GB2346457A

GB2346457A - Air/fuel ratio control

Info

Publication number: GB2346457A
Application number: GB0002343A
Authority: GB
Inventors: Ranendranath Rudra
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-02-02
Filing date: 2000-02-02
Publication date: 2000-08-09
Anticipated expiration: 2020-02-02
Also published as: GB9902308D0; GB2346457B; GB0002343D0

Abstract

A method of controlling the air/fuel ratio supplied to a combustion chamber having a exhaust gas sensor, selects a desired air/fuel ratio 31, selects a suitable air/fuel ratio variation function waveform 32 which is applied to the combustion chamber, measures the time that the sensor output is in a LEAN state and a RICH state, determines the ratio of these times, and uses the ratio to adjust the applied function waveform. Several different air/fuel ratio functions may be defined and the adjustment may comprise selecting an appropriate function. The sensor may be an oxygen sensor.

Description

Enaine Management Svstem The present invention relates to a method for controlling the air/fuel ratio (AFR) supplied to a combustion chamber. It also relates to a device for performing that method and to an internal combustion engine fitted with such a device.

The present invention will be described with reference to its primary application, but the method of controlling the air/fuel ratio could also be applied to other combustion processes, for example a boiter or furnace.

For a number of years, engine management systems have been used to control the fuelling of automotive engines. As the regulation of exhaust emissions has become more critical, closed loop feedback control of the fuelling of combustion processes is normal. In order to do this an exhaust sensor is used to provide feedback information of the actua AFR at which the engine is running. Since the exhaust gas composition changes with operating AFR, being able to sense some of the exhaust gases gives an indication of the operating AFR. This allows fuelling to be corrected in response to production variability, wear, ageing etc.

It is possible to use a sensor that provides a linear response to the AFR, but these sensors are prohibitively expensive. Instead it is normal to use a sensor that is extremely non-liner in its response to the point that it really only indicates whether the exhaust gases are RICH (excess fuel) or LEAN (excess air). What is more, these devices may suffer from hysteresis, such that the point at which they change state (the switch-point) when going from RICH to LEAN is may not be the same as the switch-point when going from LEAN to RICH.

In most circumstances, and for most purposes (especially to minimise emissions), it is desirable to run the engine at a specific AFR known as stoichiometric, which is when there is just enough air to burn all the fuel completely. For such simple purposes the existing technology will suffice, but is far from ideal. As the sensor is non-linear, it cannot be used to provide a simple error signal in a closed loop controller.

The mode of operation of conventional systems is as follows. The nominal fuelling requirement is calculated to achieve stoichiometric AFR and this AFR is delivered to the engine (fuelling AFR). The fuelling is controlled to increase or decrease the AFR dependant on whether the exhaust sensor indicates RICH or LEAN. This increase or decrease is continued until the exhaust sensor switches state, after which time the change in fuelling AFR is reversed. This new direction of change continues until the exhaust sensor switches to the other state. For example, if the exhaust sensor indicates LEAN, the fuelling would be changed to cause the fuelling AFR to get richer (ie more fuel) until the exhaust sensor switches to RICH. Then the fuelling would be changed to cause the fuelling AFR to get leaner (i. e. less fuel) until the exhaust sensor switches to LEAN. In this way the AFR is generally maintained between the two switchover points of the exhaust sensor.

This system is the basis of most automotive closed loop fuelling controllers. However, there are major limitations to this system. The amount by which the AFR goes richer or leaner beyond the exhaust sensor's switching point depends on the length of time it takes the sensor to respond as well as the hysteresis of the switch. This makes critical the distance of the sensor from the point of combustion (usually a cylinder). If it is closer then the time delay between changing the fuelling of the engine and the resultant exhaust gases passing the sensor is shorter. Unfortunately, the sensor cannot be positioned too close to the point of combustion else it will not operate at all, or its accuracy degrades too quickly and it must be replace.

What is more, even when placed sufficiently far from the point of combustion, as the sensor ages or becomes contaminated, its rate of response slows. This means that the time delay between the fuelling and the feedback of data concerning the resultant exhaust gases gets even longer.

This has the knock on effect of making the AFR go far too rich or lean (as appropriate) before each switch. Effectively, the engine starts to run at AFRs fluctuating between those that are far too rich and far too lean. This not only results in a higher level of emissions, but in poor efficiency and damage.

