Combustion engine with NH3 sensor in the exhaust system

Description

[0001] The invention relates to a method for operating an internal combustion engine.

[0002] Controlling the mass ratio of the air and fuel components of a mixture fed into the combustion engine of an internal combustion engine for combustion is not only crucial for the operation of the internal combustion engine, but also for the aftertreatment of exhaust gases produced by the engine. This mass ratio is typically described by the dimensionless number λ (lambda), also known as the combustion air ratio. The combustion air ratio specifically indicates the mass ratio of the mixture components, air and fuel, relative to the stoichiometrically ideal mass ratio for a theoretically complete combustion process.

[0003] Spark-ignition and quantity-controlled gasoline engines are often operated with a generally stoichiometric air-fuel ratio, i.e., with λ ≈ 1. To achieve such a stoichiometric air-fuel ratio, the residual oxygen content in the exhaust gas is analyzed using a so-called lambda probe and used to control the air-fuel mass ratio for subsequent combustion processes. Such reactive control results in the actual air-fuel ratio fluctuating within a usually relatively narrow range around the stoichiometric air-fuel ratio. In this regard, it is known to actively implement a so-called natural frequency control for the air-fuel ratio, in which the combustion engine is defined to operate alternately slightly rich, i.e., with a slightly substoichiometric air-fuel ratio (λ < 1), and slightly lean, i.e.,with a slightly over-stoichiometric combustion air ratio (λ > 1). Switching between rich and lean operation occurs at defined deviations from the stoichiometric combustion air ratio.

[0004] A typical design of an exhaust aftertreatment system integrated into the exhaust system of a gasoline engine comprises, in the direction of exhaust flow, a first lambda sensor, then a first three-way catalyst, then a second lambda sensor, and finally a second three-way catalyst. A three-way catalyst is characterized by the fact that it converts carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC) into carbon dioxide (CO2), nitrogen (N2), and water (H2O). Furthermore, such a three-way catalyst typically also has a so-called oxygen storage capacity. It can therefore store oxygen to a limited extent.During lean operation of the internal combustion engine of an internal combustion engine which includes such an exhaust gas aftertreatment device, excess oxygen contained in the raw exhaust gas is stored in the first three-way catalyst, whereas during rich operation the stored oxygen is used for the aforementioned conversions. If during such rich operation of the internal combustion engine the oxygen storage capacity is almost completely exhausted, rich breakthroughs can occur in the first three-way catalyst, i.e. significant quantities of unburned hydrocarbons are present in the first three-way catalyst which cannot be converted, which can lead to the formation of ammonia (NH 3 ) and, via the ammonia, to the formation of nitrous oxide (N 2 O). Such formation of ammonia and nitrous oxide should be avoided wherever possible.

[0005] US 7 003 943 B2 discloses a method for the controlled adjustment of the combustion air ratio λ in an internal combustion engine by means of a NOx sensor, which is intended to replace the otherwise usual two lambda sensors.

[0006] US 10 598 063 B2 discloses a method for testing the functionality of a three-way catalyst using a NOx sensor arranged downstream of the three-way catalyst.

[0007] US 9 115 660 B2 discloses an internal combustion engine with an internal combustion engine and an exhaust system in which a three-way catalyst, a NOx storage catalyst, an SCR catalyst and a NOx sensor are integrated, viewed in the flow direction of the exhaust gas.

[0008] US 2019 / 0101033 A1 describes an internal combustion engine with an internal combustion engine and an exhaust system in which, extending from the internal combustion engine, a first lambda sensor, a first catalyst, a NOx sensor, a second lambda sensor, and a second catalyst are integrated. The catalysts are three-way catalysts.

[0009] DE 10 2018 218 138 A1 discloses an internal combustion engine with an internal combustion engine and an exhaust system in which a first lambda sensor, then a first three-way catalyst, and then a second three-way catalyst are arranged. A NOx sensor used as an NH3 sensor is integrated into the first three-way catalyst, and a second lambda sensor is integrated into the second three-way catalyst.

[0010] EP 2 853 725 A2 describes an internal combustion engine with an internal combustion engine and an exhaust system into which a lambda probe, a 3-way catalyst, an NH 3 sensor and an ammonia slip catalyst are integrated starting from the internal combustion engine.

[0011] DE 10 2013 211 521 A1 discloses an internal combustion engine with an internal combustion engine and an exhaust system in which, extending from the internal combustion engine, a first lambda sensor, a three-way catalyst, a second lambda sensor, and an SCR catalyst operating with ammonia as a reducing agent are integrated. Furthermore, NOx sensors are provided.

