Exhaust aftertreatment methods in a motor vehicle

Abstract

Method for exhaust aftertreatment in a motor vehicle in which an exhaust gas flow (22) coming from an internal combustion engine (12) operating in a high load range is cooled in a braked exhaust gas turbocharger (20) and The cooled exhaust gas flow (28) coming from the exhaust gas turbocharger (22) is subsequently fed to an exhaust gas catalyst (24), wherein the exhaust gas turbocharger (20) is braked by an electric machine coupled to the exhaust gas turbocharger (20) and operated in generator mode characterized by the fact that the electrical energy generated in the generator operation of the electric machine is supplied to an electric traction machine (14) for the electric drive of the motor vehicle.

Description

[0001] The invention relates to a method for exhaust aftertreatment in a motor vehicle, with the help of which combustion gases produced in an internal combustion engine can be cleaned.

[0002] From DE 10 2019 126 611 A1 it is known to couple an exhaust gas turbocharger provided in a drive train of a motor vehicle with an electric machine in order to be able to drive the exhaust gas turbocharger electrically in engine operation and to recuperate electrical energy in generator operation in such a way that the actual turbine power of the exhaust gas turbocharger provides a requested boost pressure as precisely as possible.

[0003] From DE 10 2019 101 508 A1 it is known to operate an electric motor for driving a compressor of an exhaust gas turbocharger also in generator mode for energy recovery into a battery in order to reduce the compressor speed.

[0004] From DE 10 2017 122 895 A1 it is known to use a water injection system to cool compressed fresh air from an exhaust gas turbocharger before combustion in an internal combustion engine in order to reduce the tendency to knock.

[0005] There is a constant need to ensure good exhaust gas purification in various operating situations of a motor vehicle.

[0006] The purpose of the invention is to demonstrate measures that enable good exhaust gas purification in various operating situations of a motor vehicle.

[0007] The problem is solved by a method having the features of claim 1. Preferred embodiments of the invention are specified in the dependent claims and the following description, each of which can individually or in combination represent an aspect of the invention.

[0008] One embodiment relates to a method for exhaust aftertreatment in a motor vehicle, in which an exhaust gas stream coming from an internal combustion engine operating in a high-load range is cooled in a braked exhaust gas turbocharger and / or by water injection, and subsequently the cooled exhaust gas stream coming from the exhaust gas turbocharger is fed to an exhaust gas catalyst.

[0009] For exhaust aftertreatment in the catalytic converter, a relatively high operating temperature is generally required to effectively convert the exhaust gases. The operating temperature of the catalytic converter typically ranges between 250°C and 1000°C, particularly between 400°C and 800°C. Such an operating temperature can usually be easily reached and maintained by the temperature of the exhaust gas flow exiting the combustion engine. To prevent the operating temperature of the catalytic converter from dropping too low, especially below its activation temperature, cooling of the exhaust gas flow is generally considered detrimental and should therefore be avoided.

[0010] However, if the combustion engine is required to deliver particularly high power, it can be operated with a particularly high fuel quantity in the high-load range. This results in a particularly large exhaust gas mass flow with a correspondingly high energy content being emitted from the combustion engine. Furthermore, it can be designed so that in such an operating situation the combustion engine is not operated at a point geared towards particularly low fuel consumption, but rather at a point geared towards high engine power, so that the combustion engine is operated not with a lean fuel / air mixture, but with a stoichiometric or rich fuel / air mixture in the high-load range. This allows for the afterburning of unburned fuel in the combustion engine in an exothermic reaction within the catalyst.The particularly high energy content of the exhaust gas stream and / or the unburned fuel in the exhaust gas stream from a combustion engine operating at high load, especially at full load, could lead to excessive heating of the exhaust catalyst. This could result in operating temperatures within the catalyst that could damage it or impair its service life. However, by cooling the exhaust gas stream—a practice otherwise considered detrimental—in the specific operating conditions of a combustion engine operating at high load, damage to or impairment of the catalyst's service life can be avoided. This ensures effective exhaust gas purification and a long service life in various vehicle operating situations.

