Misfire detecting device for internal combustion engine

In a misfire detecting device for an internal combustion engine, a crank angle detector detects the crank angle of the engine, a speed detector detects the speed of the engine from the crank angle thus detected, a load detector detects the load of the engine from the speed thus detected; and a misfire detector detects the misfire rate of the engine from the load thus detected, and outputs a misfire detection signal when the misfire rate thus detected exceeds a misfire rate determining value set according to a certain amount of load.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a misfire detecting device for an internal 
combustion engine which detects a misfire which occurs in the engine for 
instance when the ignition system is out of order. 
2. Prior Art 
For instance when, in an internal combustion engine, the ignition system 
becomes out of order, a large quantity of gas is discharged, so that the 
catalyst generates an abnormally high temperature. In order to prevent 
this difficulty, a device has been proposed in the art which detects the 
variation of an operating parameter, such as the angular speed of the 
crank shaft of the internal combustion engine or the cylinder internal 
pressure, representing the state of combustion, thereby to inform the 
operator of the abnormal condition by means of an alarm lamp or the like. 
Conventional misfire detecting devices of this type have been disclosed by 
Japanese Patent Application (OPI) No's 19532/1983 and 26345/1987 (the term 
"OPI" as used herein means an "unexamined published application"). 
The device disclosed by the aforementioned Japanese Patent Application 
(OPI) No. 19532/1983 operates as follows: When the difference between the 
crank shaft's angular speeds before and after the stroke of expansion in 
the internal combustion engine is smaller than a predetermined value, it 
is determined that a misfire has occurred. On the other hand, the device 
disclosed by the Japanese Patent Application (OPI) No. 26345/1987 operates 
as follows: The cylinder internal pressure of the engine is detected with 
a cylinder internal pressure sensor, and a crank angle is obtained with 
which the cylinder internal pressure is maximum. When the crank angle is 
within a predetermined crank angle range, it is determined that the 
combustion is normal. 
That is, it is determined from the angular speed of the crank shaft or the 
cylinder internal pressure every combustion cycle whether the combustion 
is normal or the misfire occurs, and when a misfire occurrence rate (the 
frequency of occurrence of misfires per 100 combustion cycles or 100 
revolutions of the engine) is larger than a predetermined value, a misfire 
detection signal is produced to turn on an alarm lamp or the like. 
The conventional misfire detecting device is designed as described above. 
Hence, in the device, in order to prevent the generation of the abnormally 
high temperature by the catalyst due to misfires, or in order to make 
protection even in the operation highest both in speed and in load which 
is most greatly affected by misfires, a low threshold value is provided 
for determination of the misfire occurrence rate; that is. the sensitivity 
of detection is decreased. 
Therefore, even in a low speed and low load operation in which the rotation 
is liable to be changed by factors other than the misfire for instance as 
in the case where an internal combustion engine operated vehicle is 
shocked or vibrated during traveling, and in which no actual damage occurs 
even when misfires occur to the extent that the misfire occurrence rate 
exceeds the criterion, the misfire detection signal is erroneously 
produced. That is, the misfire detecting operation is low in reliability. 
Furthermore, the misfire detecting device disclosed by the aforementioned 
Japanese Patent Application (OPI) No. 19532/1983 is so designed as to 
detect misfires in all the operating conditions. When the load of the 
internal combustion engine is considerably light as in the case where it 
is run at high speed with no load, the friction loss of the internal 
combustion engine is small, or the inertial energy is large, and therefore 
the variation in angular speed of the crank shaft due to misfires is 
smaller than in the case where the load is heavy. As a result, it is 
impossible for the device to accurately detect misfires, or, even if the 
detection sensitivity is increased, misfires are erroneously detected 
because of external noises. That is, the result of detection provided by 
the misfire detecting device is low in reliability. Hence, for instance in 
the case where the device is coupled to a display unit, to indicate a 
failure in the ignition system according to the result of detection, to 
inform the operator of it thereby to request for repair, the following 
difficulties may be involved: Although the combustion is abnormal, it is 
not indicated; or although the combustion is normal, it is indicated as 
abnormal. That is, the operator cannot rely on the indication based on the 
result of detection provided by the device. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to eliminate the 
above-described difficulties accompanying a conventional misfire detecting 
device. 
