Engine knock detecting system

An engine knocking detecting system is disclosed, which comprises an engine vibration sensing device which produces a signal which represents a vibration of an engine; an engine speed sensing device which produces a signal which represents the speed of the engine; parallelly arranged band-pass filters to which the vibration representative signal from the engine vibration sensing device is fed simultaneously, the band-pass filters having respective pass-bands which are not overlapped; and a knocking judging device for judging whether the engine is under knocking or not by analyzing the output signals issued from the band-pass filters and the engine speed representative signal from the engine speed sensing device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to systems for detecting 
undesirable conditions of an internal combustion engine, and more 
particularly to systems for detecting knocking of the engine. 
2. Description of the Prior Art 
As is known, engine knocking caused by rapid preflame reactions within the 
end gases lowers the output and the thermal efficiency of the engine. 
In order to reduce the knocking, various measures are commonly employed, 
which are, for example, usage of a fuel having a higher octane rating, 
lowering of combustion pressure and/or combustion temperature, reducing of 
the time during which the end gases are under high pressures and 
temperatures. However, these measures have failed to exhibit satisfied 
results. 
Thus, recently, there have been proposed systems in which a knocking sensor 
is employed for detecting the undesired engine knocking, and in which when 
an engine knocking is detected by the knocking sensor, a spark advancer is 
actuated to delay the ignition timing thereby suppressing or at least 
minimizing the engine knocking. 
One of such systems is shown in Japanese Patent First Provisional 
Publication No. 64-54227. In the system of this publication, an engine 
vibration representative signal issued from a knocking sensor is passed 
through a band-pass filter means. The filtered signal from this filter 
means is compared with a reference signal to determine whether the engine 
is under knocking or not. The band-pass filter means is so controlled as 
to narrow the pass-band thereof for minimizing noise possessed by the 
filtered signal. In fact, if the pass-band of the filter is not narrowed 
enough, the S/N ratio of the filtered signal increases and thus the 
detection of a small knocking of the engine becomes difficult. 
In the disclosed system, the filter means consists of a knocking 
controlling band-pass filter whose pass-band is variable and two 
additional band-pass filters which control the pass-band of the knocking 
controlling band-pass filter. The pass-band change is carried out based on 
the peaked outputs of the three band-pass filters which are obtained after 
the knocking detection by the knocking controlling band-pass filter, and 
thus, the frequency pass-bands of these three filters are overlapped. 
That is, for the pass-band change, with reference to the peaked levels of 
the engine vibration representative signals from the filters, the center 
frequencies of the three band-pass filters are weighted to obtain a mean 
frequency which is treated as a practical knocking frequency. In fact, the 
mean frequency is used as the center frequency of the knocking controlling 
filter. 
Although, as is described hereinabove, the disclosed system can reduce the 
noise of the filtered signal by narrowing the frequency pass-band, the 
same has the following drawback due to its inherent construction. 
That is, in the disclosed system, the knocking occurrence within the 
narrowed pass-band is absolutely necessary. However, as is known, a 
knocking representative frequency is not always contained within the 
narrowed pass-band, and thus satisfied knocking detection is not expected 
from such conventional system. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a measure 
which is free of the above-mentioned drawback. 
According to the present invention, there is provided an engine knocking 
detecting system which has a satisfied knocking detecting ability. 
According to the present invention, there is provided an engine knocking 
detecting system which comprises an engine vibration sensing means which 
produces a signal which represents a vibration of an engine; an engine 
speed sensing means which produces a signal which represents the speed of 
the engine; parallelly arranged band-pass filters to which the vibration 
representative signal from the engine vibration sensing means is fed 
simultaneously, the band-pass filters having respective pass-bands which 
are not overlapped; and knocking judging means for judging whether the 
engine is under knocking or not by analyzing the output signals issued 
from the band-pass filters and the engine speed representative signal from 
said engine speed sensing means.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2 of the drawings, there is shown an embodiment of the 
present invention. 