[0012] The object of the invention was to avoid, as far as possible, the formation of ammonia and nitrous oxide in the exhaust gas of an internal combustion engine.

[0013] To achieve this object, a method for operating an internal combustion engine is provided according to patent claim 1. Preferred embodiments of the method according to the invention are the subject matter of the further patent claims and / or emerge from the following description of the invention.

[0014] According to the invention, an internal combustion engine is provided which comprises at least one internal combustion engine and an exhaust system. The internal combustion engine is preferably at least temporarily spark-ignited and furthermore preferably also operable with quantity control (i.e., as a gasoline engine). Integrated into the exhaust system in the flow direction of the exhaust gas, i.e., starting from the internal combustion engine, are a (first) lambda probe, then a first three-way catalyst, and subsequently a second three-way catalyst. Furthermore, an NH 3 sensor, i.e., a sensor by means of which ammonia can be detected directly or indirectly, is integrated into the exhaust system between the first three-way catalyst and the second three-way catalyst.

[0015] Such an internal combustion engine enables the implementation of a method according to the invention, according to which the internal combustion engine is operated alternately (slightly) rich and (slightly) lean (preferably with defined, fixed or variable deviations from the stoichiometric combustion air ratio (λ = 1), for example in the range λ = 1 ± 0.05 or λ = 1 ± 0.03 or λ = 1 ± 0.01), which can serve in particular to achieve an average stoichiometric operation of the internal combustion engine. In this case, switching from rich operation to lean operation takes place when a fundamentally detectable presence of ammonia in the exhaust gas is detected by means of the NH 3 sensor.Such determination can be made relatively quickly, whereby the production of ammonia and, via the ammonia, of nitrous oxide in the first three-way catalytic converter can be minimized because the combustion engine switches to lean operation early and in good time. This ensures that sufficient oxygen is available in the raw exhaust gas, i.e. in the exhaust gas produced by the combustion engine and not yet aftertreated (upstream of the first three-way catalytic converter), to prevent further rich breakthroughs in the first three-way catalytic converter. Corresponding control of the combustion air ratio can be significantly faster than if a switch from rich operation to lean operation were determined based on the signal from a lambda probe, as can be provided according to the prior art.

[0016] A switch from lean operation to rich operation can preferably take place within the scope of a method according to the invention when a defined presence (i.e. a fundamentally detectable presence or a presence in a defined minimum amount) of oxygen is detected in the exhaust gas downstream of the first three-way catalytic converter (and preferably upstream of the second three-way catalytic converter). The presence of oxygen in the exhaust gas can be detected in a fundamentally known manner by means of a second lambda probe which is integrated into the exhaust system between the first three-way catalytic converter and the second three-way catalytic converter. Alternatively or additionally, a NOx sensor, i.e. a sensor by means of which at least nitrogen oxides can also be detected directly or indirectly, can also be arranged in this section of the exhaust system for this purpose.Such a NOx sensor can also advantageously be designed integrally with the NH 3 sensor, ie as an NH 3 -NO X combination sensor.

[0017] According to a preferred embodiment of an internal combustion engine operable according to the invention, at least one particulate filter can further be integrated into the exhaust system in order to minimize the emission of particulates with the exhaust gas. Such a particulate filter can be assigned to the first three-way catalytic converter and / or the second three-way catalytic converter, wherein assignment is understood to mean integration into the respective three-way catalytic converter (to form a so-called four-way catalytic converter) or arrangement downstream thereof, in particular directly downstream thereof (and with regard to the first three-way catalytic converter, upstream of the second three-way catalytic converter). To form a four-way catalytic converter, it can preferably be provided that a particulate filter is provided with a coating that acts as a three-way catalytic converter.

[0018] According to an advantageous embodiment of an internal combustion engine operable according to the invention, the first lambda probe can be designed as a broadband lambda probe, whereby a high degree of control accuracy for the basic control of the combustion air ratio can be achieved by means of the first lambda probe. The second lambda probe, if present, can advantageously be designed as a step-change lambda probe (also known as a two-point lambda probe), whereby the incipient presence of oxygen in the exhaust gas can be determined relatively quickly and accurately. In principle, however, the first lambda probe can also be designed as a step-change lambda probe.

