Heating of an exhaust gas after-treatment device during engine warm-up

Abstract

Method for operating an internal combustion engine with an internal combustion engine (1), a fresh gas stream (5) and an exhaust gas stream (7), wherein an exhaust gas turbine (6) with variable turbine geometry (23) of an exhaust gas turbocharger and an exhaust gas aftertreatment device (11) are integrated into the exhaust gas stream (7) and a fresh gas compressor (22) of the exhaust gas turbocharger is integrated into the fresh gas stream (5) and wherein a low-pressure exhaust gas recirculation line (19) branches off from the exhaust gas stream (7) downstream of the exhaust gas turbine (6) and opens into the fresh gas stream (5) upstream of the fresh gas compressor (22), wherein during a warm-up phase the variable turbine geometry (23) is adjusted such thatthat the compression of the fresh gas by means of the fresh gas compressor (22) is greater compared to normal operation at a corresponding operating point and - exhaust gas is routed via the LP exhaust gas recirculation line (19) and - a post-injection of fuel is carried out in at least one combustion chamber (4) of the internal combustion engine (1) during a working stroke in this combustion chamber (4).

Description

[0001] The invention relates to a method for operating an internal combustion engine during a warm-up phase, which may in particular follow a cold start of the internal combustion engine.

[0002] To achieve the lowest possible pollutant emissions from an internal combustion engine, the exhaust aftertreatment components integrated into the engine's exhaust system, which may include one or more catalytic converters and a particulate filter, should ideally always operate at temperatures above their respective activation temperatures (also known as "light-off temperatures"), at which sufficient effectiveness of the exhaust aftertreatment can be assumed. After a cold start of the engine, during which not only the combustion engine but also the exhaust aftertreatment components operate at temperatures significantly below their respective activation temperatures, the operating temperatures of at least some of the exhaust aftertreatment components should reach their activation temperatures as quickly as possible.To ensure this, it is known to actively heat individual exhaust aftertreatment components. This can be achieved, firstly, by means of dedicated heating devices, which may include, for example, electric heating elements or be designed as burners. Secondly, so-called in-engine measures can be implemented, which aim to generate relatively hot exhaust gas by deliberately operating the combustion engine at a relatively low efficiency, thus enabling relatively rapid heating of the exhaust aftertreatment components via the exhaust gas.

[0003] German patent application DE 10 2018 124 869 A1 discloses a method for the exhaust aftertreatment of an internal combustion engine with at least one combustion chamber. The internal combustion engine is connected to an air supply system via its intake and to an exhaust system via its exhaust. The exhaust system contains at least one exhaust aftertreatment component for converting pollutants in the exhaust gas of the internal combustion engine. The system is designed to determine the load condition of the internal combustion engine and the distance-related emissions, in particular the distance-related NOx emissions. If the distance-related emissions approach a defined threshold and it is foreseeable that these distance-related emissions cannot be met with unchanged engine parameters, the power and / or torque of the internal combustion engine is reduced to lower the raw emissions of the internal combustion engine.

[0004] German patent DE 10 2018 220 121 A1 describes an exhaust aftertreatment system for an internal combustion engine, comprising an exhaust system with an exhaust channel. Within this exhaust channel, an oxidation catalyst is arranged, followed by a metering element for adding a reducing agent, and then an exhaust aftertreatment component for the selective, catalytic reduction of nitrogen oxides. The oxidation catalyst is provided with an electric heating element, which heats a section of the catalyst. After a cold start, the oxidation catalyst is heated to its activation temperature, and further heating is then supported by in-engine measures and an exothermic reaction of unburned exhaust gas components on the oxidation catalyst.

[0005] German patent DE 10 2017 123 492 A1 discloses an exhaust aftertreatment system for an internal combustion engine. The exhaust aftertreatment system comprises an exhaust gas duct in which an exhaust gas turbine of an exhaust gas turbocharger, followed by a first catalyst, in particular an oxidation catalyst, and then a second catalyst, in particular an SCR catalyst or a particulate filter with an SCR coating, are arranged. A bypass duct connects the exhaust gas duct upstream of the exhaust gas turbine to the exhaust gas duct downstream of the first catalyst and upstream of the second catalyst, with a third catalyst, in particular an oxidation catalyst, being arranged in the bypass duct. During low-load operation of the internal combustion engine, a bypass valve of the bypass duct opens, thereby diverting an exhaust gas flow past the exhaust gas turbine and the first catalyst, and cleaning it by the third catalyst located in the bypass duct.

