Abstract:
An apparatus for, and method of, detecting the uneven operation of the individual cylinders in an internal combustion engine, and for diagnosing the cause thereof. The engine is operated at any convenient idle speed. Power cycle time periods between successive ignition times are measured, the deceleration rates between successive time periods are then computed, and finally the average deceleration rates for the respective cylinders are computed. Misfires occurring randomly in cylinders are detected when individual deceleration rates exceed the average deceleration rate for the corresponding cylinder by a predetermined limit amount. Repeated malfunctioning in an individual cylinder is detected when the average deceleration rate for the weakest cylinder exceeds that for the strongest cylinder by a predetermined limit amount.

Description:
The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army. 
    
    
     BACKGROUND OF THE INVENTION 
     There is an undisputed need for rapid, accurate, reliable and inexpensive means for and method of testing and diagnosing malfunctioning in internal combustion engines of both the spark-ignition and the compression-ignition (diesel) types. The need can best be met by using electronic means, particularly by means including a minicomputer or microprocessor. One troublesome diagnostic task is determining whether the cause of irregular or inferior operation of an engine is due to causes affecting all cylinders in a random fashion, or to causes affecting only a particular one or ones of the cylinders. That is, parts of the ignition system and of the fuel-supplying system affect all cylinders randomly or equally, and other parts affect solely one cylinder. For example, a carburetor and an ignition timer each affect all cylinders, whereas a spark plug, a fuel injector, and a valve each affect only one cylinder. 
     SUMMARY OF THE INVENTION 
     A diagnostic apparatus and method for internal combustion engines relies on the fact that the speed of an engine crankshaft experiences an almost instantaneous increase during the firing of the fuel in each fully operative individual cylinder; and experiences an almost instantaneous decrease whenever a cylinder fails to fire or for any reason operates marginally. The deceleration rate during each power period (time from when one cylinder should ignite fuel to when next cylinder should ignite fuel) is compared with an average deceleration for the respective cylinder to identify a random malfunction affecting any cylinder. The average deceleration rates of the respective cylinders are compared to identify a malfunction affecting solely one cylinder in a very repetitive manner. Deceleration rates during a misfire are substantially constant at all engine speeds, so that the test comparisons validly can be made at any desired or convenient engine speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of an internal combustion engine diagnostic system; 
     FIG. 2 is a detailed diagram of the elapsed time device in FIG. 1; 
     FIG. 3 is a chart of instantaneous speed changes in a normal engine; 
     FIG. 4 is a chart of instantaneous speed changes in an engine suffering a random misfire in one cylinder; 
     FIG. 5 is a chart of instantaneous speed changes in an engine suffering repeated misfiring in one cylinder; 
     FIGS. 6a and 6b are a flow chart of a program used in the computer in FIG. 1 to control the test procedure and compute the test results; and 
     FIG. 7 is a block diagram of an alternative apparatus for diagnosing malfunctions in individual cylinders of an internal combustion engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in greater detail to the drawing, FIG. 1 shows an internal combustion engine 10, equipped with a tachometer 12, from which electrical pulses are applied over line 13 to an elapsed time device 14. The elapsed time device (shown in detail in FIG. 2) operates under the control of a computer 16 to measure time intervals. The computer 16 computes the test results for display by a display device 18. 
     If the internal combustion engine 10 is a spark ignition engine, the tachometer 12 may be a conventional commercially available unit for obtaining one pulse per power period from the spark ignition system of the engine. If the internal combustion engine 10 is a compression-ignition or diesel engine, the tachometer 12 may be simply a housing with a shaft driven by the engine, and a tooth or teeth on the shaft which passes or pass a magnetic pick-up to produce one electrical pulse in the winding or coil of the pick-up for each tooth on the shaft. The pulse tachometer 12 produces one or more electrical pulses per power period of the engine, and these pulses are applied to an elapsed time device 14 which, if necessary reduces the number of pulses to one pulse per power period. 
     The engine test to be described utilizes one electrical pulse per power period. One full engine cycle is defined as the time taken for the engine to accomplish intake, compression, power and exhaust in one cylinder. One engine cycle occurs in one crankshaft revolution of a two stroke engine because all four functions are accomplished in two strokes of the piston. On the other hand; one engine cycle occurs during two crankshaft revolutions of a four-stroke engine because the four functions are accomplished in four strokes of the piston. There are as many power periods per engine cycle as there are cylinders in the engine being tested. 
