WO1995002174A1

WO1995002174A1 - Improved misfire detection in automobile engine

Info

Publication number: WO1995002174A1
Application number: PCT/US1994/007656
Authority: WO
Inventors: Kyong M. Park; Niraj Gupta
Original assignee: Kavlico Corporation
Priority date: 1993-07-07
Filing date: 1994-07-07
Publication date: 1995-01-19

Abstract

An improved misfire detection in automobile engines may include, in a first embodiment, an automobile engine (12) which has a pressure transducer (16) in its exhaust system (14), and misfires are identified by the pressure drop accompanying the misfire and the timing of the pressure drop relative to the cylinder identification signal. This may be accomplished digitally, sampling the pressure level at a rate of 1,000 cycles per second or more, and statistically analyzing the samples representing relative pressure. The sharp pressure drop and the range of pressures are important parameters in identifying the misfiring cylinder. A second embodiment of the present invention may include a simple, reliable, compact and inexpensive fluid pressure discontinuity analysis system which includes a basic capacitive pressure transducer (120), made of two closely spaced insulating plates whose opposed faces containing conductive layers, and one of the plates is a flexible diaphragm of low mechanical hysteresis.

Description

IMPROVED MISFIRE DETECTION IN AUTOMOBILE ENGINE

Related Patent Applications This invention is related to the prior U.S. Patent Applications Serial No. 08/188,580, filed January 28, 1994, entitled "Improved Misfire Detection in Automobile Engine" (Docket 97-229), and Serial No. 08/088,172, filed July 7, 1993, entitled "Pressure Discontinuity Analysis System," (Docket 95-127) with the foregoing applications being assigned in whole or in part to the assignee of the present invention.

FIELD OF THE INVENTION

This invention relates to improved misfire detection in an automobile engine and, to the practical detection of large abrupt changes in magnitude, called discontinuities, of fluid pressure in mechanical systems involving gasses or liquids.

BACKGROUND OF THE INVENTION

Many systems (such as electrically-operated heat-pump air-conditioning systems and natural-gas powered heating and air-conditioning systems) , industrial processes (such as petro-che ical refining processes or steam-boiler systems) , and various pumping systems, involve the use of flowing gases or liquids which under normal operating conditions have pressure levels that are substantially constant or that vary within relatively small ranges. Thus, a discontinuity in such a fluid pressure level is indicative of some kind of malfunction, which for safety or efficiency reasons should be detected whenever it occurs, in order that appropriate correction measures may be taken.

Further, when an automobile engine misfires, it would be desirable to quickly and accurately identify the cylinder which is misfiring, so that repairs or correction of the problem could be quickly accomplished. In the past, various methods have been employed to accomplish this goal. The most common technique is to run the engine at a fairly low speed and successively disable the cylinders, one-by- one, and check for changes in motor sound and/or vibration, with disabling of the misfiring cylinder not causing any change, while the disabling of other cylinders producing further (intentional) misfiring and a change in engine sound or vibration. However, the foregoing method is time consuming; and, with intermittent misfiring conditions, it is difficult to identify the source of the problem.

Accordingly there has been a need for a simple, compact, reliable and inexpensive real-time pressure discontinuity analysis system capable of detecting such discontinuities and alerting either a human operator or another automatic correction system to the occurrence of a pressure discontinuity. While relatively expensive alarm systems of this type are known, these tend to be complicated and to operate by indirect sensing methods which require sophisticated data-processing procedures and techniques in order to be of practical utility. For example, the "Method of and System for Detecting Misfire in Internal Combustion Engine", U.S. Patent 4,083,234, issued April 11, 1978 (and assigned to Nissan Motor Co., Ltd. of Japan), involves two separate indirect transducers, namely an acoustic transducer, such as an earphone receiver placed near the output of the engine exhaust gas, together with a rotary electromechanical engine-speed responsive frequency generator which produces a frequency variable proportional to the output speed of the engine. Furthermore, the signals from these two separate transducers must be processed by a fairly complicated electromechanical frequency analyzer (involving four separate band-pass filters, and two rotary contact mechanical elements operated at variable frequencies dependent upon engine speed) . Accordingly there has been a need for a more direct pressure-discontinuity analysis system, which utilizes only one transducer, namely a pressure transducer, and which operates successfully regardless of the values of other related variables (such as engine speed, in the case of automobile engine misfiring detection systems) . Also there has been a need for an analysis system which does not include any rotating mechanical parts, and whose mechanical aspects are limited to the simple flexing of a diaphragm of low mechanical hysteresis, which simplification greatly improves reliability and extends lifetime durability without need of scheduled service and essentially eliminates the possibility of breakdown-mandated repairs.

Accordingly, a principal object of the present invention is to identify the particular cylinder causing misfiring conditions in engines.

