System and method for monitoring engine performance characteristics

A system and method for monitoring engine performance characteristics such as knock and misfire. The system includes a pressure sensor coupled to an exhaust manifold of an engine being monitored by the system, a motion sensor operatively coupled to the engine's block, an engine position sensor, a load sensor and a processor adapted to independently process the outputs of the pressure sensor and the motion sensor to determine an occurrence of a knock event. The outputs of the pressure sensor and engine position sensor are processed to identify which cylinders are knocking. The outputs of the pressure sensor, load sensor and engine position sensor are processed to determine an occurrence of a misfire condition and to identify which cylinders are misfiring.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a system and method for monitoring engine 
performance characteristics and, more particularly, pertains to a system 
and method for monitoring occurrences of knocks in an internal combustion 
engine and identifying knocking cylinders of the engine. 
2. Description of the Related Art 
Under wide-open throttle conditions, an engine is most susceptible to 
knock. A direct constraint on engine performance, knock also constrains 
engine efficiency by limiting the engine compression ratio. Knock depends 
upon both the quality of fuel and the engine design. 
FIGS. 11A, 11B and 11C illustrate the in-cylinder pressure variations 
during normal combustion, light knock, and heavy knock, respectively. With 
light knock (FIG. 11B), knock occurs late in the burning process and the 
amplitude of the pressure fluctuations is relatively small. With heavy 
knock (FIG. 11C), knock occurs closer to the top-center earlier in the 
combustion process and the initial amplitude of the pressure fluctuation 
is significantly higher. 
Prior knock detection systems include systems employing piezoelectric 
pressure transducers positioned at the end-gas region of the combustion 
chamber. There are several problems with such systems. First, it requires 
a pressure sensor for each cylinder of the engine which drives up the cost 
and complexity of the system. Second, precise positionings of the pressure 
transducers within the combustion chambers are required because 
transducers positioned at different points in the chamber will record 
different pressure levels at a given time due to the shock wave 
propagation phenomena (caused by the spontaneous release of much of the 
chemical energy of the end-gas fuel) characteristic of knock. 
Other prior knock detection systems employ a vibration-driven accelerometer 
mounted on the block of the engine. The performance of such systems, 
however, is plagued by a high incidence of "false knock" indications. 
Furthermore, the performance of such systems is highly dependent upon the 
placement of the accelerometer on the engine block. 
SUMMARY OF THE INVENTION 
A system and method for monitoring engine performance characteristics 
employing a single pressure sensor coupled to an exhaust manifold of the 
engine and a motion sensor mounted to the block of the engine, operating 
in parallel, has been found to detect engine knock with greater 
reliability than prior systems which only employ a vibration-driven 
accelerometer. The deficiencies of the prior systems employing a plurality 
of in-cylinder pressure transducers are also addressed by the system and 
method disclosed herein. 
In this era of on-board and other vehicular diagnostic systems, it is 
important from a cost and complexity standpoint to reduce the number of 
sensors employed in such systems. The disclosed system and method is able 
to monitor both engine knock and engine misfiring with the same pressure 
sensor that is coupled to the exhaust manifold, and, thus, is also 
responsive to this need. 
In accordance with a specific illustrative embodiment of the present 
invention, a system for monitoring engine performance characteristics 
includes a pressure sensor, a motion sensor, an engine position sensor and 
a processor. The pressure sensor is coupled to an exhaust manifold of an 
engine being monitored by the system and generates a pressure signal 
indicative of a pressure within the exhaust manifold. The motion sensor is 
operatively coupled to a block of the engine and generates an engine 
motion signal indicative of a movement of the engine block. The engine 
position sensor generates a cam signal indicative of a position of a cam 
shaft of the engine. The processor is programmed to process the pressure 
signal, the engine motion signal and the cam signal, to determine when a 
magnitude of the pressure signal changes in a predetermined manner, to 
determine when characteristics of the engine motion signal satisfy 
predetermined criteria, and to detect the occurrence of a knock in the 
engine when the magnitude of the pressure signal changes in the 
predetermined manner and the characteristics of the engine motion signal 
satisfy the predetermined criteria within a predetermined time interval. 
In a further aspect of the present invention, the processor of the system 
is also programmed to process the cam signal to associate a cylinder of 
the engine with the knock and to generate a knocking cylinder 
identification signal which identifies the cylinder associated with the 
knock. 
In yet a further aspect of the present invention, the system further 
includes a load sensor operatively coupled to the engine. The load sensor 
generates an engine load signal indicative of a load condition of the 
engine. The processor is programmed to process the pressure signal and the 
engine load signal to determine an occurrence of a misfire in the engine. 
