Method for operating a diesel engine

Abstract

Method for operating a diesel engine that uses NO in the exhaust aftertreatment system x -storage catalyst, in the substoichiometric range (fat operation) for efficient operation of the NO x -storage catalyst by carrying out a post-injection (3) which is only partially combusted in the combustion chamber, with the following steps: (10) Determining lambda in the combustion chamber (combustion chamber lambda) from a subset of the total amount of fuel required for the working cycle, excluding the post-injection quantity relevant for rich operation; (11) Determining a target value for lambda in the exhaust gas (exhaust gas lambda); and (12) Determining the required quantity of post-injection for the current work cycle from these values ​​to achieve the target value for the exhaust lambda.

Description

[0001] The present invention relates to a method for operating a diesel engine which uses a NOₓ in the exhaust aftertreatment system. x -storage catalyst, in the substoichiometric range (fat operation) for efficient operation of the NO x -storage catalyst by carrying out a post-injection that is only partially burned in the combustion chamber.

[0002] Diesel engines that use a NOₓ in the exhaust aftertreatment system x Engines equipped with a storage catalyst require special engine operating modes to ensure efficient operation of the catalyst. These include deoxidation and desulfurization. When these modes are active, the engine operates in the substoichiometric range at approximately lambda 0.95.

[0003] This operation is achieved by performing a post-injection, which is only partially burned in the combustion chamber.

[0004] So that the NO xFor the storage catalyst to operate efficiently, the corresponding lambda value must be maintained. Deviations from this value have a considerable impact on the efficiency of the denoxification and desulfurization cycles. In particular, an excessively lean lambda value, in combination with the timing of the post-injection, leads to a significant increase in exothermicity, potentially resulting in the destruction of exhaust system components. An excessively rich lambda value leads to significantly increased HC emissions.

[0005] To maintain or regulate the appropriate lambda value, it is known to implement lambda control using a lambda sensor in the exhaust gas, which controls the amount of post-injection to achieve the desired target lambda value. After the engine cycle, the post-injection quantity may be adjusted.

[0006] From DE 10 2004 032 537 A1, a method for controlling fuel injection is known, wherein a total quantity is divided into at least one first partial injection quantity and at least one second partial injection quantity. The total quantity is determined based on a desired lambda value and an air quantity, and the first partial injection quantity is determined based on a desired moment. The second partial injection quantity is then calculated as the difference between the total quantity and the main injection quantity.

[0007] From DE 10 2008 059 698 A1, a method for operating a diesel engine with an exhaust gas purification system comprising a nitrogen oxide storage catalyst is known. The method employs a first operating mode in which the air-fuel mixture has a first lambda value of > 1, and a second operating mode for the regeneration of the nitrogen oxide storage catalyst, wherein a transition phase is inserted before the second operating mode is activated, in which the diesel engine is operated in a third operating mode.

[0008] DE 10 2006 020 675 A1 describes a method for lambda and torque control of an internal combustion engine, whereby a main injection quantity is influenced for torque control and a post-injection quantity is influenced for lambda control.

[0009] DE 20 2014 007 675 U1 describes an internal combustion engine with a control unit designed to operate during a regeneration phase of a NOₓ x -The process involves determining an air mass flow rate for lean-burn operation, calculating the difference between the determined value of the air mass flow rate and a target value thereof, using the calculated difference to determine a fuel quantity value, and injecting the determined fuel quantity value by means of a post-injection. The present invention provides a further solution for carrying out the efficient operation of such a NO x -storage catalyst. Its underlying task is to provide a method for operating a diesel engine whose NO x -storage catalyst can be operated particularly efficiently.

[0010] According to the invention, this problem is solved in a method of the type described above and according to claim 1 by the following steps: Determining lambda in the combustion chamber (combustion chamber lambda) from a subset of the total fuel quantity required for the operating cycle, excluding the post-injection quantity relevant for rich operation; Determining a target value for lambda in the exhaust gas (exhaust gas lambda); and Determining the required amount of post-injection for the current operating cycle from these values ​​to achieve the target value for the exhaust lambda.

[0011] Advantageous further developments of the invention are the subject of the dependent claims.

[0012] In the solution according to the invention, the lambda value in the combustion chamber is determined from a subset of the total amount of fuel required for the combustion cycle. Furthermore, a target value for the exhaust gas lambda is determined. From these two values, the required amount of post-injection for the current combustion cycle and the target value for the exhaust gas lambda can then be determined.

[0013] Since the injection and the subsequent post-injection involve successive partial quantities, it is possible to calculate the combustion chamber lambda after each injection.

[0014] In the method according to the invention, lambda in the combustion chamber (combustion chamber lambda) is preferably determined from the fresh air mass available before the combustion cycle and the fuel fraction. The invention specifically provides that lambda in the combustion chamber (combustion chamber lambda) is determined from the injection quantity for a pre-injection and a main injection. This does not preclude the method according to the invention from being carried out on the basis of only a main injection.

[0015] By calculating the combustion chamber lambda from the known combustion chamber filling (cylinder filling, model-based air path) and the amount of fuel to be injected, minus the post-injection quantity relevant for rich operation, it is possible to calculate the post-injection quantity that leads to the desired exhaust gas lambda.

[0016] One variant of the method according to the invention is characterized in that the combustion chamber lambda is determined after each main injection. This allows the desired post-injection quantity to be determined and adjusted accordingly for each injection process.

