Abstract:
Means for and method of testing the power of an internal combustion engine. A tachometer means coupled to the engine generates one electrical pulse per engine cycle of the engine. While the engine is full-throttle accelerated, a computer means measures the time periods between successive pulses, each time period constituting the reciprocal of the average engine speed during the particular engine cycle. One-half of the time period corresponding with a low speed and one-half of the time period corresponding with a high speed are added to the intermediate time periods to determine an acceleration time period which is a measure of engine power. The acceleration rate is determined by subtracting the reciprocal of the time period corresponding with the low speed from the reciprocal of the time period corresponding with the high speed to determine the speed change, and dividing the speed change by the acceleration time period. Brake torque is determined by multiplying the acceleration rate by the known inertia of the engine. The engine is then shut off and the deceleration rate is determined and is multiplied by the known inertia of the engine to determine the friction torque of the engine. The brake torque is added to the friction torque to determine the indicated (total) torque of the engine.

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 full-load, full-speed testing of internal combustion engines for indicated power, brake power and friction power can be accomplished by connecting the engine to a motoring dynamometer capable of absorbing the full-load output of the engine and capable of driving the maximum frictional load of the engine while measuring speed and torque. Such dynamometers are very large, very expensive, and very inconvenient to use. 
     A much more economical and convenient apparatus for testing spark-ignition internal combustion engines under simulated full-load conditions is apparatus for operating the engine with all but one of n ignitions interrupted, where n may be greater than the number of cylinders, so that all cylinders are operated in sequence under full power conditions. The engine operates at full speed driving frictional and pumping loads, without danger of overspeeding and damaging the engine. 
     An economical and convenient brake power test of compression-ignition internal combustion engines under simulated full-load conditions is the acceleration burst test in which an engine initially operating at idle speed is suddenly given full throttle and caused to accelerate to a maximum governed speed. The inertia of the engine is the load on the engine, and the time taken to accelerate through a low speed to a high speed is a measure of the full-power output capability of the engine. This test is particularly useful for testing diesel engines, and provides a somewhat less accurate indication of the condition of a spark-ignition engine equipped with a carburetor instead of fuel injectors. 
     The accuracy with which the full output power capability of the engine is given by the time taken to accelerate from a low speed to a high speed depends on the accuracy of the speed measurements, which are in units of angular displacement (such as revolutions) divided by units of time (such as minutes or seconds). An accurate measurement of speed is complicated by the fact that the burst acceleration from a low speed to a high speed takes only about one second or less. Another complication has been found to be due to pulsations in instantaneous speed which are due to explosions and compressions in individual cylinders of the engine. The pulsations in instantaneous speed are particularly disturbing if they are non-uniform due to malfunctioning of one or more individual cylinders of the engine. 
     An additional problem associated with this type of test is that of interpreting the results. A low output power indication may be caused by relatively high power absorption of the engine and its accessories. Large variations in frictional load can be expected from test to test on different vehicles in different conditions with different accessory loads since the frictional load on an engine is dependent on oil type and temperature, accessory load, and other effects. Thus, a very significant engine power test parameter is the indicated (total) power developed within the engine. Knowing this and the brake (output) power capability of an engine allows for differentiation between internal and external engine problems, and sometimes differentiation between real and apparent engine problems. 
     SUMMARY OF THE INVENTION 
     In a test of the power of an engine, tachometer means generates one electrical pulse per engine cycle of the engine (one pulse per revolution of a two stroke engine, or one pulse per two revolutions of a four-stroke engine). An engine-cycle time period between successive electrical pulses is the reciprocal of the average speed during the engine cycle. Instantaneous speed fluctuations, which occur due to power and compression strokes in individual cylinders, follow the same pattern during all engine cycle time periods. The electrical pulses, which always occur at the same relative time in each engine cycle, are used to accurately measure the time taken to accelerate from a low engine speed to a high engine speed without errors due to instantaneous speed fluctuations. One half of the time period between successive pulses in a predetermined low engine speed range and one half of the time period between successive pulses in a predetermined high engine speed range are added to the intermediate time periods to provide an accurate time measure of the full-power condition of the engine. 
