it drops below this curve, a combustion misfire is mistakenly 
recognized. Conversely, if in this range one looks at the upper lack of 
smoothness curve, in which the change component LUK.sub.n is also taken 
into account, then it becomes clear that this curve, because it extends 
farther upward, does not drop below the curve of the limit value of the 
lack of smoothness LUG.sub.n. Mistakes in recognition are thus reliably 
avoided, even in this highly unsteady operating state.