ENGINE MISFIRE DETECTION AND ENGINE EXHAUST SYSTEMS

Abstract: Misfire is detected in an internal combustion engine by monitoring the voltage (u lambda ) of a lambda sensor disposed in the exhaust system upstream of a catalyser. This voltage is either compared with the voltge of another lambda sensor downstream of the catalyser or is processed, in the case of lambda control, to detect troughs in the sensor voltage (u lambda ). The phase relationship between the sensor voltage fluctuations and the engine timing identifies when the gases exhausted from a misfiring cylinder reach the lambda sensor. Knowing the expected gas travel time in relation to TDC, the misfiring cylinder is identified and the fuel injection to the misfiring cylinder can be terminated. This prevents unburnt fuel/air mixture from reaching the catalyser where it would combust and overheat the catalyser.

Description: DE5CRIPTI0N ENGINE MISFIRE DETECTION AND ENGINE EXHAUST SYSTEMS The present invention relates to a method of detecting misfire in a multi-cylinder internal combustion engine and to an apparatus for protecting an exhaust system of a multi-cylinder internal combustion engine, which exhaust system includes a catalyser. It is known that exhaust gas catalysers and even the exhaust manifolds of internal combustion engines can be damaged or destroyed due to overheating. The danger of overheating arises in the event of a misfire, that is to say, the fuel/air mixture in one or more of the combustion chambers fails to ignite through faulty ignition. The uncombusted mixture when it reaches the catalyser whose working temperature is around 550°C is immediately ignited, rapidly heating the catalyser to a temperature at which the catalyst is destroyed. There is even the danger of the motor vehicle in whi h the engine is installed being set alight. It is an object of the invention to provide a method and apparatus whereby faulty combustion or misfire can be immediately identified so that steps can be taken to prevent consequential overheating of the exhaust system, particularly of the catalyser. Advantages of the Invention These disadvantages are avoided by the method claimed in claim 1 and the apparatus claimed in claim 5. Not only is a isfire promptly detected through the oxygen content of the uncombusted gases leaving the engine, but measures can be taken to enable the faultily operating cylinder or cylinder group to be identified. In the case of petrol injection individual to the cylinder or the cylinder groups, the feeding of further fuel to the misfiring combustion chamber can be prevented. Further advantageous features of the invention are mentioned in the dependant claims. Drawings The invention is further described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a diagrammatic representation of a four stroke internal combustion engine whose exhaust system contains a catalyser, fitted with lambda sensors in accordance with one embodiment of the invention; Fig. 2 is a graph showing the output voltages of the lambda sensors; Fig. 3 is a schematic circuit diagram of an electronic fuel injection system operable with lambda control in accordance with another embodiment of the invention; Fig. 4 is a series of graphs relating to the embodiment of Fig. 3; Fig. 5 is a logic diagram relating to one mode of operation of the embodiment of Fig. 3; Fig. 6 is a series of graphs relating to the mode of operation of Fig. 5; Fig. 7 is a logic diagram relating to an alternative mode of operation; Fig. 8 is a graph showing the use of window discriminators; and Fig. 9 is a diagram explaining the expected gas travel time. Description of the Preferred Embodiment Referring first to Fig. 1, a four-stroke petrol engine 10 for a vehicle has an exhaust system 12 containing at least one silencer 14 and an exhaust gas catalyser 16. The catalyser 16 is so disposed in the exhaust system that it runs at about 550°C, this being the optimum temperature for catalysing the reduction of nitrogen oxide and the si ultaneous oxidation of carbon monoxide and hydrocarbons, using up any residual oxygen in the exhaust gases. Should there be a misfire in one of the engine cylinders, the uncombusted gases exhausted from that cylinder are caused to ignite in the catalyser. This causes the catalyser to heat up rapidly to a temperature at which the expensive catalyst is destroyed and there is a danger * of the vehicle itself being set on fire, unless appropriate counter measures are taken. According to the embodiment of Fig. 1, lambda sensors 18 and 20 are fitted in the exhaust system 12 upstream and downstream of the catalyser 16. The lambda sensors comprise a solid electrolyte and, as is well known, they deliver an output voltage which is dependant upon the residual oxygen content of the exhaust gases. The sensor voltages are shown very diagrammatically in Fig. 2. The output voltage u, of the upstream sensor 18 fluctuates slightly in synchronism with the exhaust strokes of the engine cylinder. On the other hand, residual oxygen is consumed in the catalyser 16 so that the output voltage u_ of the downstream sensor 20 is substantially constant. Assuming the fuel/air mixture fed to the engine is in stoichio etric relationship or slightly lean, the average of the slightly rippling voltage u, is about the same as the voltage u„ . In Fig. 2, it is assumed that one cylinder commences to misfire so that uncombusted fuel and air reach the upstream sensor 18 and the oxygen in the air causes a sudden drop in the voltage u, . On the other hand, the fuel and oxygen are combusted in the catalyser 16 so that the output voltage u„ of the downstream sensor 20 does not fall and it may even rise slightly . To detect the occurrence of the sudden change in u„-u, , the outputs of the sensors are connected to a comparator 22 which delivers an output signal S when the voltage u„-u, exceeds a predetermined value .