GB2283576A - Engine analyser using a current probe - Google Patents

Abstract

A diagnostic system for a multiple cylinder internal combustion engine includes a digital engine analyzer 11 having an oscilloscope display 13 controlled by microprocessors operating under menu-driven stored program control. A clamp-type, Hall-effect pickup probe 31 non-invasively detects a current flowing through a conductor in the engine to generate a current detection signal which is fed to an amplifier adapter 41 which is, in turn, coupled to an input terminal of the analyzer. The analyser processes the amplified signal to produce a waveform display signal which is displayed on the oscilloscope in either of two different display formats. Another input lead of the analyzer senses the number 1 cylinder firing and the analyzer synchronizes the waveform display signal to the number 1 cylinder signal. <IMAGE>

Description

APPARATUS AND METHOD FOR ENk DIAGNOSIS USING CURRENT WAVEFORM ANALYSIS Cross-Reference to Related Application This is a continuation-in-part of U.S. application serial no. 08/145,589, filed November 4, 1993, entitled "Apparatus and Method for Engine Diagnosis Using Current Waveform Analysis." Background of the Invention Field of the Invention The present invention relates to methods and apparatus for electronically diagnosing and analyzing the performance of internal combustion engines. The invention relates particularly to digital engine analyzers of the type which display digitized information on an oscilloscope screen.

Description of the Prior Art The present invention relates to an improved application for the digital engine analyzer disclosed in U.S. patent no. 5,245,324. That analyzer, as is true of virtually all engine analyzers, analyzes voltage waveforms generated by various components of an internal combustion engine. Accordingly, automotive service personnel who use engine analyzers tend to think only in terms of voltages in the engine rather than amperage, where voltage is understood to be electrical potential and amperage is understood to be flow of electricity or current. Some analyzers have current pickup probes for measuring such parameters as starter cranking current and alternator or generator output current, but these are relatively large currents, anywhere from 1 to 100 amps, and most analyzers do not display the current signals for waveform analysis.

Commonly, the current in a circuit is detected by connecting an impedance in series with the circuit and measuring the voltage drop across the impedance. But this technique is inconvenient in an internal combustion engine, since it requires disconnection and reconnection of the circuitry in question.

Non-invasive current pickup probes are known. One such probe is sold by Snap-on Tools Corporation under the designation MT3000-410, and it measures current levels for display on an oscilloscope of an MT3000A engine analyzer, also sold by Snap-on Tools Corporation, and operating in accordance with the disclosure of the aforementioned U.S.

patent no. 5,245,324. However, such a current probe measures current levels from 1 through 500 amps and is not suitable for detecting low-level currents less than 1 amp.

A low-level current probe is sold by Tektronix Corp.

under the designation AM503 for measuring current levels for display on any laboratory oscilloscope. That probe can measure current levels from 1 milliampere to 20 amps, but it is very expensive and its use in connection with an engine analyzer would be cost prohibitive.

U.S. patent no. 3,603,872 teaches non-invasive pickup of the current flowing in the secondary of an ignition coil of an automotive engine and display of the current on an oscilloscope, but it does not teach any means for relating the current display to specific points in the engine cycle.

Summary of the Invention It is a general object of the invention to provide an improved engine diagnosing system which avoids the disadvantages of prior diagnosing systems while affording additional structural and operating advantages.

An important feature of the invention is the provision of a diagnostic technique which utilizes analysis of current waveforms derived from an internal combustion engine.

In connection with the foregoing feature, another feature of the invention is the provision of diagnostic techniques of the type set forth, which synchronizes the current waveform to a specific event in the engine cycle.

Still another feature of the invention is the provision of a technique of the type set forth, which affords noninvasive detection of low-level current signals less than 1 ampere.

In connection with the foregoing features another feature of the invention is the provision of apparatus for performing the diagnostic technique set forth.