In another aspect of the present invention, a method for monitoring engine 
performance characteristics includes the steps of: providing a pressure 
signal indicative of a pressure within an exhaust manifold of an engine; 
providing an engine motion signal indicative of a movement of a block of 
the engine; providing a cam signal indicative of a position of a cam shaft 
of the engine; and employing a processor to process the pressure signal, 
the engine motion signal and the cam signal, to determine when a magnitude 
of the pressure signal changes in a predetermined manner, to determine 
when characteristics of the engine motion signal satisfy predetermined 
criteria, and to detect the occurrence of a knock in the system when the 
magnitude of the pressure signal changes in the predetermined manner and 
the characteristics of the engine motion signal satisfy the predetermined 
criteria within a predetermined time interval. 
In a broader aspect of the present invention, a system for monitoring 
engine performance characteristics includes: a pressure sensor coupled to 
an exhaust manifold of an engine being monitored by the system with the 
pressure sensor generating a pressure signal indicative of a pressure 
within the exhaust manifold; and a processor adapted to process the 
pressure signal to detect an occurrence of a knock in the engine and to 
generate a knock indication signal. 
In another aspect of the present invention, a method for monitoring engine 
performance characteristics includes the steps of: generating a pressure 
signal indicative of a pressure within an exhaust manifold of the engine; 
and employing a processor to process the pressure signal to detect an 
occurrence of a knock in the engine. 
In another aspect of the present invention, a system for monitoring engine 
performance characteristics includes a pressure sensor and circuitry. The 
pressure sensor is coupled to an exhaust manifold of an engine being 
monitored by the system and generates a pressure signal indicative of a 
pressure within the exhaust manifold. The circuitry indicates a knock when 
a pressure pulse for a cylinder as indicated by the pressure sensor is 
substantially greater than a normal level for the engine operating 
condition, and indicates a misfire when the pressure pulse for a cylinder 
is substantially less than a normal level for the engine operating 
condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an engine 20 and a system 30 for monitoring performance 
characteristics of the engine. The system 30 includes a plurality of 
sensors for measuring various engine performance parameters. In the 
illustrated preferred embodiment, the plurality of sensors include an 
engine position sensor 32 which is operatively coupled to a camshaft of 
the engine 20. The engine position sensor 32 generates a cam signal on 
leads 34, 34' indicative of a position of the camshaft. In an exemplary 
preferred embodiment, the cam signal is in synchronism with revolutions of 
the camshaft. The cam signal can, for example, be provided in analog form 
on lead 34 or digitally on lead 34'. An exemplary engine position sensor 
32 comprises an inductive coil sensor positioned near the engine flywheel 
generating an analog pulse in response to the spark plug signal. An 
alternative position sensor 32 comprises an optical encoder. Other methods 
and apparatuses for measuring the rate at which the engine's camshaft is 
turning and the position of the camshaft relative to the engine block are 
also contemplated as being within the scope of the present invention. 
The plurality of sensors also includes a motion sensor 36 which is 
operatively coupled to the top of the block of engine 20. The motion 
sensor 36 comprises, for example, a conventional accelerometer, and 
generates an engine motion signal on lead 38 indicative of a movement of 
the engine block. In an exemplary preferred embodiment, the motion sensor 
36 is characterized by a 120 msec response time and additionally includes 
conventional noise sensor and/or acoustic sensor elements. 
The aforementioned sensors also include a load sensor 42 coupled to an 
intake manifold of the engine for providing an engine load signal on lead 
44 responsive to and indicative of a load condition of the engine. The 
load sensor 42 measures the intake manifold pressure (manifold absolute 
pressure (MAP)) which is a good overall indicator of the load condition of 
the engine. As a greater load is applied to the engine, the pressure 
measured by the load sensor 42 proportionally increases. A preferred load 
sensor 42 is a 15 psi absolute sensor such as the Kavlico P-15 absolute 
pressure sensor manufactured by Kavlico Corporation of Moorpark, Calif. 
Alternatively, the engine load signal on lead 44 may be generated by 
providing a throttle position sensor. 
The system 30 additionally includes a pressure sensor 52 coupled to an 
exhaust manifold of the engine 20. The pressure sensor 52 generates a 
pressure signal on lead 54 indicative of a pressure within the exhaust 
manifold. Preferably, the pressure sensor 52 comprises a ceramic variable 
capacitive pressure sensor, e.g., 5 psig, 10 msec response time, at 1 time 
constant. The pressure sensor 52 may be of the type described for example 
in U.S. Pat. No. 4,388,668, granted Jun. 14, 1983, and assigned to the 
Assignee of the present application. 
The pressure transducer includes two disks of ceramic material, and in one 
operative embodiment, the disks are approximately 1.27 inches in diameter 
and the diaphragm is approximately 14.5 thousandths of an inch thick. The 
two ceramic disks are preferably spaced apart by glass frit by a 
relatively small distance such as a few thousandths of an inch, and they 
have space conductive plates on their inner surfaces, which form a 
capacitor. As the diaphragm is flexed with pressure changes, the 
capacitance of the spaced plates changes. 