[0011] In one aspect, the exhaust gas flow can be cooled by passing it through the turbine of a decelerated exhaust gas turbocharger. This allows energy to be extracted from the exhaust gas flow to the extent of the braking power applied to the turbocharger. The enthalpy extracted from the exhaust gas flow in the decelerated turbocharger can result in a lower exhaust gas temperature at the turbocharger outlet than would be possible without deceleration. With a high exhaust gas mass flow rate in a combustion engine operating at high load, cooling the exhaust gas flow in the decelerated turbocharger can achieve a total exhaust gas enthalpy comparable to that of a typical exhaust gas flow in a combustion engine operating at partial load.If necessary, the total enthalpy of the exhaust gas flow can be reduced even further to compensate for a heating effect from the exothermic afterburning of unburned fuel in the exhaust catalyst. Excessive heating of the exhaust catalyst can be reliably prevented by cooling the exhaust gas flow in the throttled exhaust turbocharger. Additionally or alternatively, cooling of the exhaust gas flow can be achieved by injecting water that evaporates within the exhaust gas flow.

[0012] The exhaust gas turbocharger is braked by an electric machine coupled to it and operating in generator mode. The braking power acting on the exhaust gas turbocharger can thus be supplied by the electric machine operating in generator mode, thereby simultaneously recuperating the braking energy into electrical energy. At the same time, the exhaust gas turbocharger, electrified by the electric machine, can be driven by the electric machine operating in engine mode when needed, for example, when fresh air demand increases and / or turbo lag is overcome.Outside of periods when the combustion engine is operating at high load and a deliberate braking effect is provided by the electric machine to cool the exhaust gas flow, the electric machine can switch appropriately between motor operation, generator operation or idle operation to achieve optimized operation of the exhaust gas turbocharger, for example to regulate a desired boost pressure for the compressed fresh air.

[0013] The electrical energy generated in the generator mode of the electric machine is supplied to an electric traction motor for the electric propulsion of the vehicle. If the vehicle is designed as a hybrid vehicle with the aid of the electric traction motor, generating additional electrically generated drive power ("boost mode") using the electric traction motor can be advantageous in operating situations where the combustion engine is operating at high load. For example, a particularly high drive power demanded at any given time can be provided very quickly, and / or fuel-inefficient operation of the combustion engine can be mitigated.The enthalpy used solely in the exhaust gas stream can thus be at least partially supplied via the electric machine of the exhaust gas turbocharger, which operates in generator mode, to the electric traction machine operating in engine mode, and still be used for the propulsion power of the motor vehicle.

[0014] It is particularly preferred that the braking power and / or throttle position of the exhaust gas turbocharger be adjusted as a function of the exhaust gas flow from the combustion engine such that, given the exothermic energy expected in the exhaust gas catalyst at this mass flow rate, the catalyst temperature is kept below the upper limit temperature specified for the conversion of the exhaust gas flow in the catalyst, and, in particular, a required minimum boost pressure of the exhaust gas turbocharger is maintained. The catalyst temperature can be measured directly or indirectly with a temperature sensor, and its signal can be used to control the braking power applied to the exhaust gas turbocharger. This avoids the risk of accidentally cooling the exhaust gas flow too much.Furthermore, it is possible to account for a time delay in the warm-up and cool-down of the exhaust gas catalyst and, taking into account the heat storage effects of the exhaust gas catalyst, to implement a feed-forward control for the exhaust gas catalyst's braking power, depending on the catalyst's temperature profile. The upper limit temperature is specifically between 800°C and 1000°C. By adjusting the throttle position of the exhaust gas turbocharger's throttle valve, it can be ensured that sufficient boost pressure is maintained for the current operating situation, even during turbocharger deceleration.

[0015] In particular, fresh air supplied to the combustion engine during high-load operation and / or the exhaust gas flow exiting the combustion engine during high-load operation are cooled by water injection. This water injection provides additional cooling of the exhaust gas flow, supplementing the cooling effect in the turbocharger. Especially if the braking force applied to the turbocharger is insufficient to achieve the desired cooling of the exhaust gas flow, the additional water injection can ensure that the desired cooling is achieved. Alternatively, it is also possible to achieve exhaust gas cooling solely by braking the turbocharger, i.e., without water cooling.