These and other features are attained by providing diagnostic apparatus for a multiple cylinder internal combustion engine in which all of the cylinders are fired in a predetermined firing order during each engine cycle, the apparatus comprising: a non-invasive pickup probe for detecting a current signal in the engine having a current as low as about 100 milliamperes and generating a detection signal, means for sensing the firing of the first cylinder in the firing order and producing in response thereto a first cylinder signal, an amplifier coupled to the pickup for amplifying the detection signal to produce an amplified signal, processing means operating under stored program control coupled to the amplifier for processing the amplified signal to generate a waveform display signal which has a waveform corresponding to that of the detected current signal, oscilloscope display means coupled to the processing means for displaying the waveform display signal, and means for synchronizing the waveform display signal to the first cylinder signal.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

Brief Description of the Drawings For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIG. 1 is a front elevational view of a diagnostic system in accordance with the present invention incorporating an engine analyzer and a current pickup assembly; FIG. 1A is an enlarged fragmentary front elevational view of the keyboard of the engine analyzer of the system of FIG. 1; FIG. 2 is a functional block diagram of the circuitry of the engine analyzer of FIG. 1; FIG. 3 is a top plan view of the adapter of the pickup assembly of the system of FIG. 1; FIG. 4 is a view in vertical section taken along the line 4-4 in FIG. 3; FIG. 5 is a schematic circuit diagram of the circuitry of the adapter of FIGS. 3 and 4; FIG. 6 is an illustration of a screen display provided by the engine analyzer of FIG. 1; FIG. 7 is an illustration of another screen display provided by the engine analyzer of FIG. 1; and FIG. 8 is a schematic circuit diagram of another version of the circuitry of the adapter of FIGS. 3 and 4.

DescriDtion of the Preferred Embodiments Referring to FIG. 1, there is illustrated a diagnostic system, generally designated by the numeral 10, constructed in accordance with and embodying the features of the present invention. The system 10 includes a digital engine analyzer, generally designated by the numeral 11, which may be of the type disclosed in U.S. patent no. 5,245,324, the disclosure of which is incorporated herein by reference, and sold by Snap-on Tools Corporation under the designation MT3000A. Accordingly, only so much of the analyzer 11 is disclosed herein as is necessary for an understanding of the present invention.

The analyzer 11 is disposed in a cabinet 12 and includes a cathode ray tube monitor screen 13 in the form of a digital oscilloscope. Arrayed along the bottom edge of the screen 13 is a set 15 of six "soft" keys, F1 through F6, the functions of which are software-controlled and vary with the mode of operation of the analyzer 11, as is explained in greater detail in the aforementioned U.S. patent no.

5,245,324. More specifically, the software for controlling the operation of the analyzer 11 causes an indication of each soft key's function to be displayed on the screen 13 immediately adjacent to the key.

Referring to FIG. 2, there is illustrated a functional block diagram of the analyzer 11, illustrating its two communication ports which are adapted to be coupled to a variety of associated peripheral devices 17, 18. Referring also to FIG. 1A, the analyzer 11 also has a main keyboard 20, which includes: a numerical keypad 21; four directional keys 22 for the directions up, down, right and left; four function keys 23 for, respectively, actuating SET POINT, FREEZE, PRINT, and SELECT functions; an ENTER key 24; six menu keys 25; a RESET key 26 and a HELP key 27.

In the operation of the analyzer 11, the numerical key pad 21 is used for selecting cylinders, inputting engine information and specifying the rpm set point. The ENTER key 24 is used for entering information input with the numerical keypad 21. The directional keys 22 serve to move the cursor and expand or position waveforms. The FREEZE function freezes any "live" test screen, i.e., a screen which follows varying input information. The SET POINT function calls up an automatic freeze feature when the engine reaches a keyedin rpm. The PRINT feature prints the displayed screen on an associated optional printer. The SELECT function selects between two horizontal and two vertical cursors when measuring a waveform.