A small printed circuit board employs the changes in capacitance to 
generate a varying electrical signal transducer output. A typical circuit 
employed for the aforementioned purpose is disclosed in U.S. Pat. No. 
4,398,426, granted Aug. 16, 1983, and assigned to the Assignee of the 
present invention. Such transducers are also available from Kavlico 
Corporation of Moorpark, Calif., as 10 psi pressure transducers. 
As shown in FIG. 1, the engine monitoring system 30 also includes 
electronics 60 which receive and process the signals on leads 34, 38, 44, 
54. Exemplary preferred electronics 60 comprise a controller or processor 
62, a clock 64, a read only memory (ROM) 66, a random access memory (RAM) 
68, an A/D converter 70, a multiplexer 72, and a signal filter/amplifier 
74. 
A key aspect of the present invention is that the engine monitoring system 
30 employs a single pressure sensor 52 coupled to an exhaust manifold of 
an engine and a motion sensor 36 mounted to the block of the engine, 
operating in parallel, to detect engine knock. Another key aspect of the 
present invention is that a preferred embodiment of the system 30 is 
adapted to monitor both engine knock and engine misfiring with the same 
pressure sensor 52 that is coupled to the exhaust manifold and identifies 
individual knocking or misfiring cylinders. 
Thus, and again referring to FIG. 1, a preferred engine monitoring system 
30 further comprises a display unit 81 which receives a signal 83 
identifying a knocking cylinder of the engine when a knock condition is 
detected. For a V-8 engine, the display unit 81 includes eight knock 
indicators 85 corresponding to the eight cylinders of the engine. A 
similar display unit 80 receives a signal 82 identifying a misfiring 
cylinder of the engine when a misfire condition is detected by the system 
30. The display unit 80 for a V-8 engine includes eight misfire indicators 
84 corresponding to the eight cylinders of the engine. The indicators 84, 
85 may, for example, comprise red light emitting diodes (LED) or any other 
suitable visible and/or audible indicator which identifies a 
knocking/misfiring cylinder to the mechanic performing a diagnostic check 
of the engine, a driver of the vehicle, etc. The display units 80, 81 may, 
but do not necessarily, include a normal/engine OK indicator 86 such as a 
green LED, for example. 
As may be readily appreciated, other display configurations are 
contemplated. For instance, the display units 80, 81 could be assembled 
within a single display unit. 
Consistent with the goal of providing a system 30 with flexible diagnostic 
capabilities, the electronics 60 also include accessible terminals for 
measuring voltages of interest such as terminals 88, 90 which respectively 
allow a technician to monitor the cam signal on lead 34 and a filtered 
pressure sensor signal with an oscilloscope 92, for example. 
As shown in FIG. 1, the processor 62 and the A/D converter 70 receive 
timing signals from the clock 64. The processor 62, in turn, controls the 
multiplexer 72 which receives the cam signal, motion signal, pressure 
signal and load signal. Where the engine position sensor 32 is an optical 
encoder or other device generating a digital cam signal 34', such an 
output signal may be provided directly to the processor 62 as shown. 
Additionally, a signal filter/amplifier 74 is provided between the 
pressure sensor 52 and the multiplexer 72. 
Components of the signal filter/amplifier 74 (FIG. 1A) are appropriately 
selected to filter out any undesired noise and such that an unacceptably 
high amount of propagation delay is not introduced into the signal path. 
The processor 62 is electrically connected to ROM 66 via a read-only 
interconnect as is known in the art. RAM 68 conventionally interfaces to 
the processor 62 via a read/write interface. A program to be executed by 
the processor 62 may be stored in either the ROM 66 or the RAM 68. 
Generally, the system and method for monitoring engine performance 
characteristics disclosed herein pertain to a knock detection feature, a 
misfire detection feature, or a combination of these features. With regard 
to the knock detection aspect, FIGS. 11A, 11B and 11C illustrate typical 
in-cylinder pressure variations during normal combustion, light knock, and 
heavy knock, respectively. While the pressure at the exhaust manifold is 
not identical to the pressure within the cylinders, it has been observed 
that a sharp rise in the pressure measured by the pressure sensor 52 is a 
good indicator of knock. Accordingly, the processor 62 is programmed to 
process the pressure signal to detect an occurrence of a knock in the 
engine when the magnitude of the pressure signal changes in a 
predetermined manner which has been empirically observed to correspond to 
an observed and verified knock event. A particular advantage of the 
present invention, with the pressure sensor 52 simply coupled to the 
exhaust manifold, is that it eliminates the requirement in some prior 
systems of precisely positioning a plurality of pressure transducers 
within the engine's combustion chambers. 
