METHOD FOR OPERATING AN ENGINE AND ENGINE

Description

[0001] The invention relates to a method for operating an internal combustion engine according to the preamble of claim 1 and to an internal combustion engine designed for carrying out the method.

[0002] In automotive engineering, it is common practice to filter soot particles from the exhaust gas, particularly from diesel engines, using a particulate filter. The aim is to prevent excessive accumulation of filtered particles in the particulate filter. Soot particles accumulated in the particulate filter can be continuously removed at temperatures above approximately 250°C through oxidation using NO₂ and H₂O as oxidizing agents. This effect is also known as the CRT effect (CRT = Continuous Regeneration Trap). Since the nitrogen oxides contained in the exhaust gas typically consist almost entirely of NO, it is common practice to increase the NO₂ content by oxidizing this NO on an oxidation catalyst located upstream of the particulate filter. A corresponding system consisting of a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) is known as a CRT system.

[0003] The formation of NO₂ at the oxidation catalyst is improved by high proportions of precious metals such as platinum and platinum group metals, although this is associated with corresponding costs and is therefore disadvantageous. If the soot removal rate achieved by the CRT effect is insufficient, the particulate filter is actively regenerated by thermally burning off the soot using the oxygen contained in the exhaust gas. However, this thermal regeneration requires significantly higher temperatures, typically exceeding 550°C. Such high temperatures rarely occur during normal driving, especially with diesel engines, and therefore often have to be generated by internal and / or external means.This can be achieved, for example, by enriching the exhaust gas with fuel, particularly in the form of unburned or partially burned fuel, by means of post-injection of fuel into the engine and subsequent exothermic oxidation of this fuel in an oxidation catalyst upstream of the particulate filter and / or in the particulate filter itself. The heating of the particulate filter can be influenced by appropriately adjusting the post-injection parameters, such as the timing and quantity of the post-injection. A wide variety of variations are described, for example, in WO 2005 / 003537 A1.

[0004] From EP 1 384 879 A2, a device for exhaust gas purification is known which comprises a diesel particulate filter and an oxidation catalyst arranged upstream. The heat generated during the oxidation of post-injected fuel is used to heat the particulate filter. When the temperature of the oxidation catalyst reaches or falls below a certain threshold, the post-injection is throttled.

[0005] Furthermore, DE 10 2006 028436 A1 discloses a method for operating a device for exhaust gas purification of an internal combustion engine with a particulate filter and a catalytic layer, in which the air flow through a combustion chamber of the internal combustion engine is reduced during certain operating phases of the regeneration of the particulate filter.

[0006] The object of the present invention is to provide an operating method for an internal combustion engine which effectively supports particulate filter regeneration even under unfavorable operating conditions. Furthermore, it is an object of the invention to provide an internal combustion engine in which particulate filter regeneration is enabled over the widest possible operating range, in particular also under partial or low load.

[0007] These problems are solved by a method with the features of claim 1 and by an internal combustion engine with the features of claim 6. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims.

[0008] In the method according to the invention, to support the regeneration of a particulate filter arranged in the exhaust system of the internal combustion engine and downstream of an oxidation catalyst, post-injections of fuel are carried out by means of an injection valve into at least one cylinder of the internal combustion engine. According to the invention, it is further provided that an advance is made for the opening and closing times of an exhaust valve of a cylinder of the internal combustion engine. The advance of the opening and closing times is carried out in a first temperature range for a temperature of the oxidation catalyst, and the post-injections take place in a second temperature range for the temperature of the oxidation catalyst, wherein an upper limit of the first temperature range has a lower value than an upper limit of the second temperature range.

[0009] Furthermore, the lower limit of the second temperature range is chosen so that it corresponds at least approximately to the start-up temperature of the oxidation catalyst.

[0010] The upper limit of the first temperature range falls into the second temperature range, with the upper limit of the first temperature range being no more than about 20K above the start-up temperature.

[0011] As the temperature of the oxidation catalyst increases, within the first temperature range there is essentially a reduction in the early shift for the opening and closing times of the exhaust valve.

[0012] In the range of the oxidation catalyst's start-up temperature, the early shift of the opening and closing times decreases significantly with increasing temperature of the oxidation catalyst.

[0013] When the upper limit of the first temperature range is reached, the advance of the control timing is completely reversed.

[0014] The invention is based on the understanding that, particularly at low engine loads, it may be necessary to raise the exhaust gas temperature to a correspondingly required level, both for carrying out discontinuous particulate filter regenerations through thermal soot combustion and for carrying out continuous particulate filter regeneration through soot oxidation using NO₂. However, post-injection of fuel has proven to have two disadvantages. Firstly, oxidation of fuel introduced into the exhaust gas by post-injection cannot occur if the oxidation catalyst is at a temperature below its activation temperature, i.e., if it cannot yet oxidize the fuel, which is present in the form of unburned or partially burned fuel, to any significant extent.On the other hand, it has been shown that unburned or partially burned fuel components in the exhaust gas can inhibit the oxidation catalyst's ability to oxidize NO to NO₂, particularly at relatively low temperatures. These disadvantages can be avoided by implementing the temperature-increasing measure of advancing the exhaust valve closing time at a significantly lower temperature than the post-injections. Starting from low temperatures, the exhaust gas is initially heated by advancing the exhaust valve timing. Once the oxidation catalyst has reached a suitable temperature level, further heating occurs through the oxidation of fuel introduced into the exhaust gas by the post-injections.

[0015] Post-injections preferably occur in a range of approximately 60° crank angle to approximately 170° crank angle, and particularly preferably in a range of 90° crank angle to 150° crank angle after top dead center during the power stroke in at least one cylinder. This results in a negligible influence of the post-injections on the torque delivered by the internal combustion engine. Essentially unburned, but optionally cracked, fuel is then expelled from the corresponding cylinder. This fuel consists predominantly of short- to medium-chain hydrocarbons, carbon monoxide, and hydrogen. If post-injections are performed, at least one such post-injection typically occurs after each torque-effective main injection.

[0016] Up to five individual post-injections per power stroke of a four-stroke internal combustion engine can occur within a time window of approximately 20 ms. Specifically, one or more injections can be performed within a power stroke over a period of 0.2 ms to 3 ms. This duration, for example, 0.2 ms to 3 ms, represents the speed-dependent time window per power stroke or cycle of each cylinder operating with post-injection.

