Patent Publication Number: US-3968425-A

Title: Measuring ignition timing using starter current

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 
     The testing of the timing of spark ignition in an internal combustion engine is normally accomplished by means of a mark on the engine flywheel or vibration damper which, when lined up with a reference mark, indicate the top dead center position of a piston in its cylinder. The mark on the rapidly rotating flywheel is observed by a stroboscopic light which is controlled by the engine ignition system. The test requires considerable skill, and is undesirably time consuming. 
     SUMMARY OF THE INVENTION 
     A very rapid and convenient determination of ignition timing is accomplished by comparing the times of the peaks of the starter motor current while cranking the engine with the ignition inhibited. The times of current peaks are compared with the times of ignition points opening. The times of current peaks have a known relation with times of top dead center, so that the lead or lag of ignition relative to top dead center in degrees can be derived. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagram of hard wired electrical and electronic hardware for determining the timing of the spark ignition of an internal combustion engine; 
     FIG. 2 is a chart of electrical waveforms which will be referred to in describing the operation of the apparatus of FIG. 1; 
     FIG. 3 is a diagram of an alternative apparatus, including a computer, for determining the timing of the spark ignition of an internal combustion engine; and 
     FIGS. 4A, 4B and 4C are flow charts of a computer program used in the computer to control the operation of the computer and other apparatus of FIG. 3. 
    
    
     DESCRIPTION OF FIG. 1 EMBODIMENT 
     Reference is now made to FIGS. 1 and 2 for a description of apparatus for determining the time of spark ignition relative to top dead center (TDC) by observing the starter motor current waveform while cranking the engine with the ignition sparks inhibited. A starter motor 12 is supplied with electric current through a switch 14 from a battery 16. The current path includes a low value resistor 18 across which a voltage waveform, FIG. 2a, representative of the current to the starter motor is produced. The voltage waveform is amplified in a conventional operational amplifier 22 having filter means to favor frequency components of interest. 
     The amplified voltage waveform is applied to a first pulse wave generator 24 which generates starter current peak pulses, FIG. 2b, extending from each starter current peak to the following time of ignition points closing. Pulse wave generator 24 includes a conventional peak detector 26 which retains the highest amplitude of the starter current waveform applied to it until reset by a points closing signal applied over line 28. A portion of the output of the peak detector 26 is applied from a voltage divider 32 to the reference input of a conventional comparator 34. The comparator provides an output at 35 when the current waveform applied over line 33 exceeds the reference input. The output at 35 is as shown in FIGS. 2b and 2f. 
     Current from the battery 16 is also applied to a conventional spark ignition system 36 including an ignition ON-OFF switch 38, an ignition coil 42, a pair of ignition points 44 operated by a cam (not shown) and a capacitor 46 connected across the points 44. The high voltage energy normally supplied through coil 42 and the distributor (not shown) to the engine spark plugs may be inhibited by a signal applied over line 47 to control a transistor 48 connected in shunt with the points 44. However, regardless of whether or not the ignition is thus inhibited, a voltage waveform is present at 49 from which a second pulse wave generator 52 can generate pulses, FIGS. 2c or 2g, corresponding with the periods of time that the points 44 are open. 
     The second or points-open pulse wave generator 52 includes an input circuit 54 designed to filter out negative voltage spikes occurring when the ignition points initially open. A two-transistor circuit 56 is a conventional threshold amplifier and clipper which may produce zero volts output when the input signal is below 0.7 volts, and 5 volts output when the input signal is above 0.7 volts. The points 44 are open for about as long as they are closed, so that the output at 59 following inverter 62 is a square wave having substantially equal positive and negative half cycles, as shown by FIGS. 2c or 2g. A monostable multivibrator or one-shot 64 is triggered by the waveform at 57 to produce a short pulse on line 65 whenever the points 44 close. This short pulse acts over line 28 to reset the peak detector 26 already described. 