In a more refined embodiment, the processor 62 is programmed to effect the 
knock detection feature by processing both the pressure signal and the 
engine motion signal. In this embodiment, the processor 62 is also 
programmed to determine when a magnitude of the pressure signal changes in 
a predetermined manner, to determine when characteristics of the engine 
motion signal satisfy predetermined criteria, and to detect the occurrence 
of a knock in the engine 20 when the magnitude of the pressure signal 
change in the predetermined manner and the characteristics of the engine 
motion signal satisfy the predetermined criteria within a predetermined 
time interval. Exemplary predetermined criteria are met when the 
difference between knock and noise measured by the motion sensor 36 and 
its noise sensor exceeds a predetermined threshold. It has been observed 
that such a system, which provides a knock indication only when the 
processed pressure and engine motion signals both indicate an occurrence 
of a knock event, generates fewer false indications of knock than prior 
knock detection systems which employ only a vibration-driven accelerometer 
mounted on the block of the engine. 
Preferably, the processor 62 is also programmed to process the cam signal 
to associate a cylinder of the engine 20 with the knock and to generate 
the knocking cylinder identification signal 83 which identifies the 
cylinder associated with the knock. 
The electronics 60 are programmed to implement the knock detection aspect 
according to a program such as the one represented in the flow chart of 
FIGS. 12A and 12B. The program begins at block 200 wherein the cam signal, 
motion signal, load signal, and pressure signal are received in digital 
form into the processor 62. After the processor 62 verifies at 202 that 
the engine is running and that the inputs are present, block 204 and 
diamond 206 reset the system along line 208 if the spark timing is not 
advanced from normal. Block 210 represents the combined functions of the 
multiplexer 72 and the A/D converter 70 which provide the signals on leads 
34, 38, 44, 54 to blocks 212, 213, 214, 215, respectively. As discussed 
above, the cam signal provides engine rate (or RPM) information at block 
218 depending upon the distance between successive cam pulses. Block 220 
is an optional statistical analysis processing stage, the details of which 
are described below in greater detail with reference to block 120 of FIG. 
9A and incorporated herein. Block 222 of FIG. 12A reflects that the load 
signal corresponds to the present engine load condition. Block 223 
reflects that the motion signal corresponds to a movement of the engine 
block. 
FIG. 12B illustrates at block 224 a knock condition detection technique 
wherein the engine motion signal and the pressure signal are processed and 
compared as discussed above. Block 224 additionally (and optionally) 
provides that samples of the pressure signal are compared to stored 
empirical data taken under normal engine operating conditions. The 
comparison with stored empirical data is described below with reference to 
block 124 of FIG. 9B and incorporated herein. 
After a knock condition is detected, the processor 62 at 226 of FIG. 12B 
attempts to identify which particular cylinder has knocked. A preferred 
system 30 additionally is programmed to include block 228 which provides 
correction information as discussed below in greater detail with reference 
to block 128 of FIG. 9B and incorporated herein. 
If a knock match is found, blocks 230, 232 of FIG. 12B indicate that the 
processor 62 may be programmed to actuate the appropriate knock indicator 
85 of the display unit 81 (FIG. 1). 
FIG. 13 is a plot showing observed sensor outputs of the system 30 over 
time for a 6 cylinder engine at 0 degrees, 3,000 rpm, and 400 mg load. 
FIG. 14 is a plot showing observed sensor outputs of the system 30 over 
time for a 6 cylinder engine at 8 degrees, 3,000 rpm, and 400 mg load. 
Turning to the misfire detection aspect of the present invention--where the 
system 30 detects and identifies misfiring engine cylinders under varying 
engine cycle rates and varying engine load conditions in real time--one or 
several pressure sensors 52 may be included in the detection system 30 as 
shown in FIGS. 1-3. Pressure sensors employed for misfire detection are 
preferably coupled to the exhaust manifold system 22 "upstream" of the 
catalytic converter 24 with each sensor 52 being positioned substantially 
equidistantly from the engine block. The system 30 detects a misfire 
condition when the pressure signal is determined to be outside a range of 
values which have been empirically determined to be acceptable for the 
present operating speed and load condition of the engine. A misfire 
condition is characterized by a sharp decrease in the pressure in the 
exhaust manifold system 22. If the voltage of the pressure signal falls 
below the aforementioned acceptable range of values, for example, the 
system 30 detects a misfire condition. FIGS. 4 and 5 show the pressure 
signal on lead 54 under normal operating conditions and under a misfire 
condition, respectively. The sharp periodic dip in the voltage plot shown 
in FIG. 5 is indicative of a misfiring engine cylinder which causes the 
pressure in the exhaust manifold system 22 and thus the voltage of the 
pressure signal on lead 54 to decrease. 
The "upstream" positioning of the pressure sensor 52 in the exhaust 
manifold system 22 relative to the catalytic converter 24 also allows the 
system 30 to detect an unacceptably high increase in the pressure measured 
within the exhaust manifold system 22 which may, for example, be caused by 
a blockage in the catalytic converter 24. As before, the system 30 
utilizes the empirically determined range of misfire sensor signal values 
which are known to be acceptable under normal operating conditions (when 
the engine is operating at a particular rate and under a particular load 
condition) to determine if the pressure in the exhaust manifold system 22 
is unacceptably high. The pressure signal may also be employed by the 
system 30 to provide noise control for the engine's exhaust system. 
