EP0387254A1

EP0387254A1 - Engine misfire detection and engine exhaust systems

Info

Publication number: EP0387254A1
Application number: EP19880907670
Authority: EP
Inventors: Hans Heim; Hans Klein; Manfred Homeyer
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1988-09-10
Filing date: 1988-09-10
Publication date: 1990-09-19
Also published as: JP2885813B2; JPH03501148A; KR900702200A; WO1990002874A1

Abstract

Les ratés d'allumage sont détectés dans un moteur à combustion interne par contrôle de la tension (ulambda) d'un détecteur lambda disposé dans le système d'échappement en amont d'un catalyseur. Cette tension est soit comparée avec la tension d'un autre détecteur lambda en aval du catalyseur soit traitée, dans le cas d'une commande lambda pour permettre la détection de creux dans la tension (ulambda) du détecteur. La relation de phase entre les fluctuations de la tension du détecteur et le réglage du moteur identifient le moment où les gaz d'échappement provenant d'un cylindre présentant des ratés d'allumage atteignent le détecteur lambda. La durée prévue de parcours des gaz en relation avec TDC étant connue, le cylindre ayant des ratés d'allumage est identifié et l'injection de carburant dans le cylindre ayant des ratés d'allumage peut être arrêtée, ce qui permet d'empêcher le mélange air/carburant non brûlé d'atteindre le catalyseur où il viendrait brûler et surchaufferait le catalyseur.The misfires are detected in an internal combustion engine by checking the voltage (ulambda) of a lambda detector placed in the exhaust system upstream of a catalyst. This voltage is either compared with the voltage of another lambda detector downstream of the catalyst or treated, in the case of a lambda command to allow the detection of dips in the voltage (ulambda) of the detector. The phase relationship between the detector voltage fluctuations and the engine setting identifies when exhaust gases from a misfire cylinder reach the lambda detector. The expected duration of the gas journey in relation to TDC being known, the cylinder having misfires is identified and the injection of fuel into the cylinder having misfires can be stopped, which makes it possible to prevent the unburnt air / fuel mixture to reach the catalyst where it would burn and overheat the catalyst.

Description

DESCRIPTION

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 which 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 misfire 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 simultaneous 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 u₁ 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 u₂ of the downstream sensor 20 is substantially constant. Assuming the fuel/air mixture fed to the engine is in stoichiometric relationship or slightly lean, the average of the slightly rippling voltage u₁ is about the same as the voltage u₂.

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 u₁. On the other hand, the fuel and oxygen are combusted in th e c a tal ys e r 16 s o th a t th e ou tpu t voltage u₂ of the downstream sensor 20 does not fall and it may even rise slightly. To detect the occurrence of the sudden change in u₂-u₁, the outputs of the sensors are connected to a comparator 22 which delivers an output signal S when the voltage u₂-u₁ exceeds a predetermined value Δu.

To reduce the likelihood of the fluctuations in the voltage u₁ 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 inj e ction v alve clo sed . One way o f dete ctin g 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 of 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

unconsumed oxygen in the event of a misfire and the consequent lowering of the output voltage u₁ cause the lambda control to regulate the engine to a

leaner mixture so that the undulating output voltage u ₂ 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 diagrammatically) 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 individual 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 ignition coil 46 in accordance with operating parameters to provide the optimum injected fuel quantities, injection timing and ignition timing in conventional manner. The operating parameters include a reference pulse BM₁ inlet pressure p (vacuum), load L (pedal position), engine speed n, the air number λ, 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 λ 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 λ>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 T₁, 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 λ=1, as shown in Fig. 4 by curve C. However, bec ause of th e now over - ric h mi xture an d the unc o nver ted 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_λ of the lambda sensor is fed to a window discriminator to ascertain whether the voltage u_λ both rises above an upper threshold u_max and falls below a lower threshold w_min 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.

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 u_λ voltage first falls below the upper threshold u_max and then below the lower

threshold u_min within 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

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 ampl i f i er 36 .

S p ur i o us 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 cylinder, it is checked whether within the respective window of 180° crankshaft angle u_λ < u_max and u_λ < u_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 is counted out, the alarm signal s is delivered.

However, 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 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 n 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 crankshaft 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 u_λ of 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 T₁, the sensor voltage u_λ falls below u_max = 110mV and then below u_min = 50mV to step the fault counter on 1. Over the following four working cycles (2x4=8 revolutions), the sensor voltage u_λ does not rise above u_max 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 u_λ rises to significant values between the troughs and the fault counter is rapidly counted out to trigger the alarm at T₂. 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_λ would not follow the course shown after T₂.

Another possibility is for the amplitude of the fluctuations in the output voltage u_λ 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_λ 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_λ 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. 9.

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

1. Method of detecting misfire in a multicylinder 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 misfired, taking into account the expected time for the exhaust gases to travel from the engine exhaust valve to the lambda sensor.

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 o f the c a tal yser wi th a re ferenc e va lue no t no rmal ly 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

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 voltage of the lambda sensor within a working cycle of the engine.

8. Apparatus according to claim 7, ch arac terised in that the trou ghs in the o utput v oltage (u_λ) of the lambda probe are detected to detect a misfire.

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 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,

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_λ) fluctuates beyond an upper

threshold (u_max) and a lower threshold (u_min) or

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 misfiring cylinder to be identified.

15. Apparatus according to any of claims 7 to 14, characterised in that the expected gas travel time as measured between 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.