[0016] Another unclaimed embodiment relates to a method for exhaust aftertreatment in a motor vehicle, in which an exhaust gas stream from an internal combustion engine operating in a high-load range is cooled by water injection, the exhaust gas stream is fed to an exhaust gas turbocharger driven by an electric motor, and subsequently the cooled exhaust gas stream is fed to an exhaust gas catalyst. The electric motor can be powered by a motor vehicle battery, which can be recharged, in particular, by recuperating braking energy.

[0017] In this system, the exhaust gas flow is cooled not, or not exclusively, by slowing down the turbocharger, but by water injection. The injected water can evaporate, and the energy required for this evaporation extracts energy from the exhaust gas flow, thereby lowering its temperature. If necessary, the total enthalpy of the exhaust gas flow can be reduced even further to compensate for any heating effect from the exothermic afterburning of unburned fuel in the catalytic converter. Excessive heating of the catalytic converter can be reliably prevented by cooling the exhaust gas flow in the slowed turbocharger.The cooling achieved through water injection even makes it possible to drive the exhaust gas turbocharger using the coupled electric motor operating during engine operation, without the energy input into the exhaust gas stream by the electric motor causing excessive heating of the exhaust gas catalyst. The upper temperature limit is specifically between 800°C and 1000°C. The following explanations apply to both embodiments described above.

[0018] Preferably, the cooling capacity of the water injection is adjusted as a function of the exhaust gas mass flow and / or the electrical drive power of the electric machine coupled to the exhaust gas turbocharger such that, given the exothermic energy and inlet temperature of the exhaust gas expected at this mass flow rate in the exhaust gas catalyst, the catalyst temperature is maintained below the upper limit temperature specified for the conversion of the exhaust gas in the catalyst. The catalyst temperature can be measured directly or indirectly with a temperature sensor, and its signal can be used to control the amount of water injected. This avoids the risk of accidentally cooling the exhaust gas too much.Furthermore, it is possible to take into account a time delay in the heating and cooling of the exhaust gas catalyst and, knowing the heat storage effects of the exhaust gas catalyst, to provide a feed-forward control for the braking performance of the exhaust gas catalyst depending on the temperature profile of the catalyst temperature of the exhaust gas catalyst.

[0019] The combustion engine is particularly preferably operated in the high-load range at a lambda value λ for a ratio of residual oxygen content in the exhaust gas stream to oxygen content in the environment of λ ≤ 1.0, especially 0.80 ≤ λ ≤ 0.95. The additional cooling of the exhaust gas stream makes it possible to use a rich fuel / air mixture for the operation of the combustion engine and to achieve a particularly high-performance operating point of the combustion engine without having to worry about excessive heating of the exhaust gas catalyst due to exothermic reactions of unburned fuel components in the exhaust gas stream.

[0020] In particular, the exhaust catalyst features an electrically operated heater and / or a burner for heating the catalyst during a cold start phase. Specifically, the electrically operated heater is powered by a battery charged by the electric motor coupled to the exhaust turbocharger. This allows the exhaust catalyst to be heated quickly above its operating temperature during the cold start phase, thus minimizing the period during which the catalyst is unable to convert exhaust gases. If the electrical energy for the electric heater is supplied by the electric motor coupled to the exhaust turbocharger, heating the exhaust catalyst can even be achieved without an additional energy source. In this case, the use of a vehicle battery charged by an alternator and / or an electric traction motor is either unnecessary or can be significantly reduced.

[0021] Preferably, the heating output of the heater and / or burner is adjusted during high-load operation of the combustion engine such that, given the expected exothermic energy of the cooled exhaust gas stream, the catalyst temperature is maintained above a lower limit temperature specified for the conversion of the exhaust gas stream within the catalyst. This lower limit temperature is, in particular, between 250°C and 400°C. If excessive cooling of the exhaust gas should occur, a drop in the catalyst temperature below the lower limit temperature can be prevented in a timely manner by applying additional heating. This allows any adjustment of the cooling capacity within the catalyst, which may only take effect with a time delay when the exhaust gas stream cools, to be compensated, at least temporarily, by the heating output generated directly within the catalyst. This improves the control of the catalyst temperature.

[0022] The invention is explained below by way of example with reference to the accompanying drawing and a preferred embodiment, wherein the features shown below can represent an aspect of the invention, either individually or in combination. It shows: Fig. 1: A schematic diagram of a powertrain.