SUMMARY OF THE INVENTION

In accordance with the present invention, improved misfire detection in automobile engines may include, in a first embodiment, a pressure-to-electrical signal transducer is employed to sense the exhaust pressure from an internal combustion engine. The timing of significant pressure discontinuities which accompany cylinder misfiring conditions is compared against a standard signal such as a C.I.D. (Cylinder Identification) signal which occurs at an accurate time once for each engine firing cycle; and the timing of the misfire relative to the C.I.D. signal uniquely identifies the misfiring cylinder.

In accordance with this first embodiment of the invention, the output from the pressure transducer is sampled at a fairly high rate and converted from analog signals to an extensive set of digital signals or numbers, corresponding to the relative pressure at successive sampling instants, extending over a firing cycle, as measured from one C.I.D. signal to the next, and including the firing times for all engine cylinders. The statistical deviation is calculated to determine whether there is a misfire, and in the event of a misfire, the time of occurrence of the misfire is identified. This time is "normalized" by taking the ratio of the time of occurrence of the misfire and dividing it by the C.I.D. period, or the time interval from one C.I.D. signal to the next. This C.I.D. ratio, or "normalized" time period is then compared with a "Look-Up Table" or Library of C.I.D. ratios each identifying one of the cylinders, the misfiring cylinder is identified, and output signal indicating this result is provided.

One important advantage of the first embodiment of the present invention is that intermittent misfires can be identified. The matter of whether or not a misfire occurs may be determined through statistical analysis, with the magnitude of the statistical deviation in pressure signals indicating whether or not misfires are occurring. One text which describes the techniques employed in statistical analysis which may be employed in a preferred implementation of the invention is "Numerical Recipes in C" or "The Art of Scientific Computing," Second Edition, by William H. Press et al., Cambridge University Press.

A further aspect of the first embodiment of the present invention, an oscilloscope, or monitor may be provided to receive both C.I.D. and pressure transducer signals, for determining visually the presence of misfires. The pressure transducer signals may be amplified and applied directly to the oscilloscope, or may be processed as described in the above-identified prior patent applications, and then applied to the oscilloscope.

It is also noted that there is a significant pressure drop at the engine exhaust associated with a misfire. Further, the "range" or the difference between maximum and minimum pressure reachings at the engine exhaust is an important indication of misfire occurrences. SUMMARY OF THE INVENTION

In a second embodiment of the present invention, there is provided a simple, reliable, compact and inexpensive fluid pressure discontinuity analysis system which includes a single basic capacitive pressure transducer, made of two closely spaced insulating plates whose opposed faces contain conductive layers, and one of which plates is a flexible diaphragm of low mechanical hysteresis. The transducer is coupled to a variable pressure source which under normal operating conditions remains at a substantially constant pressure level. The output signal from the transducer has its high-frequency alternating current (AC) component attenuated by a low-pass filter, whose output in turn has its direct current (DC) component substantially removed by capacitive blocking, after which the remaining AC output is then amplified by a circuit which may include an operational amplifier. This AC- amplified signal is then compared with a "reduced-magnitude average" reference signal (produced by an AC-to-DC conversion side-circuit followed by magnitude-level adjustment) . The comparator output triggers a one-shot monostable multivibrator used to produce an on-off switching signal which operates an LED alarm signal. A further aspect of the second embodiment of the present invention is that a pressure discontinuity analysis system may include a pressure transducer for providing electrical signals corresponding to input pressure, a comparator and a detection use circuit. Applied to the inputs of the comparator are an AC signal corresponding to the AC component of the output of the pressure transducer, and a DC reference signal which is a function of the average magnitude of the AC component of the output of the pressure transducer. Circuitry is also provided for energizing the detection use circuit only when a pressure discontinuity occurs, causing the AC component to exceed a predetermined level. Brief Description of the Drawings Fig. 1 is a block diagram of an overall system illustrating the principles of the present invention;

Fig. 2 is a plot of relative pressure against time extending for several engine cylinder firing cycles for a normal eight cylinder engine operating at approximately 1,000 revolutions per minute;

Fig. 3 is a plot of relative pressure against time extending for several cycles of cylinder firing of an engine so operating at approximately 1,000 revolutions per minute, but with one of the cylinders misfiring; and

Figs. 4A and 4B together show the mode of operation of the circuit of Fig. 1, in detecting misfiring cylinders, and identifying them. FIG. 5 is a partly schematic perspective view of a discontinuity analysis system illustrating the principles of the invention;

FIG. 6 is a schematic diagram of the discontinuity analysis system of FIG. 5; FIG. 7 is a typical measurement of normal fluid pressure as a function of time, illustrating minor fluctua¬ tions in a substantially constant level of pressure;

FIG. 8 is a time-plotted measurement of abnormal fluid pressure in the presence of level discontinuities, such as by a repeated malfunction in a pumping device;

FIG. 9 is a plot versus time of a reference voltage representing average pressure, and a signal representing pressure, including pressure discontinuities;

FIG. 10 is a plot versus time of the corresponding output of a comparator of a reference signal and a filtered pressure signal;

FIG. 11 is a plot versus time of a filtered pressure signal, measured during normal pressure conditions;

FIG. 12 is the same as FIG. 11 except that the measurement has been done during abnormal pressure conditions; and FIG. 13 is a schematic block diagram which depicts the information-theoretic architecture of the analysis system of FIG. 6.