More specifically, an object of the invention is to provide a reliable 
misfire detecting device for an internal combustion engine which detects 
misfires only when necessary, and which is free from the difficulty that a 
detection output is erroneously provided when the engine is liable to be 
varied in rotation; that is, when misfires are liable to be erroneously 
detected. 
A misfire detecting device according to an aspect of the invention 
comprises: misfire detecting means for setting a misfire rate determining 
value according to a load given to the internal combustion engine, and 
outputting a misfire detection signal when a misfire rate detected exceeds 
the misfire rate determining value. 
In the device, the misfire detecting means controls the misfire rate 
determining value in such a manner that it is increased when the load of 
the engine is light and decreased when heavy; that is, it sets the misfire 
rate determining value to a most suitable value according to the load of 
the engine, and outputs the misfire detection signal when the misfire rate 
of the engine exceeds the most suitable value thus set. 
A misfire detecting device according to another aspect of the invention 
comprises: load detecting means for detecting a load given to the internal 
combustion engine; and means for inhibiting misfire detecting means to 
detect a misfire or nullifying the detection of a misfire done by the 
misfire detecting means when the load is lower than a predetermined value. 
Another example of the misfire detecting device of the invention comprises: 
load detecting means for detecting a load given to the internal combustion 
engine; display means for displaying the detection of a misfire performed 
by the misfire detecting means; and means for inhibiting the display of 
the detection of a misfire when the load is smaller than a predetermined 
value. 
In the misfire detecting device of the invention, when the load of the 
internal combustion engine is lower than the predetermined value; that is, 
when the load is light, the display of the detection of a misfire is 
inhibited. Hence, the device is free from the difficulty that the 
occurrence of a misfire is erroneously detected. 
The nature, utility and principle of the invention will be more clearly 
understood from the following detailed description and the appended claims 
when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of this invention will be described with reference to 
the accompanying drawings. 
First, an example of a misfire detecting device for an internal combustion 
engine, which constitutes a first embodiment of the invention, will be 
described with reference to FIGS. 1 and 2. 
In FIGS. 1 and 2, reference numeral 1 designates an engine; and 2, crank 
angle detecting means for outputting a pulse signal when the crank angle 
is at a reference angular position (for instance the top dead center) or 
angular positions forming a predetermined angle (for instance 45.degree. 
CA (crank angle)) before and after the reference angular position. 
The crank angle detecting means 2, as shown in FIG. 2, is provided on the 
engine so as to detect the rotation of the crank. 
Further in FIG. 1, reference numeral 3 designates load detecting means; and 
4, misfire detecting means. The load detecting means is to detect the load 
of the engine 1. More specifically, the load detecting means may be an air 
flow meter 31, a suction pipe pressure sensor 32, or a throttle 
degree-of-opening sensor 33 which are shown in FIG. 2, or engine speed 
(number-of-revolutions per minute) detecting means 34 which measures the 
period of the predetermined crank angles, for instance, from a reference 
angular position signal provided by the crank angle detecting means 2, to 
detect the number of revolutions per minute of the engine. The misfire 
detecting means 4 operates to determine, based on a signal provided by the 
load detecting means 3, whether or not it is necessary to perform a 
misfire detecting operation. The misfire detecting means 4 further 
operates according to a signal from the crank angle detecting means 2, to 
calculate, with the crank angle's reference angular position as a 
reference, the ratio of the times required for the predetermined angular 
intervals before and after the reference angular position, thereby to 
detect the occurrence of a misfire. 