There are various techniques for improving the situation but they are still limite, and usually consist of altering the fuelling ratio by a step function on each switch. This is designed in a small degree to counteract the effect of the time delay.

It is an aim of the present invention to overcome the above mentioned problems associated with the existing technology, and provide a more efficient and flexible method of managing the engine whilst using existing sensor technology.

Therefore according to the present invention there is provided a method of controlling the air/fuel ratio supplied to an internal combustion engine having a exhaust gas sensor, comprising the steps of : defining a sensor LEAN state, a sensor RICH state, and a periodically-varying air/fuel ratio function by which the ratio of air to fuel supplied to the engine is controlled ; controlling the air/fuel ratio supplied to the engine in accordance with the air/fuel ratio function monitoring the sensor output and determining the periods that the instantaneous periodically-varying air/fuel ratio causes the sensor to determine the engine is running in the LEAN state or in the RlCH state; and, adjusting the air/fuel ratio function to bring the ratio of the RICH state time to the LEAN state time to a desired value.

For convenience, the ratio of the RICH state time to the LEAN state time will be referred to as the LEAN: RICH ratio, and the periodically-varying air/fuel ratio function will be termed the AFR function.

It is preferred that several different AFR functions are defined, and said adjustment of the AFR function comprises selecting an appropriate function dependent upon the relationship between the actual LEAN: RICH ratio and the desired LEAN: RICH ratio.

As mentioned above it is often desirable to operate the engine at a stoichiometric air/fuel ratio, but it is not always appropriate or beneficial to do so. For example if might be preferred, to improve performance or to protect a catalyst, to run the engine in a generally richer or leaner state. Often the desirability of operating at these various states is only short lived, and is in response to the conditions in which the engine is running. Preferably, therefore, a desired average operating condition is selected, and this is used to select a desired LEAN: RICH ratio that should result from this. External parameters are preferably used to determine the desired average operating conditions. These external parameter could include engine temperature, external temperature, desired engine performance (e. g. efficiency or power), catalyst performance and drivability.

The exhaust gas sensor will advantageously measure the amount of oxygen in the exhaust gases.

According to the present invention there is also provided a device for controlling the air/fuel ratio supplied to a combustion chamber having a exhaust gas sensor, by a method which device comprises: an input adapted to receive data from the exhaust gas sensor; a processor which defines at least one periodically-varying air/fuel ratio function and controls the fuelling of the engine in accordance with that function, the processor further receiving data from the sensor, and adjusting the air/fuel ratio function dependant upon the received data.

It is desirable that at least two inputs are provided and at least one of these inputs is adapted to receive data from the exhaust gas sensor and the remaining inputs communicate with other sensors.

It is yet more desirable that the remaining inputs are adapted to receive information concerning external engine parameters.

According to present invention there is yet still further provided an internal combustion engine fitted with a device for controlling the air/fuel ratio supplied thereto as previously described.

The attributes of the periodically varying AFR function are controlled by the engine management system. As long as the fuelling AFR crosses a sufficient range, the exhaust sensor will switch in a periodic fashion from RICH state to LEAN state. The LEAN: RICH ratio will give an indication of the fuelling AFR with respect to the switching AFR of the sensor. In practice the switching AFR may be different dependant on the direction of change, but for convenience the switching AFR is assumed to be that of a perfect sensor or the average of the different switch-points. If the AFR function is symmetrical about the switching AFR of the exhaust sensor, the LEAN: RICH ratio will be 1 (unity). Thus if the engine management system were varying the fuelling AFR with such an AFR function, a LEAN: RICH ratio of 1 would indicate that the average fuelling AFR was that corresponding to the switchpoint of the exhaust sensor. On the other hand, if the LEAN: RICH ratio was not 1, it would indicate that the average fuelling AFR was rich or lean with respect to the sensor's switch-point. The value of the LEAN: RICH ratio indicates not only whether rich or lean but for a given AFR, by how much.

Knowing how rich or lean the average fuelling AFR is with respect to the sensor's switch-point has various advantages. These include being able more effectively to control the fuelling to achieve the desired operating AFR, as well as the fact that it becomes possible to operate at an average fuelling AFR other than the actual switching AFR of the sensor whilst still maintaining closed loop feedback control. There are many circumstances where this is desirable.

