Method of analysing cylinder performance in an internal combustion engine

Abstract: An engine (10) equipped with a low resolution crankshaft rotation encoder (12) is diagnosed by measuring the average speeds for each encoder period, deriving estimated instantaneous speeds (V(L) and V(H)) for top dead centre positions of each cylinder and for the points midway between top dead centres. Speed changes calculated from the estimated speeds for the compression and combustion phases of each cylinder provide measures of the compression and power output of each cylinder. An engine speed signature trace (34) derived from the estimated speeds is displayed to reveal speed changes and a composite display of several consecutive signatures provides further information.

Description: This invention relates to a method of analyzing cylinder performance in an internal combustion engine. The diagnostic connector on automobiles permits service personnel to diagnose quickly problems related to the operation of on-board sensors and microcomputers. There is a need, however, for the capability to diagnose individual cylinder performance, that is, to identify misfiring or otherwise under-performing cylinders. Such information, in the form of compression pressure and power output values for each cylinder, can be obtained from engine speed variations caused by the compression and power strokes of each cylinder. US Patent No. 4,539,841, entitled "Method of Determining Engine Cylinder Compression Pressure and Power Output", teaches how for a given engine, at a predetermined engine speed and load, the peak compression pressure and the cylinder power output can be closely estimated from instantaneous engine speed at properly selected crank angles. The method taught in this patent, however, requires a fairly high resolution encoder to generate acceptable instantaneous speed data (e.g., the speed should be measured over a crank angle smaller than about 4°). In practice it means a separate speed sensor has to be used to collect speed data from the ring gear. It is strongly preferred to obtain the necessary information without a separate speed sensor. Low resolution sensors are commonly used on engines for ignition timing and are capable of delivering a speed sample each 60° or 30°, for example. The present invention seeks to provide an improved method of analysing cylinder performance. Accordingly, an aspect of the present invention provides a method of analysing cylinder performance of an internal combustion engine as defined in claim 1. It is possible with the invention to derive meaningful compression and power output information without a high resolution sensor and particularly to obtain such information from standard speed sensors used for timing purposes. It is also possible to present the diagnostic data in a useful way and to store the data easily. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an engine and an embodiment of diagnostic tool and display for determining compression and power parameters of each cylinder; Figure 2 is a graph of a high resolution speed trace and a low resolution speed trace for several cylinder events; Figure 3 is a graph of a high resolution speed trace and a low resolution speed trace for one cylinder event; Figure 4 is a graph of a high resolution speed trace for one cylinder event and an estimated approximation of the speed trace derived from a low resolution speed trace, according to an embodiment of the invention; Figure 5 is a graph of a high resolution speed trace and an engine speed signature for an engine cycle; Figure 6 is a display of an engine signature for a condition of one spark plug disconnected; Figure 7 is a display of a family of engine signatures for a condition of one spark plug disconnected; Figure 8 is a display of an overlay of several engine signatures for the condition of one intermittently misfiring cylinder; Figure 9 is a display of an overlay of several normalized engine signatures based on the same data used for the display of Figure 9; and Figure 10 is a flow chart of an embodiment of program for use in the computer of Figure 1. The diagnostic method set forth in the ensuing description was specifically developed for a four cylinder gasoline engine with a fuel injector for each cylinder and having a sensor which delivers six pulses per crankshaft revolution (6X encoder). It will be understood, however, that the method applies equally well to engines with six or eight cylinders, for example, using 6X, 12X, 18X or 24X encoders, with suitable adjustments to suit the number of cylinders and the type of encoder. Referring to Figure 1, an engine 10 includes an encoder 12 mounted on and rotated by the engine crankshaft 14 and which has teeth 16 equally spaced around its circumference at 60° intervals. An electromagnetic speed sensor 18 positioned adjacent the path of the teeth 16 senses the passing of the teeth as the crankshaft 14 is rotated and provides an alternating output to a zero crossing responsive square wave amplifier 20 whose output is a square wave signal at