     FIG. 2 is a circuit diagram of the elapsed time device 14 of FIG. 1. Device 14 receives electrical pulses from tachometer 12 over line 13 and applies them through a divide-by-N-counter 15 to a one-shot multivibrator 114. The divider 15 is provided if the tachometer used produces more than one pulse per power period. The output 29 from the divider 15 is one pulse per power period. 
     The elapsed time device 14 includes a 16-bit counter each consisting of four 4-bit integrated circuits 102. The counter counts the pulses applied over clock line 104 from a clock (not shown). The 16 outputs from the counter are coupled to 16 stages of a corresponding count latch consisting of integrated circuits 106. The count latch 106 receives and holds the count in the counter 102 when enabled by a transfer signal on line 108 from the transfer latch 112. Transfer latch 112 receives relatively infrequent pulses having a duration greater than the 0.1 msec duration of one cycle of the 10 kHz clock from a one-shot multivibrator 114, which responds to input pulses on line 29 from the divide-by-N counter 24. 
     The elapsed time unit 14 also includes a 16-bit buffer 126 consisting of four integrated circuits, which can be enabled over line 128 to transfer the 16-bit count in the count latch 106 to the computer 16 via the 16-conductor data bus 132. The buffer 126 is enabled by signals through inverter 134 from nand gate 136. Gate 136 provides an output when it receives both a device select signal over line DS from the computer and an appropriate &#34;data in&#34; control signal over line DI from the computer. In this way the computer can sample the data stored in the counter latches under program control as required. From the counter latches, the computer periodically receives the count which represents the time period between two pulses representative of the engine speed. 
     In normal operation the elapsed time device 14 is initialized by the computer 16 by a &#34;start&#34; signal applied over line 138 to nand gate 142, simultaneously with a device select signal over line DS. The output of gate 142 causes the third latch 124 to assume a &#34;busy&#34; state. The latch 124 remains in the busy state until set to the &#34;done&#34; state by a signal through inverter 144 from the one-shot 118 when the count in counter 102 is transferred to the count latch 106. The busy or done status of the counter of the timing unit is available to the computer 41 through line B and D whenever the gates 146 and 148 are enabled by a &#34;device select&#34; signal on line DS from the computer. 
     In summary, the elapsed time device 14 continually measures and latches the time periods between successive pulses occurring once per engine power period, and sets its own state to &#34;done&#34; each time an engine power time period is stored. The computer can then cause a transfer of the stored count in the latch through the buffer to the computer. The computer sets the timing device to the &#34;busy&#34; state whenever continued measuring of time periods is needed. 
     The elapsed time device 14 is not needed if the computer 16 employed includes a real time clock, and the program for the computer causes the computer to perform the time period measuring and storing function performed by the device 14. 
     The computer 16 may, by way of example only, be a &#34;Nova 1200&#34; minicomputer manufactured and sold by Data General Corporation, Southboro, Mass. 01772. The Nova 1200 is a low cost minicomputer designed for general purpose applications. It has a 16-bit word, multi-accumulator central processor, and a full memory cycle time of 1200 nanoseconds. It executes arithmetic and logical instructions in 1350 nanoseconds. The entire Nova 1200 central processor fits on a single 15-inch-square printed circuit subassembly board. The basic computer includes four thousand 16-bit words of core memory, a Teletype interface, programmed data transfer, automatic interrupt source identification, and a direct memory access channel. User programming conveniently can be in the BASIC language. 
     The display device 18 (FIG. 1) for use with the Nova 1200 computer may be a conventional Teletypewriter, a printer, a 4-digit display such as one including Numitron character display tubes, or any other similar display device. 
     OPERATION 
     The operation of the system of FIG. 1 will now be briefly described with references to the charts of FIGS. 3, 4, and 5, and later will be described in greater detail with references to the flow chart of FIG. 6. 
     In the initial cndition, the engine 10 is operated at an idle speed which may be any speed in the range of about 500, 600 or 700 rpm, the tachometer 12 supplies pulses to the elapsed time device 14 which is continuously counting the time periods between power period pulses after receiving a &#34;start&#34; signal from the computer 16. If the engine is operating normally at an idle speed of about 620 rpm, the data accumulated and processed by computer 16 may be as shown in Table 1-3 below, and as plotted in FIG. 3. The Input Time Periods listed in Table 1-3 correspond with the power periods bounded by the vertical lines in FIG. 3. The Δ Time Measurements in Table 1-3 represent changes in Input Time Periods from one engine power period to the next engine power period. The Average Speed in Table 1-3 is the reciprocal of the Input Time Period for the corresponding engine power period multiplied by a constant so that the Average Speed is given in revolutions per minute (RPM). The Average Speed is represented in FIG. 3 by the horizontal lines in each power period at appropriate levels relative to the RPM scale at the left of the chart. The wavy line in FIG. 3 is a hand-drawn approximation of the instantaneous and continuous fluctuations in engine speed due to time-spaced explosions in individual cylinders in a normally functioning engine under test. 
