GB2357153A

GB2357153A - Monitoring of engine air / fuel ratio

Info

Publication number: GB2357153A
Application number: GB0023900A
Authority: GB
Inventors: Oliver Hild; Thomas Pfeufer; Stefan Schneider
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1999-09-30
Filing date: 2000-09-29
Publication date: 2001-06-13
Anticipated expiration: 2020-09-29
Also published as: FR2799234A1; GB2357153B; FR2799234B1; DE19946962C1; GB0023900D0

Abstract

A method of monitoring the air / fuel ratio of an internal combustion engine capable of operating with either lean or stoichiometric air / fuel mixture. In lean operation a check is performed to determine whether the air / fuel mixture is compatible with a predefined range of engine operation. If not, then the engine is switched to stoichiometric operation. In stoichiometric operation the torque is monitored and a comparison is made between the actual and required torque.

Description

2357153 METHOD FOR THE MONITORING OF AN INTERNAL COMBUSTION ENGINE The

invention relates to a method for the monitoring of an internal combustion engine which can be operated both with a stoichiometric fuel/air mixture and also with a lean fuel/air mixture,, in other words with an excess of air. In particular, the engine in question can be an internal combustion engine with direct fuel injection.

In order to further reduce the fuel consumption of internal combustion engines, ever increasing use is being made of internal combustion engines employing lean combustion. In the case of this type of lean operation, a distinction is made between two basic operating modes.

In a lower load range, the internal combustion engine is operated with a heavily layered cylinder loading and a high excess of air (referred to as layered-lean operation in the following). This is achieved by means of a late injection into the compression stroke shortly before the ignition point. In this situation, the internal combustion engine is operated largely with an open throttle valve, avoiding throttle losses. In order to reduce the NOx emissions, a high exhaust gas recirculation rate is set in this situation.

In an upper load range, the internal combustion engine is operated lean and with homogeneous cylinder loading (referred to as homogeneous-lean operation in the following). Injection actually takes place during the intake section in order to obtain a good intennixture of fuel and air. The intake air mass is set in accordance with the required torque, which is demanded by a driver using an accelerator pedal for example, by way of a throttle valve.

2 Finally, the internal combustion engine can also be operated with a stoichiometric fuel/air mixture (referred to as stoichiometric operation in the following). In this case, as is already known, the required quantity of fuel is calculated from the intake combustion air mass, taking the rotational speed into consideration, and where necessary corrected by way of a lambda regulation facility.

A method for the monitoring of an internal combustion engine with direct injection of the fuel and largely throttle-free load control is known from WO 99/18343. In this method, an estimated value is calculated for the fuel mass which is measured out to a cylinder for each working cycle. This fuel mass is the decisive influencing variable relating to the value for the torque produced by the internal combustion engine. The estimated value for the fuel mass is calculated depending on an air ratio which is determined by a lambda probe located in the exhaust gas section of the internal combustion engine. Emergency operation of the internal combustion engine is initiated if the estimated value for the torque produced and the required value for the torque satisfy predetermined conditions, for example if they differ from one another by a given extent.

However, the quality of this estimated value for the fuel mass and thus the calculation of the required torque are then decisively dependent on the quality of the lambda probe in the exhaust gas section. If the air ratio provided by this lambda probe is not available with a suitable resolution, it is sometimes not possible to calculate the estimated value for the actual torque with a sufficient degree of accuracy.

3 The object of the invention is therefore to specify a method which makes it possible to monitor the lean operation of an internal combustion engine independently of the resolution of the lambda probe.