Detailed Description of Pref-»τr-»n Fit-hodime ts

Referring more particularly to the drawings, Fig. 1 shows the first embodiment of the present invention which includes an automobile engine 12 having an exhaust system 14, to which the pressure transducer 16 is coupled. The pressure transducer 16 may be of the type described for example in U.S. Patent No. 4,388,668, granted June 14, 1983, and assigned to the Assignee of the present application. The pressure transducer 16 includes two disks of ceramic material, and in one operative embodiment, the disks were approximately 1.27 inches in diameter and the diaphragm was 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 spaced 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. Forming a part of the pressure-to-electrical signal transducer 16 is a small printed circuit board which transforms the changes in capacitance to a varying electrical signal. One typical circuit which is employed for this purpose is disclosed in U.S. Patent No. 4,398,426, granted August 16, 1983, and assigned to the Assignee of the present invention. These transducers are available from Kavlico Corporation, 14501 Los Angeles Avenue, Moorpark, California 93021, as ten

PSIG (Pounds Per Square Inch - Gauge) pressure transducers.

Returning to Fig. 1 of the drawings, the output signal from the transducer 16 is routed to the analog-to-digital converter 18 where it is sampled at a rate, such as 1,000 or 5,000 or more samples per second, under the timing control of the clock 20. From the engine 12, the cylinder identification signal for (C.I.D.) signal on lead 22 is supplied to the microcomputer and controller 24, along with the output from the analog-to-digital converter 18. The program control for the system is stored in the memory 26. Memory 26 may be, for example, an EPROM or an EEPROM. The additional random access memory 28 is provided to receive and store data being processed by the microcomputer and controller 24, and may store additional information as a "library" or "Look-up Table." The display unit 30 provides an indication of the misfiring signal by the illumination of one of the red LED signal elements 32. When the green LED 34 is illuminated, this indicates that there are no misfiring cylinders. The output from the transducer 16 is also supplied to the signal amplification and/or processing circuit 36. The C.I.D. signal is routed on lead 38 to the oscilloscope 40. Similarly, the output from the circuit 36 is applied to the oscilloscope 40. The electrical signals from the transducer 16 may be merely amplified by the circuit 36 and applied to the oscilloscope 40. With these conditions, the signal appearing on the screen of the oscilloscope 40 be substantially of the form shown in Fig. 2, with minor variations in pressure, as the successive cylinders of an eight cylinder engine fire. On the other hand, in the case of a misfire, the trace will be of the type shown in Fig. 3 of the drawings. More specifically, note the successive minima appearing at points 52, 54 and 56, for example. These minima represent points of reduced pressure which occur when a misfire takes place.

Referring now to the analog-to-digital converter 18, signals of the type shown in Fig. 3 of the drawings are applied to the analog-to-digital 18, and these signals are sampled at a rate sufficiently high to follow the changes in input pressure, perhaps 1,000 to 5,000 or more times per second. Tables 1A and IB are the result of a typical numerical processing from the analog-to-digital converter 18. The first and third columns of Tables 1A and IB represent successive time intervals, while the second and fourth represent relative pressure. At the top of the first and second columns is the standard deviation for a normal non-misfiring engine running at 1,000 rpm. In this case, this standard deviation was approximately 0.039. At the top of the third and fourth columns, representing a misfiring engine running at the same revolutions per minute, is the standard deviation of 0.121. Thus, it may be seen that the standard deviation is in the order of three times as great for a misfiring engine, as for a normal non-misfiring engine. Table IB is a continuation of Table 1A, placed on a separate sheet for convenience in attaching to the patent application. The first two columns of the combined tables generally correspond to the beginning of Fig. 2, while the last two columns correspond to the beginning of Fig. 3. In reviewing the final column of relative pressure values included in Tables 1A and IB, it may be noted that the relative pressure drops from the initial figure of 0.4979, down to a minimum of 0.1855, about one-third of the way down Table IB. Thereafter, the relative pressure values start to increase.

