Claims:We Claim:
1. A method of controlling NOx in a storage catalyst of an exhaust system, said method comprising:

detecting (100), by a control unit, the amount of NOx stored in said storage catalyst;

verifying (102), by said control unit if said amount corresponds to a maximum storage capacity of said storage catalyst;

comparing (104), by said control unit if said amount corresponds to a maximum storage capacity of said storage catalyst; and

changing (106), by said control unit a combustion mode of an engine for enabling a rich-burning in dependence of regeneration capacity of storage catalyst.

2. The method of claim 1, wherein said combustion mode is a function engine load, gear position and temperature of said engine.
, Description:Complete Specification:

The following specification describes and ascertains the nature of this invention and the manner in which it is to be performed.
Field of the invention
[0001]This invention to the field of controlling the NOx regeneration when using a NOx Storage Catalyst in an exhaust system.

Background of the invention
[0002] A NOx-Storage Catalyst in the exhaust path of an engine is a means of exhaust gas after-treatment usually employed in lean-burning engines, such as diesel engines. It can store NOx across a wide range of exhaust temperatures in normal and lean engine operation. While SCR-based NOx reduction requires catalyst temperatures above 200deg C in order to work at high efficiency, the storage capacity of an NSC is already considerable between 100 – 200deg C. However, this storage capacity is limited to a few grams of NOx since the storage material itself gets converted to a metal- nitrate in the process. Thus, the catalyst periodically needs to be regenerated in order to maintain its storage capability. Regeneration is accomplished by changing the combustion mode of the engine to rich operation. This engine operation produces an excess of reductants, such as CO, HC and H2 over the residual oxygen in the exhaust gas. These reductants will then react in the catalyst with the NOx in its stored form – Metal-Nitrates – to form N2 and CO2.

[0003] In suitable conditions, a NOx Storage catalyst system can store NOx for several minutes before it requires rich engine operation for a few seconds to “clean out the Nitrates” and restore its previous storage performance. While the storage of NOx is efficient at temperatures below 200deg C, the efficient regeneration of the catalyst requires temperatures above 200 deg C. Much lower catalyst temperatures during rich engine operation lead to spontaneous desorption of NOx. Nitrates then release NOx faster than the subsequent reduction with CO (or H2) can convert NOx to N2. In addition, the fraction of hydrocarbons (HC) in the rich exhaust gas require higher temperatures to react with NOx. Consequently, at low temperatures a fraction of HC will leave the catalyst unconverted as HC-slip. Rich-burning cannot be done continuously as it leads to increased emissions and fuel consumption.

Brief description of the accompanying drawing
[0004] Different modes of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:

[0005] FIG. 1 illustrates a method of controlling NOx in a storage catalyst of an exhaust system.

Detailed description of the embodiments
[0006] FIG. 1 illustrates a method of controlling NOx in a storage catalyst of an exhaust system. The method comprises, detecting (100), by a control unit, the amount of NOx stored in storage catalyst, verifying (102), by control unit if amount corresponds to a maximum storage capacity of the storage catalyst. The control unit compares 104 if the amount corresponds to a maximum storage capacity of the storage catalyst, and the control unit changes the combustion mode of the engine for enabling rich-burning in dependence of regeneration capacity of the storage catalyst. In an embodiment, the combustion mode is a function of engine load, gear position and temperature of the engine.

[0007]The working of the above mentioned method for controlling NOx will be explained in further detail. An engine operates under two conditions, first learn burning condition, and under rich condition. Rich-burn engines have an air-to-fuel ratio that is balanced, resulting in an exhaust O2 content of about 0.5%. Lean-burn gas engines have an exhaust O2 content typically >8%. The rich operation of an otherwise lean burning engine is typically achieved by throttling of excess air, modification of the injection parameters in timing and quantities and addition of a post-injection. The latter will react with residual oxygen in the cylinder leading to increase in concentration of CO (typically >2%). There is need to reduce the amount of residual oxygen to a minimum, while maintaining a low concentration of hydro carbons (HC) and smoke. Such conditions can only be achieved in part– under normal or lean conditions – possible engine operation area in terms of engine speed and load. Particularly at low engine load stable rich combustion with acceptable emission parameters is difficult and closer to the engine limits. The associated additional fuel consumption and the impact on oil dilution is highest at the lowest engine load.

[0008]Often the trade-off for stability and emissions is a limitation to the lower engine load limit at which NSC regeneration is possible. At middle and higher engine load stable rich operation is achieved more robustly and with a more favourable compromise of the combustion parameters. Since excess air in
normal operation of engine at higher engine load is less, compared to low load – the amount of additional injected quantity for the production of CO in Rich operation is considerably less. In addition, since the exhaust mass flow at higher engine speed and load is higher, it requires a consequently shorter time for rich operation to clear the same amount of NOx from a Storage catalyst.

[0009]Considering only the storage of NOx, the NSC is ideal to treat NOx at continuous low engine load operation – which prevails during urban driving – particularly in congested traffic conditions. While SCR would require continuous warm-up to reduce NOx efficiently, elevated engine load is required only intermittently – for several seconds- to regenerate the NSC. In fact, in most normal on-road driving situations in the course of several minutes there are occasions where enough engine load is maintained for long enough to regenerate the NSC under ideal conditions.

[0010]Currently a regeneration of the NSC is demanded by the control logic of the engine control unit. Whenever the quantity of the already stored NOx is too high to maintain high efficiency for storage of further NOx, the demand for a regeneration is placed by the control logic – mainly based on the current NOx load and a few corrective parameters, such as temperature of the NSC – since the capacity of the catalyst is temperature dependent.