the frequency of the alternating input from the speed sensor 18. This square wave signal is provided to a pulse generator 22 which provides a pulse output with the passing of each tooth of the encoder 12. Therefore the time interval between pulses is inversely proportional to engine speed and the frequency of the pulses is directly proportional to engine speed. A computer 24, of conventional type, receives the pulses developed by the pulse generator 22. The computer 24 comprises a microprocessor, a clock, a read-only memory, a random access memory, a power supply unit, an input counter interface and an output interface. The computer 24, upon a manual input command or upon sensing certain engine conditions, executes an operating program stored in its read-only memory. This program includes steps for reading input data and timing intervals via the input counter interface, processing the input data and providing for an output such as to a display 26 via the output interface. The display 26 may take the form of a printer or a video monitor for displaying various information relating to the diagnostic procedure. By measuring the interval between pulses and calculating the average speed for each interval, a low resolution trace of engine speed is obtained as shown in Figure 2 by the trace 30 which is registered with scale markings for the engine rotation angle; where 0° coincides with the top dead centre of one cylinder and each 180° increment corresponds to the top dead centre of another cylinder. The trace 30 is made up of a succession of 60° samples each aligned with an angular period and representing the average speed over that 60° period. The speed samples cover intervals immediately before and after top dead centre, respectively denoted S(-L) and S(+L), since they are relatively low values for a normally operating engine, and a relatively high sample S(H) which occurs in an interval midway between top dead centre of successive cylinders. A high resolution speed trace 32 derived from speed samples taken every 4° of crank rotation is superimposed on the Figure 2 graph to reveal the relationship of the low resolution trace 30 to actual engine speed. For normal operation the lowest speeds shown by the trace occur at about the top dead centre of each cylinder and the highest speed is about midway between top dead centres, or 90° before or after, top dead centre. More precisely, the actual speed peaks shortly before the 90° position and the speed at the 90° position is about equal to the measured average speed S(H) at the midway point. The method involves estimating the instantaneous speed of the engine at top dead centre and midway between top dead centres from the low resolution trace 30. As described above, the estimated speed V(H) at the midway position can be set equal to the average speed for that period, or V(H)=S(H). This is better shown in Figure 3 which is an enlarged view of the speed traces from -120° to +120° crankshaft rotation. When referring to a single cylinder, the two midway speeds V(H) are conveniently denoted as V(-H) for the point before top dead centre and as V(+H) for the point after top dead centre. Of course the speed V(+H) for one cylinder is the same as the speed V(-H) for the next cylinder. In the same manner the notation for S(H) may be specified as S(-H) or S(+H). In Figure 3 it should be noted that the speed curve 32 at -60° is fairly close to the midway average speed S(-H) and that the trace 32 between -60° and 0° is nearly linear. Since the average speed for the period between -60° and 0° is the measured value S(-L), the low value on the curve 32 can be estimated from the relationship S(-L) = [S(-H)-V(L)]/2, where V(L) is the estimated low speed at top dead centre. The equation may be restated as V(L) = 2*S(-L) - S(-H). Slightly different results are obtained by using the estimated speed S(+H) for 90° and the average speed S(+L) for the period 0° to 60° to arrive at the equation V(L) = 2*S(+L) - S(+H). Both equations for estimating V(L) provide very good estimates, especially in a properly operating engine, in which it has been found that the difference between the estimated and actual values is only 1 or 2 RPM, amounting to an error of a fraction of a percent. If one of the cylinders malfunctions, the value V(L) for that cylinder is over-estimated by about 1%. The values of V(L) for the remaining, properly functioning cylinders are, as a consequence, under-estimated by about 1%, except for the cylinder which is next in the firing order after the malfunctioning one. For this cylinder, it has been found that the value V(L) can be up to 3.5% below the actual value. These estimation errors are quite tolerable for diagnostic purposes especially since they tend to accentuate the fault-related information. For a single cylinder, as shown in Figure 4, the actual speed trace 32 is replaced by a V-shaped estimated trace 34. The midway speed V(-H) before top dead centre, the speed V(L) at top dead centre and the midway speed V(+H) after top dead centre define the trace and contain