[0006] In order to achieve rapid warm-up of an internal combustion engine and an exhaust aftertreatment device arranged in an exhaust stream of the internal combustion engine after a cold start of the internal combustion engine, it may be provided according to US 2017 / 0314486 A1 to increase the exhaust back pressure in the exhaust stream by means of an exhaust valve, which requires the internal combustion engine to be operated with an increased load, which in turn leads to an increased exhaust temperature.

[0007] The invention was based on the objective of achieving the lowest possible pollutant emissions during the warm-up phase of an internal combustion engine.

[0008] This problem is solved by carrying out a method according to claim 1. Advantageous embodiments of this method are the subject of further claims and will become apparent from the following description of the invention.

[0009] According to the invention, a method for operating an internal combustion engine is provided, wherein the internal combustion engine comprises at least one combustion engine, in particular a compression-ignition combustion engine, a fresh gas stream, and an exhaust gas stream. At least one exhaust gas turbine with variable turbine geometry (VTG), which is part of an exhaust gas turbocharger, and an exhaust gas aftertreatment device are integrated into the exhaust gas stream. The exhaust gas aftertreatment device, or at least individual exhaust gas aftertreatment components thereof, can preferably be arranged downstream (with respect to the flow direction of exhaust gas through the exhaust gas stream originating from the combustion engine) of the exhaust gas turbine. At least one fresh gas compressor of the exhaust gas turbine is integrated into the fresh gas stream.Furthermore, an ND exhaust gas recirculation line is provided, which branches off from the exhaust gas stream downstream of the exhaust gas turbine and preferably also downstream of the exhaust gas aftertreatment device or at least downstream of some of the exhaust gas aftertreatment components thereof, and which leads upstream (with respect to the flow direction of fresh gas through the fresh gas stream towards the combustion engine) of the fresh gas compressor into the fresh gas stream.

[0010] The method according to the invention is characterized in that during a warm-up phase, which may in particular begin with a cold start of the internal combustion engine, (at least temporarily and / or simultaneously) The VTG is adjusted in such a way that the compression of the fresh gas by means of the fresh gas compressor is greater and, in particular, maximized compared to normal operation at a corresponding operating point (i.e., at the same operating load and operating speed), which can result in maximum combustion chamber filling, and (direct) post-injection of fuel into at least one combustion chamber of the internal combustion engine is carried out (at least also) during an expansion stroke in this combustion chamber, and exhaust gas is routed via the LP exhaust gas recirculation line.

[0011] According to the invention, the introduction of fuel is a post-injection if it is carried out additionally and preferably also at a time interval from a previously carried out main injection.

[0012] Preferably, it can be provided that the post-injection takes place in a range of the rotation angle (crankshaft angle) of an output shaft of the internal combustion engine, preferably designed as a four-stroke reciprocating piston engine, which lies between 10°KW and 180°KW after top dead center in a working stroke cycle.

[0013] If the internal combustion engine has multiple combustion chambers, post-injection of fuel can take place in one, several or, preferably, all combustion chambers.

[0014] To realize a "variable turbine geometry (VTG)", an exhaust gas turbine comprises a device by means of which a flow cross-section, through which exhaust gas can be guided to a turbine runner of the exhaust gas turbine, can be changed, at least with regard to its effectiveness, for which purpose in particular the size of the free flow cross-section and / or the angle of the flow to blades of the turbine runner can be changed.

[0015] According to the invention, a "warm-up phase" is defined as the operation of the internal combustion engine in which at least one exhaust aftertreatment component of the exhaust aftertreatment system has an operating temperature that is below the corresponding start-up temperature. A "cold start" is defined, according to the invention, as the commissioning of the internal combustion engine in which at least one exhaust aftertreatment component of the exhaust aftertreatment system has an operating temperature that is approximately (i.e., even with a deviation of up to 10%, 20%, or 30%) the ambient temperature.A warm-up phase does not always have to follow a cold start or the commissioning of the internal combustion engine with an operating temperature below a corresponding start-up temperature of at least one exhaust aftertreatment component; rather, a warm-up phase can also begin when the internal combustion engine, and in particular the combustion engine itself, has previously been operated in such a way that the start-up temperature, which had already been exceeded, is again (for a defined minimum period) below the threshold, as may be the case with a longer period of idling or overrun operation of the combustion engine.