[0017] The second temperature range, i.e., the temperature range in which post-injection is provided, preferably extends from a lower limit approximately in the range of the oxidation catalyst's activation temperature to the upper limit, which can be considered the optimal temperature for the development of the CRT effect and is approximately 450 °C for the oxidation catalyst. The activation temperature of a catalyst, also referred to as the light-off temperature, is generally understood to be the temperature at which a significant amount, typically about 50%, of the exhaust gas components to be treated is converted. A typical value for the activation temperature is around 230 °C to 250 °C.

[0018] At higher temperatures near the upper limit of the second temperature range, the downstream particulate filter is also advantageously heated by heat transfer. However, heating the oxidation catalyst by enriching the exhaust gas with fuel via in-cylinder post-injection, resulting in an exhaust gas temperature downstream of the oxidation catalyst exceeding 330°C and thus a particulate filter temperature also exceeding 330°C, can be quite sufficient. Typically, the upper limit of the second temperature range does not exceed 480°C for the oxidation catalyst or for exhaust gas temperatures on the exhaust side of the oxidation catalyst, in order to keep the effort as low as possible.

[0019] By adjusting the temperature of the oxidation catalyst through post-injection, particularly effective oxidation of nitrogen monoxide (NO) contained in the exhaust gas to nitrogen dioxide (NO₂) and particularly effective soot oxidation by the generated NO₂ are enabled. This, in turn, makes it possible to use an oxidation catalyst with a reduced loading of the precious metals platinum and / or palladium. Furthermore, particulate filter regenerations due to thermal soot combustion can be largely avoided. This keeps the thermal stress on the oxidation catalyst and adverse aging effects low. Therefore, little or no precious metal content is required to compensate for an age-related decrease in activity.It is therefore preferred that an oxidation catalyst be used whose precious metal content, based on a volume of the oxidation catalyst, is in the range of 170 g / m 3< to 700 g / m 3<, in particular a content of about 350 g / m 3<.

[0020] Precious metal loadings of catalysts are also given in g / ft³; in this unit, the aforementioned values ​​correspond to values ​​of 5 g / ft³ to 20 g / ft³, or preferably 10 g / ft³. Such precious metal loadings are comparatively low compared to typical values, which are in the range of about 40 g / ft³ to 70 g / ft³ (1,412 g / m³ to 2,472 g / m³).

[0021] Precious metals can include, in particular, platinum and / or palladium. Even with such a low precious metal loading of the oxidation catalyst compared to usual values, a significant oxidation activity of NO to NO₂ can be achieved. With an oxidation catalyst exhibiting the comparatively low precious metal content described here, a significant soot conversion rate based on the CRT reaction can be achieved even at temperatures above approximately 230°C.

[0022] Regarding the advance of the closing point of an exhaust valve in a cylinder, this results in the exhaust valve closing before the exhaust stroke of the respective cylinder has ended. This causes an intermediate compression of the exhaust gas in the cylinder. During this intermediate compression, compression work is performed, which leads to an increase in the temperature of the exhaust gas. In this way, the oxidation catalyst can be heated up in a particularly cost-effective manner and, in particular, brought to its activation temperature. It is especially preferred if the closing point of at least one exhaust valve is advanced in all cylinders of the internal combustion engine.

[0023] The advancement of the exhaust valve closing point can be achieved in particular by means of a so-called phaser, i.e., a variable camshaft adjustment. The phaser ensures that the exhaust valve of the corresponding cylinder of the internal combustion engine closes earlier and, in typical phaser designs, also opens earlier.

[0024] Advancement of the exhaust valve closing timing is a highly effective method for exhaust gas heating, which also works when post-injection is ineffective because the oxidation catalyst is still below its activation temperature. The initial temperature range for this advancement therefore preferably extends from very low temperatures of around 0 °C or even lower to an upper limit in the region of the oxidation catalyst's activation temperature.

[0025] By selecting the lower limit of the second temperature range to be approximately equal to the activation temperature of the oxidation catalyst, fuel enrichment of the exhaust gas as a result of post-injection only occurs once the oxidation catalyst has reached its activation temperature. This ensures that the oxidation catalyst can convert the fuel introduced into the exhaust gas to a particularly high degree and thus reach a temperature suitable for producing NO₂. In particular, it can be stipulated that exhaust gas enrichment or fuel post-injection is only activated when a specific threshold or trigger value of the oxidation catalyst temperature, for example, approximately 230 °C, is reached as the lower limit for the second temperature range.Generally, it is preferable for the exhaust gas to be enriched with fuel within a temperature range of 250 °C to 450 °C for the oxidation catalyst. This ensures that the oxidation catalyst reaches its activation temperature, but remains within a range where thermal damage to the catalyst is largely avoided.

[0026] By selecting the upper limit of the first temperature range within the second temperature range, a smooth transition occurs between the operation of the internal combustion engine with an advanced exhaust valve timing and the operation in which post-injection is performed. For example, particularly in the area of ​​the oxidation catalyst's activation temperature, the amount of post-injection fuel delivered to each cylinder of the internal combustion engine can be increased with increasing temperature, while simultaneously the advancement of the exhaust valve closing point(s) is reduced, especially compared to the pure heating operation of the oxidation catalyst. Thus, at the beginning, when the exhaust valve opens relatively early, only a small amount of fuel can be introduced into the second cylinder via post-injection. The further the closing or exhaust valve timing is advanced, the greater the amount of fuel that can be injected.The earlier the exhaust valve opening time is advanced, the greater the amount of fuel that can be injected into the cylinder via post-injection. This largely prevents cylinder wall wetting with post-injected fuel.

[0027] Undesirable cylinder wall wetting can occur if the exhaust valve closes early and also opens relatively early. In this case, the pressure in the combustion chamber drops during the power stroke of the respective cylinder. If post-injection occurs at this point, there is a risk of fuel wetting the cylinder wall. This is because, with reduced pressure in the combustion chamber, the fuel jet has a particularly long range during post-injection. Cylinder wall wetting can, in turn, lead to undesirable dilution of the engine lubricating oil. To prevent this, it can be ensured that the exhaust valve timing is not advanced when post-injections are performed, and conversely, that the closing or opening point of the exhaust valve is only advanced when no post-injection is (yet) taking place.Post-injection only begins once the exhaust valve timing returns to that of normal operation without re-compression. In this case, the upper limit of the first temperature range coincides with the lower limit of the second temperature range.

[0028] By reducing the advance of the exhaust valve opening and closing timing with increasing oxidation catalyst temperature, this advance is rapidly reset to zero, particularly in the vicinity of the starting temperature or shortly before it is reached, thus restoring normal operating conditions. Further heating of the oxidation catalyst then occurs through the oxidation of post-injected fuel.