     A random single misfire in one cylinder of a four-cylinder, four-cycle engine is represented by the data in Table 1-4 and the chart of FIG. 4. The misfire results in an abrupt reduction in instantaneous speed during the power period in which the misfire occurs. 
     
                                           Table 1-3__________________________________________________________________________Idle Speed Data - Normal EngineInput Time  Δ Time         Time to  Average                       DecelerationPeriod Measurements         End of Period                  Speed                       Rate(0.1 msec)  (0.1 msec)         (Seconds)                  (RPM)                       (RPM/sec)__________________________________________________________________________490           0.0490   612482     8     0.0972   622  -209.1483    -1     0.1455   621  26.7474     9     0.1929   633  -246.5483    -9     0.2412   621  246.5473    10     0.2885   634  -274.7477    -4     0.3362   629  112.0467    10     0.3829   642  -285.3470    -3     0.4299   638  87.5469     1     0.4768   640  -29.0476    -7     0.5244   630  199.0470     6     0.5714   638  -170.1476    -6     0.6190   630  170.1__________________________________________________________________________ 
    
     
                                           Table 1-4__________________________________________________________________________Idle Speed Data - 1 Random MisfireInput Time  Δ Time         Time to  Average                       DecelerationPeriod Measurements         End of Period                  Speed                       Rate(0.1 msec)  (0.1 msec)         (Seconds)                  (RPM)                       (RPM/sec)__________________________________________________________________________391           0.0391   767392    -1     0.0783   765  50.0391    1      0.1174   767  -50.0388    3      0.1562   773  -152.3 Misfire397    -9     0.1959   756  446.6 ←412    -15    0.2371   728  680.2412    0      0.2783   728  0.0406    6      0.3189   739  -263.1402    4      0.3591   746  -182.0401    1      0.3992   748  -46.4398    3      0.4390   754  -141.2390    8      0.4780   769  -392.4389    1      0.5169   771  -50.8__________________________________________________________________________ 
    
     
                                           Table 1-5__________________________________________________________________________Idle Speed Data - 1 Cylinder MisfiringInput Time  Δ Time         Time to  Average                       DecelerationPeriod Measurements         End of Period                  Speed                       Rate(0.1 msec)  (0.1 msec)         (Seconds)                  (RPM)                       (RPM/sec)__________________________________________________________________________643           0.0643   467  -430.8605    38     0.1248   496  -469.6 Misfire633    -28    0.1881   474  354.3 ←679    -46    0.2560   442  489.4622    57     0.3182   482  -622.4586    36     0.3768   512  -490.6 Misfire608    -22    0.4376   493  310.3 ←639    -31    0.5015   469  384.0590    49     0.5605   508  -634.5558    32     0.6163   538  -508.0 Misfire576    -18    0.6739   521  296.3 ←__________________________________________________________________________ 
    
     Repeated misfires in the same one cylinder of the engine are represented by the data in Table 1-5 and the chart of FIG. 5. The three abrupt speed reductions shown occur in every fourth power period of the four cylinder engine. 
     The three engine conditions illustrated are distinguished by computing and comparing the deceleration rates between power periods during the test, which may extend over about 200 engine cycles. The three conditions are accurately and positively distinguished without regard for the actual idle speed of the engine during the test. It will be noted that the three illustrated tests were performed with the engine operating at average speeds of about 620 rpm, 750 rpm and 470 rpm. The actual speed reductions during misfires vary in accordance with engine speed, and therefore a simple comparison of speed change from one engine cycle to the next is not a reliable way to detect misfires at all idle speeds normally encountered in practice. 
     The last columns in each of Tables 1-3, 1-4 and 1-5 contain calculated deceleration rates between the corresponding engine cycle and the preceding engine cycle. Positive values represent deceleration, and negative values represent acceleration. A misfire is indicated whenever the deceleration rate exceeds 250. A deceleration rate exceeding 250 in the next following engine cycle is ignored because it is due to the preceding misfire. 