[0016] The invention aims to achieve a targeted increase in the temperature of the exhaust gas produced by the internal combustion engine through the fuel injected into at least one combustion chamber via post-injection. This relatively hot exhaust gas can then be used to heat the exhaust aftertreatment system as quickly as possible. Due to the relatively late post-injection, this fuel, which is still being burned in the respective combustion chamber, contributes only a small proportion to the generation of drive power from the internal combustion engine. As a result, a relatively large amount of the chemical energy bound in the fuel is carried away as heat energy from the exhaust gas by the internal combustion engine and thus introduced into the exhaust system. This additional heat energy is therefore available for heating the exhaust aftertreatment system.

[0017] The further enlargement and, in particular, maximization of the compression of the fresh gas provided for in the invention by means of a corresponding adjustment of the VTG ensures that sufficient oxygen is available to the combustion engine for the conversion of the fuel introduced via the post-injection.

[0018] The exhaust gas recirculation via the low-pressure exhaust gas recirculation line, as further provided according to the invention, ensures, on the one hand, that the peak temperatures occurring during combustion remain relatively low, which has a beneficial effect on the raw nitrogen oxide emissions of the combustion engine. In addition, an extra inert gas mass is introduced into at least one combustion chamber via the exhaust gas recirculation and thus also discharged from the combustion engine as part of the exhaust gas, so that a relatively large exhaust gas mass or enthalpy flow can be achieved overall, which has a beneficial effect with regard to the desired heating of the exhaust aftertreatment system.

[0019] To ensure that the largest possible exhaust gas mass flow is routed through the exhaust aftertreatment system and also through the exhaust gas turbine, it is preferably provided that the internal combustion engine either does not have a high-pressure exhaust gas recirculation line that branches off from the exhaust gas stream upstream of the exhaust gas turbine and leads into the fresh gas stream downstream of the fresh gas compressor, or that, if the internal combustion engine has such a high-pressure exhaust gas recirculation line, no or only relatively little exhaust gas is routed through the high-pressure exhaust gas recirculation line during the warm-up phase of the internal combustion engine compared to normal operation at the corresponding operating point.This ensures that as much heat energy as possible can be transferred from the exhaust gas to the exhaust gas aftertreatment system and that as much exhaust gas enthalpy as possible is available to achieve the highest possible compression power of the fresh gas compressor via the exhaust gas turbine.

[0020] According to the invention, it can further be preferably provided that the main injection of fuel into at least one combustion chamber of the internal combustion engine (at least also) during a compression stroke in this combustion chamber is carried out with a delay compared to normal operation. This is intended to ensure that the fuel introduced via the main injection is specifically converted in such a way that a relatively large proportion of the chemically bound energy is not converted into drive power of the internal combustion engine but into heat energy of the exhaust gas.

[0021] To prevent the increase in exhaust gas temperature provided for in the invention, which is caused in particular by the late post-injection of fuel, from leading to excessive raw pollutant emissions from the internal combustion engine, it is preferably provided that the fuel quantities introduced into the at least one combustion chamber during the post-injections are adjusted depending on the air-fuel ratio at which the internal combustion engine is operated. In particular, it is possible to adjust the fuel quantities such that an air-fuel ratio (λ) of between 0.98 and 1.02 or between 1.1 and 1.2 is achieved.A combustion air ratio of between 0.98 and 1.02 may be particularly appropriate when the focus is on keeping raw nitrogen oxide (NOx) emissions low, while a combustion air ratio of between 1.1 and 1.2 can be used to keep raw emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and (soot) particles low.

[0022] The oxygen content of the exhaust gas can advantageously be measured using a suitable exhaust gas sensor, for example a conventional NOx sensor or a lambda probe, which can be integrated into the exhaust system. Alternatively or additionally, a model-based, computational determination of the air-fuel ratio is also possible; that is, it is calculated from the operating parameters of the internal combustion engine, in particular the injected fuel quantities, the operating speed, the boost pressure, etc.

[0023] The adjustment of the combustion air ratio can be controlled in particular, whereby in the corresponding control loop the setpoint of the combustion air ratio is the reference variable, the actual value of the combustion air ratio is the controlled variable and the amount of fuel introduced into the combustion chamber via post-injection is the manipulated variable.