[0029] In a further embodiment of the invention, the post-injections in at least one of the cylinders of the internal combustion engine are timed such that first periods of a predetermined duration, in which post-injections are carried out in each working cycle of a respective cylinder, alternate in direct succession with second periods of a predetermined duration, in which the post-injections are prevented.

[0030] Depending on the heating requirements, a greater or lesser number of alternating, directly successive first periods with exhaust gas enrichment or fuel post-injection into at least one cylinder, and second periods without exhaust gas enrichment or fuel post-injection, are provided. This pulsed fuel enrichment of the exhaust gas, or the intermittent implementation of post-injection into at least one cylinder, allows for a particularly high NO₂ formation at the oxidation catalyst and a particularly high oxidation rate of soot deposited in the particulate filter by NO₂ and H₂O. The CRT effect is therefore particularly pronounced.This is partly due to the fact that during fuel enrichment of the exhaust gas, i.e., during the first phase, the oxidation catalyst is brought to a temperature that allows for the highest possible activity with regard to the oxidation of NO contained in the exhaust gas to NO₂. This oxidation can then occur unhindered and without inhibition of the oxidation catalyst by CO and / or HC during the second phase. Pulsed exhaust gas enrichment with fuel via in-engine post-injection, in which phases or initial periods of exhaust gas enrichment alternate with phases or second periods without exhaust gas enrichment, thus leads to effective, passive soot oxidation of the particulate filter, i.e., to effective passive regeneration of the filter.

[0031] During the initial periods of exhaust gas enrichment or post-injection, the oxidation catalyst, the exhaust gas, and the downstream particulate filter are heated by the exothermic oxidation of unburned and / or partially burned fuel within the oxidation catalyst. During periods without exhaust gas enrichment or post-injection, NO₂ is formed and soot oxidation occurs through the CRT effect. Due to their heat capacity, the oxidation catalyst and the particulate filter cool only slightly during this process. Therefore, the temperature required for particulate filter regeneration can be significantly reduced compared to active regeneration using thermal soot combustion via oxidation with oxygen. This results in a reduction of the thermal stress on the oxidation catalyst and the particulate filter, which may be a coated, i.e., catalytically active, particulate filter.

[0032] Due to the low temperature stress, the aging of catalysts installed in the exhaust system, such as the oxidation catalyst and the particulate filter, as well as any SCR catalyst (SCR = Selective Catalytic Reduction) or an ammonia slip catalyst, is reduced. This makes it possible, in particular, to significantly reduce the precious metal loading of the oxidation catalyst and / or the particulate filter. The SCR catalyst and / or the ammonia slip catalyst, which oxidizes any ammonia that may escape from the SCR, can be dimensioned accordingly smaller. Furthermore, particularly cost-effective materials can be used for the SCR catalyst.

[0033] In the second phase, during which the exhaust gas is typically free of unburned fuel and comparatively rich in oxygen, the surface of the oxidation catalyst is cleaned of absorbed and inhibiting fuel molecules. This increases the availability and the catalytically effective surface area of ​​the oxidation catalyst.

[0034] This process achieves the overall cleaning of all engine components and exhaust system components through intermittent exposure to fuel-enriched exhaust gas. Cleaning and detoxifying the oxidation catalyst also lowers its activation temperature. Furthermore, the temporal separation of the heating phase (i.e., exhaust gas enrichment or post-injection of fuel during the first phase) from the passive regeneration phase results in a particularly good passive soot combustion rate.

[0035] The duration of the first time interval and / or the duration of the second time interval is specified as a function of the temperature of the oxidation catalyst. The temperature can be measured, in particular, downstream of the oxidation catalyst.

[0036] With a comparatively cold oxidation catalyst, fuel can be introduced into at least one cylinder for a longer initial period than with a comparatively hot oxidation catalyst. The initial period itself, i.e., the period during which exhaust gas enrichment or post-injection occurs, is preferably in the range of 1 s to 300 s, particularly from 3 s to 30 s. The duration of the second period, during which fuel enrichment or post-injection into a second cylinder is prevented, can be in the range of 0.5 s to 200 s, particularly from 10 s to 60 s. This second period, or post-injection pause, is variable, and its duration is, in principle, independent of the duration of the first period.Therefore, shortening or lengthening the first time period does not necessarily lead to a shortening or lengthening of the second time period. With the time periods described here, a particularly thorough passive regeneration of the particulate filter can be achieved using the CRT effect.

[0037] With a relatively cold oxidation catalyst, when it has only been heated slightly above its activation temperature, it is also advantageous to provide no post-injection pause or only a very short one, whereas with a relatively hot oxidation catalyst, a correspondingly long post-injection pause may be necessary. In particular, within a temperature range of 300°C to 350°C of the oxidation catalyst temperature or downstream of this range, no post-injection pause or at most a very short one may be required.

[0038] Furthermore, the duration of individual post-injections of fuel into at least one cylinder per combustion cycle can be adjusted depending on the temperature. A short duration per combustion cycle is particularly advantageous when the temperature is already high. This allows the high temperature to be maintained easily, while the short duration largely prevents fuel from coating the cylinder walls of at least one other cylinder. Conversely, to raise the temperature of the oxidation catalyst considerably, a longer duration of post-injections per combustion cycle can be provided, during which at least one post-injection is performed in at least one cylinder. This results in large injection quantities, leading to a correspondingly rapid temperature increase.

[0039] In a further embodiment of the invention, a split recirculation of exhaust gas from the cylinders of the internal combustion engine into an intake air tract of the internal combustion engine is provided such that exhaust gas from only a first part of the cylinders can be fed to the intake air tract of the internal combustion engine to a significant extent, and exhaust gas from the remaining second part of the cylinders is fed to the oxidation catalyst essentially completely and without recirculation, with post-injection of fuel occurring only in at least one of the cylinders of the second part of the cylinders. In other words, exhaust gas is enriched only in one or more cylinders whose exhaust gas is not recirculated. Or, put another way, fuel enrichment of exhaust gas by means of post-injection in one or more cylinders whose exhaust gas is recirculated is generally avoided.

[0040] This is based on the understanding that if the exhaust gas is enriched with unburned and / or partially burned fuel, undesirable fouling of the intake air tract can occur if this exhaust gas is recirculated. This is avoided in this system because exhaust gas enrichment only occurs in the cylinder(s) whose exhaust gas cannot, or can only to a negligible extent, enter an exhaust gas recirculation line through which exhaust gas can be returned to the intake air tract of the internal combustion engine. By preventing the intake air tract, and in particular any charge air cooler located within the intake air tract or intake air path, from being exposed to unburned fuel, reliable operation of the internal combustion engine can be ensured in a particularly simple manner.