sufficient information to analyse cylinder performance. The compression of the cylinder determines the slope of the trace before top dead centre and thus the difference of V(-H) and V(L) reveals information about compression. A variety of equations can be constructed to extract that information in one form or another. For example, relative compression RCP can be determined by RCP = 100%*[V(-H)-V(L)]/V(L). In a similar manner the difference between V(+H) and V(L) is a function of the power output of the cylinder. Relative power output RPO can be expressed as RPO = 100%*[V(+H)-V(L)]/V(L). Negative, or distinctly lower values of RCP and/or RPO for a particular cylinder in comparison with those of the other cylinders indicate a compression or a combustion problem in that cylinder. In a well running engine the values of RCP and RPO are very close, and are in the range of 2% to 8% depending on engine calibration and operating conditions. Calculating and displaying the compression and power output values reveals useful information about cylinder performance especially for grossly malfunctioning cylinders. A speed trace available for visual inspection, however, would reveal less distinct faults. A diagnostic speed trace can be built from the estimated speeds V(H) and V(L). A speed trace 34 for a complete engine cycle during proper operation is shown in Figure 5 overlaid on the actual speed trace 32. The diagnostic speed trace is thus a series of V shaped forms and is hereinafter called the engine speed signature. Only nine data points comprising the high and low speed points V(H) and V(L) are required to define the signature for an entire engine cycle in contrast to 180 points needed to define a high resolution speed trace. It is therefore relatively easy to build a library of such signatures linked to a variety of known engine problems. The small number of data points in each signature makes an automatic pattern search very fast and simple. An original speed signature taken when the engine is new could be stored in the engine computer for reference for future use. A trace of a speed signature is shown in Figure 6 for an engine having the spark plug disconnected in cylinder 2. The RCP and RPO parameters for each cylinder are shown as Comp. and Power. The speed after top dead centre falls off instead of increasing and the power parameter RPO drops to -4 from a normal of 5 for the preceding cylinders. The drop in speed also affects the next cylinder, No. 1, which shows an RPO of 10 due to the gain in engine speed. The same condition is shown in Figure 7 which is a display of an overlay of 25 consecutive signatures. The same signature shape is shown by each signature, revealing consistent engine operation. The narrow band indicates relatively little speed variation from cycle to cycle. Figure 8 is an overlay of 25 signatures for an engine having intermittent misfire for cylinder 2. Each misfire results in a speed drop during the power stroke (after top dead centre) which leads to lower speeds for subsequent cycles. The composite RPO value for cylinder 2 is -2 compared to 4 and 7 for the other cylinders. The overlay of several signatures clearly reveals the intermittent failures of either injectors or ignition. A particularly useful form of display of speed signatures is provided by normalizing the data for each cylinder to the estimated top dead centre speed, V(L), and displaying the trace of several normalized speed signatures. This gives the operator very clear indication of the engine operation, especially in the case of intermittent misfires. Figure 9 is the overlay of normalized signatures for the same data used to generate Figure 8 signatures. The normalized trace is prepared by dividing V(-H), V(L) and V(+H) by V(L) for each individual cylinder. Then the display value at each top dead centre is always 1 and the display for each cylinder has a "butterfly" pattern with the spread of each wing pattern revealing the speed variance over the period displayed, so that a very smooth running engine will have a tight pattern with very little wing "flutter". Misfires are instantly revealed as a pattern below the V(L) level, as shown for cylinder 2 in Figure 9. The method is carried out in the computer 24 by a program represented by the flow chart shown in Figure 10. The engine speed is sensed (block 40) by receiving the pulses initiated by the crankshaft encoder 12, the average speed is calculated for each period between pulses (block 42), the high speeds V(H) at the midway points are determined from the average speeds S(H) (block 44), the low speeds V(L) at top dead centres are calculated (block 46), compression and power out parameters for each cylinder are calculated based on the differences between V(H) and V(L) for the compression and combustion phases, respectively (block 48), the engine speed signature is displayed for one cycle (block 50) or an overlay of several signatures is displayed (block 52). Alternatively, the estimated data is normalized (block 54) and normalized speed signatures are displayed (block 56).