[0024] The internal combustion engine of a combustion engine according to the invention can, in particular, be a (compression-ignition and quality-controlled) diesel engine. However, it is also possible that the internal combustion engine is a (spark-ignition and quantity-controlled) gasoline engine or a combination of a compression-ignition and a spark-ignition engine, e.g., an internal combustion engine with homogeneous compression ignition. The internal combustion engine can be operated with either liquid fuel (i.e., diesel or gasoline) or a gaseous fuel (in particular, natural gas, LNG, or LPG).

[0025] An internal combustion engine operated according to the invention can be used, in particular, for the (direct or indirect) provision of propulsion power for a motor vehicle. The motor vehicle can, in particular, be a wheeled and not rail-bound vehicle (preferably a passenger car or a truck).

[0026] The invention is explained in more detail below with reference to an embodiment illustrated in the drawing. In this drawing, the Fig. 1 shows a simplified representation of an internal combustion engine suitable for carrying out a method according to the invention.

[0027] The Fig. 1 The figure shows, in simplified form, an internal combustion engine suitable for carrying out a method according to the invention for a motor vehicle.

[0028] This comprises a (four-stroke) internal combustion engine 1, which is exemplified as a reciprocating piston engine with four cylinders 2 arranged in a row. Each cylinder 2, with its piston 3 and cylinder head, defines a combustion chamber 4. During operation of the internal combustion engine 1, fresh gas is supplied to these combustion chambers 4 via a fresh gas line 5. The fresh gas consists, at least in part, of air drawn in from the environment and subsequently passed through a fresh gas compressor 22. This fresh gas compressor 22 is part of an exhaust gas turbocharger, which also includes an exhaust gas turbine 6 integrated into an exhaust gas line 7 of the internal combustion engine.Exhaust gas produced during the combustion of mixtures consisting of fresh gas and fuel injected directly into the combustion chambers 4 via fuel injectors (not shown) is discharged via the exhaust system 7 of the internal combustion engine. In addition to the exhaust turbine 6, the exhaust gas also flows through an exhaust aftertreatment system 11, which is designed to remove components of the exhaust gas that are considered pollutants and / or convert them into harmless components.

[0029] Each of the 4 combustion chambers is, according to the [document / section], [equated / conducted / etc.]. Fig. 1 In the illustrated embodiment example, two inlet valves 8 and two exhaust valves 9 are assigned, which are actuated via a valve actuating device 10, which may, for example, each comprise a camshaft (not shown) for the inlet valves 8 on the one hand and the exhaust valves 9 on the other.

[0030] During operation of the internal combustion engine 1, the pistons 3, guided by a crankshaft 21 to which they are connected via connecting rods (not shown), oscillate within the combustion chambers 4 between top dead center (TDC) and bottom dead center (BDC), alternately performing a charge exchange stroke cycle and a power stroke cycle. The charge exchange stroke cycle comprises an exhaust stroke of the respective piston 3 (corresponding to an exhaust stroke in the associated combustion chamber 4) and an intake stroke (corresponding to an intake stroke in the associated combustion chamber 4). The power stroke cycle comprises a compression stroke of the respective piston 3 (corresponding to a compression stroke in the associated combustion chamber 4) and a power stroke (corresponding to a power stroke in the associated combustion chamber 4).The four stroke movements of the pistons 3 or the four corresponding cycles of the cyclic processes taking place in the combustion chambers 4 correspond to one operating cycle, which takes place in the respective combustion chamber 4 of the internal combustion engine 1.

[0031] Depending on the rotation angle of the crankshaft 21 during the charge exchange stroke cycles of the individual pistons 3, the inlet valves 8 and the exhaust valves 9 are opened and closed at defined timings by means of the valve actuation device 10, thereby controlling the introduction of fresh gas into the combustion chambers 4 or the removal of exhaust gas from them.

[0032] To achieve optimal utilization of the exhaust gas enthalpy for generating compression power via the exhaust gas turbocharger during operation of the internal combustion engine 1 under varying operating loads and speeds, the exhaust gas turbine 6 includes a variable turbine flow (VTG) device 23, controllable by a control device (not shown). This VTG device can comprise a plurality of guide vanes (not shown) arranged in an inlet of the exhaust gas turbine 6, which are individually rotatable and can be adjusted collectively by means of an adjustment device (not shown). Depending on the rotational positions of the guide vanes, they narrow the free flow cross-section in the inlet of the exhaust gas turbine 6 to a greater or lesser degree and also influence the section of the primary flow to the turbine impeller and the direction of this flow.