     A waveform comparator 66 compares the outputs of the first or current-peak and the second or points-open pulse wave generators 24 and 52, and provides a positive output pulse having a duration from the time that the points open until the time of the starter current peak, or, provides a negative output pulse having a duration from the time of the starter current peak until the time that the points open. 
     The starter current peak waveform 2b on lead 35 is inverted in inverter 68 and applied to one input of an &#34;and&#34; gate 72. The points open waveform 2c is applied to the other input of gate 72. When the two inputs are both positive, the gate 72 is enabled providing an output which closes switch 74 and connects the +10v. terminal to the output line 75. The signal on line 75 is then as shown by FIG. 2d. 
     The starter current peak waveform 2f on lead 35 is applied to one input of an &#34;and&#34; gate 76. The points open waveform 2g after inversion by inverter 58 is applied to the other input of the gate 76. When two inputs to the gate 76 are both positive, the gate is enabled providing an output which closes switch 78 and connects the -10v. terminal to the output line 75. The signal on line 75 is then as shown by FIG. 2h. 
     The output line 75 from comparator 66 is connected to one input of an integration and speed compensation circuit 76 including, in order, a unity gain voltage follower 78, an integrator 82 and a voltage controlled amplifier 84 having an output line 85. The integrator 82 smooths the pulse 2d or 2h into a voltage having a positive or a negative amplitude dependent on the duration of the respective pulses 2d or pulses 2h. Such signals can be applied to any suitable display device (not shown). 
     The circuit 76 also includes means to compensate the output signal on lead 85 for the effect on the timing of the starter current peak due to the speed at which the engine is being cranked by the starter motor. The speed compensation means includes a one-shot multivibrator 86 which receives short pulses from the one-shot multivibrator 64 at the times of points closings. The pulses, which have a repetition rate exactly proportional to engine speed, are smoothed in an integrator 88 to provide a voltage amplitude at 89 which is proportional to engine speed. This signal is applied to the control input terminal of the voltage controlled amplifier 82 to compensate the output signal at 85 in accordance with engine speed. 
     OPERATION OF FIG. 1 
     In the operation of the ignition timing tester of FIG. 1, the starter motor switch 14 is closed to supply battery current to the starter motor 12 and crank the engine. The ignition switch 38 is closed, but an ignition spark inhibiting signal is applied on lead 47 to prevent explosions of fuel in the cylinders of the engine. The starter motor current waveform developed across resistor 18 may be as shown by FIG. 2a. Circuit 24 translates the waveform to a rectangular wave 2b or 2f having pulses starting at the peaks of the starter current waveform and ending at the times of points closings. Circuit 52 produces a rectangular wave 2c or 2g having pulses starting when the points open and ending when the points close. The comparator circuit 66 compares the outputs from circuits 24 and 52 and produces a positive output pulse 2d having a duration corresponding with the lead time of the ignition relative to the starter motor current peak, or produces a negative output pulse 2h having a duration corresponding with the lag time of the ignition relative to the starter motor current peak. An integration and speed compensation circuit 76 translates the positive or negative pulses from circuit 66 to positive or negative voltages having amplitudes corresponding with the durations of the pulses. 
     The signals have amplitudes representing how much the spark timing leads or lags the starter motor current peaks. The starter motor current peaks occur at fixed times relative to the times that the pistons are at top dead center at a given speed of engine cranking. The time relationship varies with speed of cranking. Therefore the signals are modified or compensated in accordance with the actual cranking speed so that the signals represent how much the spark timing leads or lags the times that the pistons are at top dead center. A speed correction signal provided by one-shot 86 and integrator 88 is used to control the voltage-controlled amplifier 84. The corrected or speed-signals compensate from amplifier 82 are then applied to a display device, such as a voltmeter calibrated to show the measured spark timing in degrees before or after top dead center. 