Claims: A method of analysing cylinder performance of an internal combustion engine (10)comprising the steps of obtaining for each cylinder a first value (S(H)) representative of the engine speed in a period between top dead centre (TDC) of the cylinder and a sequentially adjacent cylinder in the firing order (CYL.1, CYL.3, CYL.4, CYL.2) and a second value (S(L)) representative of engine speed in a period proximate top dead centre (TDC) of the cylinder; estimating a midway engine speed (V(H)) on the basis of the first value (S(H)), and a top dead centre engine speed (V(L)) on the basis of the first value (S(H)) and the second value (S(L)); and analysing the cylinder performance on the basis of the midway and top dead centre engine speeds. A method according to claim 1, wherein the top dead centre speed (V(L)) is obtained on the basis of the equation V(L) = 2*S(L) - S(H). A method according to claim 1 or 2, wherein the second value (S(L)) is representative of the average engine speed (S(-L)) in a period prior to top dead centre, and the top dead centre engine speed (V(L)) is derived on the basis of the equation V(L) = 2*S(-L) - S(H). A method according to claim 1, 2 or 3, wherein the first value (S(H)) is representative of the average engine speed in a period including the beginning of a compression phase of the cylinder or in a period including the end of the compression phase (S(+H),S(-H)). A method according to any preceding claim, comprising the step of obtaining for each cylinder a value (V(-H)) representative of the engine speed prior to top dead centre, and deriving a relative compression parameter (RCP) for the cylinder on the basis of the obtained value (V(-H)) and the top dead centre engine speed (V(L)). A method according to claim 5, wherein the relative compression parameter (RCP) is obtained on the basis of the equation RCP = [V(-H)-V(L)]/V(L). A method according to any preceding claim, comprising the step of obtaining for each cylinder a value (V(+H)) representative of the engine speed after top dead centre, and deriving a relative power output (RPO) for the cylinder on the basis of the obtained value (V(+H)) and the top dead centre engine speed (C(L)). A method according to claim 7, wherein the relative power output (RPO) is obtained on the basis of the equation RPO = [V(+H)-V(L)]/V(L). A method according to any preceding claim, comprising the step of forming an engine speed signature from the derived engine speeds. A method according to any preceding claim, wherein the step of analysing the cylinder performance comprises the step of displaying a waveform based on the derived engine speeds of each cylinder for comparative observation thereof. A method according to claim 10, comprising the step of displaying in overlaying relationship a plurality of waveforms from a plurality of engine revolutions. A method according to any preceding claim, comprising for each cylinder the steps of normalising the derived engine speeds relative to the top dead centre engine speed (V(L)) for a plurality of engine cycles, and displaying in overlying relationship a plurality of images each based on the normalised speeds in one engine cycle.

Inventors: DALLEY ROBERT ALLEN, EVANS ROGER BRUCE, MARTABANO CRISTI ANN, SCHROEDER THADDEUS, DALLEY, ROBERT ALLEN, EVANS, ROGER BRUCE, MARTABANO, CRISTI ANN, SCHROEDER, THADDEUS
Applicants: ELECTRONIC DATA SYST CORP, GEN MOTORS CORP, SATURN CORP, ELECTRONIC DATA SYSTEMS CORPORATION, GENERAL MOTORS CORPORATION, SATURN CORPORATION
Application year: 1991
Main classification: G01M15
IPC codes: G01M15, F02B77