It has been observed that the pressure and resultant output voltage of the 
pressure sensor 52 varies depending upon the rate at which the engine is 
running and upon the load applied to the engine. Under normal operating 
conditions, ranges of pressure signal values may be measured and used by 
the electronics 60 to determine when the pressure in the exhaust manifold 
system 22 is inappropriately high or low at a particular engine speed and 
load condition. The processor 62 which, for example, comprises a MC68HC11 
type controller manufactured by Motorola Corporation serves several 
functions. First, the processor 62 receives and utilizes the pressure 
signal via lead 54 to determine whether a misfire has occurred in any of 
the cylinders during a particular cycle of the engine. The beginning and 
end of an engine cycle are respectively designated by two consecutive 
output pulses of the cam signal at lead 34. Second, the processor 62 
monitors the range of the pressure signals received during a frame of 
data, (i.e., the range of all pressure signals sampled by the processor 62 
between consecutive cam pulses). The difference between the maximum 
voltage and minimum voltage (P.sub.max and P.sub.min, respectively) of the 
pressure signals at lead 54 detected during such a time interval provides 
a range signal (RANGE) corresponding to a particular engine cycle. Thus, a 
lowest peak value (P.sub.min) is measured and a range (RANGE) of the 
pressure signal at lead 54 is calculated by the processor 62 during an 
engine cycle and used to detect a misfire condition occurring during that 
particular cycle in real time by comparing these values to empirically 
derived misfire signal signatures. The system 30 provides a simplified 
detection scheme which does not require but may utilize statistical 
processing techniques. 
FIG. 6 shows a group of misfire signal signatures or characteristics which 
illustrate the observed range, maximum minus minimum pressures of the 
pressure signal at lead 54 plotted against the rate at which the engine is 
running; each of the misfire signal plots corresponding to a different 
load condition of the engine. When no load is applied to the engine, the 
misfire signal signature 94.sub.1 of solid square data points plots 
observed normal operating condition pressure signal range values along the 
vertical axis against the rate of the engine along the horizontal axis. 
Under a "light load" condition (e.g., the engine is idling with the air 
conditioner on) the misfire signal signature 94.sub.2, of hollow square 
data points was observed. Similarly, the misfire signal signatures 
94.sub.3, 94.sub.4, 94.sub.5 show the correlation between normal operating 
condition pressure signal range values and different cycle rates of the 
engine as observed at a 50 ft-lb, 100 ft-lb, and 150-200 ft-lb load 
condition of the engine. The misfire signal characteristics 94 are stored 
in a suitable memory device accessible by the processor 62, such as the 
ROM 66 or the RAM 68 (FIG. 1). 
Real time processing is difficult utilizing prior art techniques which 
require data to be collected for at least an entire frame for averaging 
and which utilize computationally intensive calculations such as square 
root operations. The processor 62 may alternatively employ a predicted 
average scheme thereby eliminating some of the burdens associated with 
statistical processing techniques. 
Referring to FIG. 1A, the signal filter/amplifier 74 receives the pressure 
signal at lead 54 and removes any direct current signal component 
therefrom. The amplifier U1 provides a filtered, alternating current 
component of the pressure signal 54 to the processor 62 which uses this 
output of U1 to obtain the RANGE, Pmin and Pmax values for each engine 
cycle. The reference voltage V.sub.REF is a conventional power supply 
voltage at +5.0 Vdc, for example. 
It has been additionally observed that the voltage V.sub.REF /2 provides a 
good approximation of an average of the AC component of the pressure 
signal at lead 54 which is used in the standard variance of the average 
(SVAR) calculation, discussed below. When analog circuit components such 
as those illustrated in FIG. 1A are selected, the gain of the operational 
amplifier U1 is set very precisely, thereby allowing the foregoing 
predicted average scheme to be employed in lieu of more involved 
statistical calculations. 
If the electronics 60 and the processor 62, in particular, have sufficient 
processing capability, then the processor 62 may also be programmed to 
calculate a standard variance of the average (SVAR) of the pressure signal 
at lead 54 in real time in making the misfire determination. As with the 
plots of the misfire signal characteristics, FIG. 7 shows that the 
standard variance of the average of the pressure signal at lead 54 over 
different engine speeds differs under different engine load conditions. 
More specifically, FIG. 7 shows SVAR signatures or characteristics 
96.sub.1, 96.sub.2, 96.sub.3, 96.sub.4, 96.sub.5 which respectively 
correspond to calculated SVARs over engine rate as observed under a no 
load condition, a "light load" condition, 50 ft-lbs, 100 ft-lbs, and 
100-200 ft-lbs. As with the misfire signal characteristics 94, the SVAR 
characteristics 96 may also be stored in a memory device such as the ROM 
66 or the RAM 68. Tables 1 and 2 which follow form a part of this patent 
application and respectively include the observed data points plotted in 
FIGS. 7 and 8. One text describing statistical analysis techniques which 
may be employed in a preferred implementation of the present invention is 
"Numerical Recipes in C" or "The Art of Scientific Computing," 2nd Ed., by 
William H. Press, et al., Cambridge University Press. 