[0041] In a further advantageous embodiment of the invention, a control current of the injection valve is adjusted to actuate a valve needle of the injection valve as a function of the temperature of the oxidation catalyst to carry out the post-injections. By changing the control current, particularly precise adjustment, especially of small post-injection quantities, is possible, and the temperature control of the particulate filter is thus improved. The method according to the invention is particularly suitable for supporting continuous particulate filter regeneration by soot oxidation with NO₂ according to the CRT effect. The injection valve can be an electromagnetic or piezoelectric injection valve.In the preferred case of an electromagnetic valve, the control current flows through a solenoid coil, causing the injection valve needle to lift and thus opening the valve for post-injection of fuel. The duration and height of the needle lift, together with the fuel injection pressure and the flow coefficient of the injection valve, determine the amount of fuel injected per post-injection and thus the amount of chemical energy introduced into the exhaust gas. This energy, after conversion into heat energy through exothermic oxidation at the oxidation catalyst or in the particulate filter, is available for heating.

[0042] In a further embodiment of the invention, above a predefinable temperature for the oxidation catalyst, the post-injection quantity is limited by setting a reduced needle stroke amplitude of the injection valve needle during post-injections compared to the maximum needle stroke amplitude. In this case, the valve needle is not fully moved to its end position in the open state of the injection valve, thus allowing for precise adjustment of small post-injection quantities. For this purpose, it can be provided that a reduced control current is set below a nominal control current value. The magnetic force generated for the opening movement of the valve needle is reduced in this case compared to control with the nominal current, and the opening of the injection valve occurs somewhat more slowly and with a delay.Typically, the valve needle drops immediately after reaching the reduced stroke amplitude required to terminate the post-injection. The duration of a post-injection within a power stroke is therefore comparatively short. This is particularly advantageous when the temperature is already high. This allows the high temperature to be maintained easily, and at the same time, the short duration largely prevents the cylinder walls of at least one cylinder operating with post-injection from being wetted by the injected fuel.

[0043] A similar behavior occurs in another advantageous variant when the actuation time of the injection valve is reduced along with the actuation current to achieve the reduced needle stroke amplitude. Below a lower limit actuation time, the valve needle of an injection valve actuated with the nominal value of the actuation current, or even with a reduced actuation current, no longer reaches the maximum possible stroke, but only a reduced stroke, and after reaching this reduced stroke amplitude position, it quickly returns to the closed position.

[0044] The internal combustion engine according to the invention has an exhaust system in which an oxidation catalyst and a particulate filter flow-downstream of the oxidation catalyst are arranged, and is equipped with control means for carrying out a process according to at least one of the variants described above. In particular, appropriate control means such as an electronic control unit and corresponding actuators and sensors are provided for performing the control functions.

[0045] The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the figure description and / or shown in the figures alone, can be used not only in the combination specified in each case, but also in other combinations or on their own, without leaving the scope of the invention.

[0046] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 An arrangement of an exhaust system on an internal combustion engine of a vehicle, wherein fuel is only injected post-injection into two cylinders of the internal combustion engine, from which no exhaust gas is recirculated; Fig. 2 The shifting of an opening and closing time of an exhaust valve of one of the cylinders of the internal combustion engine and the change in pressure in the combustion chamber associated with this shift; Fig. 3 A graph illustrating the procedure for an advance of exhaust valve timing in conjunction with an adjustment of post-injection quantities; Fig. 4 A graph showing an advantageous course of the dependence of an injection valve of the internal combustion engine on the temperature of an oxidation catalyst associated with the internal combustion engine; Fig.5. Curves illustrating the pulsed post-injection of fuel, the associated heating of an oxidation catalyst in the exhaust system, and the changes in NO2 content in the exhaust gas associated with the pulsed post-injection; Fig. 6. a graph illustrating the time course of a control current and a needle opening during post-injection; Fig. 7. another graph in which the full control current is applied for a shorter period of time than in the graph according to . Fig. 6 ; Fig. 8 a graph according to Fig. 7 , in which, however, an additional amplitude of the control current is shown in relation to the graph according to Fig. 7 is reduced; Fig. 9 a graph according to Fig. 7 , however, with a further reduced duration over which the control current is applied; Fig. 10 the increase of the post-injection pause between time periods during which post-injections are carried out, as a function of the temperature of the oxidation catalyst; and Fig. 11 a diagram to illustrate an advantageous dependence of the injection valve control duration and the post-injection pause on the temperature of the oxidation catalyst.

[0047] Fig. 1 Figure 1 shows an arrangement 10 of an exhaust system 12 on an internal combustion engine 14 of a vehicle, in particular a commercial vehicle, designed as a direct-injection 4-stroke diesel engine. The diesel engine 14 has four cylinders 16, 18, 20, 22. A group of two first cylinders 16, 18 is fluidically coupled to a turbine 26 of an exhaust gas turbocharger via an exhaust line 24. An exhaust gas recirculation line 28 branches off from the exhaust line 24, into which the exhaust gas from the two first cylinders 16, 18 enters. The exhaust gas recirculation line 28 is designed as a high-pressure exhaust gas recirculation line, but a low-pressure exhaust gas recirculation system can also be provided. The amount of exhaust gas recirculated into an intake air tract of the internal combustion engine 14 (not shown) can be adjusted via an exhaust gas recirculation valve 30, which is shown only schematically.

[0048] The two additional, second cylinders 20, 22 of the internal combustion engine 14 are fluidically coupled to the turbine 26 via a further exhaust line 32. However, no exhaust gas recirculation line branches off from this second exhaust line 32. Therefore, exhaust gas emitted from the second cylinders 20, 22 cannot, or at most can only to a negligible extent, enter the exhaust gas recirculation line 28 and reach the intake side of the internal combustion engine 14. The exhaust lines 24 and 32 can be components of a two-part exhaust manifold.