     The computer program, to be described in detail, first collects Input Time Periods for about 200 power periods in a four cycle engine. The average Deceleration Rate for the 50 power periods of each cylinder are calculated, and each individual Deceleration Rate is compared with the average Deceleration Rate for the respective cylinder to detect individual random misfires. If the number of random misfires exceeds a predetermined limit value, the conclusion EXCESSIVE RANDOM MISFIRES is displayed on the display device 18. 
     A comparison is also made of the average Deceleration Rates of the 50 power periods in the four different cylinders to determine the weakest cylinder having the largest average Deceleration Rate and the strongest cylinder having the smallest average Deceleration Rate. If the difference between the largest and smallest average Deceleration Rates exceeds a predetermined value, the conclusion CYLINDER POWER UNBALANCE is displayed on the display device 18. Otherwise, the conclusion PASSED IDLE PERFORMANCE TEST is displayed. 
     The operation of the system in performing the test of an internal combustion engine will now be described in detail with references to the flow chart of FIGS. 6a and 6b. 
     
                       Program Description______________________________________StatementNos.            Statement and Function______________________________________10      DIM T(201), D(200), B(4). This statement   simply allocates memory space within the - computer for data   arrays to be used by the - computer during test operation.20      CALL 1. Execution of this instruction by the   computer sends a START pulse to the Elapsed   Time Device (14). This sets the device to   the Busy state which essentially initializes   the system preparing it for time period   (speed) measurements.30      FOR I = TO 201.40      CALL 2, T(I).50      NEXT I. This set of instructions forms an   instruction loop which executes the middle   instruction 201 times, starting with a value   of I=1 and incrementing the value of I by 1   each time.   The middle instruction (4) causes the system   to wait for the next Elapsed Time Device (14)   input pulse (one per engine firing) and then   the computer inputs the time period between   the last two pulses (in 0.1 msec units). This   value is saved as T(I). Thus, the first   pulse period measured is saved as T(1), the   next as T(2), etc. until T(201) is input and   saved. When all 201 time periods have been   measured and saved the program continues on   to the next block of instructions. Typical   input values are shown in Tables 1-3, 1-4,   and 1-5.60      FOR I = 1 TO 200.70      D(I) = 6E + 9*(1/T(I) - 1/T(I+1))/(T(I)+T(I+1)).80      NEXT I. As with the last set of instructions,   this set forms an instruction loop which re-   peatedly executes the middle instruction, while   incrementing the value of I. However, in   this case the loop is only repeated 200 times   as indicated by instruction 60.   This middle instruction (70) calculates the   average deceleration rate (in units of RPM/   SEC) between each successive pair of input   time periods saved as T(I). The actual   function calculated is       1            1              -       T(I)         T(I+1)D(I) =                         (6 × 10.sup.9)       T(I)   +     T(I+1)   Tables 1-3, 1-4, and 1-5 list typical values   for D(I) calculations for a nonmisfiring   engine, randomly misfiring engine, and   periodically misfiring engine, respectively.90      FOR I = 1 TO 4.100     B(I) = 0110     NEXT I. The array B(1), B(2), B(3), B(4) is   used later in the program to calculate the   average deceleration rates corresponding to   each cylinder (4 elements in the array for a   4 cylinder engine). This program loop   initializes the array and sets all four elements   of the B array to zero.120     FOR I = 1 TO 50.130     J = * I - 3.140     B(1) = B(1) + D(J)150     B(2) = B(2) + D(J + 1)160     B(3) = B(3) + D(J + 2)170     B(4) = B(4) + D(J + 3)180     NEXT I. This program loop executes instruc-   tions 130 to 170 fifty times while incrementing   the counting parameter I from 1 to 50. Upon   completion of this program loop every fourth   deceleration rate in the D array (200 decelera-   tion rates), starting with the first, is   summed together and stored in B(1). Similarly,   every fourth value starting with the second,   is summed with B(2), and the resulting total   sum left in B(2), and the same for B(3) and   B(4) with respect to every fourth value   starting with the third and fourth elements   of the D array.190     FOR I = 1 TO 4.200     B(I) = B(I)/50.210     NEXT I. This program loop simply performs   four divisions to calculate the average   deceleration for each cylinder. The functions   performed are:   B(1) = B(1)/50   B(2) = B(2)/50   B(3) = B(3)/50   B(4) = B(4)/50   where the four average decelerations are now   stored in B(1) through B(4).   NOTE: At this point in the test sequence the   test system has measured or calculated all of   the data that it requires for misfire detec-   tion. The remainder of the test sequence   checks each deceleration data point for   excessive deviation from the average for the   corresponding cylinder, checks for excessive   variation between average decelerations, and   evaluates and prints out the results of the   vehicles idle performance test.220     I = 0230     J = 0240     M = 0. Program statements 220 through 310   control the test system during the test for   excessive deviation from the average.   Parameters I, J, and M are used as counting   variables in this part of the program. I   counts deceleration data points from 1 to 200.   J counts cylinder position so that decelera-   tion data can be correlated with the average   deceleration for the appropriate cylinder.   