In the drawing, denoted by numeral 1 is an internal combustion engine. 
Denoted by numeral 3 is a crank angle sensor which detects the speed of 
the engine 1, and denoted by numeral 5 is a knocking sensor which senses 
the vibration of the engine 1. The knocking sensor 5 is, for example, of a 
piezoelectric type which can transduce the engine vibration to an electric 
signal. Denoted by numerals 7a, 7b, 7c, 7d and 7e are five parallelly 
arranged band-pass filters which serve as a band-pass means. The engine 
vibration representative signal from the knocking sensor 5 is fed to the 
fie band-pass filters 7a, 7b, 7c, 7d and 7e. Signals filtered by these 
filters are fed to an arithmetic circuit 9. The engine speed 
representative signal from the crank angle sensor 3 is fed also to the 
arithmetic circuit 9. Denoted by numeral 11 is a control circuit which 
controls an ignition timing of the engine 1 in accordance with an 
instruction signal from the arithmetic circuit 9. 
The five band-pass filters 7a, 7b, 7c, 7d and 7e have respective pass-bands 
which are not overlapped. The different filtered signals from these five 
filters are treated or used as knocking representative signals. By 
analyzing the knocking representative signals, the arithmetic circuit 9 
judges whether the engine is under knocking or not, and in accordance with 
this judgement, the ignition timing control circuit 11 controls the 
ignition timing of the engine 1. 
The operation steps for controlling the ignition timing of the engine will 
be described with reference to the flowchart of FIG. 3. 
By the knocking sensor 5, engine vibration is sensed (Step 101). The engine 
vibration representative signal from the knocking sensor 5 is fed to the 
five band-pass filters 7a, 7b, 7c, 7d and 7e, so that, as is seen from the 
graph of FIG. 4, five different frequency bands of the signal are 
respectively filtered by the five band-pass filters 7a, 7b, 7c, 7d and 7e 
to obtain knocking vibration modes f1, f2, f3, f4 and f5 of the respective 
bands, and respective power spectrum gains Pf1, Pf2, Pf3, Pf4 and Pf5 of 
these modes f1, f2, f3, f4 and f5 are read (Step 103). 
It is to be noted that the graph of FIG. 4 shows a frequency power spectrum 
at the time when the engine 1 runs at 2000 rpm with knocking. 
Then, the power spectrum gains Pf1, Pf2, Pf3, Pf4 and Pf5 are added to 
obtain the sum total P of them (Step 105). Then, a difference Q between 
the sum total P and a reference gain Po is obtained (Step 107), the 
reference gain Po being based on an output signal of the knocking sensor 5 
issued when the engine runs without knocking. By analyzing the difference 
Q, judgement is carried out as to whether the engine is under knocking or 
not. That is, when the difference Q is greater than a reference value R, 
it is judged that the engine is under knocking. Upon this judgement, the 
ignition timing is delayed by the ignition timing control circuit 11 for 
suppressing the knocking (Step 109). 
Referring to FIGS. 5 and 6, there are shown flowcharts showing operation 
steps carried out in a second embodiment of the present invention. 
In this embodiment, the outputs of the five band-pass filters 7a to 7e are 
weighted in accordance with the engine speed detected by the crank angle 
sensor 3 for achieving much reliable knocking detection. 
As is seen from the graph of FIG. 7, the weighting coefficients at the 
vibration modes f3 and f4 are constant irrespective of engine speed, the 
weighting coefficients at the vibration modes f1 and f2 decrease with 
increase of engine speed, and the weighting coefficient at the vibration 
mode f5 increases with increase of engine speed. 
The reason why the weighting coefficients at the vibration modes f3 and f4 
are constant is as follows. 
