Method and control unit for reducing cold start emissions in a spark-ignition internal combustion engine

Abstract

Method for reducing the cold-start emissions of a spark-ignition internal combustion engine (10) in whose exhaust system (12) a three-way catalyst (14) is arranged, comprising the following steps: - Determining the starting conditions of the internal combustion engine (10), wherein - upon detection of a cold start of the internal combustion engine (10) the internal combustion engine is operated with a lean air-fuel ratio (λ > 1) or with a stoichiometric air-fuel ratio (λ = 1), - during the cold start of the internal combustion engine (10) multiple ignition takes place in the combustion chambers (52, 54, 56, 58) of the internal combustion engine (10) in order to increase the ignitability of the lean or stoichiometric combustion mixture, and - during the cold start phase the maximum possible number of ignition sparks is generated.

Description

[0001] The invention relates to a method for reducing cold-start emissions in a spark-ignition combustion engine and to a control unit for carrying out such a method.

[0002] The continuously tightening of emissions legislation places high demands on vehicle manufacturers, which are met through appropriate measures to reduce raw engine emissions and through suitable exhaust aftertreatment. With the introduction of EU6 legislation, a limit for particle number is prescribed for gasoline engines, which in many cases necessitates the use of a gasoline particulate filter. Such soot particles are formed particularly after a cold start of the combustion engine due to incomplete combustion in combination with a superstoichiometric air-fuel ratio after the cold start, cold cylinder walls, and the heterogeneous mixture distribution in the combustion chambers of the engine. The cold start phase is therefore crucial for compliance with the legally prescribed particle limits, both with regard to particle mass and particle number.Furthermore, a cold start with a substoichiometric, rich air-fuel ratio leads to higher emissions of carbon monoxide (CO) and unburned hydrocarbons (HC), as conversion to carbon dioxide and water vapor is not yet possible due to the cold catalyst. During operation, in vehicles equipped with a gasoline particulate filter (GPF), this filter then becomes further loaded with soot. To prevent the exhaust backpressure from increasing too much, the GPF must be regenerated continuously or periodically. An increase in exhaust backpressure can lead to increased fuel consumption, loss of power, and impaired engine smoothness, potentially resulting in misfires. To achieve thermal oxidation of the soot trapped in the GPF with oxygen, a sufficiently high temperature level combined with the presence of oxygen in the exhaust system of the gasoline engine is necessary.Since modern gasoline engines are normally operated without excess oxygen at a stoichiometric air-fuel ratio (λ=1), additional measures are required. These measures include, for example, increasing the temperature by adjusting the ignition timing, temporarily leaning out the mixture, injecting secondary air into the exhaust system, or a combination of these measures. Currently, retarding the ignition timing in combination with leaning out the mixture is preferred, as this method requires no additional components and can supply a sufficient amount of oxygen at most operating points of the gasoline engine.

[0003] From DE 101 31 937 A1, a device for reducing vehicle emissions and a method for reducing the cold start emissions of an internal combustion engine are known, in which the valve timing, the ignition angle and the air-fuel ratio are adjusted to improve the combustion temperature and combustion efficiency in the cold start phase and to reduce cold start emissions.

[0004] The invention is based on the objective of reducing the emissions of a spark-ignition internal combustion engine based on the Otto cycle during the cold start phase of the engine and thus improving the exhaust gas behavior of the engine. According to the invention, this objective is achieved by a method for reducing the cold start emissions of a spark-ignition internal combustion engine in whose exhaust system a three-way catalytic converter is arranged, which comprises the following steps: - Determining the starting conditions of the internal combustion engine, whereby - when a cold start of the internal combustion engine is detected, the internal combustion engine is operated with a lean air-fuel ratio (λ > 1) or a stoichiometric air-fuel ratio (λ = 1), and wherein - During the cold start of the internal combustion engine, multiple ignitions occur in the combustion chambers of the internal combustion engine to increase the ignitability of the lean combustion mixture.