It is to be understood that Tables 1A and IB are merely illustrative of the type of signals which are transmitted by the A-to-D converter 18 to the microcomputer and controller 24. Of course, Tables 1A and IB are written in decimal form, while the numerical or digital information transmitted from the A-to-D converter 18 to the microcomputer 24 is in binary form. It is also noted that each period for the firing of a complete set of eight cylinders would take many more samples than are shown in Tables 1A and IB, but these are merely exemplary of a small portion of the start of one cycle. Reference will now be made to Figs. 4A and 4B which show the sequence of operations of the microcomputer and controller 4, as shown in Fig. 1. Initially, block 62 indicates start-up and the acquisition of data from both the C.I.D. lead 2 and from the pressure transducer 16 (processed by the analog-to-digital converter 18) . Diamond 64 inquires as to whether or not these two inputs are present, with an indication producing a recycling of the system along the path indicated by the line 66 (discussed below) . If both inputs are present, we proceed to the blocks 68 and 70, indicating the recording of both the C.I.D. signal and also the sensor data. Below block 70 is the diamond 72 indicating the evaluation of successive C.I.D. pulse periods to see if the pulse periods are decreasing in time indicating deceleration of the engine. A "yes" response to this inquiry 72 causes recycling of the system (as indicated by line 74) , as some misfires are to be expected during deceleration and the system is not intended to track such misfires. A negative response to the question raised by diamond 72 indicates that the recording process will continue, as indicated by the line extending to block 68. The next steps of statistical analysis are indicated by the blocks 76 and 78. The statistical analysis is accomplished as discussed in the text citation cited hereinabove, using data of the type set forth in Tables 1A and IB. The output is of the type indicated in Fig. and, as set forth in the second column of Tables 1A and IB, these are normal, non-misfiring conditions, which cause the system to be recycled as indicated by the line 80. However, if there is a large standard deviation, as is present in the pressure signals of Fig. 3 and the fourth column of Tables 1A and IB, then a misfire is indicated as it is different from known normal conditions, and we proceed to block 82 toward the bottom of Fig. 4A. Following analysis as indicated in block 82, line 84 indicates the next step, as identified in the diamond 86 (Fig. 4B) , is to determine whether there is "any match for misfire?" Library information 88 is input to diamond 86 as a "Look-Up Table" for the timing of discontinuities. If a misfire match is found, then the next step is to indicate energization of signals to control the display 30 of Fig. 1 as indicated in block 90. Further, the appropriate red light or LED is turned on to identify the misfiring cylinder, and the green LED light is turned off, as indicated in block 92.

Returning to the diamond 86, if no match is found for a single cylinder misfire, the next step, shown in diamond 94 is to determine whether there is a multi- cylinder misfire. A determination from this statistical analysis leads to recycling of the program back to the starting block 62. A positive response will lead to optional blocks 96 and 98, or directly to block 100. Relative to block 00, a special multi-misfire indication may be provided which would light up all of the red lights or LEDs 32 in the output display 30. Alternately, more detailed examination may be provided inducing known intentional misfires as indicated by the block 96, analyzing the response as indicated by block 98, and identifying the specific multiple misfiring cylinders in the display 30 of Fig. 1.

It is noted in passing that the oscilloscope 40 and the direct or amplified input to the oscilloscope 40 from the pressure transducer 16 may be employed in order to further identify misfires. It is further noted that, by using the oscilloscope and the circuit 36 along with the direct input 38 to the oscilloscope 40, the oscilloscope may be used directly without the microcomputer 4 to detect misfire. In this connection, the signal amplification and processing circuit 36 may be patterned after those disclosed in the patent applications cited hereinabove to provide a single pulse, occurring at the point of the misfire. This single pulse may be routed to the oscilloscope and may be displayed along with a pair of C.I.D. signals defining a frame. By the location of the misfire pulse between a successive C.I.D. pulses, the misfiring cylinder may be identified. This technique involves the provision of a set of reference time intervals, one for each cylinder, so that the position of the misfire pulse within the C.I.D. period may be associated with the misfiring cylinder.

Referring now back to Figs, and 3 of the drawings, it may be noted that, with no misfiring, in Fig. , the maximum relative pressure is about 0.54 and the minimum is about 0.35, giving a "range" or difference of about 0.19. On the other hand, with the misfiring condition as shown in Fig. 3, the maximum relative pressure reading is about 0.66 and the minimum is about 0.18, giving a much greater range of about 0.48. This very substantial pressure drop, and related greatly increased range is utilized in defecting the existence and timing of the misfire.