     DESCRIPTION OF FIG. 3 EMBODIMENT 
     FIG. 3 shows an alternative apparatus for determining the time of spark ignition relative to top dead center by observing the starter motor current waveform while cranking the engine with the ignition sparks inhibited. Ignition system parts are given the same numerals as corresponding parts have in FIG. 1. A starter current transducer 98 has its output connected together with the outputs of other transducers 102 to a multiplexer, from which the signals are applied in time-shared sampling fashion to an analog-to-digital converter 106. The resulting digital signals are transferred over bus B in  to a general purpose minicomputer 108. The converter 106 is controlled in its operation by control signals from computer 108 over bus B out . 
     The computer 108 may, by way of example only, by a &#34;Nova 1200&#34; minicomputer manufactured and sold by Data General Corporation, Southboro, Mass., Zip 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. 
     A Real Time Clock 112 may be a standard option available with the Nova 1200 computer, and the A/D converter 106 and multiplexer 104 may be one of the standard Nova peripherals such as the 4141/CPU eight channel, 12-bit analog to digital conversion system. 
     A pulse shaper 52, like the one in FIG. 1, responds to the ignition system 36 and supplies an interrupt pulse from one-shot 64 through bus B in  to the computer 108 every time the ignition distributor points 44 open. 
     An ignition control unit 122 includes a flip-flop 124 from which an output on line 47 can render transistor 48 conductive and thus prevent the sending of spark pulses from ignition system 36 to the spark plugs when the distributor points 44 open. The flip-flop 124 is set by an output from a gate 126 when it is enabled by a &#34;device select&#34; signal on line 125, and a &#34;stop ignition&#34; signal on line 127 from the computer 108. The flip-flop 124 is reset by an output from a gate 128 when enabled by a &#34;reset&#34; signal over line 129 from computer 108. 
     The test results computed by the computer 108 are displayed by a display device 132 which may be a conventional Teletypewriter, a printer, a 4-digit display such as one including Numitron character display tubes, or any other suitable display device. 
     The apparatus shown in FIG. 3 is used for testing the ignition timing of an engine, and it is also useful concurrently or sequentially for making other engine performance tests. The operation of the system is controlled by the computer and by the program stored in the computer. The portion of the computer program concerned with the testing of ignition timing will be described with references to the flow charts of FIGS. 4A, 4B, and 4C. 
     OPERATION OF FIG. 3 
     In the operation of FIG. 3, the engine (not shown) is cranked by the starter motor with the ignition switch &#34;on&#34;. The computer 108 supplies signals to the ignition control unit 122 which prevent the spark plugs of the engine from firing, while permitting the pulse shaper 52 to respond to the ignition system and send a pulse to the computer every time the points 44 open. 
     The computer receives frequent samples of the starter current amplitude from starter current transducer 98 through multiplexer 104 and A/D converter 106. The computer continuously compares the starter current sample amplitudes and determines the time at which a peak amplitude occurs. The computer computes the time interval between the current peak and the points opening. 
     The computer computes the time displacement of the starter current peak relative to the piston top dead center at the particular measured cranking speed of the engine. The computer thus determines the lead or lag of the ignition in relative to top dead center. 
     The computer also makes the non-linear translation from ignition lead or lag in time to ignition lead or lag in angular degrees relative to top dead center. The calculated ignition lead or lag in degrees relative to top dead center is then displayed by display device 132. 
     FLOW CHART OF FIG. 4 
     A detailed description of the operation of the apparatus of FIG. 3 is given with references to the program flow chart of FIG. 4. See also the Appendix for subroutines and program variables. 
     It is assumed that the engine is already cranking and the ignition is turned on prior to starting this test. Under these conditions the program depicted in FIG. 4 will control the test operation. A detailed explanation of each part of this program is given below. Because of the special nature of the Interrupt Program, this part of the program is discussed first. 
