GB2442797A

GB2442797A - Engine Diagnostic System

Info

Publication number: GB2442797A
Application number: GB0620131A
Authority: GB
Inventors: Erik Haggstrom
Original assignee: PICO TECHNOLOGY Ltd
Current assignee: PICO TECHNOLOGY Ltd
Priority date: 2006-10-11
Filing date: 2006-10-11
Publication date: 2008-04-16
Anticipated expiration: 2026-10-11
Also published as: GB2442797B

Abstract

System and method for diagnosing conditions or problems in an internal combustion engine, particularly while the engine is running. An engine diagnostic system for diagnosing a condition of an internal combustion engine such as mis-fire, cylinder balance, said internal combustion engine having a plurality of cylinders driving a rotating shaft coupled to an alternator, the diagnostic system comprises: an input of a signal constituting a DC signal from said alternator or alternatively a battery or sensor; a system coupled to said input to determine from said alternator signal, data indicating for each said cylinder a contribution of the cylinder to said rotation of said shaft; and an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation. Preferably the output is a waveform representing cylinder performance. Additionally an envelope of the alternator signal is established and an AC ripple is superimposed thereon.

Description

Engine Diagnostic Systems This invention is generally eoiicemed with

systems, methods and computer program code br diagnosing conditions or problems in internal combustion engines, iii particular whilst the engine is running.

Modem cars generally have some form of engine control unit (ECU) or engine management unit (EMU) for monitoring and controlling the performance of the vehicle's engine. Specialist garages often have diawiostic equipment which is able to interface with the ECU or EMLI of a specific type of vehicle and extract data relating to the engine's performance. However, this equipment is generally specific to a manufacturer, and often to a model of vehicle and there is a need for engine diagnostic systems which do not rely on such a proprietary interface. One possibility would be to directly connect to the various sensors which an ECU or EMU employs but in practice these are often not readily accessible. The compression of the cylinders of a car engine can be determined indirectly by measuring the load on the starter motor whilst the engine is cranking, but this technique provides limited information and, moreover, can only be used when starting the engine.

An example of an on-board diagnostic monitor for internal combustion engine is described in US 6,415,656, but this is of the ECU-type described above, and suffers from the aforementioned drawbacks. The possibility of measuring engine speed by detecting a "noise" signal from a generator or alternator or from the engine ignition is described in GB 2,420,179, which uses this signal to determine the vehicle speed for use with a vehicle navigation system. Other background prior art can be fbund in US 2002/0050810, IF 2004-226164, WO 02/01698, WO 95/05606, JP 61-247867, and iF 61-46975.

Flowever, the inventors have recognised that diagnostic information relating to the functioning of' individual cylinders of an internal combustion engine may be derived from the signals from an alternator.

According to one aspect of the invention there is therefore provided an engine diagnostic system lbr diagnosing a condition of an internal combustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shaft coupled to an alternator, the diagnostic system comprising: an input to receive a signal from said alternator; a system, coupled to said input, to determine from said alternator signal data indicating for each said cylinder a contribution of the cylinder to said rotation of said shalt; and an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.

In embodiments of the system the contribution or more particularly the relative contribution, of each separate cylinder to the rotation of the shaft can be distinguished allowing the contribution of one cylinder to be evaluated with respect to itself (at a different time) and/or one or more other cylinders. The fhniier comparison can he used to identify a mis-fire; the latter can be used to determine whether or not the cylinders are balanced with respect to one another.

Broadly speaking the inventors have recognised that as each cylinder fires a rotational impulse is delivered to the shaft, generally the crank shaft, of the engine, and in response the rotation of the shaft briefly speeds up and then slows down once again until the next impulse is delivered by the next cylinder to fire. The inventors have ffirther recognised that these changes in what might be termed instantaneous rotational speed of the shaft (although in practice averaging is employed to determine this "instantaneous" speed) can be detected indirectly using the output of the alternator, and this itself can be picked up, for example, on almost any part of the wiring loom of a vehicle bearing the vehicle's dc power supply. More particularly the inventors have understood that the signals from the alternator contains a sufficiently high frequency information to detect and analyse the contribution of each cylinder in driving the rotation of the shaft and, still further, that this information can be reliably extracted from the high levels of background noise typically arising not only in part from the alternator itself but also from sources such as ignition and injection systems and the like.

In some embodiments the envelope (or average envelope) oithe ac signal from the alternator may be detected to detect a variation in the rotational speed of the shall.