The menu keys 25 include: a PRIMARY MENU key which is used to display a menu of primary ignition tests; a SECONDARY MENU key, used to display a menu of secondary ignition tests; a DIAGNOSTIC WAVEFORM MENU key, used for displaying a menu of diagnostic waveforms, including a Lab Scope Waveform; a CYLINDER TEST MENU key, used for displaying a menu of cylinder tests, including a Vacuum Waveform; an OPTION MENU key, used to display a menu of options, including identification of the devices, if any, connected to ports A and B and a Scope Setup screen for user definition of the devices connected to port A and port B; and a MEMORY MENU key used to display a menu of screens in memory which can be cleared or recalled. The RESET key 26 clears the current screen display and returns the system to a start-up Engine Information Screen. The HELP key 26a displays either a help menu or information about the current screen.

The analyzer 11 includes an AC power cord (not shown) adapted to be plugged into an associated 120 or 240-volt, 50 or 60 Hz, AC supply. The analyzer 11 is also provided with suitable conductors (not shown) for connection to an associated source of DC power, such as a battery, which may be the battery of the vehicle under test. It will be appreciated that the analyzer 11 is also provided with a lead set, including an auxiliary lead 28, provided at its distal end with a suitable connector 29 for coupling to a current pickup assembly 30, constructed in accordance with and embodying the features of the present invention, or to other associated adapters, probes or pickups. The lead set includes a number of other leads (not shown), including an inductive pickup lead, a secondary lead, a primary/fuel injection lead, an alternator/battery lead and a ground lead.

Referring in particular to FIG. 2, the signals acquired by the several leads are applied to analog circuits. More specifically, there is input to the analog circuits a signal 1CYL from the inductive pickup lead, a signal PRIM from the primary/fuel injection lead, a signal VOLTLD from the alternator/battery lead and one or more of three secondary signals, respectively labeled ALTSEC, MAINSEC, and HIGHSEC, from the secondary lead, depending upon the type of engine being analyzed and the type of pickup coupled to the inductive lead. In this regard, the secondary lead is preferably a multi-conductor cable which connects to a multi-conductor pickup device, three of the conductors serving to provide a three-bit digital ID signal indicating to the analog circuits an identification of the specific pickup being used and, thereby, an indication of the type of ignition system being analyzed. The auxiliary lead 28 can also be coupled to multiple probe or pickup devices, and it is preferably a multi-conductor cable which will similarly provide signals identifying the particular probe or pickup device used. In the present invention, the auxiliary lead 28 will be used to provide CURRENT signals to the analog circuits.

The analog circuits are connected by a number of lines and buses to digital circuits. The analyzer 11 also includes communication circuits having ports A and B to which peripheral devices 17 and 18 may be coupled by bidirectional lines. The communication circuits are connected to video display circuits, the latter also being connected to the screen monitor 13 and to the digital circuits. The soft key set 15 and the main keyboard 20 are connected to the digital circuits.

Referring to FIG. 1, the current pickup assembly 30 includes a pickup probe 31 and an adapter 40. The pickup probe 31 is preferably a Hall-effect probe, which may be of the type manufactured by F.W. Bell Company, under model no.

P-100. The pickup probe 31 is sensitive to static and dynamic magnetic fields, and uses a semiconductive Halleffect device to convert magnetic energy (created by the current in a conductor) into potential energy, all in a known manner. The pickup probe 31 is of the clamp-on type, including a base 32 having a notch 33 formed therein and dimensioned to accommodate an associated electrical conductor. The probe 31 has a cover 34 which is pivotally coupled to the base 32 at a pivot joint 35 for pivotal movement with respect thereto between an open position (not shown) accommodating insertion of an associated conductor into the notch 33 and a closed position (illustrated in FIG.

1) cooperating with the base 32 to clamp around the associated conductor. The probe 31 is provided with a cable 36 having a connector 37 at the distal end thereof.