[0033] The exhaust aftertreatment device 11 of the internal combustion engine comprises, viewed in the direction of exhaust gas flow, a first exhaust aftertreatment component 12, which may be a nitrogen oxide storage catalyst or a combination of an oxidation catalyst and a nitrogen oxide storage catalyst.

[0034] This first exhaust aftertreatment component 12 is followed by a second exhaust aftertreatment component in the form of a particulate filter 13. It is provided, by way of example, that the particulate filter 13, or the filter body forming it, has a catalytically active coating, whereby the particulate filter 13 simultaneously constitutes a first SCR catalyst of the exhaust aftertreatment system 11. The presence of a reducing agent in the exhaust gas flowing through the particulate filter 13, which acts as the first SCR catalyst, is necessary for the catalytic reduction of pollutants contained in the exhaust gas, in particular nitrogen oxides. This presence is achieved by means of a first metering device 14 for such a reducing agent, which is arranged upstream of the particulate filter 13. The reducing agent can, in particular, be ammonia or an ammonia-containing solution.Between the first metering device 14 and the particle filter 13 a first mixing device 15 is arranged, which can be designed, for example, in the form of flow guide elements that cause turbulence in the flow of the exhaust gas already mixed with the reducing agent.

[0035] The exhaust aftertreatment system 11 comprises, in an arrangement downstream of the particulate filter 13, a (further) SCR catalyst 16 as a third exhaust aftertreatment component. Upstream of the SCR catalyst 16, a second metering device 17 for a reducing agent is arranged, as well as, in an arrangement between this second metering device 17 and the SCR catalyst 16, a second mixing device 18. The second metering device 17 is provided because a low-pressure exhaust gas recirculation line 19 branches off from the exhaust stream 7 between the particulate filter 13 and this second metering device 17, via which, as required and controlled by an EGR valve 24, at least a portion of the exhaust gas can be recirculated into the fresh gas stream 5. The low-pressure exhaust gas recirculation line 19 leads into a section of the fresh gas line 5, which is located upstream of the fresh gas compressor 22.If such low-pressure exhaust gas recirculation (LPR) is performed, it should be ensured that the recirculated exhaust gas does not contain any unreacted reducing agent. Therefore, dosing by the first metering device 14 should, at least when LPR is used, be carried out in such a way that, as far downstream of the particulate filter 13, as possible no reducing agent remains in the exhaust gas. This, however, requires the additional introduction of reducing agent into the exhaust gas upstream of the SCR catalyst 16 in order to achieve a reduction of nitrogen oxides.

[0036] A barrier catalyst 20 is provided as the last component in the direction of flow, or as the fourth exhaust aftertreatment component of the exhaust aftertreatment system 11. This is an oxidation catalyst that converts reducing agents (in particular ammonia to N2 and H2O) that were not converted in the SCR catalyst 16, thus preventing the release of these reducing agents into the environment.

[0037] According to the invention, during a warm-up phase, which may follow a cold start of the internal combustion engine, fuel is introduced into the combustion chambers 4 both via a main injection, which takes place during the respective compression strokes in the combustion chambers 4, and via a post-injection, which takes place during the respective power strokes. In contrast to normal operation of the internal combustion engine, which is always or at least mostly the case when the operating temperatures of at least some, and in particular all, of the exhaust aftertreatment components are above the respective start-up temperature, the main injection during the warm-up phase of the internal combustion engine occurs relatively late. As a result, a relatively large proportion of the energy chemically bound in this fuel is converted into heat energy of the exhaust gas and not into drive power of the internal combustion engine 1.The same applies to the fuel introduced via post-injection, which contributes only a small portion to generating drive power for the internal combustion engine 1 and thus mainly results in relatively high exhaust gas temperatures. Since this obviously has a negative impact on the efficiency during operation of the internal combustion engine 1, the invention provides that this post-injection of fuel is carried out exclusively during the warm-up phase, but not during normal operation of the internal combustion engine.

[0038] The method of introducing the fuel into the combustion chambers 4 according to the invention generates relatively hot exhaust gas, which contributes to the fastest possible heating of the exhaust aftertreatment device 11 during the subsequent flow through the exhaust gas stream 7.