Claims

This object is achieved by the invention described in Claim 1. In accordance with the invention,, the current fuel/air mixture used for lean operation of the internal combustion engine is subjected to a plausibility check. Plausibility checking in this context is understood to consist of checking whether a particular variable lies within a particular operating window or whether the behaviour of the internal combustion engine is plausible given certain input variables. This checking procedure can include a check on whether the exhaust gas composition indicated by a lambda probe in the exhaust gas section corresponds to a fuel/air mixture which lies outside a predefined operating window. It is naturally also possible, instead of a lambda probe signal, to use a different signal for plausibility checking purposes, which should fall within a predefined range in the case of lean operation, for example the exhaust gas temperature or the NOx concentration in the exhaust gas. It is also possible not to directly measure the variable which is subjected to plausibility checking but to derive it from other operating parameters. Implausible behaviour of this type can, for example, also be recognised whenever an accelerator pedal controlling the operation of the internal combustion engine is returned to zero above a certain rotational speed and/or torque threshold, but the internal combustion engine still remains above the rotational speed and/or torque threshold. In the event of implausible conditions, a switchover is then performed to stoichiometric operation of the internal combustion engine, in which mode the known 4 lambda regulation facility is active. In stoichiometric operation, the actual torque can be determined through measurement of the intake air mass. Therefore, torque monitoring is then performed, during which the required torque is compared with the actual torque. The method according to the invention thus departs from a detailed monitoring of the internal combustion engine in its lean operation, and simply performs plausibility checks on signals and operating states, and in the event of implausible conditions immediately switches over to lambda regulated operation in which conventional monitoring measures are known to have a good effect and where diverse diagnostic capabilities are available. Useful further embodiments of the invention are set down in the subclaims. An embodiment of the invention will be described in detail in the following with reference to the drawings. In the drawings: Figure I shows a schematic representation of an internal combustion engine with direct fuel injection, Figure 2 shows a signal flowchart for monitoring of the internal combustion engine, and Figure 3 shows a finiher signal flowchart for monitoring of the internal combustion engine. Fig. I shows a schematic representation of an internal combustion engine with direct petrol injection, which can be operated both with a stoichiometric fuel/air mixture and also with a lean fuel/air mixture. For reasons of clarity, only those components of the internal combustion engine which are necessary for comprehension of the invention are included in the drawing; in particular, only one cylinder of a multi-cylinder internal combustion engine is shown. The internal combustion engine has a piston 10 which restricts a combustion chamber 12 in a cylinder 11. An induction port 13 opens out into the combustion chamber 12 at an inlet valve 14 through which the combustion air flows into the combustion chamber 12. An outlet valve 15 connects the combustion chamber 12 to an exhaust gas section 16, in whose continuation are located an oxygen sensor in the form of a broadband lambda probe 17 and also a NOx storage catalytic converter 18. By reverting to the signal from the lambda probe 17, the fuel/air mixture is regulated by a control unit 21 in accordance with the predetermined values in different operating modes of the internal combustion engine. For example, a known lambda regulation takes place in stoichiometric operation. To enable such lambda regulation to occur, a further lambda probe 32, which is used for regulation of reference and desired values, is located downstream of the NOx storage catalytic converter 18. The oxygen probe in this case is a binary lambda probe 32 (two-point lambda probe) which exhibits a step function response at a lambda value of X = 1. Instead of the lambda probe 32, it is also possible to use a NOx sensor. A temperature detector 33 is also located in the exhaust gas section. 6 The purpose of the NOx storage catalytic converter 18 during lean operation of the internal combustion engine is to ensure that the required exhaust gas limit values in respect of NOx compounds can be observed. Thanks to its coating, it adsorbs the NOx compounds in the exhaust gas which are produced during lean combustion. In order to reduce the NOx emissions occurring during layer charging operation specifically in the case of internal combustion engines with direct fuel injection, an exhaust gas recirculation facility is provided. In this instance, the temperature of the combustion is reduced by mixing exhaust gas with fresh intake air, whereby the NOx emissions are reduced at the same time. Therefore an exhaust gas recirculation line 19 is routed from the exhaust gas section 16 upstream of the NOx storage catalytic converter 18 to the induction port 13, which line opens into the induction port between a throttle valve 20 and the inlet valve 14. A controllable valve 22 which is non-nally referred to as an exhaust gas recirculation valve is connected into the exhaust gas recirculation line 19. The quantity of exhaust gas which is recirculated can be set by controlling the valve 22. The combustion air for the cylinder I I flows by way of an air mass meter 23 into the induction port 13. The throttle valve 20 located therein is a throttling element powered by an electric motor (E-gas system) whose cross section of opening can also be influenced by the control unit 21 in addition to activation by a driver (accelerator pedal position). It is possible in this way, for example, to reduce disturbing reactions to load changes. Furthermore, the throttle valve 20 is opened completely by the control unit 21 during lean layer charging operation. In addition, through appropriate control of the throttle 7 valve 20, the control unit 21 provides for a soft transition from stoichiornetric to lean-homogenous operation, and from the latter to layer charging operation. Finally, a temperature sensor 24 which is connected to the control unit 21 is also situated in the induction port 13. The temperature sensor 24 can naturally also be integrated into the air mass meter 23. Located in the combustion chamber 12 are a sparking plug 25 and also jection valve 26 which for the purpose of fuel injection is fed from a an in high-pressure storage facility 27, which is part of a known fuel supply system for direct petrol injection. The control unit 21 is finally also connected to a temperature sensor 28 which provides a signal indicating the temperature of the internal combustion engine, for example by measuring the coolant temperature. The rotational speed of the internal combustion engine is determined by way of a sensor 29 sensing the crankshaft or a transmitter wheel secured to the crankshaft. Further control parameters required for operation of the internal combustion engine such as accelerator pedal position, signals from knocking sensors etc. are similarly fed to the control unit 21 and are identified generally by the reference character 30 in Fig. 1. Finally, a block 31 whose function will be described in more detail below is provided in the control unit 21 for torque determination and monitoring. In addition, the control unit 21 is connected to a memory 34 in which different threshold values TQI-SW1, TQ1-SW2 as well as at least one characteristic field KFI are stored, the meaning of which will be described 8 below. Depending on the operating conditions, the control unit 21 then determines whether the internal combustion engine will be operated in its stoichiometric, lean-homogenous or layered-lean mode. When the internal combustion engine is running lean, this produces the problematical situation whereby the known monitoring of the