     An interrupt input to a computer is a special input. When a pulse occurs on such a line it allows the computer to finish the instruction which it is executing and then causes it to jump to another location of memory and start executing a special program located there. This type of input is used in this device where the output of Pulse Shaper (52) gives an interrupt to the computer causing it to jump to the special Interrupt Program which simply does three things as shown below: 
     
         Interrupt Program                                                         
Statement                                                                 
Number  Statement and Description                                         
______________________________________                                    
1010    CALL 3, T3(I). As indicated in the subroutine                     
        descriptions, this statement simply inputs                        
        a real time value and saves it as T3(I)                           
        where I is controlled in the main program.                        
1020    LET F1 = 1. This instruction sets the                             
        flag variable F1 equal to 1 to indicate to                        
        the main program that an interrupt has                            
        occurred (points have opened).                                    
1030    RETURN. This instruction simply defines                           
        the end of the interrupt program. This                            
        causes the computer to resume operation                           
        wherever it left off in the main program.                         
______________________________________                                    
 
    
     Thus, this part of the program causes the system to automatically record the time each time the ignition points open. Where the data is stored is controlled by the main program since the variable I (which points to a specific T3 array location) is set only by the main program. 
     
         ______________________________________                                    
Main Program                                                              
Statement                                                                 
Number     Statement and Description                                      
______________________________________                                    
10      CALL 1, 0. As indicated in the subroutine                         
        descriptions this statement inhibits the                          
        ignition from firing. This prevents a                             
        firing or partial firing of the engine from                       
        distorting the starter current waveform.                          
        Functionally, the ignition is inhibited by                        
        causing the computer to output both a                             
        Stop Ignition and a Device Select signal                          
        to the Ignition Control Unit (see FIG. 3)                         
        which sets flip-flop 124 and turns on                             
        transistor 48 until flip-flop 124 is reset                        
        by a CALL 1, 1 instruction.                                       
20      LET I = 0. This instruction simply                                
        initializes the firing cycle counter, I,                          
        to zero.                                                          
NOTE: The next part of the program (instructions 30-100)                  
      synchronizes the test system with the starter                       
      current variations and essentially sets the system                  
      up for taking timing data on the first or next                      
      points opening relative to Top Dead Center. It                      
      does this by first detecting a continuing nega-                     
      tive slope characteristic and then by detecting                     
      the waveform minimum value.                                         
30   CALL 2, C(1). As indicated in the sub-                               
     routine descriptions this statement                                  
     measures an average value of starter                                 
     current (averaging filters out starter                               
     commutation ripple) and saves the value                              
     as C(1).                                                             
40   LET J = 2. This instruction sets the                                 
     counting variable to 2 so that the next                              
     instruction will store its measured value                            
     as the second point in the C array.                                  
50   CALL 2, C(J). As indicated in the sub-                               
     routine descriptions this statement                                  
     measures an average value of starter                                 
     current and saves the value as C(J).                                 
60   IF J &gt; 4 then GO TO 90. This statement                               
     compares the value of the counting                                   
     variable J to 4 and if it is larger the                              
     computer will next execute instruction                               
     90. Otherwise it will execute the next                               
     sequential instruction, 70. For J to be                              
     greater than 4 the system must have found                            
     4 sequential decreasing average values for                           
     starter current (see instructions 70-85).                            
70   If C(J) &gt;= C(J - 1) THEN GO TO 30. This                              
     is the instruction which actually checks                             
     the waveform slope characteristic. It                                
     causes the computer to compare the last                              
     two average measurements and if the last                             
     is not the smallest (i.e. it is greater                              
     than or equal to the previous measurement)                           
     the computer jumps back to instruction 30                            
     and starts taking negative slope data                                
     all over again.                                                      
80   LET J = J + 1.                                                       
85   GO TO 50. When the last two measured values                          
     were found to have a negative slope                                  
     characteristic by instruction 70, these                              
     instructions cause the computer to incre-                            
     ment the value of J and jump back to                                 
     instruction 50 to take the next measurement.                         