Δu. To reduce the likelihood of the fluctuations in the voltage u, triggering a false alarm, a smoothing circuit 24 can be arranged between the sensor 18 and the comparator 22. The alarm signal s can be used to trigger an audible and/or visible alarm or to stop or restrict the fuel supply to the engine. If the misfiring cylinder can be identified, in the case of an internal combustion engine with petrol injection, the fuel supply to the faulty cylinder can be cut off by holding the respective injection valve closed. One way of detecting the faulty cylinder is to provide, instead of a single upstream sensor 18, several separate sensors in respective branch tracts of the exhaust system leading from the engine exhaust valves. The output of voltage o each such sensor is then compared with that of the downstream sensor. In the case of an engine fitted with lambda control, in which the injected fuel quantity is adjusted in response to the air number Λ of the exhaust gases as measured by the upstream sensor 18, the unconsu ed oxygen in the event of a misfire and the consequent lowering of the output voltage u, cause the lambda control to regulate the engine to a leaner mixture so that the undulating output voltage u - then climbs back substantially to its previous value, as shown, in the event that the engine is not immediately stopped. Fig, 3 shows an electronic fuel injection system for a four-stroke, four cylinder internal combustion engine having electronically controlled fuel injection and ignition systems. The four injection valves 26,28,30,32 (shown diagram atically ) are opened and closed in timed relation to the crankshaft rotation under the control of a computer 34 and via end stage amplifiers 36. The injection valves 26-32 are indi idual to the -*- * four cylinders of the engine in that they inject the fuel into the engine inlet manifold branches adjacent the respective engine inlet valves. The computer 34 also controls the four spark plugs 38,40,42 and 44 (shown diagrammatically) via an ignition coil and ° distributor 46. The computer 34 controls the injection valves 26-32 and the ignition trigger voltages suppled to the igni ion coil 46 in accordance i h operating parameters to provide the optimum injected fuel quantities, 5 injection timing and ignition timing in conventional manner. The operating parameters include a reference pulse BM, inlet pressure p (vacuum), load L (pedal position), engine speed n, the air number X, as measured by a lambda sensor in the engine exhaust system, and engine cooling water temperature T. The reference pulses BM are obtained in timed relation to rotation of the crankshaft and are for synchronisation purposes . During normal running, the engine operates witn lambda control and the lambda sensor (like the sensor 18 of Fig. 1) is arranged upstream of a catalyser in the exhaust gas system of the engine. Lambda control is control in accordance with the composition of the exhaust gases, and in particular, upon the oxygen content of the exhaust gases, whereby the fuel/air ratio is that which provides for optimum combustion, i.e., close to stoichiometric and minimum pollutants in the exhaust gases reaching the catalyser . Fig. 4 shows at A the output of the lambda sensor and at B, the instants of ignition of the individual cylinders so that the timed relationship between the changes in the measured air number X and the individual combustion operations can be seen. In normal operation as seen at the left of Fig. 4, the air number λ fluctuates between two extremes in accordance with on-off control by the servo loop forming part of the electronic control. In other words, a high air number X>1 denotes a lean mixture whereby the fuel injection system operates to increase the injected fuel quantity until the air number λ<1 whereupon the injected fuel quantity is decreased. Each on-off fluctuation lasts for several, e.g., six, revolutions of the crankshaft. At the instant T, , one cylinder starts to misfire and in Fig. 4 continues to do so at every working stroke. The result is that the lambda sensor or oxygen sensor erroneously measures that the mixture is, on average, too lean so that the computer 34 . operates to increase the injected fuel quantity until the lambda sensor again indicates an average air number X=l, as shown in Fig. 4 by curve C. However, because of the now over-rich i ture and the unconverted oxygen from the misfiring cylinder, the output of the lambda sensor exhibits a sudden dip once for every two revolutions of the crankshaft as shown at the right—hand side of Fig. 4. To detect this situation, the output of the lambda probe is evaluated in a misfire detector 48 as will now be described. In the simplest version, the output voltage U of the lambda sensor is fed to a window discriminator to ascertain whether the voltage u. both rises above an upp f^ er threshold umax and falls below a lower threshold wmi.n within 720° of crankshaft rotation. If it does ', a pulse is sent to a misfire counter, whose count is shown at curve D in Fig. 4. When the counter has counted, say eight misfires an alarm signal s is delivered. 