[0017] The method according to the invention can be carried out, for example, when a lambda sensor is not or not yet operational. The method according to the invention can therefore provide a backup functionality in the event of a lambda sensor malfunction. According to the invention, denoxification and desulfurization become possible, for example, when a lambda sensor is defective or during the warm-up phase. The method according to the invention can also be used to relieve the load on a lambda controller. It can also be used to adapt a post-injection quantity map.

[0018] In any case, this results in particularly efficient denoxification and desulfurization, especially during dynamic operation.

[0019] For the method according to the invention, the motor is also preferably operated in the substoichiometric range at approximately lambda 0.95.

[0020] The invention is explained in detail below with reference to an exemplary embodiment in conjunction with the drawing. The drawing shows: Fig. 1. A schematic representation of exemplary injection processes of a diesel engine with corresponding lambda values; and Fig. 2 a flowchart of an embodiment of the method according to the invention.

[0021] In Fig. Figure 1 schematically depicts pre-injections 1, a main injection 2, and a post-injection 3 in a diesel engine. The hatched areas show the torque-relevant fuel mass injected in each case, while the unburned fuel during post-injection 3 is indicated by area 4.

[0022] The diesel engine in question has an exhaust aftertreatment system with a NOₓ filter. x -Storage catalyst. This storage catalyst requires special engine operating modes, namely deoxidation and desulfurization, to operate efficiently. When these modes are activated, the engine operates in the substoichiometric range with a lambda value of 0.95. This operation is achieved by performing a post-injection 3, which is only partially combusted in the combustion chamber (cylinder), as shown in Fig. 1 shown.

[0023] In Fig. 2 are the individual process steps for operating a diesel engine that produces NO in the exhaust aftertreatment system x The storage catalyst is represented in this substoichiometric range by a lambda value of 0.95. As mentioned, this operation is achieved by performing a post-injection, which is only partially combusted in the combustion chamber.

[0024] In step 10 of the process, the lambda value in the combustion chamber (combustion chamber lambda) is determined from a subset of the total fuel quantity required for the combustion cycle, excluding the post-injection quantity relevant for rich operation. The lambda value in the combustion chamber is derived from the fresh air mass available before the combustion cycle and the injection quantity for the... Fig. The pre-injection 1 and main injection 2 shown in Figure 1 are determined.

[0025] In step 11, a target value for lambda in the exhaust gas (exhaust gas lambda) is determined. In step 12, the required amount of post-injection for the current combustion cycle is calculated from these values ​​to achieve the target value for the exhaust gas lambda. The determined post-injection quantity is then injected during the current injection process (step 13), resulting in the desired target value for the exhaust gas lambda, here 0.95. This lambda value enables efficient operation of the storage catalyst with regard to denoxification and desulfurization.

[0026] The following is an example of how to determine the post-injection quantity.

[0027] The task here is to determine the required amount of post-injection to achieve a target lambda of 0.95. The following are assumed to be known: the fresh air mass in the cylinder (e.g., 390 mg) and the partial quantities up to the post-injection. mf4 = Fresh air mass * Stoichiometric fresh air mass * Target lambda −mf1−mf2−mf3=390 mg −23 mg=5 mg 14.7mg airmg fuel*0.95

[0028] The amount of post-injection required to achieve the target lambda value corresponds to the quotient of the fresh air mass in the cylinder and the product of the stoichiometric air mass and the target lambda value, minus the partial amounts injected up to that point.

[0029] If one further assumes that torque is generated from the injected fuel up to a lambda of 1, it is also possible to calculate what proportion of the post-injection does not contribute to the torque.

[0030] In the example above, the remaining air mass in the cylinder before post-injection is: mAirRest=390mg Air−23 mg*14.7mg Airmg Fuel=51.9 mg This means that for the required post-injection quantity of 5 mg, 51.9 mg of fresh air are still available. Therefore, more 51.9mg air / 14.7mg air / mg fuel = 3.5mg fuel be burned.

[0031] 1.5 mg of the post-injection quantity does not contribute to the torque.

Claims

[1] Method for operating a diesel engine which produces NO in the exhaust aftertreatment system x -storage catalyst, in the substoichiometric range (fat operation) for efficient operation of the NO x -storage catalyst by carrying out a post-injection (3) which is only partially combusted in the combustion chamber, with the following steps: (10) Determining lambda in the combustion chamber (combustion chamber lambda) from a subset of the total amount of fuel required for the working cycle, excluding the post-injection quantity relevant for rich operation; (11) Determining a target value for lambda in the exhaust gas (exhaust gas lambda); and (12) Determining the required quantity of post-injection for the current work cycle from these values ​​to achieve the target value for the exhaust lambda. [2] Method according to claim 1, characterized by, that lambda in the combustion chamber (combustion chamber lambda) is determined from the fresh air mass available before the working cycle and the fuel partial quantity. [3] Method according to claim 2, characterized by , that lambda in the combustion chamber (combustion chamber lambda) is determined from the injection quantity for a pre- and main injection (1, 2). [4] Method according to any of the preceding claims, characterized by , that the combustion chamber lambda is determined after each main injection (2). [5] Method according to any of the preceding claims, characterized by , that it is carried out when a lambda sensor is not or not yet operational. [6] Method according to any of the preceding claims, characterized by that it is carried out to relieve the load on a lambda controller. [7] Method according to any of the preceding claims, characterized by , that it is used to adapt a post-injection quantity map. [8] Method according to any of the preceding claims, characterized by , that the engine operates in the substoichiometric range at approximately lambda 0.95.

Images