The relationship between the LEAN: RICH ratio and the average fuelling AFR depends on the characteristics of the AFR function by which the fuelling AFR is varied. The AFR function can be defined to suit the operating conditions and may be changed by the engine management system. For example engine temperature, power requirements, air temperature, control of emission levels, fuel economy and drivability can all make it desirable to operate at LEAN: RICH ratios other than 1.

Methods according to the present invention are not affected by a slow sensor response, as all this does is to shift the output signal in time but not alter the ratio. For the same reason, it is not as sensitive as prior art systems to the time delay caused by the distance from combustion chamber to the sensor, which in consequence makes the siting of the sensor less critical.

The present invention also allows improved fuelling AFR control in response to failed combustion. In a prior art system, if combustion fails the sensor will indicate strongly LEAN even if the fuelling AFR was correct, because the sensor is detecting the oxygen in the exhaust. This caused the fuelling AFR to be made richer and richer (usually up to some limit). By contrast, in the present invention, this can be limited or prevented because the output state of the exhaust sensor does not immediately control the fuelling AFR. If a misfire occurs it would result only in a relatively small change in the average LEAN: RICH ratio derived from the sensor's readings, which should not effect the instantaneous control of the running of the combustion process. However, the misfire would cause a change to the output from the exhaust sensor that could be used by the engine management system, perhaps in a special diagnostic mode, to detect the occurrence of that misfire.

The exhaust sensor signal could be analysed to detect components not associated with the periodic AFR function and that this comparison could be used to infer misfires.

If the invention were applied to an internal combustion engine with multiple cylinders there would not normally be one exhaust sensor per cylinder. Typically on a 4 cylinder engine, the one sensor would be located where the four individual exhausts join into one. On an engine with a V arrangement of two banks of cylinders, there would preferably be one sensor per bank of cylinders, and so a total of two.

It may be desirable to provide some form of diagnostic function, wherein the fuelling AFR of one cylinder is varied differently with respect to the others. The LEAN: RICH ratio, as well as giving an average AFR, could itself be averaged to give a more general AFR picture. If one cylinder is running differently from the others, this should show up in an average taken over a longer period of time. As the diagnostic function would know which cylinder was altered, and by how much, this could give an indication of how that cylinder is performing with respect to the others. This diagnostic process could be repeated for each cylinder. It is likely that the feedback controlling the AFR function could be inhibited while this diagnostic was being performed. Therefore it would be best to carry out such a diagnostic test when the engine has settled into a relatively stable operating condition, say once a journey has commenced.

By way of example only the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a simplifie representation of an internal combustion engine controlled according to the present invention; Figure 2 is graph showing a comparison of fuelling AFR, varied by an AFR function, to the LEAN state RICH state reading of the sensor; Figure 3 is a graph similar to Figure 2, but with the fuelling AFR varied by a different AFR function; Figure 4 is a graph similar to Figures 2 and 3, but with the fuelling AFR varied by a third AFR function; and Figure 5 is a flow diagram representing method of controlling the combustion process to operate at a desired LEAN: RICH ratio.

Referring initially to Figure 1, a combustion chamber 10 having an exhaust pipe 11 and a fuel/air inlet 12 is shown. The chamber in the example represents a cylinder or number of cylinders of a petrol driven internal combustion engine. The ratio of air to fuel (AFR) supplied to the combustion chamber 10 is controlled by a processor 13 (which would in practice be an engine management system) which receives a signal from an exhaust gas oxygen sensor 14 (a so called lamda sensor). The sensor 14 responds to the oxygen content of the exhaust gases, and this indicates the state that the engine is actually running at. The sensor output is non-liner and in effect the sensor switches between two states indicating RICH or LEAN of stoichiometric operation. This is a typical system found in automotive applications currently.

The AFR introduced into the combustion chamber is termed the fuelling AFR, and as mentioned above this is controlled by the processor 13.

The processor communicates with a fuelling controller 15 which receives air from an air inlet 16 and fuel from a fuel line 17, and supplies the desired fuelling AFR to the combustion chamber through the fuellair inlet 12.