If a rate at which the processor 62 samples the pressure signal remains 
constant over varying engine rates, the resolution of the sample data 
necessarily decreases as rotations per minute (RPM) of the engine 
increase. Accordingly, the present invention may include increasing a 
sample rate of the processor 62 as the speed of the engine increases in 
RPMs. 
Successive cam pulses serve as reference points for a beginning and an end 
of a single engine cycle during which all cylinders fire once. Since the 
cylinders of an engine fire in a predefined order, the cam pulses serve as 
reference points in time where the processor 62 begins and ends sampling 
the pressure signal at lead 54 for a particular engine cycle. When 
different cylinders of an engine misfire, the observed pressure minimum in 
the pressure signal at lead 54 appears at different positions within the 
frame. However, the time intervals between the pressure decreases of 
successively misfiring engine cylinders are not necessarily equally 
spaced. In fact, as indicated in FIG. 8, it has been observed that the 
lowest peak values of the pressure signal at lead 54, for each of the 
different cylinders when misfiring, are not equally spaced in time from 
one successive misfiring cylinder to the next, varying in position within 
the frame depending upon the rate of the engine and the load applied 
thereto. However, it may be seen that the timing of the successive 
cylinder misfiring points as indicated in FIG. 8 is generally sequential 
with minor variations at different engine speeds. 
After the misfire detection system electronics 60 have detected a misfire 
condition, the processor 62 utilizes an index ratio to determine which 
cylinder misfired. The index ratio is determined from the rate that the 
engine is operating at and from the number of samples of the pressure 
signal at lead 54 taking during the engine cycle for which a misfire 
condition has been detected. The index ratio correlates the pressure 
signal at lead 54 with the cam signal at lead 34 to generate a signal 
identifying a misfiring cylinder of the engine after a misfire condition 
is detected. More specifically, the index ratio is determined by dividing 
the number of sample intervals between the cam signal and the occurrence 
of the pressure minimum point P.sub.min, by the total number of sample 
intervals between successive cam signals. 
The accuracy with which the processor 62 identifies a particular misfiring 
cylinder is enhanced by adjusting the index ratio with index ratio 
correction information which compensate for the effects of different 
engine cycle rates and varying engine load conditions on the position of 
P.sub.min relative to the cam pulses. FIG. 8 shows corrected index ratios 
for each cylinder of a V-8 style engine where 360 samples per frame were 
taken by the processor 62 and where the engine was subjected to a 50 ft-lb 
load. In FIG. 8, the 360 samples were rescaled to 256 data points. The 
corrected index ratios for each cylinder are plotted over engine speed and 
have been adjusted by the index ratio correction information which 
compensate for the effects of different load conditions. The index ratio 
correction information may be stored in a memory device such as the ROM 66 
or the RAM 68. Table 3 which follows forms a part of this patent 
application and includes the observed data points plotted in FIG. 8. 
The electronics 60 are programmed to implement the misfire detection aspect 
of the present invention according to a program such as the one 
represented in the flow chart of FIGS. 9A and 9B. The program begins at 
block 100 wherein the cam signal, the load signal, and the pressure signal 
are received in digital form into the processor 62. After the processor 62 
verifies at 102 that the engine is running and that the inputs are 
present, block 104 and diamond 106 reset the system along line 108 since 
decreasing periods between successive cam pulses indicate a deceleration 
of the engine during which some misfires are to be expected. Block 110 
represents the combined functions of the multiplexer 72 and the A/D 
converter 70 which provide the signals on leads 34, 44, 54 to blocks 112, 
114 and 116 respectively. As discussed above, the cam signal provides 
engine rate (or RPM) information at block 118 depending upon the distance 
between successive cam pulses. If the processor 62 employs the above 
described statistical analyses, block 120 represents, for example, a SVAR 
calculation of the pressure signal. Block 122 reflects that the load 
signal corresponds to the present engine load condition. 
FIG. 9B illustrates at block 124 the aforedescribed misfire condition 
detection technique wherein samples of the pressure signal are compared to 
stored empirical data taken under normal engine operating conditions. 
After a misfire condition is detected, the processor 62 at 126 attempts to 
identify which particular cylinder has misfired. The index ratios are 
adjusted with the stored index ratio correction information as represented 
in block 128. If a misfire match is found, blocks 130, 132 indicate that 
the processor 62 may be programmed to actuate the appropriate misfire 
indicator 84 of the display unit 80 (FIG. 1). If no match is found for a 
single cylinder misfire, the processor 62 may also be programmed at 134 to 
determine via statistical analyses whether there is a multi-cylinder 
misfire condition. Furthermore, optional blocks 136, 138, 140 reflect that 
the program which is executed by the processor 62 may also include a 
variety of self-tests or diagnostic features for the engine misfire 
detection system 30. 