[0005] Additionally or alternatively, combustion can be stabilized by maintaining the ignition spark. Likewise, it is also possible, alternatively or additionally, to introduce more ignition energy into the combustion chambers by extending the closing time of the ignition distributor.

[0006] In a known prior art cold-start method, the internal combustion engine is operated with a substoichiometric, rich combustion mixture during the cold-start phase to improve the mixture's ignitability. According to the invention, a cold-start of the internal combustion engine with a lean, superstoichiometric air-fuel ratio or a stoichiometric air-fuel ratio can significantly reduce both gaseous and organic solid emissions. The primary objective of the proposed method is to reduce the raw emissions occurring during engine combustion in the cold-start phase. Superstoichiometric operation of the internal combustion engine reduces both the number and size of soot particles.The residual oxygen present in the combustion chambers can be used to oxidize the soot particles that occur during combustion due to the cold combustion chamber walls and a heterogeneous mixture distribution, thus reducing the raw emissions of the combustion engine. Furthermore, the superstoichiometric combustion air mixture prevents the local formation of unburned fuel droplets, which contribute significantly to the generation of particulate emissions. Another advantage of the proposed method is that the three-way catalyst heats up faster during a superstoichiometric cold start than during a substoichiometric cold start, since no excess fuel evaporates and therefore no additional heat is extracted from the combustion air or exhaust gas through fuel vaporization.To prevent misfires during a cold start of an internal combustion engine with a superstoichiometric air-fuel mixture, multiple ignitions of the air-fuel mixture are performed in the combustion chambers during the cold start phase. This increases the ignitability of the lean air-fuel mixture and thus prevents misfires. A stoichiometric air-fuel ratio after the cold start offers the advantage of reducing raw emissions, particularly particulate emissions, but without the additional oxidizing agent required to oxidize the resulting soot particles. Therefore, primary emissions are slightly higher with a stoichiometric air-fuel ratio than with a lean air-fuel ratio, but the ignitability of the mixture is higher, and nitrogen oxide emissions and the risk of misfires are lower.To reduce the risk of misfires after a cold start with a stoichiometric or lean air-fuel ratio, at least two sparks are generated in each combustion chamber per combustion cycle. This ensures that at least one spark ignites a combustible air-fuel mixture, thus guaranteeing combustion even during the cold start phase despite the lean air-fuel ratio. In some cases, this allows vehicles with spark-ignition combustion engines to meet stricter emissions standards without an additional particulate filter, or to reduce the number of particulate filter regeneration cycles. This not only reduces emissions but also improves fuel consumption, as the need for fuel-increasing particulate filter regeneration is diminished.

[0007] The features listed in the dependent claims enable further improvements to the method for reducing cold-start emissions specified in the independent claim.

[0008] In a preferred embodiment of the method, multiple ignition is achieved using a spark band ignition system. With a spark band ignition system, two or more ignition sparks can be generated in a combustion cycle of the respective combustion chamber of the internal combustion engine using a single spark plug in a relatively simple manner. A spark band ignition system can emit multiple ignition sparks within a short period, i.e., within one combustion cycle per combustion chamber. The number of sparks depends on the engine speed, as the number of possible ignition sparks per combustion cycle decreases with increasing engine speed. A spark band ignition system stabilizes the smooth running of the internal combustion engine and prevents misfires during the cold start phase.Alternatively, multiple ignition can also be achieved through corona ignition or laser ignition; however, spark band ignition offers by far the most cost-effective solution for multiple ignition. Furthermore, spark band ignition generally stabilizes combustion and thus reduces the need for enriching the combustion air mixture at cold starting temperatures.

[0009] According to a preferred embodiment of the method, the internal combustion engine is operated with a stoichiometric air-fuel ratio after the three-way catalytic converter has reached its light-off temperature. To minimize the stress on the spark plugs during the cold start phase and to keep secondary emissions, particularly nitrogen oxide (NOx) emissions, low, it is advantageous to operate the internal combustion engine with a stoichiometric air-fuel ratio from the moment the three-way catalytic converter reaches its light-off temperature and thus 50% of its maximum conversion capacity for harmful exhaust gas components. This enables highly effective conversion of the raw emissions generated during fuel combustion in the combustion chambers of the internal combustion engine by the three-way catalytic converter.Furthermore, once the catalyst's light-off temperature is reached, multiple ignitions can be avoided, which keeps the periods of high stress on the spark plugs low and thus only slightly reduces the service life of the spark plugs.