[0049] The exhaust system 12 comprises an oxidation catalyst 34, in particular a diesel oxidation catalyst, arranged downstream of the turbine 26, followed by a particulate filter 36, in particular a diesel particulate filter. The particulate filter 36 can, in particular, be an oxidation-catalyzed particulate filter. Downstream of the particulate filter 36, the exhaust system 12 can include an SCR catalyst 44, which can be followed by an (optional) catalyst for oxidizing ammonia, i.e., a so-called ammonia slip catalyst 46. If an SCR catalyst 44 is provided, a reducing agent solution for selective nitrogen oxide reduction, preferably an aqueous urea solution, is added upstream of it via a metering device 48, from which ammonia is formed in the hot exhaust gas.This is then converted in the SCR catalyst 44 (SCR = Selective Catalytic Reduction) in a selective catalytic reduction reaction with nitrogen oxides contained in the exhaust gas to nitrogen and water.

[0050] To operate the diesel engine 14, fuel is injected into cylinders 16, 18, 20, and 22 via injection valves (not shown). These injection valves are controlled by a control unit 50 with a corresponding control current. Without limiting the generality of the description, it is assumed that the injection valves are electromagnetically actuated valve needles. Multiple individual fuel injections can be performed per operating cycle. In this case, an in-cylinder post-injection of fuel is used to raise the temperature of the oxidation catalyst 34, enabling it to oxidize NO present in the exhaust gas to NO₂ with maximum efficiency. A single post-injection of fuel can comprise several, in particular up to five, individual post-injection events within a single operating stroke of a cylinder equipped with post-injection.For simplicity, the following is referred to as post-injection. Soot particles that have accumulated on the particulate filter 36 can be oxidized by the NO₂ formed on the oxidation catalyst 34 and by H₂O. This passive regeneration of the particulate filter 36 by soot oxidation with NO₂ preferably takes place in a temperature range above approximately 300°C, i.e., in a temperature range in which thermal aging of the oxidation catalyst 34 is limited to a comparatively small degree. An ideal and particularly preferred temperature is approximately 450°C.

[0051] If the temperature of the oxidation catalyst 34 is below the start-up temperature, post-injection for heating is not practical and preferably not provided or only to a very limited extent. In such a case, the oxidation catalyst 34 is preferably heated by shifting the timing of at least one exhaust valve in at least one of the cylinders 16, 18, 20, 22 of the diesel engine 14. For this purpose, a phase adjuster is preferably used. By such variable camshaft control, an opening time 82 and a closing time 84 of an exhaust valve are set (cf. Fig. 2 ) in particular the respective second cylinder 20, 22 is advanced. An advance of up to approximately 60° crank angle can be provided.

[0052] This is in Fig. 2 Figure 86 illustrates the stroke of an exhaust valve during normal operation of the diesel engine 14. Accordingly, the exhaust valve closes when an intake valve opens, the stroke of which is illustrated by another curve 88. The phaser then advances the opening time 82 and the closing time 84 of the exhaust valve, as shown in Figure 1. Fig. 2 This is illustrated by another curve at 90 degrees.

[0053] Accordingly, the exhaust valve closes at point 92 before the exhaust stroke of the respective cylinder is complete. This results in an intermediate compression in the cylinder's combustion chamber, which is represented as a maximum 94 or peak of a further curve 96 indicating the pressure in the combustion chamber. Fig. 2 This illustrates the point.

[0054] Just as the closing time 92 is advanced, so too is the opening time 98 of the exhaust valve. This causes the combustion chamber pressure to drop, in this case by approximately 140° crank angle. To illustrate this drop in combustion chamber pressure, see in Fig. 2 Another curve 100 shows the pressure conditions in the combustion chamber during normal actuation of the exhaust valve, corresponding to curve 86.

[0055] The temperature increase associated with the intermediate compression, which results from the residual gas being compressed further when the exhaust valve closes earlier, allows the oxidation catalyst 34 to be brought up to its activation temperature particularly easily and with minimal effort. Preferably, the phase adjuster acts on all cylinders 16, 18, 20, 22 of the diesel engine 14 in the same way.

[0056] However, if post-injection occurs in the combustion chamber when the pressure is reduced due to the early opening of the exhaust valve, the lower back pressure in the cylinder allows the fuel jet to spray against the cylinder wall with higher momentum and a greater mass fraction. This leads to an undesirable ingress of fuel into the engine oil.

[0057] Therefore, in the present case, with active post-injection, the opening time 82 and the closing time 84 of the exhaust valve are preferably not advanced or only slightly advanced when post-injections take place. Conversely, post-injections are only carried out at all, and especially with a larger quantity of fuel 42, when the advanced opening time 98 and the advanced closing time 92 have been advanced back to their normal positions.

[0058] In this process, an overlap can be provided, particularly in the area of ​​the activation temperature of the oxidation catalyst 34, such that an increasing amount of fuel 42 is injected by means of post-injection, while the earlier timings are gradually shifted back towards the normal opening time 82 and the normal closing time 84. This minimizes the wetting of the cylinder wall with fuel 42 introduced during post-injection.

[0059] An exemplary approach is described in Fig. 3 schematically illustrated. In the graph of Fig. 3 A curve 102 is assigned to an early shift V of the opening and closing times 82, 92 of a respective exhaust valve, plotted on a left ordinate 106. An early shift V is provided in a first temperature range of the oxidation catalyst 34, designated Δϑ1.

[0060] Starting from low temperatures ϑ below the activation temperature of the oxidation catalyst 34, designated ϑ A, the advance V is reduced from 100%, corresponding to approximately 60° crank angle, to the normal operating setting with increasing temperature ϑ plotted on an abscissa 110. In particular, it is reduced increasingly sharply in the region of the activation temperature ϑ A. Upon reaching an upper limit Δϑ1 o of the first temperature range Δϑ1, the advance V of the valve timing is completely reversed, and the normal operation of the diesel engine 14 with respect to the valve timing of the exhaust valves is achieved. Above the upper limit Δϑ1 o of the first temperature range Δϑ1, no advance V of the valve timing is provided. The upper limit Δϑ1 o of the first temperature range Δϑ1 preferably corresponds approximately to the activation temperature ϑ A of the oxidation catalyst 34, but can, as in Fig. 3 The temperature may also be slightly higher. This ensures that the oxidation catalyst 34 is reliably heated to or above the start-up temperature ϑ A by means of the advance V of the exhaust valve timing. However, it is preferred that the upper limit Δϑ1 o of the first temperature range Δϑ1 is no more than about 20 K above the start-up temperature ϑ A.