Additionally or alternatively however a higher frequency component of ac (diode) ripple from the alternator superimposed on this general envelope is detected to determine data relating to the rotational speed of the shaft and, more particularly, relating to the variation of this speed. This data may then be used to determine the contribution of the each cylinder to these rotation speed variations, and this allows a comparison to identify either a complete (momentary) loss in power such a mis-fire, typically an ignition problem, and/or a change in relative power where, say, one cylinder is low on power, typically the result of a fuel problem such as a blocked injector or poor compression. The system may therefore include an arrangement for comparing the contribution of one cylinder with an average contribution from the cylinder andlor one or more other cylinders.

In some preferred embodiments the system detenmnes a waveform indicating a variation in rotational speed of the shall caused by firing of the cylinders, but in other embodiments the contribution of a cylinder may be determined from the timing of the ae ripple superimposed on the envelope of the signals from the alternator without the need to convert to a value representing a speed, for example by comparing a period of the ripple (or an average thereof) when each different cylinder is firing.

The skilled person will appreciate that embodiments of the system are not restricted to any particular order of firing of the cylinders or to any particular number of cylinders or engine configuration.

In one preferred implementation of the system the input includes a connector plugging into a cigarette lighter socket of a vehicle (or other similar power socket), this being a patti cularly convenient form of interface.

Thus in another aspect the invention provides a vehicle engine diagnostic system for diagnosing a condition of an internal combustion engine of a vehicle whilst said engine is running, the system comprising: an input including a connector to plug into a dc power socket of the vehicle; and a computer system to process data captured from said input, and to output an indication of a condition of an individual cylinder of said internal combustion engine responsive to said captured data.

hi a related aspect the invention provides a method of diagnosing a condition of an internal combustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shaft coupled to an alternator, the method comprising: inputting a signal from said alternator; determining from said alternator signal data indicating for each said cylinder a contribution of the cylinder to said rotation of said shaft; and outputting an indication of said condition of said engine, responsive to said determining, dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.

As previously mentioned in some preferred embodiments the ac signal from the alternator is derived from a battery circuit of a battery providing electrical power to the eigine. Although, potentially, a user might he able to diagnose conditions of the engine simply by looking at a filtered version of the alternator signal, preferably the method further comprises performing a comparison to determine a condition of the engine, for example according to the techniques described above. Preferably the method therefore further comprises determining changes in the rotational speed of the shaft due to each separate cylinder-firing event.

In some particularly preferred embodiments of the method any selection of' one or more of the following steps is performed: capturing the input signal; detecting a ripple frequency, for example using a time-frequency transform; low-pass filtering the input signal; calculating a differential of the input signal; detecting troughs; identifying times of the troughs; filtering the times, for example to remove spurious events; calculating a differential or difference of or between the times; calculating a value dependent on a engine speed, more particularly a rotational speed of the shaft; low pass filtering the speed; detecting an engine condition andlor average or instantaneous cylinder contribution; diagnosing a mis-lire and/or cylinder contribution imbalance.

The invention further providcs processor control codc to implement the above-described methods, in particular on a data carrier such as a disk, CD-or DYD-ROM, programmed memory such as read-only memory (Firmware), or on a data carricr such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprisc source, object or executable code in a conventional programming language (interpreted or compiled) such as Java, C#, C++, C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Mark) or VHL)L (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.

In a ffirther aspect of the invention there is provided a vehicle engine diagnostic system for diagnosing a condition of an internal combustion engine whilst said engine is ninning, said internal combustion engine having a plurality of cylinders driving a rotating shall and including a battery having a battery circuit, the system comprising: an input to input a signal from said battery circuit; a system, coupled to said input, to determine from said signal data indicating for each said cylinder a contribution of die cylinder to said rotation of said shaft; and an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.

In a still further aspect of die invention there is provided a vehicle engine diagnostic system for diagnosing a condition of an internal combustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shaft, the system being con ligured for diagnosing said engine condition without interrogating an engine management unit of said vehicle, the system comprising: an input to receive a signal from a device responsive to rotation of said shaft; a system, coupled to said input, to determine from said signal data indicating for each said cylinder a contribution of the cylinder to said rotation of said shaft; anti an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.

In some embodiments of the system the device comprises an alternator coupled to the rotating shaft, as described above. However in other embodiments rather than employ the signal Ironi the alternator a separate sensor may be employed. Thus, for example, a coil or other magnetic pick tip device such as a Hall effect sensor may be employed to directly detect rotation of the alternator, crank shaft or some other component mechanically coupled to the crank shaft to provide an input signal for the system. In still further embodiments other sensor types such as an optical sensor or (where the car is on a rolling road) a sensor sensing rotation ofa wheel may be employed.