[0010] In a further preferred embodiment of the invention, the internal combustion engine is operated with an air-fuel ratio λ of 1.02 to 1.05 during the cold start phase. Tests have shown that even a slight increase in the air-fuel ratio above a stoichiometric ratio leads to a significant reduction in raw emissions during the cold start phase of the internal combustion engine. With a small excess of air, the risk of misfires is limited, thus ensuring smooth running of the internal combustion engine even during the starting phase.

[0011] In an alternative embodiment of the method, the combustion engine is operated with an air-fuel ratio λ > 1.05 during the cold start phase. A higher excess of air further improves the raw emissions, particularly particulate emissions, of the combustion engine. However, the risk of misfires and NOx emissions also increases with the excess air. Therefore, it is particularly advantageous to adjust the air-fuel ratio during a cold start of the combustion engine depending on the temperature of the three-way catalytic converter in order to reduce raw emissions during the cold start phase until the engine reaches its light-off temperature.

[0012] In a preferred embodiment of the method, the internal combustion engine is operated with a superstoichiometric, lean air-fuel ratio for a period of 30 s to 240 s, particularly preferably for a period of 30 s to 180 s. Generally, a three-way catalytic converter reaches its light-off temperature no later than 240 s after a cold start of the internal combustion engine and can ensure efficient conversion of the restricted exhaust gas components. Depending on the size of the three-way catalytic converter and its arrangement within the exhaust system, the warm-up time of the three-way catalytic converter can be significantly reduced, for example, in the case of a small pre-catalytic converter located close to the engine, so that the method according to the invention is only necessary for a very short period.This helps to keep the increased wear on the spark plugs low and at the same time ensures an emissions-optimized cold start of the combustion engine.

[0013] According to a further improvement of the method, the maximum possible number of ignition sparks is generated during the cold start phase. To improve the ignitability of the lean combustion mixture during the cold start phase, the spark plugs are designed to emit the maximum possible number of ignition sparks per combustion cycle to improve the smoothness of the internal combustion engine and prevent misfires. The first ignition spark is preferably emitted into the combustion chamber at the same time as during stoichiometric combustion in normal engine operation, with subsequent ignition sparks staggered accordingly. This ensures efficient fuel utilization even during the cold start, thereby reducing the engine's fuel consumption compared to a conventional cold start with a rich combustion air mixture, as known from the prior art.

[0014] In a further, preferred embodiment of the method, the ignition timing specified for stoichiometric normal operation of the internal combustion engine is maintained. By foregoing ignition retardation, the efficiency of the internal combustion engine during the cold start phase can be increased. Moreover, such an adjustment of the ignition angle is unnecessary because, during the cold start phase, no unburned hydrocarbons evaporate in the exhaust gas and thus extract additional heat from the exhaust gas, which would delay the heating of the three-way catalytic converter.

[0015] A further improvement to the cold-start emission reduction process involves passing the combustion engine's exhaust gas through a particulate filter or a four-way catalytic converter. In addition to gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx), a particulate filter or a four-way catalytic converter can also remove soot particles from the exhaust gas, thus further increasing the efficiency of the exhaust aftertreatment.

[0016] According to the invention, a control unit for a spark-ignition internal combustion engine is proposed, which has a machine-readable program code with which a method according to the invention is carried out when the program code is executed on the control unit. The machine-readable program code allows the method according to the invention to be applied to an internal combustion engine control unit in a simple and cost-effective manner, so that the method according to the invention for reducing emissions during a cold start of the internal combustion engine does not require any additional components.

[0017] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0018] Unless otherwise stated in individual cases, the various embodiments of the invention mentioned in this application can be advantageously combined with one another.