Referring also to FIGS. 3 and 4, the adapter 40 has a rectangular, box-like housing 41, provided in one side wall thereof with a connector socket 42 and provided in the top wall thereof with a connector socket 43. Also mounted in the top wall of the housing 41 is a zero-adjust control knob 44 and a scale-selecting switch lever 45. Mounted within the housing 41 is a circuit board 46 which carries an electric circuit 50, the details of which are illustrated in FIG. 5. In use, the connector 37 of the probe 31 is adapted to be coupled to the connector socket 42, while the connector 29 on the auxiliary lead 28 of the engine analyzer 11 is adapted to be coupled to the connector socket 43.

Referring to FIG. 5, the probe cable 36 includes five conductors 51-55. The conductor 51 is an input conductor connected to a fixed current supply, the conductors 52 and 53 are output conductors and the conductor 54 is connected to ground. A potentiometer 56 may be connected in series with the conductor 55 for purposes of calibration adjustment, and may be disposed within the housing of the probe 31. The adapter circuit 50 includes power supplies 57 and 57A, each of which includes an integrated circuit voltage regulator 58 provided with external capacitors 59 and 59a to prevent oscillations. The power supplies 57 and 57A, respectively, supply positive and negative operating voltages for the pickup 31 and the adapter 40, each converting a 12 VDC voltage from the engine analyzer 11, which may be derived from the battery of the vehicle being serviced, to an 8 VDC supply voltage. In particular, a +8 VDC supply voltage is provided to the pickup probe 31 via the conductor 51 through a resistor 69, which regulates the current through the Hall-effect sensor to a predetermined current, preferably about 50mA.

The circuit 50 includes an input amplifier stage 60, which includes an operational amplifier 60a in the instrumentation amplifier configuration. The probe output conductors 52 and 53 are respectively connected through resistors 61 and 62 to the inverting and non-inverting input terminals of the op amp 60a, a capacitor 63 being connected across those input terminals. The amplifier is also provided with an external potentiometer 64. Connected in parallel between the output terminal and the inverting input terminal of the op amp 60a are a resistor 65 and a capacitor 66. Connected in series between the non-inverting input terminal of the op amp 60a and ground are a resistor 67 and a potentiometer 68.

The input amplifier stage 60 converts the differential output from the pickup probe 31 to a circuit ground based voltage level. The ratio of the value of the resistor 61 to that of the resistor 65, and the ratio of the value of the resistor 62 to the values of the resistor 67 and the potentiometer 68 establish an input gain, which is preferably approximately 3. The wiper of the potentiometer 68 is controlled by the control knob 44 on the adapter housing 41 to provide "zero" offset for the adapter circuit 50. The potentiometer 64 is an input offset adjustment for the op amp 60a. The capacitors 63 and 66 stabilize the op amp 60a by providing, respectively, an input shunt and a high frequency negative feedback.

The output of the input amplifier stage 60 is connected to an output amplifier stage 70, which includes an operational amplifier 70a connected in a non-inverting, single-ended amplifier configuration. More specifically, the output from the op amp 60a is coupled through a resistor 71 to the non-inverting input terminal of the op amp 70a, a capacitor 72 being connected across the inverting and noninverting input terminals of the op amp 70a. Connected in parallel across the output terminal and the inverting input terminal of the op amp 70a are a capacitor 73 and a resistor 74. Connected in parallel with the resistor 74 is the series combination of a resistor 75 and a single-pole, single-throw switch 76, which is controlled by the switch lever 45 on the adapter housing 41. The potentiometer 56 of the pickup probe 31 is connected to the junction between the resistors 74 and 75.

The gain of the output amplifier stage 70 is a function of the ratio between either the resistor 74 or the parallel combination of the resistors 74 and 75 to the resistance of the potentiometer 56. The switch 76 is a scale switch.