The above-described engine speed detecting means 34 and the misfire 
detecting means 4 are included in a microcomputer 10 shown in FIG. 2. The 
microcomputer 10 includes: an input interface for inputting signals from 
the air flow meter 31, the suction pipe pressure sensor 32, the throttle 
degree-of-opening sensor 33, and the crank angle detecting means 2; a 
single chip microcomputer incorporating an A/D (analog-to-digital) 
converter for converting input analog signals into digital signals, and a 
timer counter (or free running counter) which counts up every 
predetermined time clock signal; and memories (ROM and RAM). 
Now, the operation of the misfire detecting device thus organized will be 
described. 
First, the fundamental principle of detecting misfires will be described 
with reference to FIGS. 3, 4 and 5. FIG. 3 shows angular speeds when a 
4-cycle 4-cylinder engine operates under heavy load (the throttle being 
fully opened, with a speed of 1000 rpm). FIG. 4 shows the ratio (TU/TL) of 
the times required for the predetermined crank angle intervals before and 
after the top dead center in the stroke of compression when the engine 
operates under heavy load. FIG. 5 shows the internal pressures of the 
cylinders during the operation with heavy load. 
The ratio TU/TL shown in FIG. 4 is obtained with the top dead center in the 
stroke of compression as a reference. That is, it is the ratio of the time 
TL required for a predetermined angular interval (for instance 45.degree. 
CA interval) before the top dead center to the time TU required for a 
predetermined angle interval (for instance 45.degree. CA interval) after 
the top dead center. The waveform shown in FIG. 4 is such that, during a 
combustion cycle, combustion is made normally in the #1, #2, #3 and #4 
cylinders, and thereafter a misfire occurs in the #1 cylinder for some 
reason, for instance a failure in the ignition system. 
If it is assumed that, as shown in FIGS. 4 and 5, the #1 cylinder in which 
combustion is made normally is designated by #1a, and the time ratio is 
TUa/TLa, whereas the #1 cylinder in which a misfire occurs is designated 
by #1b, and the time ratio is TUb/TLb, then it can be seen that TUb/TLb is 
larger than TUa/TLb. This is due to the following fact: When combustion is 
made normally, the interval requiring TLa is in the stroke of compression 
with the angular speed decreased as shown in FIG. 3, and the interval 
requiring TUa is in the stroke of explosion, with the angular speed 
increased; whereas when a misfire occurs, TLb is similar to TLa, and as 
for the interval requiring TUb, the angular speed is further decreased, 
and the time TUb is increased. 
As is apparent from the above description, the time ratio TU/TL of the 
cylinder where a misfire occurs is larger than that of the same cylinder 
where combustion is made normally. This is detected, to determine it every 
combustion cycle whether or not a misfire has occurred. 
This misfire detecting operation will be concretely described with 
reference to FIGS. 6 through 9, flow charts showing the arithmetic 
operations of the computer 10. In the concrete example, with a 
predetermined crank angle (for instance the top dead center in the stroke 
of compression) as a reference angle, measurement is made for the time 
required for a predetermined angular interval (for instance 45.degree. CA 
interval) before the reference angle and the time required for a 
predetermined angular interval (for instance 45.degree. CA interval) after 
the reference angle, and a misfire is detected from the ratio of the times 
thus measured. 
In FIG. 6, an interruption is made in response to a signal from the crank 
angle detecting means 2, so that the count value of the counter which 
counts up every predetermined time clock signal is stored in a memory 
buffer MB45 whenever the cranks angle reaches 45.degree. CA before the top 
dead center in the stroke of compression. 
In FIG. 7, an interruption occurs in response to a signal from the crank 
angle detecting means 2, so that the count value of the counter which 
counts up every predetermined time clock signal is stored in a memory 
buffer MTDC whenever the crank angle reaches the top dead center in the 
stroke of compression. Those values stored in the memory buffers MB45 and 
MTDC indicate the time instants corresponding to the 45.degree. CA before 
the top dead center and the top dead center, respectively. 
FIG. 8 is a flow chart showing arithmetic operations Steps S1, S2, S4, S5 
and S6 which are carried out whenever the crank angle reaches 45.degree. 
