Patent Application
20060000200
The invention concerns a method and device for adjustment of a fuel/air ratio for an internal combustion engine.
A catalyst arranged in the exhaust system of an internal combustion engine is ordinarily used to clean the exhaust of an internal combustion engine. This converts harmful components, like hydrocarbons CH, carbon monoxide CO and oxides of nitrogen NOx essentially to nontoxic gases. It is critical to the so-called degree of conversion of the catalyst that the oxygen content of the exhaust lie within optimal values. For a so-called three-way catalyst these optimal values lie in a narrow range around the value corresponding to a fuel/air mixture of λ=1. In order to be able to maintain this narrow range, it is customary to regulate the fuel/air ratio for an internal combustion engine by means of oxygen sensors arranged in the exhaust system of an internal combustion engine.
A method for lambda control for internal combustion engine with a downstream catalyst is known from Unexamined Patent Application DE 40 24 212 A1 in which the oxygen fractions of the exhaust of the internal combustion engine are recorded by oxygen sensors upstream and downstream of the catalyst. In stipulated operating ranges a control signal with controllable amplitude is generated by coupling in an outside signal with controllable amplitude. With increasing catalyst aging the amplitude is reduced. The functional state of the catalyst in the exhaust system of the internal combustion engine can be determined with the method by means of lambda regulation and the time for replacement of an aged catalyst determined.
A method for adjustment of the fuel/air ratio for an internal combustion engine with a downstream catalyst is known from Unexamined Patent Application DE 43 37 793 A1 in which the oxygen fractions in the exhaust of the internal combustion engine are determined by oxygen sensors upstream and downstream of the catalyst. Both sensors influence regulation of the fuel/air ratio. It is initially determined with an amplitude evaluation whether the catalyst has already reached a certain degree of aging. This actual control quantity is issued by the sensor upstream of the catalyst. A switch is then made to frequency evaluation or frequency regulation in which the catalyst yields the actual control quantity downstream of the catalyst. Such evaluations are sensitive per se, but have a relatively strong influence on the operating behavior of the internal combustion engine. This is avoided by only switching to frequency evaluation when the catalyst has already reached a certain state of aging. With increasing operating time the oxygen storage capability of the catalyst declines. The control frequency therefore increases with increasing catalyst aging so that lambda regulation is adjusted to the state of aging of the catalyst. As soon as the determined control frequency downstream of the catalyst is higher than the frequency threshold, aging of the catalyst can be reliably recognized and the catalyst replaced.
The object of the invention is to improve the method for adjusting the fuel/air ratio for an internal combustion engine with a downstream catalyst according to the prior art and permit greater dynamics, as well as to devise an apparatus for execution of the method.
This object can be solved by a method for adjustment of a fuel/air ratio for an internal combustion engine with an associated catalyst, comprising the steps of determining an exhaust composition in an exhaust system of the internal combustion engine by means of sensors, generating a control signal to influence the fuel/air ratio as a function of output signals of at least one of the sensors, and making by means of a characteristic curve of the control signal a switch back and forth between an operating state with oxygen excess and an operating state with oxygen deficiency of the catalyst, wherein a shape of the characteristic curve is adjusted as a function of an oxygen and/or NOx addition and/or desorption capability of the catalyst.
A course of a transition from operating state to another and/or a course of the characteristic curve within an operating state can be adjusted as a function of oxygen and/or NOx addition and/or desorption capability of the catalyst. The characteristic curve of the control signal can be adjusted as a function of a catalyst temperature. The characteristic curve of the control signal can also be adjusted as a function of the degree of aging of the catalyst. Adjustment of the characteristic curve of the control signal may occur as a function of the operating parameters of an internal combustion engine. The characteristic curve of the control signal can be adjusted unsymmetrically to a stipulated lambda value over a time range that includes at least several periods of the control signal. The characteristic curve of the control signal may be a sawtooth. The subsequent operating states can be adjusted with different residence time of the control signal. The subsequent operating states can be adjusted with different amplitude of the control signal. The characteristic curve of the control signal may be nonlinear at least in a region. The characteristic curve of the control signal may become leaner or richer degressively. The characteristic of the control signal may initially become leaner around a stipulated amount or richer and then is degressively guided in the direction λ=1.00. The characteristic curve of the control signal can be a rectangular curve with different amplitudes and/or residence times in the adjusted operating states. During fuel cutoff in the overrun or idle of the internal combustion engine, the internal combustion engine may be operated more in the rich operating state than in the lean operating state. In a catalyst the control signal may be adjusted so that increased incorporation of oxygen and/or NOx in the catalyst occurs temporarily. In a catalyst after a stipulated operating time the control signal can be adjusted so that a phase with increased lean operation follows a phase with at least two periods with mostly rich operation. Before reaching an operating temperature of the catalyst the characteristic of the control signal may deviate from the characteristic after surpassing the operating temperature. In a catalyst at almost operating temperature, the characteristic curve of the control signal may have a sawtooth trend before reaching a stipulated operating temperature. The catalyst state and/or a state of the sensor upstream and/or downstream of the catalyst can be determined from the control signal.