internal combustion engine implemented by way of torque monitoring either cannot be applied or can only be applied with severe restrictions. In lean operation the control unit 21 therefore performs a plausibility check on the signals from the lambda probes 17 and 32, which are described on the basis of Fig. 2. Fig. 2 shows a signal flowchart. The square elements denote a logical interrogation. A square element with a "<" symbol denotes a threshold value interrogation, a square element with an "n" denotes an inversion of the logical state (NOT circuit), a square with a "v" denotes an OR operation, a square with a "+" sign denotes an AND operation, and a square with a diagonal line denotes an active element whose function is explained individually. A horizontal oval represents a signal value which is read out, and a horizontal rectangle indicates the setpoint selection for a threshold value. Finally, a triangle denotes a request for stoichiometric operation. In order to carry out monitoring of the internal combustion engine, firstly the signal from the lambda probe 17 is read out in element S 1 and compared in element S9 with a threshold value obtained from S2. In the event of a threshold value being exceeded, element S9 outputs a logic 1 level. By 9 analogy, in step S3 the signal from the lambda probe 32 is compared (step S10) with a threshold value (step S4). If one of the elements S9 or S10 provides a logic 1 level, then the OR operation in element S 14 will produce a logic 1 level at that element's output. In addition, in step S5 the operating state of the internal combustion engine is interrogated to ascertain whether it has been started. If it has been started,, the inversion performed in element S 11 and the active delay element in element S 15 result in a dead time. Only when this dead time has elapsed does the output of the delay element in element S 15 go to a logic 1 level. Only when two logic 1 levels are applied to the input of element S17 does the OR operation provided by the following element S 18 result in a request for stoichiometric operation of the internal combustion engine (element S 19). The lower path of the signal flowchart shown in Fig. 2 describes the plausibility check on the signal from the lambda probe in the event of overrun fuel cut-off. To this end, the signal from lambda probe 17 is again interrogated. Alternatively, the signal from lambda probe 32 may also be interrogated; however, for the sake of simplicity this is not shown. By way of element S12, a comparison is then performed with a threshold value originating from element S7. This threshold value from element S7 is a minimum lean limit which should exceed the lambda probe signal in the overrun fuel cut-off state. In element S8, the overrun fuel cut-off state is interrogated and forwarded via a time-delay circuit in element S 13 to an AND circuit in element S 16. If the signal from the lambda probe from step 6 exceeds the threshold value from step 7 and if an overrun fuel cut-off state exists for a certain period of time, the result at the output of element S 16 is thus a logic 1 level, which in turn results in a request for stoichlometric operation in element S 19. In addition to this plausibility checking of the lambda probe signal described above, it is also possible to consider whether the fuel/air mixture corresponds to a minimum lambda value. To this end, the request for stoichiornetric operation in step S 19 is logically 0Red as follows. Firstly, in element S20,, a check is made to determine whether the internal combustion engine is running in layered-lean operating mode (alternatively, also homogeneous-lean operation). Following a time delay introduced by element S25, a logic 1 level is applied to the input of an AND circuit in element S27. In parallel to this, the lambda value is interrogated in element S21 and subjected to a threshold value check in element S26. The threshold value used in this check originates from a characteristic field in element S24, which has been fed with the results of the interrogation of the rotational speed (step S22) and of the torque (step S23). If the lambda value falls below this threshold value, element S26 provides a logic 1 level. Therefore, a failure to attain a minimum lambda value after a certain period of time during layer charging operation results in a logic 1 level at the output of element S27 which performs a logical AND operation. If one of the inputs to element S29, which performs a logical OR operation, provides a logic 1 level, stoichiometric operation of the internal combustion engine is requested in element S31. The inputs to element S29 are elements S 19 and S27 and also an irreversibility circuit which is formed by elements S27 to S30. Whenever the output from element S29 provides a logic 1 level, a counter in element S28 is incremented by 1. This counter in element S28 is compared with the initial counter state, obtained from element S32. If the result of the threshold value monitoring in element S30 is such that the counter in element S28 is incremented, the output from element S30 provides a logic I level which is fed to the OR circuit of element S29. One of the inputs to the OR circuit of element S29 is thus always at a logic I level as soon as element S29 has supplied a logic I level to its output. The request for stoichiometric operation in element S31 is thus irreversible for the current operating phase of the internal combustion engine, in other words until the internal combustion engine is turned off again. When the internal combustion engine has been switched to stoichiometric operation, torque monitoring is performed in block 31 of the control unit 2 1. To this end, the actual torque currently being produced by the internal combustion engine is determined by means of the intake combustion air mass LMM by way of the characteristic field KF1 of the memory 34. At the same time, the required torque requested by the control unit 21 is registered. If the difference between the required torque and the actual torque exceeds a threshold value TQI-SWI, then the internal combustion engine is switched to an emergency mode of operation, for example with heavy throttling. If the difference exceeds a further threshold value TQI-SW2, the internal combustion engine is shut down. In an alternative monitoring option, a check is made to determine whether the accelerator pedal position corresponds to a zero value. If the rotational speed and/or the torque of the internal combustion engine lie(s) above a threshold which is selectable depending on operating conditions, a switchover is similarly performed from lean operation to stoichiometric 12 operation of the internal combustion engine. This monitoring option is based on the assumption that in the event of an undesired acceleration of a motor vehicle equipped with an internal combustion engine the driver will remove his or her foot from the accelerator pedal. This interrogation can also include a check to ensure that no external intervention by way of a vehicle speed control facility is present. To this end, a check is made as to whether the vehicle speed control facility value is similarly zero. The transition from lean operation to stoichiometric operation can in principle be soft, in other words with the transition taking place through gradual setting of the throttle valve 20, or it can be hard, in other words as quickly as possible. Within the framework of the monitoring facilities described, it is also possible to assess the request for stoichiometric operation of the internal combustion engine. Depending on the result of the assessment, either a hard, in other words quick, transition or a soft and thus slow transition can be initiated. Instead of the described lambda probe signals, other signals from the internal combustion engine can also be subjected to plausibility checking if these signals are intended to remain within a predefined range in the lean operating mode. The signal in question may relate, for example, to the exhaust gas temperature in the exhaust gas section 16, which is registered by means of the temperature sensor 33. It is also possible to provide a NOx sensor instead of or in addition to the lambda probe 32, and to subject the NOx concentration to a plausibility check.

