Exhaust gas recirculation under rich conditions to improve fuel economy

Abstract

Method for operating a turbocharged gasoline engine, the method comprising the following: Diluting an engine intake air charge to a first dilution level when operating at a stoichiometric air / fuel ratio and as a reaction to a condition of excessive exhaust gas temperature downstream of the engine, dilution of the engine intake air charge to a second, higher degree of dilution when operating with a rich air / fuel ratio, characterized in that the first dilution level is limited by a first tolerance limit and the second dilution level by a second tolerance limit, the second tolerance limit being greater than the first tolerance limit.

Description

[0001] This application relates to the field of automotive engineering and in particular to exhaust gas recirculation (EGR).

[0002] An internal combustion engine in a vehicle can operate over a wide range of speeds and loads. In a turbocharged gasoline engine operating with a stoichiometric air / fuel ratio (A / F ratio), the high engine load can lead to undesirably high exhaust gas temperatures. Such temperatures can accelerate the aging of materials in the engine system – particularly in the turbine and exhaust aftertreatment components.

[0003] US Patent 2011 / 0 231 081 A1 discloses a control method for an internal combustion engine. German Patent DE 103 05 451 A1 discloses a method and a device for operating an internal combustion engine. US Patent 2009 / 0 259 387 A1 discloses a device and a method for controlling a homogeneously turbocharged, compression-ignition internal combustion engine. These documents do not disclose that the first degree of dilution is limited by a first tolerance limit and the second degree of dilution by a second tolerance limit, the second tolerance limit being greater than the first tolerance limit.

[0004] One strategy for reducing the exhaust temperature of a gasoline engine during a high-load transition is to enrich the air / fuel mixture. Above the stoichiometric ratio, the liquid fuel injected into an engine cylinder does not contribute to combustion, but it does contribute to the evaporative cooling of the combustion gases contained within. Furthermore, excess fuel can be endothermically reformed in the cylinder, providing additional cooling. However, this strategy is detrimental to fuel economy, as the excess fuel provides no energy.

[0005] To address this problem, the present inventors disclose a series of solutions in which exhaust gas recirculation is aggressively employed during high-load conditions with a rich air / fuel mixture. One particular embodiment provides a method for operating a turbocharged gasoline engine. The method comprises diluting the engine intake air charge to a first dilution level when operating at a stoichiometric air / fuel ratio. Furthermore, in response to a state of excessive exhaust gas temperature downstream of the engine, the method comprises diluting the intake air charge to a second, higher dilution level when operating at a rich air / fuel ratio. According to the invention, the first dilution level is limited by a first tolerance limit, and the second dilution level by a second tolerance limit, the second tolerance limit being greater than the first.

[0006] Certain benefits of EGR in a gasoline engine under high load have already been established. For example, US 8 001 779 B2 provides a high- and low-pressure hybrid EGR system that extends the benefits of EGR over a wide range of engine speeds and loads. This document recognized that EGR used at high load can reduce the need for enrichment to protect exhaust system components. However, it did not recognize the potential synergy resulting from the combined use of enrichment and aggressive EGR during high-load transitions.

[0007] In contrast, the present disclosure shows that the application of various forms of EGR in combination with enrichment can be used to eliminate the fuel economy losses resulting from enrichment. In this way, the exhaust system can be protected during high-load transitions without excessive fuel economy losses.

[0008] The above brief description is provided for the simplified introduction of a selected part of this disclosure and not for the determination of essential or key features. The subject matter of the invention as defined by the claims is neither limited to the content of this brief description nor to implementations that offer solutions to the problems or disadvantages listed herein. Fig. Figure 1 schematically shows aspects of an exemplary motor vehicle system according to an embodiment of the present disclosure. Fig. Figure 2 shows an exemplary method for operating a turbocharged gasoline engine according to an embodiment of the present disclosure. Fig. Figure 3 is an exemplary graphical comparison of various engine parameters during the execution of the procedure. Fig. 2.

[0009] Fig. Figure 1 schematically shows aspects of an exemplary engine system 10 of a motor vehicle. In the engine system 10, fresh air is drawn into an air filter 12 and flows to the compressor 14. The compressor can be any suitable intake air compressor – for example, an engine-driven or a driveshaft-driven supercharger. In the engine system 10, however, the compressor is mechanically coupled to a turbine 16 in the turbocharger 18, the turbine being driven by expanding engine exhaust gas from the exhaust manifold 20. In one embodiment, the compressor and the turbine can be coupled within a twin-scroll turbocharger. In another embodiment, the turbocharger can be a variable geometry turbocharger (VGT), the turbine geometry being actively changed as a function of the engine speed.