When it is closed, the output of the op amp 70a is preferably approximately 0.1 volt per ampere sensed, while when the switch 76 is open, the gain is preferably multiplied by ten, giving an output of approximately 1 volt -per ampere sensed. The resistor 71 couples the output of the op amp 60a to the input of the op amp 70a. The capacitors 72 and 73 stabilize the op amp 70a by, respectively, providing an input shunt and a high frequency negative feedback. The output of the output amplifier stage 70 is supplied to the engine analyzer 11 via the connector 29, which also supplies the +12 VDC and -12 VDC voltages to the adapter power supply circuits 57 and 57A.

In operation, it will be appreciated that the pickup probe 31 is clamped around a conductor, the current through which is to be analyzed. The scale switch 76 is set to the proper scale to give the desired waveform display size, and one or more of the other leads of the engine analyzer 11 are coupled to the appropriate points of the engine under test, depending upon the nature of the test being conducted.

Also, depending upon the nature of the test being conducted, the engine analyzer 11 can be operated in either of two different waveform display modes, viz., a Lab Scope Waveform mode, which can provide single-trace or dual-trace displays, and a Vacuum Waveform mode, which can show the current waveform with reference to the cylinder ignition and valve timing marks.

Referring to FIG. 6, there is illustrated the Lab Scope Waveform display screen, which is accessed through the Diagnostic Waveform Menu which is, in turn, accessed by actuating the DIAGNOSTIC WAVEFORM MENU key 25 on the main key board 20. More specifically, the Diagnostic Waveform Menu display lists a number of soft key labels, one of which is "Lab Scope Waveform", and the actuation of the corresponding soft key brings up the Lab Scope Waveform display 80 of FIG. 6. This display screen includes a title at the top, the date and time in the upper left-hand and right-hand corners, respectively, RPM and VOLTS displays and five soft key labels 81a through 81e, respectively entitled "Grid ON/OFF", "Cursors ON/OFF", "Volts Range/Time Base", "Waveform Position" and "Menu Select". The "Grid ON/OFF" soft key 81a controls selection of the display of an internally-generated graticule for the displayed waveform.

The "Cursors ON/OFF" soft key 81b controls selection of the display of horizontal and vertical cursor lines. The "Volts Range/Time Base" soft key 81c is used to change the voltage range (volts/division) by actuating the up and down direction keys 22, and to change the time base (time/division) by using the left and right direction keys 22. Changing the voltage range alters the magnification of the waveform on the screen, while changing the time base lengthens or shortens the displayed waveform. The "Waveform Position" soft key 81d controls the vertical and horizontal positioning of the waveform on the screen, by use of the directional keys 22.

In the upper center of the screen, below the title, is a list 84 of six functions which are accessed through actuation of the "Menu Select" soft key 81e. The directional keys 22 are used to highlight the selected function on the menu. Each of those functions has two options, which are selected by actuation of the SELECT key 23. More specifically, the "Trig Slope" function options are "+" and "-" for, respectively, using a value on the rising or falling edge of the display waveform for synching the waveform. The "Default Setup" function has two options, viz., "SET" or "NOT SET". When the SET option is selected it provides the user with predetermined known starting points from which to adjust or manipulate a waveform to stabilize it. The default setup is deactivated when an option is changed for any of the other five "MENU SELECT" functions or when the Volts Range or Time Base is changed.

The "Sig Coupling" function has two options, AC and DC. The "Waveform" function has two options, SINGLE and DUAL, respectively corresponding to single-trace and dual-trace displays. The "Trig Source" function has six options which are used to select from which of the input leads of the engine analyzer 11 the trigger source will be taken. By use of this function, the displayed waveform can be synchronized to another input signal, such as the signal from the S1 spark plug wire. The "Sig Invert" function has ON and OFF options for displaying the waveform in normal or reversed polarity.