     The average acceleration rate is determined by subtracting the reciprocal of the time period corresponding with the low speed from the reciprocal of the time period corresponding with the high speed to determine the speed change, and dividing the speed change by the acceleration time period. Brake torque is determined by multiplying the acceleration rate by the known inertia of the engine. The engine is then shut off and the deceleration rate is similarly determined and is multiplied by the known inertia of the engine to determine the friction torque of the engine. The brake torque is added to the friction torque to determine the indicated (total) torque of the engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of apparatus used for an acceleration burst test of the full-power condition of an internal combustion engine; 
     FIG. 2 is a chart of engine speed vs. time showing instantaneous speed fluctuations during acceleration of a four-cylinder engine; 
     FIG. 3 is a chart of engine speed vs. time during acceleration which will be referred to in describing the operation of the invention; 
     FIG. 4 is a chart of engine speed vs. time during a test including a deceleration portion following the acceleration portion. 
     FIG. 5 is a logic diagram of the elapsed time device included in the system of FIG. 1; 
     FIG. 6 is a flow chart of a program used in the computer in the system of FIG. 1 to control the test procedure and compute the test result; and 
     FIG. 7 is a chart showing torque vs. speed curves for a typical compression-ignition engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in greater detail to the drawing, FIG. 1 shows an internal combustion engine 10, such as a diesel engine, equipped with a pulse tachometer 12, from which electrical pulses are applied over line 13 to an elapsed time device 14. The elasped time device (shown in detail in FIG. 5) 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. 
     The pulse 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 revolution of the engine crankshaft, and these pulses are applied to an elapsed time device 14 which, if necessary reduces the number of pulses to one pulse per engine cycle. 
     The acceleration burst test to be described utilizes one electrical pulse per engine cycle. One 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 fourstroke engine because the four functions are accomplished in four strokes of the piston. 
     FIG. 2 is a speed-time chart showing the acceleration characteristic 20 of an engine from 1000 rpm to 2000 rpm in a time period T. The solid vertical lines of the chart represent the boundaries of individual engine cycles and the times of electrical pulses from the tachometer 12. The line 20 in the chart shows speed fluctuations recuring similarly in each engine cycle. The engine represented is a four-cylinder engine having four power pulses per engine cycle causing four instantaneous speed peaks. It can be seen that the instantaneous speed fluctuations may result in a short-term reduction in speed while the average speed is increasing. These speed fluctuations introduce inaccuracies into any ordinary method of measuring the time T required to accelerate from a speed of 1000 rpm to a speed of 2000 rpm. The inaccuracies are even greater when the engine has one or two faulty cylinders which cause even greater irregularities in instantaneous speed during acceleration. 
     The accuracy with which the average acceleration time T can be measured is improved by deriving all time and speed measurements from tachometer pulses occurring once per engine cycle. All pulses occur at the same relative part of the respective engine cycles. Therefore, the time interval from a pulse at a low engine speed to a pulse at a high engine speed is unaffected by instantaneous speed changes during engine cycles. The average engine acceleration is assummed to be linear during an engine cycle. 
     FIG. 3 is a speed-time chart similar to FIG. 2 but with the speed characteristic 22 smoothed to average out the instantaneous speed fluctuations. The acceleration time T from 1000 rpm at time t a  to 2000 rpm at time t b  is not measured, but rather the acceleration time C 1  is measured. Time C 1  equals onehalf of the first engine cycle time period A 1  corresponding with an average speed greater than 1000 rpm, plus one-half of the first engine cycle time period A 2  with an average speed greater than 2000 rpm, plus the intermediate time periods C. The average speed S 1  during engine cycle A 1  is the reciprocal of the time period A 1  and is greater than 1000 rpm. Similarly, the average speed S 2  during engine cycle A 2  is greater than the high speed of 2000 rpm. 
     FIG. 4 is a speed-time chart similar to FIG. 3, but illustrating both the acceleration and deceleration portions of the test. In addition to the parameters shown in FIG. 3. FIG. 4 also shows the measured deceleration time F 1 , which equals one-half of the first engine cycle time period D 1  corresponding with an average speed less than 2000 rpm (after acceleration), plus one-half of the first engine cycle time period D 2  with an average speed less than 1000 rpm, plus the intermediate time periods. 
     FIG. 5 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 engine cycle. The output 29 from the divider 15 is one pulse per engine cycle. 