[0010] The compressor 14 is flow-connected to the intake manifold 22 via a charge-air cooler (CAC) 24 and a throttle valve 26. The pressurized air from the compressor flows through the CAC and the throttle valve on its way to the intake manifold. In the illustrated embodiment, a compressor bypass valve 28 is coupled between the inlet and outlet of the compressor. The compressor bypass valve can be a normally closed valve configured to open under selected operating conditions to release excess boost pressure.

[0011] The exhaust manifold 20 and the intake manifold 22 are coupled to a series of cylinders 30 by a series of exhaust valves 32 and intake valves 34, respectively. In one embodiment, the exhaust and / or intake valves can be actuated electronically. In another embodiment, the exhaust and / or intake valves can be actuated by cams. Whether actuated electronically or by cams, the timing of the opening and closing of the exhaust and intake valves can be adjusted as required for a specific combustion and exhaust gas purification performance.

[0012] Depending on the embodiment, various fuels can be supplied to the cylinders 30: gasoline, alcohols, or mixtures thereof. In the illustrated embodiment, fuel is supplied to the cylinders by the fuel pump 36 via direct injection through the fuel injectors 38. In the various embodiments considered in this document, the fuel can be supplied via direct injection, port injection, throttle body injection, or any combination thereof. In the engine system 10, combustion is initiated by spark ignition at the spark plugs 40. The spark plugs are actuated by timed high-voltage pulses from an electronic ignition unit not shown in the drawings.

[0013] The engine system 10 includes a high-pressure (HP) exhaust gas recirculation (EGR) valve 42 and an HP EGR cooler 44. When the HP EGR valve is open, a portion of the high-pressure exhaust gas from the exhaust manifold 20 is drawn through the HP EGR cooler to the intake manifold 22. In the intake manifold, the high-pressure exhaust gas dilutes the intake air charge for cooler combustion temperatures, reduced emissions, and other benefits. The remaining exhaust gas flows to the turbine 16 to drive the turbine. If reduced turbine torque is desired, all or part of the exhaust gas can instead be routed through the wastegate 46 to bypass the turbine. The combined flow from the turbine and the wastegate then flows through the various exhaust aftertreatment devices of the engine system, as described below.

[0014] In engine system 10 there is a NO xA storage catalyst (NSK) 48 is coupled downstream of turbine 16. The NSK contains an internal catalyst support structure onto which a catalytic washcoat is applied. The washcoat is designed to trap NO. x from the exhaust gas stream when the exhaust gas stream is lean, and to reduce the captured NO x , when the exhaust flow is rich, a Stage 50 three-way catalyst (TWC) is coupled downstream of the NSK. The TWC is configured to oxidize remaining CO, hydrogen, and hydrocarbons, and nitrogen oxides (NOx) present in the engine exhaust. x to reduce.

[0015] It should further be noted that the type, number, and arrangement of exhaust aftertreatment stages in the engine system may vary in the different embodiments described in this disclosure. For example, some configurations may include a particulate filter or a multi-purpose exhaust aftertreatment stage that combines particulate filtering with other exhaust purification functions, such as NO capture. x combined, included.

[0016] Continue on Fig. 1. Referring to the above, all or part of the treated exhaust gas can be released into the environment via a silencer 52. However, depending on the operating conditions, some of the treated exhaust gas can be redirected through the low-pressure (LP) EGR cooler 54. The exhaust gas can be redirected by opening the low-pressure EGR valve 56, which is coupled in series with the low-pressure EGR cooler. The low-pressure EGR cooler can contain a suitable type of heat exchanger – gas-air, gas-water, etc. From the low-pressure EGR cooler 54, the cooled exhaust gas flows to the compressor 14. By partially closing the exhaust backpressure valve 58, the flow potential for _P-EGR can be increased during selected operating conditions. Other configurations may include a throttle valve upstream of the air filter 12 instead of the exhaust backpressure valve.