[0019] The invention is explained below using exemplary embodiments with reference to the accompanying drawings. These show: Fig. 1 an internal combustion engine with an air supply, a fuel injection system, an ignition system and an exhaust system, on which a method according to the invention for reducing cold start emissions can be carried out; Fig. 2 a flowchart for carrying out a method according to the invention for reducing the cold start emissions of a spark-ignition internal combustion engine; Fig. 3. A profile of the particle concentration / particle mass in a method according to the invention for reducing cold-start emissions compared to the cold-start emissions in a conventional cold start known from the prior art; and Fig.4 a course of the particle number in a method according to the invention for reducing cold start emissions compared to the particle emissions in a conventional cold start known from the prior art.

[0020] Fig. Figure 1 shows a spark-ignited internal combustion engine 10 for a motor vehicle, with an exhaust system 12 , in which a three-way catalytic converter 14 is arranged. In addition to the three-way catalytic converter 14 The exhaust system may contain additional catalysts and exhaust aftertreatment devices, in particular a NOx storage catalyst 46 and / or a particulate filter. 16 The particulate filter 16 can have a three-way catalytically active coating, and as a so-called four-way catalyst 18 be designed, whereby the particle filter 16 in this case the functions of the three-way catalyst 14 and the particulate filter16 All of this is combined in one component: the internal combustion engine. 10 is supplied via an air supply system 30 supplied with fresh air. This involves 40 intake manifolds of the combustion engine. 10 an air filter 32 and an air mass meter 34 arranged. The fresh air can be supplied by means of a turbocharger. 48 are compressed, whereby in the exhaust system 12 a turbine 50 is arranged, which includes a compressor 36 in the intake tract 40 of the internal combustion engine 10 drives and in this way the combustion chambers 52 , 54 , 56 , 58 of the internal combustion engine 10 The supplied fresh air is compressed. This is used to control the combustion chambers. 52 , 54 , 56 , 58 The amount of fresh air supplied is in the intake tract 40 a throttle valve 38 arranged. The internal combustion engine 10It also features a fuel supply system in which fuel is drawn from a fuel tank 22 by means of a fuel pump 26 through a fuel line 24 a fuel injection system 28 supplied and fed into the intake tract as needed 40 or into the combustion chambers 52 , 54 , 56 , 58 of the internal combustion engine 10 is injected. The internal combustion engine 10 It also features a distributor. 42 on, over which spark plugs 44 at the combustion chambers 52 , 54 , 56 , 58 can be controlled and each generates one or more ignition sparks into the combustion chamber 52 , 54, 56, 58 emitted. Thus, the combustion air mixture can enter the combustion chambers. 52 , 54 , 56 , 58 will be ignited. In the exhaust system 12 of the internal combustion engine 10are in the direction of flow of exhaust gas from the combustion engine 10 through the exhaust system 12 upstream of the three-way catalyst 14 and downstream of the three-way catalytic converter, 14 lambda sensors 60 , 62 arranged, with which the combustion air ratio l of the internal combustion engine 10 This can be regulated. The lambda sensors are used for this purpose. 60 , 62 , the fuel injection system 28 and the ignition distributor is connected via signal lines to a control unit 20 of the internal combustion engine 10 connected, via which the combustion air ratio l , the amount of fuel and the ignition timing at which the spark plugs fire 44 Each emits an ignition pulse and can be controlled.