5 If it is desired to identify the misfiring cylinder the window discriminator extends over 180° crankshaft angle, once for each cylinder, the cylinders being identified by the ignition trigger pulses and it is checked whether the sensor voltage first falls below the 10 up r p r er threshold umax and then below the lower threshold umm. w'ithin each 180° window. If it does,' a counting pulse is delivered to a respective counter and when the counter is counted out, a fault signal s is delivered to indicate an alarm and to which 15 cylinder it relates. The fault signal s can be used in this case to inhibit the fuel injection signal to the respective injection valve by interrupting the corresponding output from the computer 34 of the end stage amplifier 36. 20 .Spurious misfires are not harmful to the catalyser and it is desirable to prevent the misfire detector 48 from responding to misfiring at spaced random intervals. For this purpose, a storage time counter is also used, as shown in Fig. 5. For each 25 cylinder, it is checked whether within the respective window of 180° crankshaft angle u*, < u and u * . < u . . A max Λ min If it does, a stepping pulse is applied to the fault counter. If it does not, a stepping pulse is applied to the storage time counter. Once the fault counter -- 5 0 is counted out, the alarm signal s is delivered. Ho ever, should the storage time counter be counted out before the fault counter has been counted out, both counters are re-set. In other words if fewer than the number of misfires needed to count out the 35 fault counter take place within the count-out period of the storage time counter, no fault is indicated. The total count of each of the two counters can be adjusted to suit the operating conditions of the engine. Thus, for each counter, an empirically determined characteristic can be stored in the computer 34 to determine in accordance with, say • engine speed π and engine load L, the number of misfires to be counted in the fault counter to trigger the alarm signal s and the number of crank¬ shaft revolutions without misfire to be counted in the storage time counter before detected misfires are to be disregarded. Fig. 6 illustrates this operation further. Curve A is the output voltage f the lambda sensor. It can be seen that the output is not as regular as Fig. 4(A) would suggest but the overall pattern of Fig. 4(A) is perceptible in Fig. 6(A). Curve B in Fig. 6 represents synchronising pulses obtained from the ignition system. Curve C represents the count of the fault counter. It can be seen that, following the start of misfiring at T, , the sensor voltage Uv falls below umax = HOmV and then below umi.n = 50mV to step the fault counter on 1. Over the following four working cycles (2x4=8 revolutions), the sensor voltage does not rise ab•ove umax so that it cannot fall through the upper threshold to step the fault counter further and the latter is then re-set by the storage time counter timing out (counting out). The fault counter is then counted to 2 before being re-set However, the effect of the lambda control then becomes apparent and the sensor voltage rises to significant values between the troughs and the fault counter is rapidly counted out to trigger the alarm at T„ . Curve D shows that the catalyser temperature only rises a few tens of degrees before the alarm is triggered. Fig. 6 assumes that no counter-measures are taken after the alarm is triggered or otherwise the sensor voltage u. would not follow the course shown after T„. Another possibility is for the amplitude of the fluctuations in the output voltage u. of the lambda sensor to be measured in window discriminators within a corresponding 180° crankshaft angle for each cylinder. Should the difference between the highest and lowest values of the sensor voltage U \ exceed a value predetermined in accordance with engine speed and/or load, a misfire signal is applied to a fault counter as described above. Also, as described above, the fault counter can be re-set in the absence of any further misfire signals within a predetermined, possible speed and/or load dependant interval. This is shown further in Fig. 7. Since it takes time for the gases exhausted from a misfiring cylinder to reach the lambda sensor, the gas travel time must be taken into account in order to identify a misfiring cylinder by reference to the sensor output voltage and the ignition trigger pulses. Fig. 8 shows how the uncombusted exhaust gases from a misfiring cylinder can be identified by the lambda sensor. For a four cylinder, four stroke engine, the working cycle of 720° crankshaft angle is divided into four windows of 180° each. The window in which the falling flank of the sensor voltage u. crosses the upper and lower threshold corresponds to the uncombusted gases. Therefore, to ascertain the cylinder from which these gases came, it is necessary to delay the ignition pulses by the time taken for the working stroke of a cylinder to take place and the time taken for the gases to travel through the exhaust tract as far as the lambda sensor. Fig. 9 shows the gas travel time measured from the TDC of the misfiring cylinder. It is typically 560° crankshaft angle but does vary according to engine operating parameters. Thus, the signals identifying the cylinders must be delayed by this gas travel time before correlating them with the misfire signal obtained from the lambda probe. Whilst mention is made above of using the ignition trigger signals as cylinder identification signals, inasmuch as the ignition trigger signal is close to TDC, this is not essential. It is possible to obtain cylinder identification signals in other ways, e.g., by dividing the interval between successive reference marks BM by the number of cylinders as shown in Fig. . The expected gas travel time can be derived from an empirically determined characteristic field which is stored in the computer and which gives the expected gas travel time in dependence on engine operating characteristics, such as engine speed n and/or engine load L.