[0017] The motor system 10 includes an electronic control system (ECS) 60 configured to control various motor system functions. The ECS contains memory and one or more processors configured for appropriate decision-making in response to sensor input and designed for intelligent control of the motor system assembly. Such decision-making can be implemented according to various strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. Thus, the ECS can be configured to implement any or all aspects of the methods disclosed below. Accordingly, the various method steps—for example, operations, functions, and / or actions—can be designed as code programmed into machine-readable storage media within the ECS.

[0018] ECS 60 includes a sensor interface 62, an engine control interface 64, and an on-board diagnostic unit (OBD unit, OBD = On-Board Diagnostic) 66. To evaluate the operating conditions of the engine system 10 and the vehicle in which the engine system is installed, the sensor interface 62 receives input from various sensors located in the vehicle – flow sensors, temperature sensors, pedal position sensors, pressure sensors, etc. Fig. 1 Some exemplary sensors are shown - an intake manifold pressure sensor (MAP sensor, MAP - Manifold Air Pressure) 68, an exhaust manifold air temperature sensor (MAT sensor, MAT - Manifold Air Temperature) 70, an air mass sensor (MAF sensor, MAF - Mass Air Flow) 72, a NO x-Sensor 74, an exhaust gas air / fuel ratio sensor 76, and an exhaust system temperature sensor 78. In one embodiment, the exhaust gas temperature sensor can be thermally coupled to an exhaust aftertreatment catalyst. Various other sensors can also be provided.

[0019] The engine control interface 64 is configured to actuate electronically controlled valves, actuators, and other vehicle assemblies—for example, the throttle valve 26, the compressor bypass valve 28, the wastegate 46, and the EGR valves 42 and 56. The engine control interface is operatively coupled to each electronically controlled valve and actuator and configured to control its opening, closing, and / or adjustment as needed to implement the control functions described here. The OBD unit 66 is part of the ECS and is configured to diagnose a loss of quality in various components of the engine system 10. Such components may include, for example, oxygen sensors, fuel injectors, and exhaust aftertreatment components.

[0020] The configurations described above enable various methods for operating a turbocharged gasoline engine. Some such methods are now described with continuous reference to the configurations above. However, it is understood that the methods described here, and others within the scope of this disclosure, can also be enabled by other configurations. Naturally, each execution of a method can change the access conditions for a subsequent execution and thereby invoke complex decision-making logic. Such logic is fully considered in this disclosure.

[0021] Fig. Figure 2 shows an exemplary procedure 80 for the operation of a turbocharged gasoline engine. In section 82 of procedure 80, a lean or stoichiometric air / fuel ratio λ is provided by adjusting the fuel injection quantity with respect to the throttle position, or vice versa. In section 84, the engine's intake air charge is diluted to a first degree of dilution via one or more types of EGR, as described below.

[0022] EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When the resulting air / exhaust mixture is used instead of ordinary air to aid combustion in the engine, it leads to lower combustion and exhaust gas temperatures. EGR can also improve fuel economy in gasoline engines. At medium and high loads, fuel economy is improved due to reduced knocking, which allows for more efficient combustion phasing, reduced heat loss to the engine coolant, and lower exhaust gas temperatures—which in turn reduces the need for enrichment to cool the exhaust components. At low loads, EGR provides the additional benefit of reducing throttling losses.

[0023] As mentioned above in connection with Fig. As mentioned in section 1, exhaust gas can be recirculated through a high-pressure (HP) EGR loop and / or a low-pressure (LP) EGR loop. In the HP EGR loop, exhaust gas is taken upstream of the turbine and mixed with intake air downstream of the compressor. In an LP EGR loop, exhaust gas is taken downstream of the turbine and mixed with intake air upstream of the compressor. Furthermore, some engine systems provide so-called "internal EGR," in which combustion can be initiated in one or more cylinders of the engine if exhaust gas from a previous combustion is still present in the cylinders. The extent of internal EGR can be controlled using variable intake and / or exhaust valve timing.

[0024] The high-pressure (HP) and low-pressure (LP) EGR strategies achieve optimal effectiveness in different areas of the engine's load-speed map. Each strategy also presents its own control system challenges. For example, HP EGR is most effective at low loads, where the intake vacuum provides sufficient flow potential; at higher loads, the target EGR flow rate may be unattainable due to reduced flow potential. HP EGR, which is inherently dependent on turbocharger wastegate and throttle conditions, may require a complex flow control strategy. Furthermore, HP EGR can experience poor EGR / air mixture and require a high rate of active cooling, due to the short distance between the HP EGR extraction point and the engine's intake manifold pipes.