[0021] In Fig. 2 is a method according to the invention for reducing the cold start emissions of a vehicle equipped with spark plugs 44 spark-ignition internal combustion engine 10shown. This is shown on the internal combustion engine. 10 and / or in the intake tract 40 and the exhaust system 12 Additional sensors, in particular temperature sensors, are provided, which enable a cold start of the combustion engine. 10 can be detected. This is done in a first procedural step. <100> Once cold start conditions are detected, the combustion engine 10 in a second procedural step <110> during a cold start phase for a period of 30 s to 240 s with a superstoichiometric, lean combustion air ratio l > 1 operated. Due to the cold start of the combustion engine 10 with a lean air-fuel ratio l > 1 are in the combustion chambers 52 , 54 , 56 , 58 additional oxygen molecules for the oxidation of the combustion gases in the cold zones of the combustion chambers 52 , 54 , 56 , 58The primary particles that occur are provided as reaction partners. Furthermore, for the cold start phase... <120> the ignition by the distributor 42 modified in such a way that the spark plugs 44 provide multiple ignition sparks per combustion cycle, preferably a spark band ignition, to ensure a lean start for the internal combustion engine. 10 to improve ignition conditions and eliminate misfires and the associated rough running of the internal combustion engine 10 to avoid. Furthermore, the lean operation of the combustion engine can lead to... 10 Localized, rich hotspots with a rich combustion air mixture are avoided, thereby reducing the number and mass of primary particles produced during combustion. The lean cold start can also benefit the three-way catalytic converter. 14 faster than a conventional cold start with a substoichiometric air-fuel ratio l< 1 is heated to a light-off temperature, enabling efficient and effective conversion of exhaust gas components such as carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx) more quickly after a cold start. This is achieved through the lean start of the combustion engine. 10 To emit no or only low secondary emissions, especially nitrogen oxides (NOx), a lean combustion air ratio is used. l < 1 depending on the temperature of the three-way catalyst 14 discontinued. After the three-way catalytic converter 14 in a process step <130> Once its light-off temperature has been reached and it can contribute to the efficient conversion of the pollutants contained in the exhaust gas, the combustion air ratio will be adjusted. l of the internal combustion engine 10 in a process step <140> to a stoichiometric combustion air ratio l= 1 adjusted to ensure efficient exhaust aftertreatment by the three-way catalytic converter 14 to enable this. By adjusting the combustion air ratio depending on the temperature. l of the internal combustion engine 10 During the cold start phase, secondary emissions are avoided, and at the same time, the particle number, particle mass, and emissions of unburned hydrocarbons (HC) and carbon monoxide (CO) are significantly reduced. The proposed spark band ignition stabilizes combustion during the cold start phase and prevents misfires during the lean start of the combustion engine. 10 are avoided. During the cold start phase, the maximum possible number of sparks per spark plug is achieved. 44 and combustion cycle in the respective combustion chamber 52 , 54 , 56 , 58 emitted, with this maximum number of ignition sparks depending on the speed of the internal combustion engine. 10This involves adjusting the ignition timing compared to normal operation of the internal combustion engine. 10 not delayed, so that the first spark occurs at the ignition time of the spark plug. 44 During normal operation, the respective combustion chamber 52 , 54, 56, 58 is emitted.

[0022] In Fig. 3 are the particle concentration P K during a conventional cold start of an internal combustion engine 10 with a substoichiometric combustion air ratio l < 1 as well as in the case of a lean start of an internal combustion engine according to the invention 10 in a NEDC driving cycle as well as the vehicle speed vThe curve of the vehicle undergoing the test over a test cycle of 1200 seconds is shown. The driving cycle is represented by a dashed line, the initial measurement according to the prior art by thin lines, and the emissions according to the inventive method by a thick line. It can be seen that the particle concentration P K and thus the particle mass of soot particles under otherwise unchanged test conditions in all test areas below the particle concentration P K This is the case during a conventional cold start. Therefore, the particulate mass during a cold start can be significantly reduced, resulting in less frequent regeneration of the particulate filter. 16 or four-way catalytic converter 18 in the exhaust system 12 of the internal combustion engine 10 This allows or can even lead to the particle limits being exceeded without a particle filter. 16can be achieved. Furthermore, the lower particle concentration P leads to K when using a particulate filter 16 or four-way catalytic converter 18 to a slower increase in exhaust back pressure in the exhaust system 12 , so that the fuel consumption of the combustion engine 10 can be further reduced.