It is to be understood that the foregoing detailed description relates to the first preferred embodiment illustrating the principles of the invention. Various modifications may be made without departing from the spirit and scope of the first embodiment. By way of example and not of limitation, the sampling rate for the analog-to- digital converter 18 may vary from approximately 5,000 samples per second up to a million samples per second. Instead of the simple display 30, an alphanumeric display giving results in sentence form may be provided. As noted above, the circuit 36 may be either a direct amplifier, or a signal processing circuit as disclosed in the patent applications cited at the beginning of this specification, and the circuit 36 may be switched from one mode of operation to the other. Concerning the Look-Up Table or library information as indicated at 88 and as stored in the memory associated with the microcomputer 4, the library information may take other forms than a Look-Up Table identifying the ratio as discussed above corresponding to each cylinder. Thus, these ratios may change slightly with different motor speeds, as set forth in revolutions per minute, and tables may be provided for successive speeds, or alternatively the ratios may be modified by a simple algorithm for different speeds. Alternatively, pattern matching or pattern identification may be employed in place of the Look-Up Table, to identify the misfiring cylinder. It is further noted that, in the implementation of the system of Fig. 1, alternative electronic components performing the same functions or equivalent functions may be substituted for the indicated components.

Tables 1A and IB which follow form a part of this patent specification.

FIG. 5 shows the second embodiment of the present invention which includes a cylindrical pressure transducer 111 coupled to a fluid-carrying pipe 115 by a Tee-joint 117 and providing electrical output signals to the discontinuity analysis circuit 119 shown in greater detail in FIG. 6. In this second embodiment of the present invention, the transducer 111 is of the type of a capacitive pressure transducer, such as are manufactured by Kavlico Corp., 14501 Los Angeles Ave. , Moorpark, CA 93021 under such patents as U.S. Patent 4,329,732, issued May 11, 1982 to Fred Kavli et al for "Precision Capacitance Transducer," and U.S. Patent 4,388,668, issued June 14, 1983 to Fred Kavli et al for "Capacitive Pressure Transducer." In these transducers there is an insulating plate closely spaced from a flexible insulating diaphragm of low mechanical hysteresis. The plate and diaphragm are coated on facing surfaces with a conductive layer; thus the capacitance between the plate and the diaphragm varies with the diaphragm's flexing, which is proportional to changes in the fluid pressure on the non-coated side of the diaphragm. This type of sensor is preferred because of its superior signal to noise ratio and because of its tailored time response characteristics. The sensor response time is roughly 5 to 10 milliseconds, which indicates that it would take about 5 to 10 milliseconds to shift 63% of the way from indicating one pressure level to indicating a new pressure level. The output waveform of such a sensor in a normally running fluid pressure system (such as in an automobile exhaust gas output stream) is presented in FIG. 7. The same sensor, with abnormal pressure conditions (such as are produced by an engine misfiring) , gives the output waveform presented in FIG. 8.

Before discussing the particular details of the second embodiment to the pressure discontinuity analysis circuit provided in FIG. 6, it may be helpful to the reader to consider the information-flow architecture of this circuit as presented in block-diagram form in FIG. 13. Here a variable pressure source 110 presents pressure changes 112 to a pressure transducer (or sensor) 120 (also shown in Fig. 6) whose output consists of time-varying electrical signals 130. These signals are operated on by low-pass filter 140, which suppresses any extraneous high-frequency oscillations which may be present. The resultant smoothed signal is passed to a signal-conditioning means 150 (also shown in Fig. 2), which consists of DC blocking means 152, producing a DC-attenuated signal 156, which is the input to AC-amplifying means 154. The result is the AC-amplified signal 157, which is both the input to comparator means 180 and averaging and level-adjusting means 160. The averaging and level-adjusting means 160 provides reference signal 170, which is compared with AC-amplified signal 157 by comparator circuit 180. The difference signal 188 triggers switch means 190, which provides on/off signal 192 to detection use means 198. The preceding information- processing algorithm could be implemented by a digital filter or a hybrid digital-analog filter, but for simplicity and cost-effectiveness the preferred embodiment comprises the analog-circuit of FIG. 6, which will now be discussed in greater detail. (Operational amplifiers will be referred to as "op amps".) The sensor output 130, denoted by "pressure voltage" V→ as shown in FIG. 6, is filtered by a signal-conditioning circuit 145. In a presently preferred embodiment, this signal-conditioning circuit 145 comprises three sub- circuits: a low-pass filter 140, a DC-blocking filter 152, and an AC-amplifying circuit 154.

The low-pass filter 140 is comprised of resistor Rl (174 kilo-ohms) and capacitor Cl (0.1 micro-farads). The output of the low-pass filter is buffered by op amp U6B, and then subjected to the DC-blocking circuit 152, which is formed by capacitor C2 (0.47 micro-farads) and resistor R2 (200 kilo-ohms) . The resultant signal has its AC component amplified by the AC-coupled inverting amplifier 154 implemented by op amp U6A and its associated circuits, including resistors R3 (1 mega-ohm) and R4 (10 kilo-ohms) .