[0025] Unlike high-pressure EGR (HP-EGR), low-pressure EGR (LP-EGR) delivers adequate flow from medium to high engine loads in areas where HP-EGR is flow-limited. It is easier to cool and can be controlled more independently of the throttle valve and wastegate. Furthermore, cooled LP-EGR provides a significant additional advantage in scenarios where exhaust gas temperature reduction is desired. Before re-entering the cylinder, the LP-EGR is cooled in the turbine, the EGR cooler, the long return line to the intake, and again in the CAC (Cooling Air Control). Consequently, the diluent temperature is lower than in HP and "internal EGR" variants. The lower-temperature diluent provides a more effective reduction in combustion temperatures and correspondingly lower exhaust gas temperatures. In the present process, the intake air charge can therefore be diluted with cooled LP-EGR – i.e.,The exhaust gas is diluted upstream of the intake air compressor with exhaust gas captured downstream of an exhaust turbine and cooled in a heat exchanger. However, other embodiments are being considered in which high-pressure and / or internal EGR can be used instead of or in addition to cooled low-pressure EGR.

[0026] Continue on Fig. 2. Referring to this, at 86 it is determined whether a tip-in condition has been detected. If a tip-in condition has been detected, the procedure proceeds to 88. However, if no tip-in condition has been detected, the procedure returns. At 88 it is determined whether an excessive exhaust gas temperature condition has been detected. Excessive exhaust gas temperature can lead to a loss of quality in various components of the exhaust system, such as the turbine or an exhaust aftertreatment catalyst. Accordingly, the excessive exhaust gas temperature condition can be detected based on a temperature sensor coupled to a turbine, an exhaust aftertreatment component, or at any point in the exhaust system.

[0027] When an excessive exhaust gas temperature condition is detected, the procedure proceeds to step 90, where the fuel content of the exhaust gas is estimated. The fuel content can be estimated, for example, based on the output of an air / fuel ratio sensor coupled to an EGR line. The procedure then proceeds from step 90 to step 92, where a richer air / fuel ratio is provided to the engine. Specifically, a predetermined degree of enrichment can be provided by increasing the fuel injection rate into the cylinders, throttle body, or intake manifold of the engine. To correct the injected quantity for any fuel that may be present in the EGR, the engine control system (ECS) can take into account the fuel content of the exhaust gas estimated as described above.

[0028] It is known that enrichment via evaporative cooling and endothermic reforming reactions cools the combustion gases in the engine cylinders. The present inventors further explain that the EGR tolerance of a gasoline engine operating under high-load conditions can be increased by enrichment. Without linking the present method to a specific theory, it is assumed that hydrogen, carbon monoxide, and / or reformed hydrocarbons discharged from the engine during rich operation, when reintroduced through the intake, can improve combustion stability even at low oxygen levels. Accordingly, higher dilution ratios during enrichment can be tolerated for improved fuel economy for the reasons mentioned above. For example, if the engine's EGR tolerance limit is 15% during stoichiometric operation, it can be 25% during enrichment.In addition to the fuel economy benefits resulting from the increased use of EGR at higher engine loads, the present method further improves fuel economy simply by recovering some of the unused fuel that would otherwise be released into the environment or destroyed by an exhaust aftertreatment catalyst. Thus, the present approach can essentially eliminate the fuel economy losses that would otherwise occur during enrichment. It can also reduce hydrocarbon and carbon monoxide emissions during enrichment and lower the requirements for the TWC (Total Water Conditioner).

[0029] Accordingly, in process 80, in 94, exhaust gas from the engine is cooled in a heat exchanger in anticipation of further dilution of the intake air charge, and in process 96, the intake air charge is diluted to a second dilution level that is higher than the first dilution level provided in process 84. In particular, the ECS can be configured to open an EGR valve in the engine to provide a first dilution level when the engine is operated at a stoichiometric air / fuel ratio, and a second, higher dilution level when the engine is operated at a richer air / fuel ratio. In some embodiments, the second dilution level can be such that, when used at the stoichiometric air / fuel ratio, it leads to combustion instability.