Without, for conciseness, repeating thc various preferred features and advantages of the embodiments of the other aspects of the invention described above the skilled person will appreciate that these features may be employed in any of the described aspects of the invention.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows an example of a vehicle alternator control device according to the prior art; Figures 2a and 2b show, respectively, three-phase voltage wavefornis Irom an alternator, and a rectified version of the three-phase waveforms of Figure 2a; Figures 3a and 3b show, respectively, a graph of voltage against time captured from a vehicle battery with the engine running, and an enlarged portion of the waveform of Figure 3a; Figure 4 shows a fast Fourier transform (fit) of the waveform of Figure 3a with frequency in hertz on the horizontal access, showing a pcak at between 1kHz and 3kHz; Figure 5 shows a filtered version of the waveform of Figure 3; Figure 6 shows a waveform illustrating variation of engine (crankshaft) speed over time; Figures 7a and 7b show, respectively, a low pass filtered version of the waveform of Figure 6, and the low pass filtered waveform over longer time scales; Figures 8a and 8b show, respectively, variations of engine speed (in rpm) over time (in seconds), and an enlargement thereof; Figure 9 illustrates identification of the turning points of the speed variation of Figure 8h; Figure 10 illustrates determination of the contribution of each cylinder event to the variation in engine speed; Figure 11 shows a screen shot of an example output from an embodiment of the system, illustrating balance between cylinders; Figures l2a and 12b show a flow diagram ola procedure for determining engine (crankshaft) speed variations from a battery circuit of a vehicle; Figures l3a and I 3b show a flow diagram of a procedure for determining rpm (revolutions per minute), power balance, and mis-fire count from the engine speed data from the procedure of Figure 12; and Figure 14 shows a block diagram of a system to implement the procedures of Figures 12 and 13 according to an embodiment of the invention.

Referring to Figure 1, this is taken from US 2002/050810, and shows an example of a vehicle alternator with control device. The figure is included to illustrate a typical configuration of an alternator fitted to the internal combustion engine of a vehicle. As shown, typically such an alternator has a three-phase winding 21 and employs a set of diodes 23 to convert ac from the alternator to de for charging the battery 3 Also shown in Figure 1 is a field coil 22 of the alternator, a power transistor 11 regulating the outp Lit voltage by controlling the supply from the main output of the alternator B to the field coil (diode 12 fly wheeling the field current when transistor 11 is off). The precise details of the power control shown in Figure 1 are not necessarily typical but this does not matter in the context of the system we describe.

We now describe, firstly steps involved in a preferred embodiment oVa procedure for filtering the battery voltage to obtain the alternator (i.e. engine or crankshaft) speed. We then describe steps in a preferred embodiment of a procedure for calculation of power balance from the speed.

IDJ2IJt. $inai The normal automotive generator is a three phase alternator with a full bridge rectifier having six rectifiers, two per winding. The output from the alternator windings is shown in Figure 2a.

This is rectified to produce a stable voltage with a supenniposed ac ripple (Figure 2b).

The ae ripple has six pulses per alternator rotation (one per diode activation). Since the alternator is driven by the crankshaft the frequency of this ripple is proportional to the speed of the engine which the alternator is connected to.

Figure 3a illustrates a capture of the battery voltage on a car with the engine running.

Zooming in on the signal (Figure 3b) gives a better view of the ac ripple. The sharp troughs of Figure 2b can be seen.

Detecting Ripple Frequency The ripple frequency JR is determined by performing an FFT on the signal (Figure 4). If detection oIthe frequency should fail a standard frequency, e.g. of 1500Hz may be used. Judging from Figure 3a one might imagine that the overall envelope of the signal could be employed but in practice this often fails and it is much better to rely on the ripple timing.

Low Pass Filtering The signal is low pass filtered to reduce high frequency noise caused by the injection and ignition systems. (Figure 5).

Find Times of troughs The absolute times of the sharp troughs are detected and saved. The difference between each time and the following is calculated. These differences are hereafter called differential times.

Filter Times The change in speed of the mechanical system of the engine and alternator is much slower than the envelope of the electrical ripple. We can therefore assume that the changes between two adjacent differential times are very small. This is used to filter out any times which have been introduced by drops in the voltage due to electrical devices connected to the battery and alternator being switched on.

Convert Times to Speed (Calculate the Reciprocal) After filtering the times we need to convert the differential times to speed according to the following formula.

foreach(diffT me[t]) rawspeed[t]=1/diffTime[t] } An example result shown in Figure 6.

Low Pass Filtering Speed The raw speed is low pass filtered to reduce noise.