     Thus, when a negative slope characteristic                           
     is found the computer will loop in instru-                           
     tions 50 through 85 until four sequential                            
     decreasing values are measured.                                      
90   If C(5) &gt; C(4) THEN GO TO 110. When the                              
     computer gets to this instruction it has                             
     already measured four sequential decreasing                          
     values of starter current, set J equal to                            
     5, and measured a fifth value C(5). The                              
     purpose of this instruction is to find                               
     the minimum point for this cycle of the                              
     waveform so this instruction causes the                              
     computer to compare the last two measured                            
     values. If the last value, C(5), is the                              
     largest, then the system continues on to                             
     instruction 110 since it knows that the                              
     minimum point has passed. Otherwise it                               
     executes the next sequential instruction,                            
     100, and continues looking for the                                   
     minimum value.                                                       
100  LET C(4) = C(5).                                                     
105  GO TO 50. At this point in the system                                
     operation the computer has not yet detected                          
     a minimum point so instruction 100 causes                            
     it to save the last measured value, C(5),                            
     as C(4) and instruction 105 causes it to                             
     branch back to instruction 50 to measure                             
     a new C(5) value.                                                    
NOTE: At this point in the system operation the starter                   
      current has just passed a minimum value so the                      
      system now has to set up for the first or next                      
      time of peak and time of points opening measure-                    
      ment. Instructions 110-140 simply set up for                        
      these tasks.                                                        
110  LET I = I + 1. This instruction simply                               
     increments the firing cycle counter by one                           
     since the system is setting up to take                               
     data on a new firing cycle.                                          
120  LET F1 = 0. This instruction sets the                                
     points opening flag, F1, to zero. When                               
     the points open this flag is set to one                              
     by the interrupt program automatically.                              
130  LET K = 1. Execution of this instruction                             
     simply initializes the peak value data                               
     counter to a value of one.                                           
140  LET M1 = 0. Execution of this instruction                            
     simply initializes the peak detector                                 
     parameter M1 to zero (see Program Variable                           
     descriptions in Appendix B.)                                         
150  CALL 2, P(K). As indicated in the                                    
     subroutine descriptions execution of this                            
     instruction causes the system to make an                             
     average measurement of the starter current                           
     and to store it as a P(K).                                           
160  IF P(K) &lt; = M1 THEN GO TO 190. This instruc-                         
     tion is the key peak detection instruction.                          
     If the newest measurement, P(K), is not                              
     greater than the peak detector parameter                             
     M1, then a peak must have been reached                               
     so the system jumps forward to instruction                           
     190. Otherwise it executes the next                                  
     sequential instructions (170, 180, 185)                              
     and sets up for a new measurement and                                
     peak test.                                                           
170  LET M1 = P(K). This instruction simply                               
     saves the highest value measured so far                              
     in this firing cycle (the last measurement)                          
     as M1 for peak detection by comparison with                          
     future P(K) measurements.                                            
180  LET K = 1.                                                           
185  GO TO 150. Since a true peak has not been                            
     detected yet, these instructions simply                              
     insure that the peak sample counter, K,                              
     is set equal to one and send the computer                            
     back to instruction 150 to take another                              
     measurement.                                                         
190  IF P(K) &lt;= (M1 - 1) THEN GO TO 210. This                             
     instruction is testing for the end of a                              
     peak. For the purpose of this apparatus                              
     the end of a peak is when an average                                 
     measurement value drops one amp below the                            
     peak detected value, M1. Thus, this                                  
     instruction compares the last measured                               
     P(K) value with (M1 - 1) and if it is less                           
     than or equal to this end of peak limit                              
     the computer branches to instruction 210.                            
     Otherwise it executes the next sequential                            
     instructions 200 and 205.                                            
200  LET K = K + 1.                                                       
205  GO TO 150. These two instructions simply                             
     increment the peak measurement counting                              
     variable K and cause the computer to branch                          
     back to instruction 150. This causes the                             
     system to take another P(K) measurement                              
     and to check if it too is part of the peak.                          