The object can also be achieved by a device for adjustment of a fuel/air ratio for an internal combustion engine with an associated catalyst, comprising exhaust composition sensors, a control unit for generating a control signal to influence the fuel/air ratio as a function of output signals of at least one of the sensors, said control signal comprising a characteristic curve for switching back and forth between an operating state with oxygen excess and an operating state with oxygen deficiency of the catalyst, and means for adjusting a form of a characteristic curve as a function of oxygen and/or NOx addition and/or desorption in the catalyst.
A sensor can be arranged in the exhaust system of the internal combustion engine upstream and downstream of the catalyst. The sensor upstream of catalyst may be a broadband lambda probe with a constant characteristic. The sensor upstream of the catalyst can also be a two-point lambda probe with a transfer characteristic. The sensor downstream of the catalyst may be a two-point lambda probe with a transfer characteristic. The sensor downstream of the catalyst may also be a broadband lambda probe with a constant characteristic. The catalyst can be a three-way catalyst. The catalyst may have a noble metal content of less than 60 g/ft3, especially less than 40 g/ft3, preferably less than 30 g/ft3, optimally less than 20 g/ft3, ideally less than 10 g/ft3. The catalyst can be an NOx storage catalyst. The catalyst may have a noble metal content of less than 80 g/ft3, especially less than 60 g/ft3. The internal combustion engine can be a directly injected internal combustion engine capable of layered charging.
One advantage of the method is that the time trend of the reference value of lambda value of the exhaust upstream of a catalyst connected after the engine is automatically adjusted to the operating states of the catalyst in which conversion is otherwise not optimum. This leads to better utilization of the catalyst and to increased reliability in maintaining emission values.
Another advantage of the invention is that because of the improved dynamics of the exhaust system in a vehicle in which the device according to the invention was implemented the driving dynamics are approved.
In addition, the catalyst during its lifetime is brought to favorable operating ranges and operated with high efficiency so that in comparison with ordinarily regulated catalysts noble metals of the catalyst can be saved and/or the catalyst itself reduced in size. This saves cost and resources.
The invention is further explained by means of drawings, in which the figures show:
The invention is particularly suited for catalysts in which the air fraction fed to the internal combustion engine of a fuel/air mixture is adjusted by means of a control signal that is periodically set between a minimal value and a maximal value of the air fraction and switched back and forth between a rich operating state with oxygen deficiency and a lean operating state with oxygen excess.
The invention is particularly favorable for a three-way catalyst in which oxygen and/or NOx are periodically introduced as oxidizers for the catalyst and desorbed and in which a control signal of a lambda control deviates essentially periodically around the lambda value λ=1. In the lean operating state the oxygen supply in the exhaust is sufficient to oxidize its HC and CO fractions, whereas in the rich operating state NOx fractions in the exhaust as oxidizers oxidize the HC and CO fractions present. A common control strategy for a three-way catalyst proposes a lambda regulation in which a λ probe is exposed to a control signal with constant frequency. In the lean operating state when λ>1 oxygen is introduced to the catalyst 2; in the rich operating state with λ<1 this oxygen is consumed for oxidation processes.
However, it is also possible to use the invention NOx storage catalyst that can be operated at higher lambda values in a three-way catalyst. Aged storage catalysts can also be operated at lower lambda values around λ=1.
In principle, ordinary lambda probes are suitable with sensors 4, 5, like broadband lambda probes, two-point lambda probes or NOx sensor with lambda probe function. As an alternative, a two-point lambda probe can also be used upstream of catalyst 2 and/or a broadband lambda probe downstream of catalyst 2 has sensors 4, 5. It is also conceivable to determine the lambda value upstream of catalyst 2 from other types of measured quantities, like injected amount of fuel and drawn in amount of air.
The oxygen storage capability of catalyst 2 varies over the lifetime of the catalyst 2. The characteristic of a lambda probe, especially a broadband lambda probe can also vary. This can be compensated by adjusting the frequency of the control signal to the state of aging. Expediently, sensor 4 is exposed to a control signal upstream of catalyst 2. Sensor 5 downstream of catalyst 2 reports to sensor 4 upstream as soon as breakthrough of rich exhaust or lean exhaust is observed behind catalyst 2. As long as lean exhaust is available up to sensor 5 downstream of catalyst 2 oxygen breakthrough is recognized via the internal combustion engine 1. A switch is made to the rich operating state of catalyst 2 until breakthrough of the rich component is observed. A switch is then made back to the lean operating state and the sequence is repeated.
With increasing age the oxygen storage capability of catalyst 2 diminishes, breakthroughs occur more quickly and the control frequency rises. The lambda control is therefore adapted to the state of aging of catalyst. Such regulation is also referred to as natural frequency regulation.