M counts misfire indications. Instructions 220,   230, and 240 simply initialize these counting   variables to zero.250     GO SUB 500. At this point in the program the   test system wants to do exactly the same   thing as it does at another point in the   program. Thus, a subprogram has been created   starting at statement 500 to perform this   task. Statement 250 simply causes the computer   to jump to the subroutine starting at 500 and   when the subroutine is completed the test   routine will continue on with the next sequen-   tial instruction (280).500     I = I + 1510     J = J + 1. Subroutine 500 is an incrementing   subroutine. Its purpose is to increment both   the I and J counting variables and to reset   J to 1 whenever its value becomes greater   than the number of cylinders in the engine   being tested (4 for the present example).   The above two instructions simply increment   the I and J parameters.520     IF J &lt; 5 THEN GO TO 540.530     J = 1. These two instructions check to see   if J has exceeded the number of cylinders (4)   and then set J as required. If J is less   than 5, it is fine as is and the computer   jumps directly to statement 540. Otherwise   (J = 5) J is reset to a value of 1 and then   the computer continues on to statement 540.540     RETURN. This statement identifies the end of   a subroutine and causes the computer to jump   back into the normal program instruction flow   and execute the instruction following the one   which called this subroutine.280     IF D(I) - B(J) &lt;= L1 THEN GO TO 310.290     M =M + 1.300     GO SUB 500. This instruction set detects   random misfires (280) and then appropriately   sets the counters before continuing testing.   Execution of statement 280 substracts the   average deceleration rate for the appropriate   cylinder (B(J)) from the individual decelera-   tion rate being tested. The result is then   compared with the limit L1 to check for a   random misfire. If the result is greater   than the limit L1, M is incremented by one   to count a misfire and I and J are incremented   (as done by subroutine 500) so that the test   system, will skip the next deceleration data   point. The reason for this data skip is   obvious from the last column of Table 1-4.   Notice that a single misfire is apt to affect   two deceleration data points. Thus, the - program automatically   skips one to insure   against incorrectly identifying too many   misfires.310     IF I &lt; 200 THEN GO TO 250. Execution of this   statement causes the computer to check if all   deceleration data has been checked for random   misfires, if not I would be less than 200 and   the computer next executes instruction 250.   If all have been processed the computer moves   on to the next sequential instruction (320).320     B1 = -1000.330     B2 = +1000. The parameter B1 is used to find   the largest average deceleration in the B   array. To initialize it for this purpose,   execution of instruction 320 sets it to a   low value (-1000). Similarly, statement 330   initializes B2 to a high value (+1000) so that   it can be used to find the smallest deceleration   in the B array.340     FOR I = 1 TO 4.350     IF B(I) &gt; B1 THEN LET B1 = B(I).360     IF B(I) &lt; B2 THEN LET B2 = B(I).370     NEXT I. These four statements form a program   loop which executes statements 350 and 360   four times for I equals 1 to 4. After execu-   tion of this program loop, the value of B1 will   be equal to the largest of the four B array   elements and B2 will be equal to the smallest   of the four B array elements. Relating this   to the engine B1 will be the largest average   deceleration rate which would correspond to   the weakest cylinder in the engine and B2   will be the smallest average deceleration   rate which would correspond to the strongest   cylinder (and would be a negative number   signifying acceleration rather than   deceleration).380     B3 = B1 - B2. This statement simply takes   the difference between B1 and B2 and stores the   result as B3 which will be used to check for   periodic misfire later in the test sequence.390     IF M &gt;= L2 THEN GO TO 430.400     IF B3 &gt;= L3 THEN GO TO 450. These two   statements perform the final engine idle   performance evaluation. In instruction 390 M   is compared to a limit L2 to see if there is   excessive random misfire. If M is greater than   or equal to the limit, the computer jumps to   statement 430 to print out the test result.   Otherwise the computer performs the comparison   of statement 400 where B3 is compared to L3.   Excessive variation of the cylinder averages   is indicated by B3 being greater than or   equal to the limit L3. If such is the case,   the computer jumps to statement 450 to print   out the test result. If both performance   tests are passed (M &lt; L2 and B3 &lt; L3) the   computer will next execute statement 410.410     PRINT &#34;PASSED IDLE PERFORMANCE TEST&#34;420     GO TO 460.430     PRINT &#34;EXCESSIVE RANDOM MISFIRES&#34;.440     GO TO 460.450     PRINT &#34;CYLINDER POWER UNBALANCE&#34;.460     STOP. The above statements simply print the   appropriate result as indicated and branch to   statement 460 to stop the test.______________________________________ 
    
     Reference is now made to FIG. 7 for a description of an alternative apparatus for detecting malfunctioning in individual cylinders of an internal combustion engine 10. The output pulses from the tachometer 12 are applied over lead 601 to a time delays unit 602 which has a plurality of outputs each of which provides output pulses having a desired delay relative to the input pulses on line 13. The purpose of time delays unit 602 is to provide gate-enabling pulses timed to insure the orderly transfer of counts from a counter 600 down through intervening units to a comparator 632. That is, data is transferred on from a unit slightly before new data is applied to the unit. 