The output 157 of the conditioning circuit 145, denoted by "signal voltage" V„, is shown in FIG. 6 as providing an input to both a comparator subsystem 185 (which includes a comparator 180 and a mono-stable multivibrator 190) and an averaging side-circuit 160.

The side-circuit 160 uses an AC-to-DC conversion circuit as a means of averaging the AC signal V. 157 to produce a reference signal 170, denoted by "reference voltage" V_t, which is the other input to the comparator 180. The side-circuit 160 includes capacitors C4 (4.7 micro¬ farads) , C6 (10 pico-farads) , C1 (150 pico-farads) , C8 (30 pico-farads) , C9 (10 micro-farads) , CIO (10 micro-farads) , and Cll (4.7 micro-farads), together with resistors R6 (20 kilo-ohms), R7 (20 kilo-ohms), R8 (10 kilo-ohms), R9 (22.6 kilo-ohms), R10 (20 kilo-ohms), Rll (15 kilo-ohms), R12 (6.2 kilo-ohms), R13 (97 kilo-ohms), and R14 (23 kilo- ohms), as well as voltage converter U2 and op amps U2, U3, U4, and U5B, and diodes Dl and D2.

The comparator 180 is based upon op amp U5A, and its output is the input to the switch circuit 190 implemented by monostable multivibrator (one-shot) U1A, which produces an output voltage pulse at the collectors of transistors Ql and Q2 for every occurrence of a pressure discontinuity detection. The pulse duration is determined by the timing components of circuit 190, namely resistor R5 (50 kilo- ohms) and capacitor C5 (10 nano-farads) . The output resistors R15 and R16 are both of 10 kilo-ohms resistivity. The resistor R17 (200 kilo-ohms) precedes diode D3, which is a Light Emitting Diode (LED) and which is illuminated for the duration of the pulse at transistor Ql. The resistor R18 (1 kilo-ohm) is connected to the collector of transistor Q2, whose output may be monitored by a digital filter or microprocessor as indicated by the output connector BNC.

As shown in FIG. 6, the sensor output V_p is filtered by a low-pass filter 140, formed by Rl and Cl, to attenuate very high-frequency oscillations of the type which may occur during normal operation and are of no consequence for discontinuity detection or reference level determination. The filtered signal is buffered by op amp U6B and coupled through capacitor C2 to an inverting amplifier U6A. AC coupling is used to block the DC level of the sensor. The gain of the amplifier 154 is set by resistor R3 to give a suitable peak-to-peak voltage at the output (U6A pin 1) , the DC level at the output of the amplifier being set by resistor R4.

The output V_t, of the amplifier U6A ,shown in the plots of FIGS. 7 and 8, is applied to the non-inverting input of the comparator 180 (U5A pin 3) . The same signal V, is coupled via capacitor C4 to an AC-to-DC converter circuit. The DC output of this circuit, appearing on pin 6 of U4, is level-shifted by op amp U5B and then applied on lead 170 to the inverting input of the comparator 180 (U5A pin ) to serve as a reference voltage V_t. The comparator output (U5A pin 1) acts as a trigger for the monostable multivibrator U1A. The one-shot circuit U1A produces a pulse output on output pins 4 and 13 on every positive-going voltage transition on its input on pin 2. For the case of normal operating conditions, not shown in the plots of the drawings, the signal V. is lower than the bottom peak of V., and the output of the comparator 180 stays high and there is no output from the one-shot. This would correspond to a plot of the type of FIG. 9 wherein the inverted signal V. would be represented by a fluctuating but almost constant-level straight line beneath the inverted reference signal V_t; in this case, the comparator 180 would never send an output pulse.

For abnormal conditions, involving a pressure discontinuity, the amplitude of V. is larger, causing V. to increase and the bottom peak of V, to move lower as shown in FIG. 9 and the corresponding FIG. 10, wherein the pulse- width is about 0.5 milliseconds and the time between rising pulse edges is about 5 milliseconds in one example tested (misfiring automobile engine at 6,000 rpm) . At the crossover points of signal voltage V. and reference voltage V_t the comparator switches state, producing a square wave at its output. The positive-going pulse on output pin 13 of the circuit U1A turns transistor Ql to its ON state, lighting up the LED for the duration of the pulse. This blink of the LED serves as a visual indicator of the abnormal pressure discontinuity event. The negative-going pulse on output pin 4 of circuit U1A is inverted by transistor Q2 to produce a positive-going pulse at its collector. This pulse can be monitored by a central processor coupled to the output collector labeled BNC. It is to be understood that the foregoing detailed description of the second embodiment, and the accompanying drawings relate to a presently preferred illustrative embodiment of the invention. However, various changes may be made without departing from the spirit and the scope of the second embodiment. Thus, by way of example and not of limitation, the transducer per se may be made of other materials than those mentioned hereinabove. Furthermore, it is possible to use a variable-resistivity sensor instead of a variable-capacitance sensor; for example, the facing surfaces of the plate and diaphragm can be coated with film resistive layers whose resistivity changes as the diaphragm is flexed. In addition, the parts need not have the precise configuration described hereinabove, but may have alternative arrangements. Further, instead of the structural parts being made of metal, they may in many cases be formed of high strength composite materials. The analog circuit of FIG. 6 may be replaced by a functionally equivalent hybrid analog-digital filter or purely digital filter having the same information-theoretic architecture, as depicted in FIG 13. Also a threshold device can be inserted between the comparator and the monostable multivibrator, in order to reduce the detection sensitivity to minor pressure discontinuities; and this threshold device can be operated either upon an absolute threshold level-setting, or upon a relative level-setting which depends upon the level of the reference signal and varies as that signal varies; and such a circuit could be used to supplement or in place of the circuit 160 of FIG. 13.