Zooming out (Figure 116) shows the result of the low pass filtering. Note that the top graph is not the battery voltage; it is engine speed.

We next describe calculation of the power balance.

Bngine Speed The signal for engine speed comprises an offset with superimposed variations (Figure 8). The variations are caused by the compression and combustion phase of each cylinder. The scale is not necessarily correct, for example if the signal is calculated from the alternator ripple. We do however know the signal is proportional to the engine speed and the scale can be compensated from in a later state if necessary.

Finding Turning Points Following determination of the engine speed variations the first step is to find the turning points of the signal i.e. local maxima and minima (the dots in Figure 9). This is done with a hysteresis to prevent small changes to introduce false data.

Filter Turning Points The turning points arc filtered to make sure they are evenly spread. This is a very important step because of the serial nature of the data. If one turning point is missed or added, the cylinder separation will produce mixed up results.

Check Density of Points To ensure that no points have been missed or added preferably a density check is made on the data. This cheek parses through the points and makes sure they are evenly spread through out the time space.

Cylinder Contribution The contribution of each cylinder event is then calculated as the di flbrenee in speed between the current maximum and the previous minimum (Figure IC)).

foredchuaax in tpmi{iJ { cozu:r1bf Jttp[i.value-tps[i-IJ.value Calculation of RPM From the turning points data we can calculate the average rpm of the engine if we know the cylinder count. This is done with the Following formula: 2(cour,1 1) rpm ttY/CflI(IL I,) Where 1,; -= time of the first maximum = time of the last maximum count -total number of maxima cylCnt = cylinder count of engine, this has to be manually entered by the user Knowing the correct average rpm we can calculate average of the signal and adjust the scale accordingly. This is however not necessary for calculating the power balance.

Separation of cylinder events To separate the cylinder events we use the cylinder count. The contribution values are rearranged and stored in an array: fortoch ceyl cycle ( contri.b[cyl $ cyclej=ccntt.b [ (c'jc!ecy1Cnt; tcy ii I Averaging The final relative cylinder contribution' value for each cylinder is the average olall cylinder contributions for that cylinder: foreach (cyl) f avqcontrih [cyl]=. averaqe (contrib [cyl]) Zero Contribution To nonnalise the result (optional) we defme the level of zero-contribution for a cylinder.

An approximate value which appears to work with most engines can however be calculated as below: maxContri btttax avgContrib) zeracontrib=z-ra.axcontribllo Normalising Result With the averaged cylinder data and the zero-contribution value we can normalise the result with the highest cylinder at 1(100%).

norrnContrih (avqContrib-zezc.Contrib) / (maxContrib-zerocont rib Figure 11 shows an example screen shot of the output from an embodiment of the system. This shows a graph illustrating the relative contribution of each cylinder, preferably as a bar chart, from which it is immediately obvious when one cylinder is consistently producing less power than the others. This may indicate, for example, an airflow or fuel restriction or for compression. Preferably an indication of a mis-fire count for each cylinder is also provided, in the example display (underneath the bar for each cylinder).

Figure 12 shows a flow diagram of a procedure for implementation on a processor, for example in stored program code, to determine the alternator (engine) speed, as described above.

Figure 13 shows a flow diaam of a procedure to calculate power balance, mis-fires, and engine rpm, again as described above.

Figure 14 shows a block diagram of an engine with an alternator and battery, and a system according to an embodiment of the invention, comprising hardware and power balance software to diagnose a condition or fault with the engine, as described above.

The software may be implemented on a general purpose or dedicated computer system provided with an analogue-digital converter. In embodiments a 12-bit aid converter may be employed at a sample rate of lOOK samples per second or greater. The software and/or raw captured data and/or intermediate results and/or output may be stored in local memory, which may non-volatile, and/or on permanent storage such as a hard disk drive. The computer system preferably includes conventional input output means such as a keyboard or mouse or other pointing device and has a screen or printer and/or network interface to provide results data. In embodiments the software may be implemented on a PC oscilloscope or data logger, for example on any of the oscilloscopes available from Pico Technology Limited, UK.

Preferred embodiments of the above-described system detect and process the voltage from a vehicle alternator or generator captured from a battery circuit of a vehicle.

However, the skiUed person will appreciate that the same techniques may also be employed to signals captured in other ways, for example from signals captured from a sensing device present in the vehicle (or engine) or a separate sensing device.