FIG. 10A shows the cam signal, with the successive pulses 152 occurring 
each time the no. 1 cylinder reaches top dead center. FIG. 10B shows a 
misfiring condition, with the minimums 154 representing the reduced 
pressure points associated with the misfiring of the same cylinder during 
successive engine cycle intervals. The misfiring cylinder may be 
identified, as discussed above, by determining the ratio of the time from 
one cam signal 152 to the next minimum point 154, and dividing this value 
by the time between successive cam pulses. 
It should be understood that the principles set forth above are also 
applicable with appropriate modifications to knock detection and have been 
omitted in the interest of economy. Although FIGS. 9 and 12 show separate 
logical flows for knock and misfire detection, the scope of the present 
invention contemplates programming the processor 62 such that the knock 
detection and misfire detection protocols are executed together, e.g., 
parallel processing, multiplexed routines, etc. 
Those skilled in the art will appreciate that various adaptations and 
modifications of the just described preferred embodiment can be configured 
without departing from the scope and spirit of the invention. Thus, the 
principles of the invention may be alternatively implemented by an analog 
or "hard-wired" circuit, preferably using comparison and reference voltage 
values which vary with engine speed and load. It is also contemplated that 
the positioning of the pressure sensor may be changed to other areas 
within an internal combustion engine or within the exhaust system of the 
engine. For example, a plurality of pressure sensors may be positioned 
each within separate exhaust manifolds. Other types of accelerometers and 
pressure sensors may be applied depending upon the application. Therefore, 
it is to be understood that, within the scope of the appended claims, the 
invention may be practiced other than as specifically described herein. 
TABLE 1 
______________________________________ 
Engine Rate 
Range Under Varying Load Conditions 
rpm no load 0 ft-lb 50 ft-lb 
100 ft-lb 
150-200 ft-lb 
______________________________________ 
100 60 78 60 78 78 
200 60 78 60 78 78 
300 60 78 60 78 78 
400 60 78 60 78 78 
500 60 78 60 78 78 
600 60 78 60 78 78 
700 60 76 61 76 76 
800 60 75 64 75 75 
900 63 76 66 76 76 
1000 65 77 68 77 77 
1100 67 81 72 81 81 
1200 69 88 74 88 88 
1300 68 93 72 93 93 
1400 67 93 71 93 93 
1500 65 90 70 90 90 
1600 62 88 70 88 90 
1700 61 86 70 87 103 
1800 62 87 70 88 115 
1900 65 85 71 89 125 
2000 68 86 72 91 132 
2100 70 89 75 94 140 
2200 73 90 78 103 147 
2300 76 99 81 109 152 
2400 80 105 85 111 157 
2500 84 108 89 115 162 
2600 86 110 93 120 164 
2700 88 112 98 124 165 
2800 89 114 101 126 166 
2900 88 111 102 127 165 
3000 86 105 101 126 183 
3100 83 103 100 125 158 
3200 80 97 95 122 153 
3300 76 93 88 119 147 
3400 73 89 84 115 140 
3500 69 84 81 111 132 
3600 65 80 78 106 120 
3700 62 78 76 100 112 
3800 61 76 75 95 111 
3900 62 76 74 90 111 
4000 65 76 75 86 113 
4100 70 78 76 85 115 
4200 75 78 78 86 119 
4300 80 80 80 88 122 
4400 82 82 82 92 125 
4500 84 83 83 97 126 
4600 87 84 84 102 126 
4700 89 84 86 104 125 
4800 91 84 87 106 125 
4900 94 84 89 107 125 
5000 96 84 90 108 125 
5100 97 84 91 109 125 
5200 98 84 91 109 125 
5300 98 84 91 109 125 
5400 98 84 91 109 125 
5500 98 84 91 109 125 
5600 98 84 91 109 125 
5700 98 84 91 109 125 
5800 98 84 91 109 125 
5900 98 84 91 109 125 
6000 98 84 91 109 125 
6100 98 84 91 109 125 
6200 98 84 91 109 125 