[0061] Upon reaching the start-up temperature ϑ A, the oxidation catalyst 34 is able to convert unburned or partially burned fuel components, which is why further heating can be achieved by means of the post-injection of fuel into at least one of the second cylinders 20, 22 as described above. A Fig. 3 The total post-injection quantity m NE per working cycle, represented as curve 104 and plotted on ordinate 108, is therefore set so that it initially increases rapidly with increasing temperature ϑ within a second temperature range Δϑ2. As the heating requirement decreases and the temperature ϑ of the oxidation catalyst 34 approaches the target temperature of approximately 450 °C, the total post-injection quantity m NE then decreases again. Thus, post-injections only occur in the second temperature range Δϑ2. The lower limit of the second temperature range Δϑ2 corresponds at least approximately to the start-up temperature ϑ A of the oxidation catalyst 34. The predefinable upper limit Δϑ2 o of the second temperature range Δϑ2 corresponds to the aforementioned target temperature at which an at least approximately optimal CRT effect can be achieved.By combining early shift V of the exhaust valve timing and post-injection, an effective and fuel-saving heating of the oxidation catalyst 34 and thus NO 2 formation can be achieved, which enables effective soot oxidation in the particulate filter 36 and therefore largely avoids thermal particulate filter regenerations through oxygen-induced soot combustion.

[0062] It is intended that, during in-engine post-injection of fuel into each cylinder operated with post-injection, the control current of the respective injection valve is adjusted as a function of the temperature ϑ of the oxidation catalyst 34. This is exemplified by a Fig. 4 The diagram shown illustrates this. In this diagram, curve 80 represents the course of the control current Iv, plotted on the ordinate 76, as a function of the temperature ϑ of the oxidation catalyst 34, plotted on the abscissa 78. Here, upon reaching the activation temperature ϑA of the oxidation catalyst 34 with respect to the oxidation of hydrocarbons, a respective injection valve is controlled with the nominal value of the control current Iv. In order to heat the oxidation catalyst 34 as quickly as possible, a comparatively large post-injection quantity is preferably injected first, and the control duration of the injection valve is also correspondingly long, so that the valve needle of the injection valve is fully open during a post-injection process.

[0063] As the temperature ϑ of the oxidation catalyst 34 increases, the heating requirement decreases, and therefore the post-injection quantity is reduced. Once a predefinable limit temperature ϑG of approximately 380 °C to 420 °C is reached, the post-injection quantity is reduced by progressively decreasing the control current Iv as the temperature ϑ continues to rise. This initially causes the valve needle to open slightly later and less rapidly, resulting in a reduced fuel quantity being injected for the same control duration. Depending on the control duration, further reductions in the control current Iv may prevent the valve needle from reaching its maximum possible stroke amplitude. This allows for particularly precise adjustment of small post-injection quantities. The oxidation catalyst 34 can therefore be maintained at an elevated temperature very effectively or heated to a desired target temperature without the risk of overheating.The aim is to reduce the control current Iv to a minimum value IV,min to ensure that the valve needle opens even with a reduced control current Iv. As the temperature ϑ of the oxidation catalyst 34 continues to increase, once the upper limit Δϑ2 o of the second temperature range Δϑ2, in which post-injections are provided, is reached (approximately 450 °C to preferably a maximum of 480 °C), no further heating is required and the post-injections are terminated. To avoid an undesirable influence on the torque-effective main injection and on any pre-injection when the control current Iv is reduced, it is preferably provided that the timing and duration of these fuel injections are adjusted accordingly.

[0064] The enrichment of the exhaust gas with unburned or partially burned fuel caused by post-injection can, however, lead to an undesirable inhibition or passivation of the oxidation catalyst 34 with respect to its NO₂ formation activity. To nevertheless oxidize the NO contained in the exhaust gas to NO₂ to the greatest extent possible, it is preferably provided to interrupt the post-injection repeatedly. This eliminates the inhibition, and the oxidation catalyst 34 can again oxidize NO to NO₂ to a greater extent. To simultaneously ensure a heating function, it is provided to operate the diesel engine 14 alternately in phases with and without post-injection. In other words, in this operating mode, initial periods are provided in which post-injection takes place, with these initial periods alternating directly with subsequent periods in which post-injection is prevented.This procedure will be illustrated below using . Fig. 5 explained in more detail.

[0065] In Fig. 5 The figure shows a section of a plurality of alternating and immediately successive time intervals in which post-injection occurs and is prevented. In a first phase or time interval 38, post-injections are carried out into the diesel engine 14 to increase the temperature of the oxidation catalyst 34. In a subsequent second phase or second time interval 40, no post-injection is carried out; thus, the execution of post-injections is prevented. The post-injections are therefore pulsed or clocked, so that the first time interval 38 with post-injections alternates with the second time interval 40 without post-injections.

[0066] However, in the present case, only the second cylinders 20, 22 are subject to post-injection of fuel 42 (see Fig. 1 ), whose exhaust gas is not recirculated, which are therefore connected to the second exhaust line 32. In contrast, no post-injection takes place in the first two cylinders 16, 18 of the diesel engine 14; they are therefore not supplied with post-injected fuel 42 during the first time period 38.

[0067] The post-injection of fuel 42 therefore takes place exclusively in the two second cylinders 20, 22 and not in the two first cylinders 16, 18, whose exhaust gas is, or can be, recirculated to a more or less significant extent into the intake air tract. This prevents the exhaust gas recirculation line 28, an exhaust gas recirculation cooler (not shown), and the intake air tract from being exposed to post-injected, i.e., unburned, fuel, thus preventing so-called coking.

[0068] Post-injection preferably occurs in the range of 60° crank angle to 170° crank angle after top dead center during the power stroke of the two second cylinders 20, 22. A range of 90° crank angle to 150° crank angle after top dead center is particularly preferred. For each individual post-injection, and depending on the temperature of the oxidation catalyst 34, a quantity of fuel 42 from 0 mg to 60 mg per cylinder 20, 22 and liter of displacement of the respective cylinder 20, 22 can be freely adjusted.

[0069] Preferably, the control unit 50 ensures that no fuel is injected into the two first cylinders 16, 18 during the first time period 38, while only exhaust gas from the two second cylinders 20, 22 is enriched with unburned hydrocarbons via post-injection.

[0070] In Fig. 5 In a first curve, bars 54 illustrate the amount of fuel 42 which is introduced into the two second cylinders 20, 22 via the in-engine post-injection during the first time period 38.

[0071] A second curve 56 in Fig. 5 The figure illustrates the ratio of NO₂ to NOₓ downstream of the oxidation catalyst 34 as a result of this intermittent or pulsed operation, i.e., the injection of post-injected fuel 42 into the two second cylinders 20, 22 and the subsequent second period 40, during which the passive regeneration of the particulate filter 36 mainly takes place. During the heating phase, i.e., during the first period 38, there is therefore virtually no NO₂ in the exhaust gas. However, a large amount of NO₂ is formed when no more fuel is injected into the second cylinders 20, 22 of the diesel engine 14 during the second period 40. Thus, during the second period 40, i.e., in regeneration mode, a comparatively large amount of NO₂ is available in the exhaust gas.