[0023] In Fig. 4 represents the number of particles P N during a conventional cold start of an internal combustion engine 10 with a substoichiometric, rich combustion air ratio λ < 1 compared to the particle number P N in a cold start according to the invention with a superstoichiometric, lean combustion air ratio l> 1 in a NEDC driving cycle and the vehicle speed of a motor vehicle completing this driving cycle over a test cycle of 1200 seconds are shown. The driving cycle is represented by a dashed line, the initial measurement according to the prior art by thin lines, and the emissions according to a method according to the invention by a thick line. It can be seen that the particle number P N The particle concentration can be reduced by approximately 30%, and in the example shown by 31%, through a cold start according to the invention using a lean combustion process. As already mentioned regarding the particle concentration... P K By means of a method according to the invention, the fuel consumption of the motor vehicle can be reduced for several reasons. Firstly, less fuel is consumed during a lean start than during a cold start with a rich fuel mixture. Secondly, fewer soot particles with a lower particle mass than the primary emissions of the combustion engine 10 are produced, resulting in fewer particles in a particulate filter. 16 or a four-way catalytic converter 18 The emissions must be held back, and the exhaust back pressure rises more slowly. Additionally, fewer fuel-consuming regeneration cycles are required for the oxidation of the soot in the particulate filter. 16 or four-way catalytic converter 18 retained soot is necessary, so that fuel can be saved again compared to a conventional cold start with higher particle emissions. Reference symbol list 10 Internal combustion engine 12 Exhaust system 14 Three-way catalytic converter 16 particulate filters 18 Four-way catalytic converter 20 Control unit 22 Fuel tank 24 Fuel line 26 Fuel pump 28 Fuel injection system 30 Air supply system 32 air filters 34 Air mass meters 36 compressors 38 Throttle valve 40 Intake tract 42 distributors 44 Spark plug 46 NOx storage catalyst 48 turbochargers 50 Turbine 52 first combustion chamber 54 second combustion chamber 56 third combustion chamber 58 fourth combustion chamber 60 Lambda sensor 62 Lambda sensor P K Particle concentration P N particle count v speed t time s seconds λ Combustion air ratio of the internal combustion engine QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10131937 A1

[0003]

Claims

[1] Method for reducing the cold start emissions of a spark-ignition internal combustion engine (10) in whose exhaust system (12) a three-way catalyst (14) is arranged, comprising the following steps: - Determining the starting conditions of the internal combustion engine (10), whereby at - Detecting a cold start of the internal combustion engine (10) the internal combustion engine is operated with a lean air-fuel ratio (λ > 1) or with a stoichiometric air-fuel ratio (λ = 1), and wherein - during the cold start of the internal combustion engine (10), multiple ignition takes place in the combustion chambers (52, 54, 56, 58) of the internal combustion engine (10) to increase the ignitability of the lean or stoichiometric combustion mixture. [2] Method according to claim 1, characterized by that the multiple ignition is achieved by means of a spark band ignition. [3] Method according to claim 1 or 2, characterized by, that the internal combustion engine (10) is operated with a stoichiometric air-fuel ratio after reaching a light-off temperature of the three-way catalyst (14) and the multiple ignition is replaced by a single ignition. [4] Method according to any one of claims 1 to 3, characterized by , that the internal combustion engine (10) is operated with a combustion air ratio λ of 1.02 - 1.05 during the cold start phase. [5] Method according to any one of claims 1 to 3, characterized by , that the internal combustion engine (10) is operated with a combustion air ratio λ > 1.05 during the cold start phase. [6] Method according to any one of claims 1 to 5, characterized by , that the internal combustion engine is operated with a lean air-fuel ratio (λ > 1) for a period of 30 s to 240 s. [7] Method according to any one of claims 1 to 6, such that the maximum possible number of ignition sparks is generated during the cold start phase. [8] Method according to any one of claims 1 to 7, characterized by , that the ignition timing provided for in a stoichiometric normal operation of the internal combustion engine (10) is maintained. [9] Method according to any one of claims 1 to 8, characterized by , that the exhaust gas of the combustion engine (10) is passed through a particulate filter (16) or a four-way catalytic converter (18). [10] Control unit (20) for a spark-ignition internal combustion engine (10), wherein the control unit (20) has a machine-readable program code with which a method according to the invention is carried out when the program code is executed on the control unit (20).

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