[0030] In the described method, the engine's intake air charge is diluted to a higher degree of dilution when operating with a rich air / fuel ratio in response to a condition of excessive exhaust gas temperature downstream of the engine. The rich air / fuel ratio is provided specifically during a tip-in condition—that is, a high-load transition. More generally, the stoichiometric air / fuel ratio can be provided during a first operating condition, and the rich air / fuel ratio during a second operating condition. The second operating condition may involve a higher engine load than the first. Under such conditions, a simultaneous increase in dilution can be expected to improve fuel economy.In other embodiments, the intake air charge can only be diluted to the second degree of dilution if it is known or can be predicted that such dilution improves fuel economy.

[0031] Furthermore with reference to Fig. At step 98, the air charge compression (i.e., boost) is increased to maintain torque when operating at a second dilution level—that is, during the tip-in condition. This measure can be taken to compensate for the increased dilution and the resulting reduction in available oxygen. As mentioned above, the ECS can be configured to run the engine at the enriched air / fuel ratio only if an output from the exhaust gas temperature sensor at step 88 of procedure 80 indicates an excessive temperature condition. However, if no excessive exhaust gas temperature is detected, the procedure instead proceeds to step 100, where the intake air charge is diluted to a third dilution level. In this scenario, no enrichment is provided. Accordingly, the third dilution level can be lower than the second dilution level.However, in some cases, due to the higher engine load during the detected tip-in state, it may still be greater than the first dilution degree.

[0032] Fig. Figure 3 presents a graphical comparison of various engine parameters during the execution of procedure 80. The upper diagram of Fig. Figure 3 shows an example of the change in exhaust gas purification catalyst temperature over time during a high-load transition. As the catalyst temperature approaches its upper limit, enrichment is initiated to prevent the catalyst temperature from exceeding the limit. The desired enrichment is achieved by increasing the fuel injection quantity, as shown by the dashed line in the middle diagram of Figure 3. Fig. Figure 3 shows that in the illustrated embodiment, an initial high stage of fuel injection increase is followed by a lower plateau that persists throughout the enrichment period. During the plateau, the enrichment results from the increased fuel injection combined with the recirculation of unburned fuel to the intake via the EGR loop. However, during the initial stage, there may be little or no unburned fuel in the EGR loop, so an even higher fuel injection level is provided. This strategy can be used to provide a relatively constant air / fuel ratio in the cylinder during enrichment, as shown by the solid line in the middle diagram. Furthermore, with reference to Fig. 3, in the lower diagram, the solid line shows the extent of the EGR valve opening and the dashed line shows the resulting EGR dilution rate in the cylinder.

[0033] Aspects of this disclosure are illustrated by way of example with reference to the embodiments described above. Components, process steps, and other elements that may be substantially the same in one or more embodiments are identified as equivalent and described with minimal repetition. However, it should be noted that elements identified as equivalent may differ to some extent. It should also be noted that the drawing figures contained in this disclosure are schematic and generally not drawn to scale. Instead, the various drawing scales, aspect ratios, and numbers of components shown in the figures may be intentionally distorted to make certain features or relationships more readily apparent.

[0034] In the procedures presented and / or described in this document, some of the indicated process steps may be omitted without affecting the scope of protection afforded by this disclosure. Similarly, the indicated sequence of process steps may not always be mandatory for achieving the desired results; it is provided solely for the sake of clarity and presentation. One or more of the described measures, functions, or operations may be repeated depending on the specific strategy employed.

[0035] It is understood that the objects, systems, and methods described above are embodiments of the present disclosure—non-limiting examples, for which numerous variations and extensions are also conceivable. The present disclosure also includes all new and non-obvious combinations and subcombinations of the above objects, systems, and methods, as well as all equivalents thereof. LEGEND

[0036] Fig. 2: 82 PROVIDING A LEAN / STOHIEMETRICAL AIR / FUEL RATIO 84 THINNING THE INTAKE AIR CHARGE TO FIRST DEGREE 86 TIP-INS RECORDED? NO 88 EXCESSIVE EXHAUST TEMPERATURE? 90 ESTIMATING THE FUEL CONTENT OF THE AGASE 92 PROVIDING AN ADDED LAMBDA RATIO 94 COOLING THE EGR IN THE HEAT EXCHANGER 96 THINNING THE INTAKE AIR CHARGE TO SECOND DEGREE 98 INCREASING AIR PRESSURE TO MAINTAIN TORQUE 100 THINNING THE INTAKE AIR CHARGE TO THE THIRD DEGREE

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