     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 lines 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 cycle, and sets its own state to &#34;done&#34; each time an engine cycle time period is stored. The computer can then cause a transfer of the store 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, Massachusetts 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 chart of FIGS. 3 and 4, and later will be described in greater detail with references to the flow chart of FIG. 6. 
     In the initial condition, the engine 10 is operated at an idle speed of about 700 rpm, the tachometer 12 supplies pulses to the elapsed time device 14 which is continuously counting the time periods between engine cycle pulses after receiving a &#34;start&#34; signal from the computer 16, and the display 18 is displaying a &#34;full throttle&#34; message received from the computer 16. 
     The human test operator applies full throttle to the engine causing it to accelerate to a governed high limit speed. 
     The computer continuously receives the count from the counter in device 14 for the time between engine cycle pulses until a count for an engine cycle A 1  is reached corresponding to an average speed greater than the predetermined low speed of 1000 rpm. The preceding engine cycle corresponds to an average speed less than the predetermined low speed of 1000 rpm. The computer then computes the time period A 1  /2 (FIG. 3) and starts adding the counts of the following engine cycle time periods. 
     The computer continuously receives the count from the counter in device 14 for the time between engine cycle pulses until a count for an engine cycle A 2  is reached corresponding to a speed greater than the predetermined high speed of 2000 rpm. The preceding engine cycle corresponds to a speed less than the predetermined high speed of 2000 rpm. The computer then stops adding counts and computes the time period A 2  /2. 
     The computer then adds the measured time period A 1  /2 and the time period A 2  /2 to the counted time C to arrive at the time period C 1 . The time period C 1  represents the time required by the engine to full-throttle accelerate from a low speed S 1  to a high speed S 2 . 
     The computer then uses time periods A1, A2, and C1 to calculate the engine acceleration rate B; where: ##EQU1## Following this calculation the result is displayed to the operator as an indication that the acceleration portion of the test is complete and that he should shut off the engine. Then the engine will decelerate, and the test system will measure or calculate D1, D2, and F1 in a manner similar to the acceleration portion of the test. FIG. 4 illustrates this portion of the test, as well as the acceleration portion of the test. From D1, D2, and F1 the computer will calculate the deceleration rate E, where: ##EQU2## 
     The computer will then calculate and display the values of indicated, brake and friction, torques (and horsepowers, if desired) for the 1000-2000 rpm speed range of the engine from the acceleration rate B and deceleration rate E. From these values, the operator can easily evaluate the power development capability of the engine as well as its frictional load in comparison with known characteristics, such as are shown in FIG. 7, of a good engine. 
     Reference is now made to the program flow chart of FIG. 6 for a description of the operation of the system of FIG. 1. 
     
         ______________________________________StatementNumberProgram Step and Description______________________________________201     CALL 1. When the computer executes this   instruction, the engine should be running   at idle. The instruction causes the   computer to send a START pulse to the   elapsed time device 14. This sets the   device to the Busy State which initializes   the system preparing it for time period   (speed) measurements.202     DISPLAY &#34;FULL THROTTLE&#34;. This instruction   outputs the message &#34;FULL THROTTLE&#34; to the   display 18 indicating to the operator   that the system is ready. At this point   the vehicle should still be operating at   idle speed.203     CALL 2, A1. This instruction causes the   system to wait for the next engine period   pulse and then the computer inputs the time   period between the last two pulses. This   input value is saved as parameter A1. For   a four cycle engine such as the LD465 with   one pulse per engine cycle (2 revolutions)   the A1 input at idle will be about 170.0   msec or A1 = 1700 (this corresponds to   about 706 RPM).204     If A1 &gt; L1 THEN GO TO 203. This instruc-   tion is testing the vehicle speed looking   for the beginning of the acceleration. If   the acceleration rate in the 1000 to 2000   RPM range is desired from one pulse per   engine cycle L1 would be 120 msec or 1200   (for 4 cycle engine) since the system   measures time in 0.1 msec units. On execu-   tion of this instruction, the computer will   compare the most recent input value of A1   with L1 and if it is greater than L1 the   computer jumps back to instruction 203   again. Otherwise it will continue on to   the next sequential instruction 205.205     LET C = 0. At this point in the program   the acceleration in the range of interest   