CA after the top dead center in the stroke of compression. 
In Step S1 of FIG. 8, the above-described count values of the timer counter 
stored through interruption are read and stored in a memory buffer MA45. 
In Step S2, the time ratio TU/TL is obtained from the following Equation 
(1): 
EQU TU/TL=(MA45-MTDC)/(MTDC-MB45) (1) 
In Equation (1), (MA45-MTDC) is the time required for the interval from the 
top dead center to 45.degree. CA after the top dead center, and 
(MTDC-MB45) is the time required for the interval from 45.degree. CA 
before the top dead center to the top dead center. 
After the calculation of the time ratio TU/TL, Step S4 is effected. In Step 
S4, it is determined whether or not the time ratio TU/TL obtained in Step 
S2 is larger than a predetermined value corresponding to the occurrence of 
a misfire. When it is larger ("Yes"), Step S5 is effected; that is, it is 
determined that a misfire has occurred with the cylinder. Thus, the 
operation is ended. When it is not larger, Step S6 is effected, so that it 
is determined that combustion is normal in the cylinder, and the operation 
is ended. 
If summarized, with a predetermined crank angle as a reference crank angle, 
the ratio of the time required for a predetermined angular interval before 
the reference crank angle to the time required for a predetermined angular 
interval after the reference crank angle is obtained, and it is determined 
from the ratio of the times thus obtained whether a misfire has occurred 
or not. 
The operation of the misfire detecting device thus organized will be 
described in its entirety with reference to FIG. 9. a flow chart. In FIG. 
9, Steps 1 through 6 are the same as those in FIG. 8. That is, after it is 
determined in Step S5 of FIG. 8 that a misfire has occurred, Steps S7 and 
the following Steps are carried out. When, in response to the result of 
comparison in Step S4, it is determined in Step S5 that a misfire has 
occurred, Step S7 is effected. In Step S7, the count value N1 of a misfire 
counter adapted to count each misfire determination is increased. On the 
other hand, when it is determined in Step S6 that the combustion is 
normal, the count value N2 of a normal combustion counter adapted to count 
each normal combustion determination. 
Thereafter, Step S9 is effected. In Step S9, the count values N1 and N2 are 
subjected to addition, and it is determined where or not the sum (N1+N2) 
reaches a predetermined number of combustion cycles, for instance 100 
combustion cycles. When it is smaller than the predetermined number of 
combustion cycles, the misfire determining operation is ended, and the 
above-described Step S1 is started with the interruption effected in 
response to the next output signal of the crank angle detecting means 2. 
When it is determined in Step S9 that the sum (N1+N2) has reached the 
predetermined number. Step S10 is effected. In Step S10, the output signal 
of the load detecting means is read. In the following Step S11, a misfire 
rate determining value is set according to an amount of load given. 
Next, in Step S12, the misfire rate determining value is compared with an 
actual misfire rate N1/(N1+N2) which is obtained from a misfire 
determination result. When it is determined that the actual misfire rate 
N1/(N1+N2) is larger than the misfire rate determining value, Step S13 is 
effected to output a misfire signal. When, in Step S12, it is determined 
that the former is smaller than the latter, Step S14 is effected to output 
a normal condition signal. 
In the above-described embodiment, when it is determined in Step S9 that 
(N1+N2) is larger than the predetermined number of combustion cycles, in 
Step S12 the misfire rate N1/(N1+N2) with respect to (N1+N2) combustion 
cycles is compared with the misfire rate determining value. However, the 
same effect can be obtained by determining the number of misfires 
occurring during a predetermined number of revolutions, for instance 100 
revolutions. 