[0019] The combustion engine of an internal combustion engine according to the invention can preferably be operated with liquid fuel (i.e., in particular gasoline). Operation with a gaseous fuel (in particular natural gas, LNG, or LPG) is also possible.

[0020] The invention further relates to a motor vehicle, in particular a wheel-based and non-rail-bound motor vehicle (preferably a car or a truck), with an internal combustion engine operated according to the invention. The internal combustion engine of the internal combustion engine can be provided, in particular, for the (direct or indirect) provision of the traction drive power for the motor vehicle.

[0021] The invention is explained in more detail below using an exemplary embodiment shown in the drawing. The drawing shows: Fig. 1: an internal combustion engine operable according to the invention in a simplified representation.

[0022] The Fig. 1shows an internal combustion engine for a motor vehicle that can be operated according to the invention and is thus suitable for carrying out a method according to the invention. This engine comprises an internal combustion engine 1, which is designed, for example, in the form of a reciprocating piston engine with four cylinder openings 2 arranged in series. The cylinder openings 2, with reciprocating pistons 3 guided therein and a cylinder head, each delimit a combustion chamber 4. During operation of the internal combustion engine 1, and thus of the internal combustion engine, fresh gas is supplied to these combustion chambers 4 via a fresh gas line 5, wherein the supply of the fresh gas is controlled by means of inlet valves 9, which are assigned to the individual combustion chambers 4. The fresh gas is exclusively or mainly air that is drawn in from the environment.Exhaust gas resulting from the combustion of mixtures consisting of fresh gas and fuel injected directly into the combustion chambers 4 via fuel injectors 6 is discharged via an exhaust system 7 of the internal combustion engine. The discharge of the exhaust gas is controlled by exhaust valves 10 assigned to the individual combustion chambers 4. Ignition of the mixtures in the combustion chambers occurs by means of electrical ignition devices 16, which generate ignition sparks (spark plugs, for example). The exhaust gas flows through an exhaust aftertreatment device 8, which is designed to remove components of the exhaust gas that are considered pollutants from the exhaust gas or to convert them into harmless components.

[0023] The internal combustion engine can be designed with turbocharging, in which case a fresh gas compressor (not shown) would be integrated into the fresh gas line 5. If the fresh gas compressor were part of an exhaust gas turbocharger, this exhaust gas turbocharger would also include an exhaust gas turbine (not shown) integrated into the exhaust gas line. Exhaust gas flowing through the exhaust gas turbine then drives a rotating turbine impeller, which is connected to a compressor impeller of the fresh gas compressor for rotational drive, so that the fresh gas compressor is ultimately driven by the exhaust gas turbine.

[0024] The exhaust gas aftertreatment device 8 of the internal combustion engine comprises, viewed in the flow direction of the exhaust gas, a first lambda probe 11, preferably in a design as a broadband lambda probe, then a first three-way catalyst 12, which can optionally be designed as a four-way catalyst and accordingly can also include a particle filter, then a NOx sensor 13 and an NH 3 sensor 14, which are designed in the form of an NH 3 -NO x combination sensor, and finally a second three-way catalyst 15, which can also optionally be designed as a four-way catalyst.

[0025] During operation of the internal combustion engine, the fresh gas to be supplied to the combustion engine 1 and the quantities of fuel to be introduced into the individual combustion chambers 4 by means of the fuel injectors 6 per working cycle are specifically adjusted depending on the specific operating state of the internal combustion engine 1, which is defined in particular by the requested load and the operating speed. For this purpose, the operating position of a throttle valve (not shown) integrated into the fresh gas line 5 and / or of a variable valve train (not shown), by means of which the intake valves 9 can be variably actuated, can be changed with regard to the fresh gas. The internal combustion engine is operated, at least temporarily, on average stoichiometrically (λ ≈ 1), so that the oxygen supplied to the combustion chambers 4 over this period essentially corresponds exactly to the amount required for thermal conversion of the fuel supplied at the same time.Since constant stoichiometric operation of the combustion engine is not feasible from a control technology perspective, the combustion engine 1 is operated alternately slightly rich and slightly lean in such a way that stoichiometric operation is achieved on average over the period under consideration.