6300 98 84 91 109 125 
6400 98 84 91 109 125 
6500 98 84 91 109 125 
______________________________________ 
TABLE 2 
______________________________________ 
Engine Rate 
SVAR Under Varying Load Conditions 
rpm no load 0 ft-lb 50 ft-lb 
100 ft-lb 
150-200 ft-lb 
______________________________________ 
100 13 17 15 15 17 
200 13 17 15 15 17 
300 13 17 15 15 17 
400 13 17 15 15 17 
500 13 17 15 15 17 
600 13 17 15 15 17 
700 13 16 14 14 18 
800 13 15 14 14 17 
900 13 14 14 14 19 
1000 13 16 14 14 20 
1100 13 20 14 14 20 
1200 13 23 14 14 21 
1300 13 24 14 14 24 
1400 13 25 14 14 26 
1500 13 20 14 14 29 
1600 13 18 14 14 35 
1700 13 17 14 15 44 
1800 13 16 15 16 54 
1900 13 16 16 19 69 
2000 14 17 17 23 79 
2100 14 18 18 31 88 
2200 14 20 20 37 99 
2300 14 26 26 42 110 
2400 15 32 32 53 118 
2500 16 36 36 62 124 
2600 17 39 39 67 130 
2700 18 43 40 69 132 
2800 19 47 41 70 133 
2900 20 49 42 70 132 
3000 21 49 42 70 130 
3100 22 48 40 69 127 
3200 23 45 40 68 124 
3300 23 42 38 67 118 
3400 22 38 36 63 109 
3500 20 33 30 56 101 
3600 19 28 21 43 88 
3700 18 24 19 30 75 
3800 17 20 18 18 63 
3900 17 18 16 16 60 
4000 17 17 16 16 58 
4100 17 17 17 17 56 
4200 17 18 18 18 54 
4300 18 20 20 20 55 
4400 18 22 22 22 56 
4500 19 23 23 23 57 
4600 19 25 24 24 56 
4700 20 26 24 24 56 
4800 20 27 24 24 57 
4900 21 27 24 24 56 
5000 23 28 24 24 59 
5100 23 28 24 24 60 
5200 23 29 24 24 61 
5300 23 29 24 24 61 
5400 23 30 24 24 61 
5500 23 30 24 24 61 
5600 23 30 24 24 61 
5700 23 30 24 24 61 
5800 23 30 24 24 61 
5900 23 30 24 24 61 
6000 23 30 24 24 61 
6100 23 30 24 24 61 
6200 23 30 24 24 61 
6300 23 30 24 24 61 
6400 23 30 24 24 61 
6500 23 30 24 24 61 
______________________________________ 
TABLE 3 
______________________________________ 
Index Ratio Corrected for 50 ft-lb 
Cyl- Cyl Cyl Cyl Cyl Cyl Cyl Cyl Cyl 
inder rpm #1 #2 #3 #4 #5 #6 #7 #8 
______________________________________ 
1 590 67 
796 66 
925 63 
1023 66 
1178 67 
1359 65 
1444 65 
1690 68 
1798 68 
1931 65 
2069 62 
2367 60 
2618 57 
2824 57 
2973 62 
3381 59 
3565 59 
3714 59 
4065 58 
2 595 154 
773 157 
914 159 
1082 158 
1254 159 
1413 159 
1614 163 
1937 158 
2006 156 
2139 154 
2315 153 
3592 155 
2747 152 
3009 152 
3259 154 
3410 155 
3585 155 
3734 155 
4035 155 
4492 157 
3 575 97 
786 95 
843 97 
980 94 
1062 96 
1212 95 
1327 96 
1393 97 
1625 98 
1812 93 
1970 94 
2295 94 
2536 93 
2770 89 
3047 92 
3286 93 
3481 93 
3724 85 
4078 87 
4410 89 
4655 88 
4828 86 
4 569 256 
808 254 
930 255 
1100 254 
1175 267 
1249 268 
1716 255 
2047 253 
2414 252 
2703 248 
2848 250 
3038 250 
3207 251 
3501 250 
3755 249 
3968 251 
4122 251 
4261 253 
5 574 223 
679 223 
735 225 
792 231 
938 223 
1054 223 
1140 224 
1272 225 
1318 226 
1424 225 
1536 228 
1661 225 
1803 223 
1982 223 
2327 220 
2515 219 
2667 219 
2772 224 
2872 222 
3162 224 
3332 224 
3491 218 
3714 219 
3865 214 
4012 214 
4321 214 
4642 217 
6 575 188 
792 189 
902 190 
1131 201 
1231 204 
1334 202 
1432 192 
1629 205 
1937 207 
2139 189 
2373 184 
2718 184 
2907 215 
2932 219 
2965 222 
3055 220 
3241 200 
3501 202 
3724 204 
3952 204 
4122 212 
4373 211 
4555 217 
4842 217 
7 587 126 
898 126 
1092 133 
1231 137 
1355 139 
1444 130 
1518 132 
1747 140 
1883 125 
1970 125 
2112 124 
2340 125 
2546 126 
2786 126 
2889 127 
3038 124 
3090 123 
3277 126 
3546 120 
3897 153 
4213 123 
4361 125 
4842 121 
8 597 32 
805 34 
912 38 
1005 38 
1127 40 
1317 44 
1531 40 
1708 43 
1942 42 
2211 24 
2508 24 
2762 23 
3081 30 
3342 32 
3501 46 
3766 23 
4024 22 
4273 18 
4629 20 
______________________________________