[0072] Another curve 58 illustrates the temperature of the exhaust gas downstream of the oxidation catalyst 34. This temperature fluctuates according to the amount of fuel applied to the exhaust gas in heating mode, and is therefore higher during the first period 38 than in the subsequent regeneration phase or second period 40. However, due to the comparatively high heat capacity of the oxidation catalyst 34, these temperature fluctuations are dampened. The temperature fluctuation on the outlet side of the particulate filter 36, which begins with a time delay, is dampened even more. Fig. 5 This is illustrated by another curve 60.

[0073] Fig. 6 Figure 38 shows in detail how, during the first time interval 38, a respective injector is actuated to inject the fuel 42 into the two second cylinders 20, 22 in a single post-injection event. Here, T1 denotes the actuation time of the injector for carrying out a single post-injection event per power stroke of the second cylinder 20, 22, whereby the actuation time T1 can be, in particular, 0.2 ms to 20 ms. However, several, typically up to four, individual post-injections can also be carried out during this time interval.

[0074] In Fig. 6 T2 corresponds to the first time interval 38, i.e., a period during which fuel 42 is injected into a respective second cylinder 20, 22 during each working cycle. This period is on the order of 1 s to 300 s. A post-injection pause corresponds to the second time interval 40, and it is calculated according to Fig. 6 from the difference of T3 - T2, where T3 is the period from the start of a first post-injection to the end of the post-injection pause.

[0075] A curve 62 gives in Fig. 6 The control current Iv, which causes the injection valve to open, lifts the injection valve needle, and thus enables the valve opening to allow post-injection of fuel, is shown. A value of the control current Iv is located on the left ordinate 65 of the graph in Fig. 6 The control current Iv is applied to the injection valve of a respective second cylinder 20, 22 during the control duration T1 of a single post-injection process. Fig. 6 Figure 64 illustrates a further time interval, which indicates the time between two working cycles of the second cylinders 20 and 22, i.e., the time between two actuation durations T1. This time interval 64 is correspondingly short at high engine speed of the diesel engine 14.

[0076] In the operating mode according to Fig. 6 The respective injection valve is supplied with a comparatively large control current Iv for a comparatively long control duration T1. Accordingly, a needle opening or valve needle lift H of 100% corresponding to the maximum needle lift amplitude is achieved over a comparatively long period, as illustrated by curve 63, where a value of the needle opening H lies on an ordinate 66 of the graph. Fig. 6 This setting is advantageous when rapid heating of the oxidation catalyst 34 is desired. Particularly at comparatively high loads of more than approximately 60% of the rated load of the diesel engine 14 and / or at highly fluctuating loads, a fuel-saving temperature setting of the oxidation catalyst 34 to achieve a high NO₂ content in the exhaust gas is possible, provided that the post-injection pause is relatively long and more than approximately twice the initial time interval T₂ is also relatively long.

[0077] A post-injection quantity adapted to the respective heating requirement for the oxidation catalyst 34 can advantageously be achieved, for example, not only by changing or reducing the control current Iv, but also by changing the control duration T1. In the graph according to Fig. 7 In the illustrated operating mode of the diesel engine 14, the control duration T1 of the control current Iv is shortened, and the time interval 64 is correspondingly longer. While a needle opening H of 100% is still achieved, it is only for a short time. Therefore, a smaller quantity of fuel 42 is injected into the two second cylinders 20 and 22. Such an operating mode is particularly advantageous when only a comparatively small temperature increase of the oxidation catalyst 34 is desired, for example, when the oxidation catalyst 34 already has a comparatively high temperature ϑ of approximately 400 °C or above. If the engine speed is comparatively low, for example, below 1500 rpm, the time interval 64 is correspondingly extended, and only a comparatively small number of post-injections can be carried out per unit of time.In such a case, it is advantageous to shorten the injection pause T 3 - T 2 and to extend the first time interval T 2 accordingly, for example to a larger value than the post-injection pause.

[0078] If the control duration T 1 is reduced according to Fig. 7 Additionally, if the control current Iv is reduced, the post-injection quantity can be adjusted even more precisely, and in particular reduced even further. In such a case, in Fig. 8 In the illustrated case, the needle stroke H no longer reaches the value of the maximum possible needle stroke amplitude, but only a more or less reduced needle stroke amplitude corresponding to the reduced height or amplitude 68 of the control current I v. Accordingly, even a smaller needle opening H than the needle opening H of 100% is only achieved for a very short duration.

[0079] In the operating mode of the internal combustion engine 14 according to Fig. 9 The reduced needle stroke H of less than 100% is achieved by applying the nominal control current I v to the injection valve analogously. Fig. 7 achieved, but with a further reduced control duration T 1 .

[0080] Thus, by reducing the control current Iv or the control duration T1, a reduced needle stroke amplitude of the injection valve needle can be set compared to a maximum needle stroke amplitude, thereby limiting the post-injection quantity. Incomplete opening of the valve needle is due to its ballistic properties, and this condition can also be described as ballistic needle stroke.

[0081] A reduced amount of post-injected fuel 42 is particularly intended when the oxidation catalyst 34 has reached a comparatively high temperature. In this case, the temperature of the oxidation catalyst 34 is maintained to a particularly high degree, while undesirable wetting of the walls of the second cylinders 20, 22 with the post-injected fuel 42 does not occur.

[0082] Within the first time interval 38, full variability of the control duration T 1 and the amplitude 68 of the control current I v is given, which allows the post-injection quantity to be adapted flexibly and almost independently of the engine operating state, but taking it into account, to a heating requirement for the oxidation catalyst 34 or the particulate filter 36 and at the same time achieve a strong CRT effect.

[0083] An example of this is in a Fig. 10 The diagram shown illustrates an example of a post-injection pause duration preferably set as a function of the temperature ϑ of the oxidation catalyst 34. The duration of the post-injection pause T3 - T2 is shown on the abscissa 70, and the temperature ϑ on the outlet side of the oxidation catalyst 34 is shown on the ordinate 72. According to the curve 74 shown in the diagram, a post-injection pause or time interval 40 is preferably, and particularly in a temperature range close to a target temperature of approximately 450 °C, set to an increasingly longer duration with increasing temperature ϑ of the oxidation catalyst 34. With a less heated oxidation catalyst 34, no post-injection pause or only a very short one is set, depending on the heating requirements, while with a comparatively hot oxidation catalyst 34, a longer second time interval 40 is present.