In FIG. 6 there is disclosed a waveform 85, which is the current through an ignition coil and, in particular, a Ford TFI ignition module, which allows current to ramp up, as at 86, to a point where the current is limited. Current continues at this limiting level for a pre-determined time, e.g., 3 ms, after which the current stops flowing and the resulting magnetic field collapses causing the high voltage which fires the spark plug. In a normal coil, the entire process, from the beginning of the ramp 86 to the firing of the spark plug, takes approximately 7 ms. The waveform 85 illustrated in solid line in FIG. 6 indicates the performance for a normal coil. If the coil has shorted turns, its resistance is reduced, causing the current to ramp up faster than normal, as at 87 in the broken-line waveform in FIG. 6. Thus, the spark plug firing occurs before it should. Recognizing the incorrect time interval in the current waveform assist the technician in making a proper diagnosis.

While the present invention may be used to measure the time functions of a waveform, as explained above, it can, of course, also be used to measure the amplitude of a current waveform. Referring to FIG. 7, there is illustrated a Vacuum Waveform display screen 90, which is accessed through the Cylinder Test Menu, called up by actuation of the corresponding key 25 on the main keyboard 20. The Cylinder Test Menu displays a number of soft key options, one of which is the Vacuum Waveform option, and actuation of the corresponding soft key brings up the screen display 90 which, as illustrated in FIG. 7, has a title and RPM and AVERAGE VACUUM displays, respectively in the upper left-hand and right-hand corners of the screen. The screen display 90 also includes a number of soft key labels 91a-91f. While six such labels are illustrated in FIG. 7, it will be appreciated that additional labels could be provided on a separate screen display, in which case the label 91f would be changed to "Next Page" to bring up the second page of labels and, on each such page after the first page, the right-hand label 91f would be "Previous Page", all in a known manner.

The soft keys corresponding with the labels 91a and 91b serve the same function as the keys for the corresponding labels 81a and 81b in FIG. 6. The "Vertical Position", "AC/DC Coupling" and "Waveform Size Select" soft keys 91d91f perform the indicated functions. The "Vacuum Probe Zero" soft key 91c calibrates the vacuum probe, when one is being used, to provide an accurate baseline for making measurements.

A significant aspect of the Vacuum Waveform display screen 90 is that it displays numbers 92 at the top of the screen indicating the ignition or firing events in firing order, starting with the #1 cylinder and, across the bottom of the screen, lists numbers 93 indicating the order of the vacuum events. Thus, the displayed waveform can be shown with reference to these cylinder ignition and valve time marks to assist in correlating the cylinders with the portions of the displayed waveform, when an entire engine cycle is displayed.

In FIG. 7, there is illustrated a waveform 95 which is the current through the ignition power line for a fuel injected engine. The waveform 95 includes a plurality of humps or peaks 96 which should be equal in number to the cylinders and injectors in the engine, indicating that all of the injectors are firing. The advantage of this technique is that it requires only one step.

The circuit 50 requires a scale switch 76, because it includes operational amplifiers which must be powered from an 8-volt supply. This means that when a direct-reading scale of one volt per ampere is used, the operational amplifiers cannot produce a 10-amp output, so it is necessary to add a second scale to get up to 10 amperes.

However, other operational amplifiers are available which can run directly from the 12-volt supply which is available in an automobile, so that it can produce a 10-ampere range of output without a change of scale. A modified adapter circuit 100 incorporating such operational amplifiers and without a scale switch is illustrated in FIG. 8. When provided with the circuit 100, the adapter 40 has a five-pin connector socket 101 similar to the socket 42, and a socket 102, which may be a 16-pin socket, similar to the connector socket 43. In use, a connector 37A of a probe 31A, similar to the probe 31, is adapted to be coupled to the connector socket 101, while the connector 29 of the auxiliary lead of the engine analyzer 11 is adapted to be coupled to the connector socket 102.

As can be seen in FIG. 8, +12 VDC and -12 VDC supply voltages are provided from the engine analyzer 11. The +12 VDC supply is coupled through a resistor 103 to the input of an IC regulator 105, which functions as a current regulator to provide 40ma of current to the probe sensor through a resistor 106. The regulator 105 has an adjustment terminal which is connected to the resistor 106 and is also connected through a capacitor 107 to ground. A capacitor 108 is connected between the +12 VDC supply and ground.