[0039] To supply the combustion engine 1 with sufficient oxygen for the combustion of all the supplied fuel, the invention further provides for adjusting the VTG 23 during the warm-up phase such that the compression of the fresh gas by means of the fresh gas compressor 22 is greater compared to normal operation at a corresponding operating point of the combustion engine 1. In particular, it is provided that the compression power of the fresh gas compressor 22 is continuously maximized during the warm-up phase, insofar as this is permitted by the operation of the combustion engine 1 and, in particular, by the drive power required by it.

[0040] Furthermore, during the warm-up phase, exhaust gas is recirculated via the low-pressure exhaust gas recirculation line 19 and thus as part of the fresh gas into the combustion chambers 4 of the internal combustion engine 1. This allows the raw nitrogen oxide emissions of the internal combustion engine 1 to be kept low. It also results in a relatively large exhaust gas mass flow, which provides a correspondingly large amount of heat energy for heating the exhaust aftertreatment system 11.

[0041] The relatively extensive and, in particular, maximum closing of the VTG 23 is intended to achieve the largest possible oxygen supply – reduced by a sensible proportion of recirculated exhaust gas – in the combustion chambers, so that more fuel (taking into account defined (lambda) limits regarding the combustion air ratio) can be injected to increase the exhaust gas temperature. B Train registration number list

[0042] 1 (Four-stroke) internal combustion engine 2 Cylinder 3 Piston 4 Combustion chamber 5 Fresh air stream 6 Exhaust turbine 7 Exhaust stream 8 Intake valve 9 Exhaust valve 10 Valve actuation device 11 Exhaust aftertreatment system 12 First exhaust aftertreatment component 13 Particulate filter 14 First metering device for a reducing agent 15 First mixing device 16 SCR catalyst 17 Second metering device for a reducing agent 18 Second mixing device 19 Low-pressure exhaust gas recirculation line 20 Blocking catalyst 21 Crankshaft 22 Fresh air compressor 23 Variable turbine geometry (VTG) device 24 EGR valve

Claims

1. Method for operating an engine comprising an internal combustion engine (1), a fresh gas line (5) and an exhaust gas line (7), an exhaust gas turbine (6) of an exhaust gas turbocharger, which exhaust gas turbine has variable turbine geometry (23), and an exhaust gas aftertreatment device (11) being integrated into the exhaust gas line (7) and a fresh gas compressor (22) of the exhaust gas turbocharger being integrated into the fresh gas line (5) and a low-pressure exhaust gas recirculation pipe (19) branching off from the exhaust gas line (7) downstream of the exhaust gas turbine (6) and opening into the fresh gas line (5) upstream of the fresh gas compressor (22), characterized in that, during a warm-up phase during which at least one exhaust gas aftertreatment component of the exhaust gas aftertreatment device (11) has an operating temperature which is below a corresponding light-off temperature, - the variable turbine geometry (23) is adjusted such that the compression of the fresh gas by means of the fresh gas compressor (22) is greater compared with a normal operation at a corresponding operating point and - exhaust gas is carried via the low-pressure exhaust gas recirculation pipe (19) and - a post-injection of fuel into at least one combustion chamber (4) of the internal combustion engine (1) is carried out during a firing stroke in this combustion chamber (4).

2. Method according to claim 1, characterized in that the internal combustion engine (1) is designed to be self-igniting.

3. Method according to claim 1 or 2, characterized in that the engine does not have a high-pressure exhaust gas recirculation pipe which branches off from the exhaust gas line (7) upstream of the exhaust gas turbine (6) and opens into the fresh gas line (5) downstream of the fresh gas compressor (22), or in that the engine has such a high-pressure exhaust gas recirculation pipe, no or relatively little exhaust gas then being carried via the high-pressure exhaust gas recirculation pipe compared with the normal operation at the corresponding operating point.

4. Method according to any of the preceding claims, characterized in that a main injection of fuel into at least one / the combustion chamber (4) of the internal combustion engine (1) during a compression stroke in this combustion chamber (4) is carried out delayed compared with the normal operation.

5. Method according to any of the preceding claims, characterized in that the fuel quantities which are introduced into the at least one combustion chamber (4) during the post-injections are adjusted on the basis of the air-fuel equivalence ratio at which the internal combustion engine is operated.

6. Method according to claim 5, characterized in that the air-fuel equivalence ratio is ascertained by measuring the oxygen content of the exhaust gas by means of an exhaust gas sensor.

7. Method according to claim 5 or 6, characterized in that the fuel quantities are adjusted such that the fuel-air equivalence ratio is between 0.98 and 1.02 or between 1.1 and 1.2.

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