[0023] The in Fig.The powertrain of a hybrid vehicle shown in Figure 1 comprises an internal combustion engine 12, which can be coupled to a vehicle transmission 16 via an electric traction motor 14. Fresh air 18 drawn in for the internal combustion engine 12 can be supplied to the engine via a compressor of an exhaust gas turbocharger 20 and an intercooler, if necessary. An exhaust gas flow 22 exiting the engine 12 can be fed to a turbine of the exhaust gas turbocharger 20 and introduced into an exhaust gas catalyst 24.

[0024] When the combustion engine 12 is operated under high load, the exhaust gas flow 22 can be additionally cooled so that the combustion engine 12 can be operated with a rich fuel / air mixture without the exhaust gas catalyst 24 being heated beyond its upper temperature limit by exothermic reactions 26 of unburned fuel in the exhaust gas flow 22. For this purpose, water injection and / or an electric machine coupled to the exhaust gas turbocharger 20 can be provided. In generator mode, the electric machine of the exhaust gas turbocharger 20 can decelerate the exhaust gas turbocharger 20 and thereby recuperate electrical energy from the exhaust gas flow 22 and cool the exhaust gas flow 22, so that in this operating situation, an intentionally cooled exhaust gas flow 28 is supplied to the exhaust gas catalyst 24.The electrical energy 30 generated in the electric machine can optionally be supplied to a battery 34 via a pulse inverter 32 and stored in the battery 34. This makes it possible to simultaneously drive the electric traction machine 14 from the battery 34 and / or directly from the electric machine of the exhaust gas turbocharger 20 and to boost an electrically generated drive torque. Furthermore, it is possible to use the energy stored in the battery 34 at a later time for the operation of the electric machine of the exhaust gas turbocharger 20 and / or for the operation of the electric traction machine 14 and / or for heating the exhaust gas catalyst 24, particularly during a cold start phase.In the event of excessive cooling of the exhaust gas flow 22, the catalyst temperature of the exhaust gas catalyst 24 can be prevented from falling below a lower limit temperature by additional heating, in particular with the aid of an electric heater and / or a burner. The catalyst temperature of the exhaust gas catalyst 24 can be measured using a temperature sensor and used to control the cooling capacity for cooling the exhaust gas flow 22 and / or to control the heating capacity for the exhaust gas catalyst 24.

Claims

[1] Method for exhaust aftertreatment in a motor vehicle in which an exhaust gas flow (22) coming from an internal combustion engine (12) operating in a high load range is cooled in a braked exhaust gas turbocharger (20) and The cooled exhaust gas flow (28) coming from the exhaust gas turbocharger (22) is subsequently fed to an exhaust gas catalyst (24), wherein the exhaust gas turbocharger (20) is braked by an electric machine coupled to the exhaust gas turbocharger (20) and operated in generator mode characterized by , that the electrical energy generated in the generator operation of the electric machine is supplied to an electric traction machine (14) for the electric drive of the motor vehicle. [2] Method according to claim 1, in which partially used enthalpy of the exhaust gas flow (28) is used for the drive power of the motor vehicle via the electric machine of the exhaust gas turbocharger (22) operated in generator mode. [3] Method according to claim 1 or 2, wherein the total enthalpy of the exhaust gas stream (28) is reduced to such an extent that a heating effect is compensated by an exothermic afterburning of unburned fuel in the exhaust gas catalyst (24). [4] Method according to one of claims 1 to 3, wherein a braking power and / or a throttle valve position of the exhaust gas turbocharger (20) is adjusted as a function of a mass flow rate of the exhaust gas flow coming from the internal combustion engine (12) such that, with an exothermic energy expected in the exhaust gas catalyst (24) at this mass flow rate of the exhaust gas flow (22), a catalyst temperature of the exhaust gas catalyst (24) is kept below an upper limit temperature provided for the conversion of the exhaust gas flow (22) in the exhaust gas catalyst (24) and, in addition, a required minimum boost pressure of the exhaust gas turbocharger (20) is maintained. [5] Method according to any one of claims 1 to 4, wherein fresh air supplied in the high-load range of the internal combustion engine (12) and / or the exhaust gas flow (22) leaving the internal combustion engine (12) operating in the high-load range is cooled by water injection.

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