This is because the vibration modes f3 and f4 take place at substantially 
all engine speeds irrespective of whether the engine is under knocking or 
not. Furthermore, since the vibration mode f3 appears much frequently at 
all engine speed as compared with the vibration mode f4, the weighting 
coefficient of the mode f3 is larger than that of the mode f4. It is to be 
noted that the power spectrum levels of the modes f3 and f4 at the time 
when the engine is under knocking are higher than those at the time when 
the engine runs without knocking. 
The reason why the weighting coefficients at the vibration modes f1 and f2 
decrease with increase of the engine speed is as follows. 
This is because the difference between the power spectrum gain Pf1 and Pf2 
and the knocking reference gain Po becomes small with increase of the 
engine speed. 
The reason why the weighting coefficient at the vibration mode f5 increases 
with increase of the engine speed is as follows. 
This is because the difference between the power spectrum gain Pf5 and the 
knocking reference gain Po increases with increase of the engine speed. 
The operation steps of this second embodiment will be described with 
reference to the flowcharts of FIGS. 5 and 6 and the correlation graph of 
FIG. 7. It is to be noted that the flowchart of FIG. 5 shows the operation 
steps carried out when the engine runs at 2000 rpm, while the flowchart of 
FIG. 6 shows the operation steps carried out when the engine runs at 4000 
rpm. As has been mentioned hereinabove, when the engine runs at 2000 rpm, 
the vibration representative signal from the knocking sensor 5 shows such 
a frequency power spectrum as shown in FIG. 4. 
As is seen from the flowchart of FIG. 5 which shows the operation steps 
carried out when the engine runs at 2000 rpm, at Step 540, the power 
spectrum gains Pf1, Pf2, Pf3, Pf4 and Pf5 of the vibration modes f1, f2, 
f3, f4 and f4 are weighted by the weighting coefficients 0.2, 0.1, 0.3, 
0.2 and 0.2 (see FIG. 7). Then, the weighted power spectrum gains are 
added to obtain the sum total P (Step 505), that is: 
EQU P=0.2Pf1+0.1Pf2+0.3Pf3+0.2Pf4+0.2Pf5 (1) 
Similar to the case of the above-mentioned first embodiment, the sum total 
P is compared with the knocking reference gain Po to judge whether the 
engine is under knocking or not (Steps 507 and 508). If judged that the 
engine is under knocking, the ignition timing is delayed by the ignition 
timing control circuit 11 (Step 509). 
As is seen from the flowchart of FIG. 6 which shows the operation steps 
carried out when the engine runs at 4000 rpm, at Step 604, the power 
spectrum gains Pf1, Pf2, Pf3, Pf4 and Pf5 of the vibration modes f1, f2, 
f3, f4 and f4 are weighted by the weighting coefficients 0.15, 0.05, 0.3, 
0.2 and 0.3 (see FIG. 7). Then, the weighted power spectrum gains are 
added to obtain the sum total P (Step 605), that is: 
EQU P=0.15Pf1+0.05Pf2+0.3Pf3+0.2Pf4+0.3Pf5 (2) 
Similar to the case of the above-mentioned first embodiment, the sum total 
P is compared with the knocking reference gain Po to judge whether the 
engine is under knocking or not (Steps 607 and 608). If judged that the 
engine is under knocking, the ignition timing is delayed by the ignition 
timing control circuit 11 (Step 609). 
As is described hereinabove, in the present invention, five parallelly 
arranged band-pass filters 7a to 7e are employed which have respective 
pass-bands which are not overlapped, and the outputs from these five 
band-pass filters 7a to 7e are all used for producing a more practical 
knocking representative signal. Thus, higher knocking detecting ability is 
obtained. Particularly, in the second embodiment, the weighting technique 
is used for obtaining much more practical knocking representative signal. 
Thus, much reliable knocking detection is obtained from the second 
embodiment. 
If desired, the power spectrum gains Pf1, Pf2, Pf3, Pf4 and Pf5 may be 
weighted by not only the engine speed but also an engine temperature. In 
this case, a coolant temperature sensor can be used for detecting the 
engine temperature.