Accordingly, it is to be understood that the detailed description of the first and second embodiments of the present invention and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents, rather than from the examples given.

TABLE 1A

Sdev : 0.039328801 Sdev : 0.121074943

Normal @1000rpm Misfire @1000rpm time data time data

0.0000 0.4491 0.0000 0.4979 0.0001 0.4442 0.0001 0.4833 0.0002 0.4393 0.0002 0.4784 0.0003 0.4393 0.0003 0.4735 0.0004 0.4491 0.0004 0.4686 0.0005 0.4491 0.0005 0.4784 0.0005 0.4491 0.0005 0.4833 0.0006 0.4540 0.0006 0.4833 0.0007 0.4540 0.0007 0.4784 0.0008 0.4638 0.0008 0.4784 0.0009 0.4491 0.0009 0.4833 0.0010 0.4442 0.0010 0.4686 0.0010 0.4491 0.0010 0.4638 0.0011 0.4393 0.0011 0.4491 0.0012 0.4393 0.0012 0.4393 0.0013 0.4296 0.0013 0.4149 0.0014 0.4393 0.0014 0.4345 0.0015 0.4491 0.0015 0.4247 0.0015 0.4442 0.0015 0.4296 0.0016 0.4491 0.0016 0.4149 0.0017 0.4393 0.0017 0.4198 0.0018 0.4442 0.0018 0.4149 0.0019 0.4442 0.0019 0.4101 0.0020 0.4296 0.0020 0.3954 0.0020 0.4345 0.0020 0.3954 0.0021 0.4296 0.0021 0.3808 0.0022 0.4296 0.0022 0.3759 0.0023 0.4247 0.0023 0.3661 0.0024 0.4198 0.0024 0.3759 0.0025 0.4345 0.0025 0.3710 0.0025 0.4393 0.0025 0.3710 0.0026 0.4393 0.0026 0.3361 0.0027 0.4442 0.0027 0.3710 0.0028 0.4540 0.0028 0.3612 0.0029 0.4540 0.0029 0.3564 0.0030 0.4393 0.0030 0.3466 0.0030 0.4345 0.0030 0.3417 0.0031 0.4345 0.0031 0.3271 0.0032 0.4296 0.0032 0.3271 0.0033 0.4296 0.0033 0.3515 0.0034 0.4101 0.0034 0.3271 0.0035 0.4345 0.0035 0.3222 0.0035 0.4296 0.0035 0.3222 0.0036 0.4393 0.0036 0.3319 TABLE IB