Figure 2 shows a graphical representation of a variation in AFR with respect to time caused by a particular AFR function. Time is measured along the x-axis and fuelling AFR is measured along the y-axis. The line indicated 20 demonstrates the varying fuelling AFR, and the line indicated 21 indicates the output of the exhaust sensor. In this particular example, the AFR function is varying the fuelling AFR in a sinusoidal waveform. When fuelling AFR gets sufficiently lean as indicated at region 22, the exhaust sensor indicates that the engine is operating in LEAN state. Conversely, when the fuelling AFR is sufficiently rich (richer than the stoichiometric 1: 1 ratio) as indicated at regions 23, the exhaust sensor output indicates the engine is running in a RICH state.

The engine management system which received data from the exhaust sensor can calculate the time spent in the RICH state 25 and the time spent in the LEAN state 26. By analysing the ratio of time spent in the LEAN state as compared to time spent in the RICH state, and averaging this over a period of time, a fairly stable measure of the engine's performance can be achieved.

In Figure 2, as the AFR applied varies around the notional stoichiometric point so that the average fuelling AFR 27 is equal to the stoichiometric point. Assuming that combustion is complete, the LEAN: RICH ratio derived from the data from the exhaust gas sensor is equal to one.

However, as demonstrated in Figure 3, by altering the parameters of the AFR function by which the air fuel ratio is varied, different LEAN/RICH profiles from the exhaust sensor can be achieved. In Figure 3, the AFR function varies the fuelling AFR to give an average fuelling AFR at a point leaner than the switch-point 28 of the sensor. Consequently, the engine spends more time running in the LEAN state 26 than it does in the RICH state 25. This causes the average LEAN: RICH ratio to alter, and the engine management system can, by comparison to a desired LEAN: RICH ratio, judge whether the engine is running at the desired condition.

It is possible to have the AFR function vary the fuelling AFR with any suitable waveform. Figure 4 demonstrates a further alternative situation, in which case the fuelling AFR is varied by a different AFR function to give the waveform shown. In this example, the average fuelling AFR 27 is significantly richer than the switch-point AFR 28. This leads the exhaust sensor output to spend the majority of the time indicating RICH state with only short periodic phases in the LEAN state.

The use of such an AFR function allows the engine to operate at an AFR richer or leaner than the switch-point AFR, furthermore, there are smaller peak deviations from the switch-point than would be required if using the AFR function in Figure 3. This is advantageous because as well as controlling average AFR, in practice there may be limitations on the range of instantaneous AFRs that an engine can operate at. The AFR function shown in Figure 4 could be used at high engine speed and/or heavy load in order to protect the engine and/or exhaust catalyst.

Figure 5 is a flow diagram that indicates a generalised scheme of operation of the present invention. In the first step 30 the present operating conditions of the engine are determined, for example, the engine speed, load requirements, temperature, etc are measured via other sensors and the data processed. The processor then uses this information to determine the desired air fuel ratio at step 31, and this is used to calculate a desired LEAN: RICH ratio. Once the desired air fuel ratio has been selected, the processor defines a suitable waveform for the periodically varying AFR function at step 32, and subsequently the parameters of that AFR function (for example amplitude and frequency of that wave) are also selected at step 33. Once the waveform and parameters of the AFR function have been selected, the fuelling AFR introduced to the combustion chamber is varied by this periodic function at step 34.

At this stage, the engine is now running with an AFR being varied with respect to the AFR function. At step 35, the output of exhaust sensor is being used to measure the time that the engine is operating in a LEAN state. This data is stored at least temporarily. Subsequently, at step 36 the time that the engine is operating in a RICH state is also measured and stored at least temporarily. At step 37 the engine management system uses this data to calculate an instantaneous LEAN: RICH ratio. This instantaneous value is then averaged over a period of time at step 38.

Step 39 is a comparison of the average measured LEAN: RICH ratio determined at step 38 to the desired LEAN: RICH ratio determined at step 31.

If the measured LEAN: RICH ratio is not greater than the desired LEAN: RICH ratio, the process goes to step 40. If however the measured LEAN: RICH ratio is greater than the desired LEAN: RICH ratio the need to increase fuelling is perceived, which information feeds back to step 32 whereat an appropriate alteration of the waveform or parameters of the AFR function is determined.

At step 40, the measured LEAN: RICH ratio is again compared to the desired LEAN: RICH ratio, and if it is found that the measured ratio is less than desired, the need to decrease the fuelling is perceived. This feeds back to step 32 whereat an appropriate alteration of the AFR function waveform or parameters thereof is determined and effected.