210  CALL 3, T1. As indicated in the subroutine                           
     descriptions execution of this instruction                           
     inputs a real time clock value (units of                             
     msec) and saves it as T1.                                            
At this point in the program the system has all                           
of the information required to determine the time of the                  
starter current peak. The peak is assumed to be in                        
the center of the samples which where taken as peak                       
samples, and one extra sample was taken after the peak                    
(to determine that the peak was over). Knowing these                      
facts the assumed time of peak is given by:                               
                   K                                                      
         T.sub.p = T1 - 6.4 -                                             
                         (6.4)                                            
                   2                                                      
220  LET T2 (I) = T1 - 6.4 - 6.4 * K/2. This                              
     instruction simply causes the computer to                            
     calculate the assumed time of an individual                          
     peak and to save that value in the T2 array                          
     location corresponding to the particular                             
     firing cycle under examination.                                      
230  IF F1 &gt;&lt; 1 THEN GO TO 230. F1 is set to                              
     zero by the main program and reset to 1                              
     by the interrupt program whenever the                                
     points open. This instruction simply                                 
     tests for points opening. If the points                              
     have already opened F1 = 1 and the computer                          
     continues on. If the engine timing is                                
     significantly retarded and the points                                
     have not opened by the time that this instruc-                       
     tion is first executed, then the computer                            
     will continually execute this instruction                            
     until F1 is set to 1 by the points opening                           
     interrupt program (i.e. it will wait for                             
     the points to open and then continue on).                            
240  LET L = (2 * 4) + 1. This instruction                                
     simply sets the firing cycle counter limit                           
     to (2 engine cycles) × (4 firings per                          
     engine cycle) + (one extra firing cycle).                            
     This limit is the minimum number of data                             
     cycles which will yield complete data on                             
     2 full engine cycles.                                                
250  IF I &lt; L THEN GO TO 30. This instruction                             
     simply compares the firing cycle counter I                           
     to its limit L and causes the computer                               
     to branch back to statement 30 if data                               
     taking is not complete. If data taking is                            
     complete (I = L) the computer continues                              
     on to instruction 260.                                               
260  LET T = (T3(L) - T3(1))/(2 * 1000). An                               
     equation for calculating engine timing is                            
     D = 10.1 + 26.3(T - 0.90) - 100 t.sub.1 /T, where                    
     T = time in seconds for 1 full engine cycle                          
     (i.e. 2 crankshaft revolutions). The T                               
     calculated by the instruction above is the                           
     same T where T3(L) - T3(1) is the time for                           
     2 full engine cycles in msec so this value                           
     is divided by 2 and 1000 to convert to                               
     1 engine cycle and seconds.                                          
270  LET T4 = 0.                                                          
280  FOR M = 2 TO L.                                                      
281  LET T4 = T4 + (T3(M) - T2(M)).                                       
282  NEXT M.                                                              
290  LET T4 = T4/(2 * 1000). In the timing                                
     equation,                                                            
     t.sub.1 = the sum of time in seconds between                         
       I.sub.MAX and points opening for 4 consecu-                        
       tive cylinders (1 engine cycle).                                   
     In the system operation T4 is equal to the                           
     t.sub.1 described above. T4 is calculated by                         
     the 5 instructions above where (T3(M) - T2(M))                       
     of instruction 281 is the actual time between                        
     the points opening and starter current peak for                      
     the Mth firing cycle. Instruction 270 initializes                    
     T4 to zero and then 280-282 form a loop which                        
     accumulates 2 engine cycles of time differences.                     
     The T4 resulting from this loop is then divided                      
     by 2 and 1000 to convert this to data for 1                          
     engine cycle in seconds (instead of msec).                           