     The power period counter 600 counts the input pulses applied to it from a clock 604. The clock 604 runs at a rate of 10 kHz or more in order to provide a sufficient resolution in the time periods measured by the counter 600 in the intervals between the applications of successive reset pulses thereto from the time delays unit 602. 
     The count in counter 600 is transferred to first register 606 through a gate 608, which is enabled over lead 609 from time delays unit 602 at a time slightly earlier than the time when a reset pulse is applied over line 13 to the counter 600. The gate symbol 608 represents an array of gates equal in number to the number of bits in the counter 600. 
     The count in the first register 606 is transferred through a gate 610 to a second register 612 every time the gate 610 is enabled over line 611, which is a time slightly earlier than the time when gate 608 is enabled. 
     The contents of the first register 606 and the contents of the second register 612 are simultaneously transferred through multi-unit gates 614 and 616, respectively, to the two inputs of a subtractor 618. These transfers are made by enabling the gates over line 617 at a time slightly prior to the time that gate 610 is enabled. The subtractor may, of course, be an adder in which the sign of one of the inputs is made to be negative, so that the addition performed is actually subtraction. The count in the second register 612 is subtracted from the count representing the following time period in the first register 606. 
     The time-difference-representing count produced by the subtractor is transferred to a comparator 620 through a multi-unit gate 622 when enabled over line 623 by a pulse occurring slightly prior to the pulse which enables gates 614 and 616. The comparator 620 also receives a count over multi-conductor line 621 which represents a time-difference reference value for comparison with the measured time difference from subtractor 618. If the measured time difference is greater than the reference value, the comparator 620 produces a misfire-representing output which is passed, if present, through multi-unit gate 624 enabled over line 625 to a misfire counter 626 at a time slightly after the time that gate 622 is enabled. 
     The misfire counter 626 is reset to zero by a pulse over line 627 from a test duration counter 628. The counter 628 receives pulses at the power period frequency over line 629 from time delays unit 602, and the counter 628 provides an output after counting a number, such as 200, if the input pulses to provide a time period during which the number of misfires are counted by the misfire counter 626. 
     The count in the misfire counter 626 is transferred out through multi-unit gate 630 when enabled by a pulse over line 631 from test duration counter 628 at a time slightly following the resetting of the misfire counter 626. The count passed by gate 630 is applied to a comparator 632 which also receives, at 633, a number-of-misfires reference value. If the measured number of misfires from misfire counter 626 exceeds the reference number of misfires entered at 633, the comparator provides a &#34;test failed&#34; output at 635. Otherwise, a &#34;test passed&#34; output is provided at 637. 
     The apparatus of FIG. 7 for detecting malfunctioning in individual cylinders of an internal combustion engine is seen to include a tachometer 12, a period counter 600 and a clock 604 for measuring the engine power time periods between crankshaft positions representing the successive ignition times in successively-fired cylinders of the engine. The subtractor 618 subtracts each measured time interval in the second register 612 from the following measured time period in first register 606 to detect whether there is a positive difference representing a momentary reduction in engine speed. The comparator 620 produces a fault output whenever a time difference exceeds a predetermined reference limit value, to provide an indication of a significant fault in an individual cylinder. The misfire counter 626 counts the number of faults occurring during a predetermined period. And, comparator 632 determines whether the number of faults exceeds a predetermined reference limit value above which engine performance is considered unsatisfactory.