[0011]Once the control logic has placed a demand for a mode change from normal to rich mode in order to regenerate the NSC, the controller will wait until such a transition is possible. This “Release” of a mode change is typically subject to a number of required conditions –firstly, the engine operates at a speed and load where rich operation is possible. As soon as all required conditions are met to allow a mode transition from normal operation to rich Mode, the controller will release the regeneration. There is currently no decision based on how favorable rich operation in the current conditions is. Typically, a target of the calibration is to make the calibrated area of rich operation as big as possible – to allow regeneration whenever it is required. The strategy for release operation must allow more unfavorable conditions as well. Otherwise regeneration under low load driving such as in congested urban traffic would not be possible. Most normal driving situations are such that the engine operation would frequently touch into higher loads – typically long enough to regenerate in an operation point where the additional fuel injection amount is small compared to the amount of NOx to be regenerated.

[0012] Once a regeneration is required, the next instance where all required conditions to release are fulfilled will trigger the regeneration. So even if more suitable conditions might occur a moment later, also very unfavorable conditions are accepted. “More favorably”, in this context typically means at higher engine load. As described above, higher load means higher combustion stability and a better ratio of additional fuel injection per reductant quantity in Rich Mode. The larger the calibrated area for rich mode, the higher is the probability to regenerate under unsuitable low load conditions – with negative impact on the resulting fuel penalty and the oil dilution. Excluding such conditions – i.e. regenerations at low engine load - would in most normal driving situations not change the effectivity of the NSC. Occasional phases of increased load will be sufficient to keep low NOx load on the NSC. Only when the higher load operation area of the engine is not reached in a long time, the consequences would be increased emissions.

[0013]In the present invention, the procedure is changed, such that the control unit calculates the fraction of time at which the engine load exceeds “the unfavorable minimum load”. Typically, if for example about 10% of time is spent at higher engine load, this is still sufficient time to regenerate the NSC soon enough before the stored NOx load exceeds acceptable limits. Under such (normal) conditions a regeneration is only started in the favorable engine operation zone. Only when the ratio of time spent in the “unfavorable low load operation zone” exceeds an acceptable limit, the demand structure will switch to an “escalation stage”.

[0014]The engine load limit for release of a regeneration will be lowered. This allows the otherwise unfavorable conditions to regenerate the NSC. Release of regeneration mode can be extended to further – otherwise adverse conditions. This can be e.g. the selection of a gear. Normally a regeneration in the first gear or second gear will be avoided since there is a high probability that such a regeneration will be interrupted by another gear change soon. Only in persistent low load it is reasonable to consider such regenerations as well. Further conditions can be: the minimum catalyst temperature can be lowered, so even the desorption of a fraction of the stored NOx is accepted.

[0015]Under normal vehicle operation – i.e. not persistently low engine load, such as congested city driving – the NSC load can be increased to several grams of NOx. While the engine load is occasionally high and the NSC temperature is not too low, larger quantities can be regenerated in a short rich operation. However, as soon as the conditions become less favorable and the probability to release an efficient rich mode gets lower, the amount of NOx on the catalyst should be kept lower. With the disclosed method this can be achieved.

[0016]The disclosed method decides, when escalation is appropriate and when to restrict the regeneration to favorable conditions only. Simulations with a large amount of vehicle driving data have been performed. Also, the detection that the desired conditions to regenerate the catalyst at elevated engine load become unlikely has to be made sufficiently fast upon changing driving conditions. For example when entering an urban area after some time of extra-urban driving, or when entering a zone of congested traffic, the decision about escalation must be revised quickly enough, to change the regeneration strategy – before NOx emissions increase. In the present proposal, the calculation of the “ratio of time spent under favourable conditions” can be reset (a) after each NSC regeneration (b) after reaching a calibrate able amount of NOx emitted from the engine which is in the order of magnitude of a typical maximum acceptable NOx load before regeneration, and (c) alternately monitor this ratio always during the past x seconds or since the last x gram of NOx emitted from the engine. Another suggested option is to monitor the ratio of low load continuously by applying time filters – such that the condition of low load is detected as a “moving average”. The same evaluation of a “favourable ratio” can be extended to the other mentioned conditions: “Ratio of time in a suitable gear” and “Ratio of time above a suitable temperature limit for regeneration”.

[0017] With the above disclosed method it is now possible to change the engine operation mode to Rich operation – before the NSC load gets excessively high. At the same time escalation should be avoided while the NOx load is low enough and more favorable conditions will arrive with a reasonable probability. The NOx reduction efficiency can be increased at lower associated fuel penalty. The oil dilution associated with rich operation is minimized. Soot emission from rich operation is minimized – with consequence of longer DPF regeneration intervals.

[0018] ‘Adapted’ or ‘arranged’, in the context of the instant disclosure, refers to the technical capability or the technical capacity of a component, in relation to which the term ‘adapted’ or ‘arranged’ is used, to carry out or executed a specified action or actions, upon the requirement of the specified action or actions to be carried out or executed. Moreover, the usage of the term ‘adapted’ or ‘arranged’ here, is in reference with the normal technical capability or technical capacity of the component, imparted by the design or the structure or the composition of the component, and not in reference with any special or extraneous capability or capacity, beyond the scope of the normal technical capability or technical capacity. Therefore there is a need to address this problem.

[0019]It should be understood that embodiments explained in the description above are only illustrative and do not limit the scope of this invention in terms of the type of high pressure pump 104 used. Many such embodiments and other modifications and changes in the embodiment explained in the description are envisaged. The scope of the invention is only limited by the scope of the claims.