300  LET D = 10.1 + 26.3 * (T - 0.90) - 100 * T4/T                        
     This instruction causes the computer to                              
     calculate the actual engine timing in degrees                        
     before top dead center during cranking.                              
310  CALL 1, 1. As indicated in the subroutine                            
     descriptions this instruction just enables                           
     ignition firings so that the engine can                              
     start. This is done by having the computer                           
     output both a Reset and Device Select                                
     pulse to flip-flop 124 (see FIG. 3)                                  
     simultaneously.                                                      
320  PRINT &#34;TIMING = &#34;, D. This instruction                               
     simply causes the computer to output the                             
     results to the Display Device (42). If                               
     the display device happens to be a teletype                          
     or printer it will also print &#34;TIMING = &#34;                            
     as well as the actual timing in degrees BTDC.                        
330  STOP. This instruction just stops the                                
     system ending the test.                                              
Appendix A -- Subroutines                                                 
CALL 1, A                                                                 
        This subroutine either enables or inhibits                        
        the igniton. To run this test the                                 
        igniton must be turned on so that the                             
        time of points opening can be determined                          
        and must be inhibited so that engine firings                      
        do not distort the normal cranking waveform.                      
        The value of parameter A determines which                         
        function is performed. If A = 0 this                              
        subroutine will inhibit all ignition                              
        firings until inhibit is reset by calling                         
        this routine again with A = 1 (or anything                        
        other than 0).                                                    
CALL 2, B                                                                 
        This subroutine inputs 80 data samples                            
        from starter current channel through the                          
        multiplexor and the A/D converter with                            
        80 μsec between samples. It then averages                      
        these samples and returns the average value                       
        to the main program as the parameter B                            
        (note that this should take 80 × 80 μsec or              
        6.4 msec).                                                        
CALL 3, C                                                                 
        This subroutine inputs a number from the                          
        real time clock and stores this value as the                      
        parameter C. For the system being described                       
        the clock is running at 1 kHz meaning that the                    
        clock is effectively counting milliseconds.                       
Appendix B -- Program Variables                                           
Measured                                                                  
C(J)   is an array of average measurements of the                         
       starter current waveform. These measure-                           
       ments are used for detection of the negative                       
       slope characteristic and the actual                                
       minimum data point for each firing cycle.                          
P(K)   is similar to C(J) except that it is used                          
       for detecting the starter current peak value.                      
T1     is a real time clock measurement. After                            
       the test system determines that the starter                        
       current peak is past T1 is input and saved                         
       for use in calculating the effective time                          
       of starter current peak.                                           
T3(I)  is an array of real time clock measurements                        
       made each time the points open during the                          
       test (measured by the interrupt part of                            
       the program).                                                      
Calculated                                                                
M1      is a variable used for detection of the                           
        starter current peak. Once peak detection                         
        has started M1 is compared with each                              
        measured P(K) value and if it is less than                        
        the measured value then it is set equal to                        
        it so that eventually it is equal to the                          
        maximum measured value (for each peak).                           
T2(I)   is an array of values assumed to be the                           
        actual times (in msec) of the peaks as                            
        calculated by subtracting off one half                            
        of the sampling time for the peak values                          
        samples.                                                          
T       is the average time in seconds for one full                       
        engine cycle during the test as calculated                        
        by taking the time required for two full                          
        engine cycles and dividing by two.                                
T4      is the average sum of time in seconds                             
        between maximum starter current and points                        
        opening for 4 consecutive cylinders.                              
D       is the actual timing of the ignition during                       
        cranking in degrees BTDC.                                         
Counting and Flags                                                        
I    counts engine firing cycles.                                         
J    counts average data points used for negative                         
     slope detection on starter current waveform.                         
K    counts average data points within 1 amp of                           
     peak starter current after peak detected.                            
F1   is reset before starting data taking                                 
     corresponding to each cylinder firing and                            
     is set by the points opening interrupt                               
     routine.                                                             
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