The circuit 100 includes a two-stage IC amplifier 110, the first stage including an operational amplifier 111, the inverting and non-inverting input terminals of which are, respectively, connected through resistors 112 and 113 to the probe output conductors 52A and 53A. The non-inverting terminal is also connected through a resistor 114 to ground.

The probe output conductor 53A is also connected to a wiper of a coarse adjust potentiometer 115 through a resistor 116, and to the wiper of a fine adjust potentiometer 117 through a resistor 118. The fine adjust potentiometer 117 is panelmounted and is preferably coupled to the remainder of the circuit 100 by a plug and socket connector. The potentiometers 115 and 117 are connected in parallel between the current supply at the probe conductor 51 and ground. A resistor 119 is connected between the output of the operational amplifier 111 and its inverting input. The probe conductor 54A is grounded, while the shield of the connector 37A is connected to the shield of the connector 101.

The output of the first stage operational amplifier 111 is also connected to the non-inverting input terminal of a second stage operational amplifier 120, the inverting input terminal of which is coupled to ground through the series connection of a potentiometer 121 and a resistor 122.

Connected in parallel between the output and the inverting input of the operational amplifier 120 are a capacitor 123 and a resistor 124, the inverting input of the operational amplifier 120 also being connected to the wiper of the potentiometer 121, so that it functions as a rheostat. The output of the operational amplifier 120 is connected through a resistor 125 to the output terminal of the circuit 100, which terminal is also connected to ground through a capacitor 126. The -12 VDC supply is connected to ground through a capacitor 127. The metal housing 41 of the adapter 40 is grounded to the cable shield through the parallel combination of a neon bulb 130 and a capacitor 131 to prevent large DC current from flowing through the engine analyzer wiring if the wiring contacts the positive battery terminal.

In operation, the first amplifier stage 111 provides some gain and allows for fine and coarse zeroing via the potentiometers 117 and 115, respectively. The second stage 120 provides gain adjustment to calibrate the probe assembly. The resistor 125 and the capacitor 126 provide an RC filter to reduce noise at the output of the amplifier.

The connector conductors ID2 and ID3 are grounded and the conductor ID1 is floating to tell the engine analyzer that a low-amps probe is connected. The circuitry 100 can be operated with the panel-mounted fine adjustment potentiometer 117 disconnected. This allows the coarse adjustment potentiometer 115 to be adjusted without interacting with the fine adjust potentiometer 117.

It will be appreciated that the present invention can be utilized for analyzing other types of current waveforms from an internal combustion engine. For example, the invention has utility in analyzing the operation of fuel injectors. In a typical, properly operating fuel injector, the current rises at a near constant rate up to a level sufficient to cause opening of the injector valve, whereupon the current drops quickly to a level sufficient to keep the valve open, and then finally stops flowing, closing the valve. The.injector circuits for certain engines have group firing, i.e., two injectors fire simultaneously. The current through each group is limited. If the coil of one injector in a group has developed shorted turns, its resistance will be reduced. The injectors in the other group are unattected by the Dad coil and will fire normally.

however, thc shorted coil draw: more current because of the lover resistance. though some turns of the coil have been lost, the increased currcnt can make up for the los: and fir the injector. Meanwhile, the other injector in that group is starved for current and will not fire. A technician, checking for only the firing of the injector, would be lead to replacc the non-firing injector, only to find that the problem still exists. By the use of current measurement, on the other hand, the technician would locate tho faulty injector by rocognizing that it pulls too uh current. If, on tlie other hand, an injector is open, it vill draw no current and can easily b identified. ln comparison, the voltage across injectors is affected vety little, whether or not current is being drawn throngh them.