0.0037 0.4393 0.0037 0.3271 0.0038 0.4442 0.0038 0.3173 0.0039 0.4345 0.0039 0.3075 0.0040 0.4296 0.0040 0.3027 0.0040 0.4296 0.0040 0.2880 0.0041 0.4296 0.0041 0.2880 0.0042 0.4247 0.0042 0.2880 0.0043 0.4198 0.0043 0.2880 0.0044 0.4149 0.0044 0.2831 0.0045 0.4296 0.0045 0.2831 0.0045 0.4296 0.0045 0.2831 0.0046 0.4345 0.0046 0.2783 0.0047 0.4393 0.0047 0.2587 0.0048 0.4345 0.0048 0.2636 0.0049 0.4393 0.0049 0.2587 0.0050 0.4345 0.0050 0.2441 0.0050 0.4345 0.0050 0.2441 0.0051 0.4296 0.0051 0.2392 0.0052 0.4296 0.0052 0.2441 0.0053 0.4198 0.0053 0.2392 0.0054 0.4296 0.0054 0.2343 0.0055 0.4345 0.0055 0.1855 0.0055 0.4393 0.0055 0.2441 0.0056 0.4393 0.0056 0.2441 0.0057 0.4442 0.0057 0.2294 0.0058 0.4442 0.0058 0.2246 0.0059 0.4491 0.0059 0.2441 0.0060 0.4345 0.0060 0.2246 0.0060 0.4345 0.0060 0.2246 0.0061 0.4198 0.0061 0.2294 0.0062 0.4198 0.0062 0.2294 0.0063 0.4247 0.0063 0.2246 0.0064 0.4101 0.0064 0.2294 0.0065 0.4198 0.0065 0.2343 0.0065 0.4149 0.0065 0.2392 0.0066 0.4296 0.0066 0.2246 0.0067 0.4247 0.0067 0.2197 0.0068 0.4296 0.0068 0.2099 0.0069 0.4345 0.0069 0.2148 0.0070 0.4247 0.0070 0.2148 0.0070 0.4296 0.0070 0.2148 0.0071 0.4247 0.0071 0.2099 0.0072 0.4247 0.0072 0.2148 0.0073 0.4101 0.0073 0.2246 0.0074 0.4101 0.0074 0.2294 0.0075 0.4247 0.0075 0.2294 0.0075 0.4296 0.0075 0.2197 0.0076 0.4345 0.0076 0.2197 0.0077 0.4296 0.0077 0.2148 0.0078 0.4345 0.0078 0.2197 0.0079 0.4491 0.0079 0.2197 0.0080 0.4393 0.0080 0.2197 0.0080 0.4393 0.0080 0.2246 TABLE IB (Continued)

0.0081 0.4247 0.0081 0.2294

0.0082 0.4247 0.0082 0.2343

0.0083 0.4345 0.0083 0.2441

0.0084 0.4247 0.0084 0.2490

0.0085 0.4247 0.0085 0.2441

0.0085 0.4296 0.0085 0.2343

0.0086 0.4296 0.0086 0.2392

Claims

CLAIMS What is claimed is:

1. A method for identifying the cylinder of an automobile engine in which a misfire occurs, wherein the engine provides cylinder identification (C.I.D.) signals which occur once each firing cycle of the engine, said method comprising the steps of: providing a pressure-to-electrical signal transducer to sense the exhaust pressure from the automobile engine; analyzing the electrical signals from said transducer to identify the occurrence of misfires and their timing relative to the C.I.D. signals; determining whether or not a misfire is occurring by discontinuities in the output signal from the transducer representing pressure discontinuities; identifying a misfiring cylinder by the time of occurrence of the misfire relative to the time period established by successive C.I.D. signals; and providing an output signal identifying the misfiring cylinder.

2. The method of Claim 1, wherein said analyzing step is accomplished digitally using a microcomputer.

3. The method of Claim 1 wherein said analysis step is accomplished visually, using an oscilloscope or comparable display of both C.I.D. and output signals derived from the transducer.

4. A system for detecting when a misfire occurs in an internal combustion automobile engine including an exhaust system, said system comprising: a pressure-to-electrical signal transducer coupled to the exhaust system of the engine; a sampling means receiving a signal output from said transducer; and circuit means coupled to receive a digitized electrical signal output from said sampling means, said circuit means including means for identifying a pressure drop representing a misfiring cylinder.

5. The system of Claim 4 wherein said circuit means includes a microcomputer.

6. The system of Claim 5 wherein said sampling means includes an analog-to-digital (A/D) converter for receiving analog signals from said transducer and for supplying output digital signals to said microcomputer.

7. The system of Claim 6 wherein means are provided for operating said A/D converter to take samples at a rate of at least 5,000 samples per second.

8. The system of Claim 6 wherein means are provided for operating said A/D converter to take samples at a rate of at least 10,000 samples per second.

9. The system of Claim 4 wherein said transducer comprises a capacitive pressure transducer, including an insulating plate and an insulating diaphragm, having facing sides coated with a conductive layer, said transducer having a time constant in the range of 8 to 12 milliseconds.

10. The system of Claim 4 further comprising: low-pass filter means for suppressing high-frequency oscillations of said signal output of said transducer, thereby producing a filtered signal; direct current (DC) blocking means, coupled to the output of said low-pass filter means for substantially removing the direct current component of said filtered signal; alternating current (AC) amplifying means, coupled to the output of said DC blocking means for amplification of the alternating current component of said filtered signal thereby producing an output AC signal; reference signal producing means coupled to the output of said AC amplifying means for generating a reference signal proportional to an average magnitude of said filtered signal during normal continuous variation of said pressure; and comparator means having a first input coupled to said

AC amplifying means and a second input coupled to said reference signal producing means, for comparing the output

AC signal of said AC amplifying means with said reference signal of the reference signal producing means, and said comparator for producing an output pulse to said sampling means when a significant pressure discontinuity occurs.