[0084] Another example of a preferred approach for setting pulsed post-injection is found in a Fig. 11 The diagram shown illustrates the setting of the injection duration T1 (curve 112) and the post-injection pause T3-T2 (curve 114) as a function of the temperature ϑ of the oxidation catalyst 34, plotted on an abscissa 120. The injection duration T1 is assigned to a left ordinate 116 and the post-injection pause T3-T2 to a right ordinate 118. Analogous to curve 74 in Fig. 10The duration of the injection pause increases with the increasing temperature ϑ of the oxidation catalyst 34. Simultaneously, once the activation temperature ϑ A of the oxidation catalyst 34 is exceeded, a rapidly increasing control duration T 1 is set. This results in a comparatively high post-injection quantity, which enables rapid heating of the oxidation catalyst 34. As the temperature ϑ of the oxidation catalyst 34 increases, the heating requirement decreases and the control duration T 1 is reduced until the post-injection operation is terminated when the predefinable upper limit Δϑ2 o of approximately 450 °C of the second temperature range Δϑ2 is reached. Below a control duration TB, the ballistic behavior of the valve needle, described above, occurs, with a reduced stroke amplitude compared to the maximum stroke amplitude. Reference symbol list

[0085] 10 Arrangement 12 Exhaust system 14 Internal combustion engine 16 Cylinder 18 Cylinder 20 Cylinder 22 Cylinder 24 Exhaust pipe 26 Turbine 28 Exhaust gas recirculation line 30 Exhaust gas recirculation valve 32 Exhaust pipe 34 Oxidation catalyst 36 Particulate filter 38 Time span 40 Time span 42 Fuel 44 SCR catalyst 46 Ammonia slip catalyst 48 Metering device 50 Control unit 52 Curve 54 Bar 56 Curve 58 Curve 60 Curve 62 Curve 63 Curve 64 Time span 65 Ordinate 66 Ordinate 68 Amplitude 70 Abscissa 72 Ordinate 74 Curve 76 Ordinate 78 Abscissa 80 Curve 82 Opening time 84 Closing time 86 Curve 88 Curve 90 Curve 92 Closing time 94 Maximum 96 Curve 98 Opening time 100 Curve 102 Curve 104 Curve 106 Ordinate 108 Ordinate 110 Abscissa 112 Curve 114 Curve 116 Ordinate 118 Ordinate 120 Abscissa

Claims

1. A method of operating a combustion engine (14), in which post-injections of fuel by means of an injector valve occur in at least one cylinder (16, 18, 20, 22) of the combustion engine (14) for supporting regeneration of a particle filter (36) disposed in an exhaust tract of the combustion engine (14) downstream of an oxidation catalytic converter (34), characterised in that an advance (V) for an opening time (82) and a closing time (92) of an exhaust valve of a cylinder (16, 18, 20, 22) of the combustion engine (14) is performed such that intermediate compression of exhaust gas occurs in the combustion chamber of the cylinder (16, 18, 20, 22), wherein the advance (V) of the opening time (82) and the closing time (92) is performed in a first temperature range (Δϑ1) for a temperature (ϑ) of the oxidation catalytic converter (34), and the post-injections occur in a second temperature range (Δϑ2) for the temperature (ϑ) of the oxidation catalytic converter (34), wherein - a range upper limit (Δϑ1o) of the first temperature range (Δϑ1) has a lower value than a range upper limit (Δϑ2o) of the second temperature range (Δϑ2); - a range lower limit of the second temperature range (Δϑ2) corresponds at least approximately to a light-off temperature (ϑA) of the oxidation catalytic converter (34); - the range upper limit (Δϑ1o) of the first temperature range (Δϑ1) falls in the second temperature range (Δϑ2), wherein the range upper limit (Δϑ1o) of the first temperature range (Δϑ1) exceeds the light-off temperature (ϑA) by no more than about 20 K; - within the first temperature range (Δϑ1), a reduction of the advance (V) for the opening time (82) and the closing time (92) of the exhaust valve substantially occurs with increasing temperature (ϑ); - in the region of the light-off temperature (ϑA) of the oxidation catalytic converter (34), an increasingly high reduction of the advance (V) of the opening time (82) and the closing time (92) occurs with increasing temperature (ϑ) of the oxidation catalytic converter (34); and - upon reaching the range upper limit (Δϑ1o) of the first temperature range (Δϑ1), the advance (V) of the timings is fully reversed.

2. The method according to claim 1, characterised in that the post-injections in at least one of the cylinders (16, 18, 20, 22) of the combustion engine (14) occur in a timed manner such that first periods (38) of predetermined duration in which post-injections are performed in each working cycle of a respective cylinder, and second periods (40) of predetermined duration in which the post-injections are disabled, alternate in direct sequence and the duration of the first period (38) and / or the duration of the second period (40) are specified in dependence of the temperature (ϑ) of the oxidation catalytic converter (34).

3. The method according to claim 2, characterised in that a divided return of exhaust gas from the cylinders (16, 18, 20, 22) of the combustion engine (14) into an inlet air tract of the combustion engine (14) is provided such that exhaust gas from only a first portion of the cylinders (16, 18, 20, 22) can be delivered to the inlet tract of the combustion engine (14) in a significant percentage, and exhaust gas of the remaining second portion of the cylinders (16, 18, 20, 22) is delivered to the oxidation catalytic converter (34) substantially in full and without any return, wherein post-injections of fuel only occur with at least one cylinder (20, 22) of the second portion of the cylinders (16, 18, 20, 22).

4. The method according to any of claims 1 to 3, characterised in that for performing the post-injections, a driving current (Iv) of the injector valve for actuating a valve needle of the injector valve is adjusted in dependence of a temperature (ϑ) of the oxidation catalytic converter (34).

5. The method according to claim 4, characterised in that above a specifiable temperature for the oxidation catalytic converter (34), a post-injection quantity (mNE) is limited by a needle stroke amplitude (H), which is reduced with respect to a maximum needle stroke amplitude of the valve needle being set for the post-injections.

6. A combustion engine comprising an exhaust tract, in which an oxidation catalytic converter (34) and a particle filter (36) downstream of the oxidation catalytic converter (34) are disposed, characterised in that the combustion engine is configured with control means for performing a method according to one of claims 1 to 5.