It may also be possible, with current analysis in accordance with tho present invention, to determine the ramp times for the openinq and closing of injector valves. This has tho advantage of requiring no disassembly and being completely non-invasive, and provides more accurate resistance measurements than can bo obtained vith ohmmeters.

Voltage waveform; do not provide accurate ramp characteristics for valve action. Current analycic can also be useful in checking for the reversing polarity, signifying proper operation, of Quad Drivers in a General Motors Quad 4 engine. failure of the current level to alternate above and below signal ground would indicate improper operation. The only other way to test for marginally bad Quad Drivers ic to use multiple test lights simultaneously. Analysis of current draw throuqh the fuel pump can also be useful to verify that the fuel pump is operating properly. here s no other way to verify this function non-intrusivoly.

While the foregoing examples are. illustrative, it wtll be appreciated that they are not exhaustive and that thc present invention may be useful in analysing other low level current signals in a combustion engine or the like.

From the foregoing, it can be seen that there has been provided a non-invasive engine diagnostic technique which utilizes analysis of relatively low-level current signal waveforms from a multi-cylinder internal combustion engine, the technique employing a low-level, clamp-on current detector, an amplifying adapter and a digital engine analyzer having a digital oscilloscope display screen and affording synchronizing of the displayed waveform to any of an number of selected signals from the engine, including the #1 cylinder firing.

Claims (13)

We Claim:

1. Diagnostic apparatus for a multiple cylinder internal combustion engine in which all of the cylinders are fired in a predetermined firing order during each engine cycle, said apparatus comprising: a non-invasive pickup probe for detecting a current signal in the engine having a current as low as about 100 milliamperes and generating a detection signal, means for sensing the firing of the first cylinder in the firing order and producing in response thereto a first cylinder signal, an amplifier coupled to said pickup for amplifying the detection signal to produce an amplified signal, processing means operating under stored program control coupled to said amplifier for processing said amplified signal to generate a waveform display signal which has a waveform corresponding to that of the detected current signal, oscilloscope display means coupled to said processing means for displaying the waveform display signal, and means for synchronizing said waveform display signal to the first cylinder signal.

2. The apparatus of claim 1, wherein said pickup probe includes a clamp-type probe for encircling a conductor carrying the current to be detected.

3. The apparatus of claim 2, wherein said probe is a Hall-effect probe.

4. The apparatus of claim 1, wherein said amplifier includes two amplifying stages.

5. The apparatus of claim 1, and further comprising power supply means for generating a predetermined DC voltage for powering said pickup probe and said amplifier.

6. The apparatus of claim 1, and further comprising display drive means coupled to said processing means and to said oscilloscope display means for displaying predetermined indicia associated with the waveform display signal.

7. The apparatus of claim 6, wherein said indicia include indications of each of the cylinder time periods during an engine cycle.

8. A method of testing a current-carrying element of a multiple cylinder internal combustion engine in which all of the cylinders are fired in a predetermined firing order during each engine cycle, said method comprising the steps of: non-invasively sensing the current through the element to produce a current detection signal, amplifying the current detection signal, displaying the amplified current detection signal on an oscilloscope, sensing the firing of the first cylinder in the firing order to produce a first cylinder signal, synchronizing the displayed current detection signal to the first cylinder signal, observing the value of a parameter of the displayed current detection signal, and comparing the observed parameter value to a predetermined normal value of the parameter.

9. The method of claim 8, wherein the amplified current detection signal is displayed on an oscilloscope of an engine analyzer.

10. The method of claim 8, and further comprising the step of displaying on the oscilloscope indicia related to the displayed current detection signal.

11. The method of claim 8, wherein the currentcarrying element is an inductive coil.

12. The method of claim 11, wherein the coil is an injection coil and the current signal rises to a peak level during a rise time, the parameter being the rise time of the current signal.

13. The method of claim 11, wherein the inductive coil is a fuel injector coil and the parameter is the amplitude of the displayed current signal.