[0026] Switching from rich operation to lean operation of the internal combustion engine 1 occurs when a defined presence of ammonia in the exhaust gas is detected by the NH 3 sensor 14. Such a presence of ammonia in the exhaust gas is due to rich breakthroughs in the first three-way catalyst 12 because unburned hydrocarbons contained in the raw exhaust gas during rich operation can no longer be largely completely converted using oxygen stored in the first three-way catalyst 12. This leads to the formation of ammonia (according to 2 NO + 5 H 2 → 2 NH 3 + 2 H 2 O) in the first three-way catalyst 12 in conjunction with nitrogen monoxide (NO) contained in the raw exhaust gas and with hydrogen (H 2 ) formed in the first three-way catalyst 12 by a water gas shift reaction (CO + H 2 O → CO2 + H 2 ).

[0027] The direct detection of ammonia by means of the NH3 sensor 14 and the use of such a detection to define the time for switching from rich operation to lean operation of the internal combustion engine 1 results in rich breakthroughs being detected as quickly as possible during operation of the internal combustion engine 1 and being counteracted by switching to lean operation of the internal combustion engine 1, since the presence of unburned hydrocarbons in the raw exhaust gas is largely avoided during lean operation. Furthermore, during lean operation of the internal combustion engine 1, oxygen contained in the raw exhaust gas is stored in an oxygen storage capacity of the first three-way catalytic converter 12. As a result, oxygen is again available in the first three-way catalytic converter 12 for converting unburned hydrocarbons during subsequent rich operation of the internal combustion engine 1.

[0028] Such a switch from lean operation to rich operation occurs when a defined presence of oxygen in the exhaust gas downstream of the first three-way catalyst 12 is detected by means of the NOx sensor 13, this presence of oxygen in the exhaust gas being due to the fact that the oxygen storage capacity of the first three-way catalyst 12 is substantially completely filled.

[0029] As a result, by controlling the combustion air ratio according to the invention during operation of the internal combustion engine 1, the formation of ammonia and, via the ammonia, of nitrous oxide in the first three-way catalyst 12 can be minimized in a relatively simple manner. LIST OF REFERENCE SYMBOLS

[0030] 1 Internal combustion engine 2 Cylinder opening 3 Piston 4 Combustion chamber 5 Fresh gas line 6 Fuel injector 7 Exhaust line 8 Exhaust gas aftertreatment device 9 Intake valve 10 Exhaust valve 11 First lambda probe 12 First three-way catalytic converter 13 NOx sensor 14 NH 3 sensor 15 Second three-way catalytic converter 16 Ignition device

Claims

1. Method for operating a combustion machine having an internal combustion engine (1) and having an exhaust tract (7), wherein, proceeding from the internal combustion engine (1), a (first) lambda probe (11), then a first three-way catalytic converter (12) and then a second three-way catalytic converter (15) are integrated into the exhaust tract (7), and wherein an NH3 sensor (14) is integrated into the exhaust tract (7) between the first three-way catalytic converter (12) and the second three-way catalytic converter (15), characterized in that the internal combustion engine (1) is alternately operated rich and operated lean, wherein a switch from rich operation to lean operation takes place if a presence, able to be ascertained in principle, of ammonia in the exhaust gas is ascertained by means of the NH3 sensor (14).

2. Method according to Claim 1, characterized in that an NOX sensor (13) and / or a second lambda probe are / is integrated into the exhaust tract (7) between the first three-way catalytic converter (12) and the second three-way catalytic converter (15).

3. Method according to Claim 1 or 2, characterized in that a / the NOX sensor (13) is integrated into the exhaust tract (7) between the first three-way catalytic converter (12) and the second three-way catalytic converter (15), wherein the NH3 sensor (14) and the NOX sensor (13) are formed integrally.

4. Method according to one of the preceding claims, characterized in that a particle filter is arranged downstream of the first three-way catalytic converter (12) or is integrated into the first three-way catalytic converter (12), and / or a particle filter is arranged downstream of the second three-way catalytic converter (15) or is integrated into the second three-way catalytic converter (15).

5. Method according to one of the preceding claims, characterized in that the first lambda probe (11) is in the form of a broadband probe.

6. Method according to one of the preceding claims, characterized in that a / the second lambda probe is integrated into the exhaust tract (7) between the first three-way catalytic converter (12) and the second three-way catalytic converter (15), wherein the second lambda probe is in the form of a step-change probe.

7. Method according to Claim 2 or one of the claims dependent on Claim 2, characterized in that a switch from lean operation to rich operation takes place if a defined presence of oxygen in the exhaust gas is ascertained by means of the second lambda probe and / or a defined presence of nitrogen oxides and / or of oxygen in the exhaust gas is ascertained by means of the NOX sensor (13) .