If it is determined that the measured LEAN: RICH ratio is equal to the desired LEAN: RICH ratio, the engine is running at the desired condition, and the process returns to calculating the value of the measured LEAN: RICH ratio starting at step 35.

The need to increase or decrease fuelling perceived at steps 39 and 40, feeds back to step 32. At step 32 by changing the AFR function a correction of the operating conditions of the engine can be affecte. In this way, closed loop feedback control of the fuelling of the engine is achieved.

At step 30, as mentioned above, the operating conditions of the engine are determined, which conditions are used to select an appropriate desired air fuel ratio. Whilst the engine is running, the conditions in which the engine is operating can change, and this will lead to a change in the desired air fuel ratio and consequently a change in the desired LEAN: RICH ratio. Therefore, not only will the present invention permit a far more effective control of the AFR that an engine operates at, it also permits a constant management of the closed loop control in order to ensure that the desired fuelling is changed according to changing circumstances.

The present invention has certain advantages in addition to those mentioned elsewhere. The present invention can give a measure of the difference between the average fuelling AFR and the switch-point AFR. What is more, it can operate in closed loop control mode at an average AFR different from the average switch point of the sensor. The use of an average value of the engine's performance means that the positioning of the sensor in the exhaust system is not as critical, and this allows the sensor to be positioned further downstream of the engine without affecting peak AFR levels.

Systems according to the present invention can be adaptive and learn the differences between the desired AFR and the actual AFR, to permit even better results. In addition, in a multi-combustion process, such as a multicylinder engine, failure of a single combustion event in one of the cylinders would not cause the system necessarily to have to lose close loop control of AFR, because the change in average values would be small. Even if a cylinder failed completely, the change in the average values might not be sufficient to completely lose closed loop control.

Claims

Claims 1. A method of controlling the air/fuel ratio supplied to a combustion chamber having a exhaust gas sensor, comprising the steps of : defining a sensor LEAN state, a sensor RICH state, and a periodically-varying air/fuel ratio function by which the ratio of air to fuel supplied to the chamber is controlled ; controlling the air/fuel ratio supplied to the chamber in accordance with the air/fuel ratio function monitoring the sensor output and determining the periods that the instantaneous periodically-varying air/fuel ratio causes the sensor to determine the chamber is running in the LEAN state or in the RICH state; and, adjusting the air/fuel ratio function to bring the ratio of the RICH state time to the LEAN state time to a desired value.
2. A method as claimed in claim 1 in which the combustion chamber is a cylinder of an internal combustion engine.
3. A method as claimed in claim 1 or claim 2, wherein several different air/fuel ratio functions are defined, and said adjustment of the air/fuel ratio function comprises selecting an appropriate function dependent upon the relationship between the actual RICH state time to LEAN state time ratio and the desired RICH state time to the LEAN state time ratio.
4. A method as claimed in any of claims 1 to claim 3, wherein the desired RICH state time to the LEAN state time ratio is selected dependent upon external parameters.
5. A method as claimed in any of the preceding claims, wherein the sensor measures the oxygen content in the exhaust gases.
6. A method of controlling the air/fuel ratio supplied to a combustion chamber and substantially as hereinbefore described with reference to the accompanying drawings.
7. A device for controlling the air/fuel ratio supplied to a combustion chamber having a exhaust gas sensor, by a method according to any of the preceding claims, which device comprises: an input adapted to receive data from the exhaust gas sensor; a processor which defines at least one periodically- varying air/fuel ratio function and controls the fuelling of the engine in accordance with that function, the processor further receiving data from the sensor, and adjusting the air/fuel ratio function dependant upon the received data.
8. A device as claimed in claim 7 which further includes a fuelling controller adapted to be regulated by the processor in response to the periodicallyvarying air/fuel ratio function.
9. A device as claimed in claim 7 or claim 8, in which at least two inputs are provided and at least one of these inputs is adapted to receive data from the exhaust gas sensor and the or each remaining input communicates with other sensors.
10. A device as claimed in claim 9, in which the or each remaining input is adapted to receive information from sensors measuring external parameters.
11. An internal combustion engine when fitted with a device for controlling the airlfuel ratio supplied thereto as claimed in any of claims 6 to claim 9.