The sensors 4, 5 are connected to a control device 6 that receives their signals and sends them to evaluation. This control device 6 is expediently a component of an ordinary engine control device used for operation of the internal combustion engine 1. In this control device 6 or via this device operating parameters of the internal combustion engine 1 or a vehicle driven by the internal combustion engine 1 are available. These operating parameters are preferably entered as maps in a corresponding storage medium. Such operating parameters include exhaust temperature upstream of catalyst 2 and/or in catalyst 2, exhaust temperature downstream of the catalyst, oxygen storage capability of the catalyst 2, exhaust flow rate, speed of the internal combustion engine 1, exhaust recirculation rate, position of a camshaft disk, charge movement flap, ignition time and/or charge pressure and the like. Information concerning the operating parameters can be linked to the sensor signals and control therefore conducted as a function of the operating parameters. This is indicated by arrows on control device 6. Individual operating parameters or different operating parameters can be used in combination with each other.
Two characteristic curves of an ordinary control signal according to the prior art for a fresh three-way catalyst (
Means are provided according to the invention in order to adjust a characteristic curve of the control signal to an actual catalyst state so that a characteristic curve shape of the characteristic curve is adjustable as a function of addition and/or desorption of an oxidizer in catalyst 2. Such a characteristic curve shape can involve a transition from one operating state to another and/or a trend of the characteristic curve within an operating state of catalyst 2. The oxidizer can be oxygen or NOx.
In this case the frequency of the control signal is not followed simply as described in the prior art according to the state of aging of catalyst 2, but the characteristic curve shape of the characteristic curve is varied by varying the amplitude and/or characteristic curve, especially a flank steepness, switching point and/or trend in an operating state. This adjustment occurs within a control cycle and can vary with increasing operating time of catalyst 2. The aging behavior of catalyst 2 can be different in rich and lean operating states so that consideration of the different behavior in the two operating states permits more efficient utilization of catalyst 2 via a corresponding adjustment of the control signal as a function of the catalyst state.
Depending on the state of aging of catalyst 2 the amplitude of the characteristic curve for a rich and lean operating state of the catalyst can be adjusted. This is shown in
Such a control strategy is advantageous for catalysts that have already been used for some time but are still useable over a longer time. Here it is favorable to operate over several control periods more strongly in the rich region, followed by a phase with mostly lean fractions and then again richer fractions and to repeat this. Because of this, increased oxygen storage in the catalyst 2 can be temporarily reached up to the limit of the regeneration capability of catalyst 2.
A trend according to
Over a time region that includes at least several periods of the control signal it can therefore be advantageous to adjust the amplitudes of the characteristic curve of the control signal unsymmetrically to a stipulated lambda value.
According to a favorable modification of the invention the characteristic curve of the control signal can be sawtooth. This is shown in
It can prescribed that the characteristic curve of the control signal is a rectangular curve or a different characteristic curve with different amplitude and/or residence time in the corresponding operating states so that the percentage of rich operating states and lean operating states can be adjusted as a function of needs to the actual state of catalyst 2.
It is also possible that adjust consecutive operating states with different duration depending on how dependent the addition process and/or desorption processes of the oxidizer in catalyst 2 are on the operating parameters of the internal combustion engine and/or the lifetime of catalyst 2.
A control characteristic curve is shown in
A control characteristic curve is shown in
With increasing age the deep storage of oxygen and/or NOx in catalyst 2 deteriorates so that the desired catalytic processes can no longer occur efficiently. The behavior of the catalyst 2 in lean operating states can be different than in rich operating states. Variation in modulation of the characteristic curve of the control signal therefore permits adjustment to these boundary conditions with a simultaneous increase in efficiency of the catalytic processes. This is advantageous to maintain emission limits.
It is particularly expedient to conduct the adjustment of the characteristic curve of the control signal as a function of operating parameters in internal combustion engine 1. This can occur via maps of operating parameters, as already described in
In addition to ensuring the emission standard, in an advantageous embodiment of the invention the catalyst state and/or state of the sensor 4, 5, especially sensor 5 downstream from catalyst 2 can be determined from a change in the characteristic curve of the control signal.
In addition to increased reliability in maintain in emission values, the invention also permits a reduction in noble metal content of catalyst 2. The catalyst 2 is mostly operated in regions in which conversion is improved. Because of this the catalyst volume can be correspondingly reduced and/or the noble metal compound of the catalyst can be reduced in order to achieve the same efficiency as in an ordinary control. It is possible to reduce the noble metal content and/or the catalyst volume during use of the method according to the invention without surpassing the pollutant emissions forming without use of the method by at least 10%, especially by at least 20%. In particular, the catalyst 2 in the case of an NOx storage catalyst has a noble metal content of less 80 g/ft3, especially less than 60 g/ft3. In a three-way catalyst a noble metal content of less than 60 g/ft3, especially less than 40 g/ft3, preferably less than 30 g/ft3, optimally less than 20 g/ft3, ideally less than 10 g/ft3 is provided.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 61 751.1 | Dec 2002 | DE | national |
| 103 10 672.3 | Mar 2003 | DE | national |
This application is a continuation of copending International Application No. PCT/EP2003/014968 filed Dec. 30, 2003 which designates the United States, and claims priority to German application no. 102 61 751.1 filed Dec. 30, 2002 and German application no. 103 10 672.3 filed Mar. 12, 2003.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP03/14968 | Dec 2003 | US |
| Child | 11169350 | Jun 2005 | US |