No doubt many othcr cffcctivc altcrnativcs will occur to the skilled person. It will he understood that the invention is hot limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto

Claims

CLMMS: 1. An engine diagnostic system for diagnosing a condition of an

internal combustion engine whilst said engine is running, said internal combustion engine having a plurality olcylinders driving a rotating shaft coupled to an alternator, the diagnostic system coniprising: an input to receive a signal from said alternator; a system, coupled to said input, to determine from said alternator signal data indicating for each said cylinder a contribution of the cylinder to saul rotation of said shaft; and an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.
2. An engine diagnostic system as claimed in claim 1 wherein said internal combustion engine has a battery charged by said alternator, arid wherein said signal from said alternator comprises an ae signal superimposed on a dc signal from a circuit connected to said battery.
3. An engine diagnostic system as clainied iii claim 1 or 2 wherein said system to determine said data indicating said contribution of said cylinder comprises a system to determine an approximate envelope of said signal from said alternator.
4. An engine diagnostic system as claimed in claim 1, 2 or 3 wherein said system to determine said data indicating said contribution of said cylinder comprises a system to determine a timing of an ac ripple superimposed on an envelope of said signal from said alternator.
5. An engine diagnostic system as claimed in any preceding claim wherein said system to determine said data indicating said contribution of said cylinder comprises a system to determine waveform indicating a variation in rotational speed of said shaft caused by firing of said cylinders.
6. An engine diagnostic system as claimed in claim 5 further comprising a system to determine an average contribution to said shall rotational spccd for each said cylinder.
7. An engine diagnostic system in any one of claims I to 6 wherein said condition comprises a reduced contribution of one of said cylinders to said driving of said rotating shaft.
8. An engine diagnostic system as claimed in claim 7 wherein said condition comprises an misfire condition and wherein said comparison comprises a comparison of a single firing event of a said cylinder to said shaft rotation with an average contribution of one or more said cylinders.
9. An engine diagnostic system as claimed in claim 7 wherein said condition comprises a halancc condition and wherein said comparison comprises a comparison of an average contribution of a said cylinder with an avenge contribution of one or more others of said cylinders.
10. A vehicle engine diagnostic system comprising an engine diagnostic system as claimed in any preceding claim, and wherein said input includes a connector to plug into a cigarette lighter socket of th. e vehicle.
11. A method of diagnosing a condition of an internal combustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shaft coupled to an alternator, the method comprising: inputting a signal from said alternator; determining from said alternator signal data indicating for each said cylinder a contribution of the cylinder to said rotation of said shaft; and outputting an indication of said condition of said engine, responsive to said determining, dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.
12. A method as claimed in claim 12 wherein said internal combustion engine has a battery charged by said alternator, and wherein said signal from said alternator comprises an ac signal superimposed on a dc signal from a circuit connected to said battery.
13. A method as claimed iii claim 11 or 12 further comprising performing said comparison.
14. A method as claimed in claim 11, 12 or 13 further comprising determining changes in rotational speed of said shaft due to each said cylinder firing, and comparing one or more of said changes in rotational speed to determine said engine condition.
15. A carrier carrying processor control code to, when running, implement the method of any one of claims 11 to 14.
16. A vehicle engine diagnostic system for diagnosing a condition of an internal combustion engine ola vehicle whilst said engine is running, the system comprising: an input including a connector to plug into a dc power socket of the vehicle; and a computer system to process data captured from said input, and to output an indication of a condition of an individual cylinder of said internal combustion engine responsive to said captured data.
17. A vehicle engine diagnostic system for diagnosing a condition of an internal combustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shaft and including a battery having a battery circuit, the system comprising: an input to input a signal from said battery circuit; a system, coupled to said input, to determine from said signal data indicating for each said cylinder a contribution oithe cylinder to said rotation of said shaft; and an output to provide an indication of said condition of said engine dependent upon a comparison of said contribution of each said cylinder to said shaft rotation.
18. A vehicle engine diagnostic system for diagnosing a condition of an internal conThustion engine whilst said engine is running, said internal combustion engine having a plurality of cylinders driving a rotating shalt, the system being configured for diagnosing said engine condition without intenogating ai engine management unit of said vehicle, the system comprising: an input to receive a signal from a device responsive to rotation of said shaft; a system, coupled to said input, to detennine from said signal data indicating for each said cylinder a contribution of the cylinder to said rotation of said shaft; and an output to provide an indication of said condition of said engine depcndent upon a comparison of said contribution of each said cylinder to said shaft rotation.
19. A vehicle engine diagnostic system as claimed in claim 18 including said device, wherein said device comprises a sensor.
20. A vehicle engine diagnostic system as claimed in claim 18 wherein said device comprises an alternator or generator coupled to said shaft.