Claims: CLAIMS 1. Method of detecting misfire in a multi- cylinder internal combustion engine, characterised in that the output voltage of a lambda sensor in the exhaust system is monitored and is compared with a reference voltage, and a departure of the difference between the sensor and reference voltages from an expected value is signalled as a misfire in at least one of the cylinders. 2. Method according to claim 1 for an internal combustion engine whose exhaust system contains a catalyser, characterised in that the outputs of two lambda sensors arranged respectively upstream and downstream of the catalyser are compared and an increase in the difference between the output voltages of the two lambda sensors is taken as signalling a misfire in at least one cylinder. 3. Method according to claim 1, for an internal combustion engine equipped with a lambda control system, characterised in that fluctuations in the output voltage of the lambda sensor of the lambda control system are monitored and, if these fluctuations exceed a reference amplitude or cross a reference threshold, this is taken as signalling a misfire in at least one cylinder. 4. Method according to claim 3, characterised in that the instant of a fluctuation in the output voltage of the lambda sensor outside a threshold is compared with the instant of a reference mark (BM) indicative of a predetermined angular position of the crankshaft and the time difference between these instants is used to determine which cylinder has mis¬ fired, taking into account the expected time for the exhaust gases to travel from the engine exhaust valve to the lambda sensor. -12- 5. Apparatus for protecting an exhaust system of a multi-cylinder internal combustion engine, which exhaust system includes a catalyser, characterised in that a comparing device is provided for comparing the output voltage of a lambda sensor disposed upstream of the catalyser with a reference value not normally reached and an output signal of the comparing device, indicative of a misfire is applied to means whereby uncombusted exhaust gases are prevented from reaching the catalyser. 6. Apparatus according to claim 5, characterised in that a second lambda sensor is disposed downstream of the catalyser and the difference between the output voltages of the lambda sensors is compared in a 5 comparator with a reference threshold. 7. Apparatus according to claim 5, for an internal combustion engine equipped with a lambda control system, characterised in that a device is provided for monitoring fluctuations in the output 0 voltage of the lambda sensor within a working cycle of the engine. 8. Apparatus according to claim 7, characterised in that the troughs in the output voltage ) of the lambda probe are detected to detect a misfire. 5 9. Apparatus according to claim 8, characterised in that the misfires are counted in a fault counter and an alarm signal (s) is provided when the fault counter is counted out. 10. Apparatus according to claim 9, characterised 30 in that a storage time counter is provided and counts the time elapsed between detected misfires and both counters are re-set in the event of the storage time counter being counted out. 11. Apparatus according to claim 9 or 10, 35 characterised in that the count-out time of either or both of the counters can be adjusted in dependence upon operating parameters of the engine. 12. Apparatus according to claim 11, characterised in that either or both counters are adjusted in accordance with a stored characteristic field. 13. Apparatus according to any of claims 8 to 12, characterised in that the troughs of the output voltage are detected by ascertaining whether the output voltage (u» ) fluctuates beyond an upper threshold (u ) and a lower threshold (u . ) or max m n fluctuates outside a predetermined amplitude at least within a working cycle of the engine. 14. Apparatus according to claim 13, characterised in that the fluctuations are measured within a window defined by the crankshaft angle for a working cycle divided by the number of cylinders to enable a is- firing cylinder to be identified. 15. Apparatus according to any of claims 7 to 14, characterised in that the expected gas travel time as measured bet-ween the TDC or ignition trigger pulses of a given cylinder and the instant at which the gases exhausted from the cylinder reach the lambda sensor is taken into account to identify a misfiring cylinder. 16. Apparatus according to claim 15, characterised in that a characteristic field is stored in the computer and gives the expected gas travel time in dependence upon at least one engine operating parameter.

Inventors: HEIM HANS, HOMEYER MANFRED, KLEIN HANS, HEIM, HANS, HOMEYER, MANFRED, KLEIN, HANS
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: F02D41
IPC codes: F02D41, F02D45, F02P17