As was described above, in the embodiment of the invention, in Step S11 of 
FIG. 9, the misfire rate determining value is set to a most suitable value 
so that, during a heavy load operation in which the catalyst generates 
abnormally high temperatures even with a low misfire rate, the misfire 
rate determining value is set to a low value, and during a light load 
operation in which no trouble occurs even with a high misfire rate, the 
misfire rate determining value is set to a high value. Hence, with the 
device of the invention, the misfire detection signal is produced only 
when necessary. Furthermore, the device produces no output erroneously 
even in the condition that, as in the case where the vehicle is run at low 
speed with light load, the rotation is liable to be changed by factors 
other than the misfire, and the misfire detection signal is liable to be 
erroneously produced. Thus, the misfire detecting device of the invention 
is high in reliability. 
Now, another example of the misfire detecting device, which constitutes a 
second embodiment of the invention, will be described with reference to 
FIG. 10, a flow chart. 
In Step S7, the output of the load detecting means 3 is read. In the 
following Step S8, it is determined whether or not the detected engine 
load is greater than a predetermined value. When it is determined that the 
detected engine load is greater than the predetermined value, Step S9 is 
effected to perform a misfire detection. Thereafter, the above-described 
misfire detecting operation, Steps S1 through S6 in FIG. 8, is carried 
out. When it is determined that the detected load is equal to or smaller 
than the predetermined value, Step S10 is effected to inhibit the misfire 
detecting operation, and the routine is ended. 
In the second embodiment shown in FIG. 10, the engine load is detected, and 
when the load thus detected is equal to or smaller than the predetermined 
value, the misfire detecting operation is inhibited. This method may be 
replaced by the following method in which the load detection and the 
misfire detection are changed in the order of execution, and the misfire 
detection is carried out every period, and in the case when the load 
detected is equal to or smaller than the predetermined value, the misfire 
detection in that period is nullified. The misfires occurring in the 
internal combustion engine which is run with light load scarcely provides 
adverse effects; for instance, they will not adversely affect the exhaust 
gas. 
Another example of the misfire detecting device, a third embodiment of the 
invention, will be described with reference to FIG. 11, a flow chart. 
In Steps S1 through S6, the above-described misfire detecting operation is 
carried out. When it is determined in Step S5 that a misfire has occurred, 
Step S7 is effected to read the output of the load detecting means 3. When 
it is determined in Step S8 that the engine load detected is larger than a 
predetermined value, Step S11 is effected to turn on an alarm lamp to 
indicate the occurrence of misfire. In the case where the engine load is 
equal to or smaller than the predetermined value, Step S12 is effected to 
turn off the alarm lamp. When it is determined also in Step S6 that the 
combustion is normal, the alarm lamp is turned off. 
That is, the third embodiment is so designed that, during a light load 
operation in which a misfire cannot be accurately detected, the alarm lamp 
is not turned on. Thus, with the device, the indication of misfire 
detections is high in reliability. 
In the above-described embodiments, the detection of a misfire depends on 
whether or not the ratio of the times required for the predetermined 
angular intervals before and after the top dead center in the stroke of 
compression is larger than the predetermined value. However, it should be 
note that the invention is not limited thereto or thereby. That is, the 
technical concept of the invention can be applied to the case where the 
detection of a misfire is performed according to the difference between 
the crank shaft angular speeds provided before and after the stroke of 
expansion in the internal combustion engine, or the case where a misfire 
is detected from detection of the angular speed of the crank shaft. 
As is apparent from the above description, based on the fact that the 
occurrence of misfires depends on the load applied to the internal 
combustion engine, the misfire detecting device is so designed that the 
misfire rate determining value is controlled according to the load, and, 
when the misfire rate of the internal combustion engine exceeds the 
misfire rate determining value, it is determined that a misfire has 
occurred. Hence, the device is free from the difficulty that the misfire 
detection signal is produced when unnecessary, or, although no misfire has 
occurred, the misfire detection signal is outputted by factors other than 
misfires. Thus, the misfire detecting device is high in reliability. 
While there has been described in connection with the preferred embodiments 
of this invention, it will be obvious to those skilled in the art that 
various changes and modifications may be made therein without departing 
from the invention, and it is aimed, therefore, to cover in the appended 
claims all such changes and modifications as fall within the true spirit 
and scope of the invention.