has just begun. The parameter C is going   to be used to accumulate the total time   period between start and stop of the   acceleration portion of the test. Thus,   C must be initialized to zero which is all   this instruction does.206     CALL 2, A2. Like instruction 203 this   instruction causes the system to wait for   the next pulse and then inputs a new time   period measurement. This time the   measurement is saved as A2.207     IF A2 &lt; L2 THEN GO TO 209. This instruc-   tion is trying to detect a speed in excess   of the upper speed limit. If the   measured time period A2 is less than the   limit L2 (for four cycle engines with one   pulse per engine cycle an L2 corresponding   to 2000 RPM would be 60 msec or L2 = 600   in system time units, 0.1 msec), then the   program jumps to statement 209 to proceed   with acceleration rate calculations. If   A2 is greater than or equal to L2 the   computer executes the next sequential   Instruction 208.208     LET C = C + A2. When the last measured   value of A2 is still within the speed   range of interest the computer will take   this branch of the program. This instruc-   tion thus, acts as a time accumulator   summing all the A2 measurements occurring   within the acceptable speed range and saving   the sum as parameter C. Each execution   of this statement simply adds the last   measured value of A2 to the previously   accumulated value of C.208&#39;    GO TO 206. This instruction simply causes   the computer to branch back to step 206   forming a program loop.209     LET C1 = C + A1/2 + A2/2. Execution of   this statement performs the following   function:             A1          A2      C1 = C +     +             2           2   This value is used by the next instruction   for the acceleration rate calculation which   needs the total time period between the   first time period (A1) and the last time   period (A2). Since the speed cross points   used within the acceleration rate   calculation (step 210) are derived from the   inverse of the time period, they are   actually an average speed measurement for   the given time period. However, by   assuming a linear speed versus time   characteristic within the time period, the   time of an instantaneous speed corresponding   to the measured average value can easily   be interpolated by division by 2. Thus,   this program statement corrects the   accumulated total time period by adding   to it one half of the first and last time   periods (A1 and A2).210     LET B = (1/A2 - 1/A1)*4*10↑8*3.14159/C1.   Execution of this instruction performs the   calculation:                1         1                      -          (4 × 10.sup.8)π                A 2       A1      B =         C1   where B is the acceleration rate in rad/sec.sup.2   (which is directly proprotional to engine   output torque). The 1/A2 factor is the   average speed of the final time period   measurement and 1/A1 is the average speed   of the initial time period measurement.   (4 × 10.sup.8)π is simply a units conversion   factor. For the test example shown in   FIG. 4:          1         1                -           (4×10.sup.8)π          581       1013B=                                      = 167.52 rad/sec.sup.2   5506211     DISPLAY &#34;ACCELERATION RATE&#34;, B. This   instruction simply outputs the message   &#34;ACCELERATION RATE&#34; and the calculated value   of B to the display 18. This informs the   operator that the acceleration portion of   the test is complete. At this time, or   soon after, he should shut off the engine so   that the system can run the deceleration   portion of the test.   NOTE: Statements 212 through 219 perform   the same function during the engine decelera-   tion as statements 203 through 210 on   acceleration except for the comparisons   of 213 and 216. These are naturally   inverted since the speed versus time   characteristic has the sign of the slope   changed. Execution of all of these steps   results in the calculation of the   deceleration rate, E. For an LD465 engine   (such as for the acceleration test example   given) a normal deceleration rate would be   about E = 73 rad/sec.sup.2.220     LET brake torque T1 = I*B.221     LET friction torque T2 = I*B. Execution of   these statements multiplies B by I, and E   by I, respectively, where I is the inertia   of the test engine (I = 1.427 ft. lb. sec.sup.2   for LD465 engine). T1 is the average   brake torque (or output torque available)   and T2 is the average friction torque of   the test engine in the test speed range.222     LET T3 = T1 + T2. This instruction sums   T1 and T2 (brake and friction torque) to   give T3 which is the indicated torque or   total torque developed within the engine.223     DISPLAY B, T1, E, T2, T3. This instruction   simply outputs the key resultant test   parameters to the operator. The values   displayed are:   B = Acceleration rate in rad/sec.sup.2   T1 = Brake torque in ft. lbs.   E = Deceleration rate in rad/sec.sup.2   T2 = Friction torque in ft. lbs.   T3 = Indicated torque in ft. lbs.   Thus, in one simple acceleration/deceleration   test the operator gets key test parameters   that in the past took considerable time   and bulky expensive equipment.______________________________________