1.0.. a.: 0:

13 In a further embodiment of the method, it is on the other hand possible to control the monitoring of the internal combustion engine oneself On this so-called control level, the correct effect of the monitored operation should be safeguarded, and an appropriate response initiated upon detection of a fault. To this end, a check is made to determine whether the lambda value for the exhaust gas at one of the lambda probes 17 or 32 falls below a rotational speed-dependent and/or a torque-dependent threshold value. If such a failure to reach a threshold value occurs, and if nevertheless no switch to stoichiometric operation is requested in the case of the type of monitoring described above, this situation is diagnosed as a misfunction of the described monitoring method.

14 Claims 1. Method for the monitoring of an internal combustion engine which can be operated either with a stoichiometric fuel/air mixture and or with a lean fuel/air mixture, in which method a) in the case of lean operation of the internal combustion engine a plausibility check is performed on the current fuel/air mixture, b) in the event of implausibility a switch is made to stoichiometric operation of the internal combustion engine, and c) during this stoichiometric operation, torque monitoring of the internal combustion engine is then performed, during which the required torque is compared with the actual torque of the internal combustion engine.

2. Method in accordance with Claim 1, characterised in that in step a) a check is made during plausibility checking to ascertain whether the lambda value for the exhaust gas upstream or downstream of a catalytic converter in the exhaust gas section of the internal combustion engine falls below a certain threshold value, in particular one which dependent on the working point.

3. Method in accordance with Claim 1, characterised in that the lambda value is obtained by detecting a lambda probe signal.

4. Method in accordance with one of the above Claims, characterised in that in step c) the actual torque is determined by measuring the supplied combustion air mass.

5. Method in accordance with one of the above Claims, characterised in that the switchover in step b) is irreversible.

6. Method in accordance with one of the above Claims, characterised in that the monitoring only commences after a certain period of time has elapsed after starting the internal combustion engine.

7. Method in accordance with one of the above Claims, characterised in that a check is made during plausibility checking to ascertain whether a pedal value controlling the operation of the internal combustion engine is zero, and whether the rotational speed and/or torque of the internal combustion engine exceed(s) a particular threshold value, in particular one which dependent on the working point, and characterised in that an implausible situation is detected when these states exist.