Abstract:
The description relates to an internal combustion engine having a turbocharger, having a first particle filter in the intake section of the internal combustion engine and having a low-pressure EGR line that comprises a second particle filter. A fast-reacting low-pressure exhaust gas recirculation system with small dimensions can be realized by virtue of the EGR line branching off from the exhaust section upstream of the first particle filter, and the second particle filter being provided with a heater. The description also relates to a corresponding EGR method.

Description:
CROSS REFERENCE TO PRIORITY APPLICATION 
       [0001]    The present application claims priority to German Patent Application No. 102008043036.0, filed Oct. 22, 2008, titled “Internal Combustion Engine Having Turbocharging and Low-Pressure Exhaust-Gas Recirculation,” the entire contents of each of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The description relates to an internal combustion engine having a turbocharger, having a first particle filter in the exhaust section of the internal combustion engine and having a low-pressure exhaust-gas recirculation (EGR) line that comprises a second particle filter, and to a method for exhaust-gas recirculation in an internal combustion engine having turbocharging and having low-pressure exhaust-gas recirculation. 
       BACKGROUND AND SUMMARY 
       [0003]    A device of said type and a method of said type are known from DE 10 2006 038 706 A1. Here, for the purpose of nitrogen oxide reduction, recirculated exhaust gas is branched off from the exhaust section downstream of a first particle filter and upstream of a catalytic converter and a silencer. The second particle filter in the low-pressure EGR line may passively assume a temperature of up to 800° C. as a result of the hot exhaust gases. 
         [0004]    In contrast to a high-pressure EGR system, in which the recirculated exhaust gas is branched off upstream of the turbine of the turbocharger, in a low-pressure EGR system, the recirculated exhaust gas is branched off downstream of the turbine of the turbocharger after having passed through a particle filter. As a result, a low-pressure EGR system has the disadvantages, in low-load operation, of a higher back pressure, and a longer build-up phase of the proportion of unburned mass in the inlet, than a high-pressure EGR system. 
         [0005]    The description is based on the object of providing an improved low-pressure EGR system. 
         [0006]    Said object is achieved with a generic device and a generic method by means of the characterizing features of the claims that follow. 
         [0007]    In one embodiment the present description provides for an EGR system for an internal combustion engine, comprising: internal combustion engine having a turbocharger, an intake system, and an exhaust system; a first particle filter located in said exhaust system at a location downstream of a turbine of said turbocharger; a second particulate filter located in a EGR line having inlet located in said exhaust system at a location upstream of said first particle filter, said second particulate filter having a heater. 
         [0008]    By having the EGR line branch off from the exhaust section upstream of the first particle filter, less mass flow need pass through the first particle filter and/or an oxidation catalytic converter, such that the exhaust-gas back pressure is reduced. Furthermore, the volume of the EGR section can be significantly reduced. This significantly improves the response speed of the EGR system. Soot and oil particles which are contained in the recirculated exhaust gas, and which may not pass into the compressor of the turbocharger, can be combusted by means of the heatable particle filter, such that said particle filter does not become easily blocked. Furthermore, a heatable particle filter of said type may be significantly smaller than a conventional particle filter, for example less than half the volume. 
         [0009]    It is expedient for the heatable particle filter not to be held constantly at a temperature at which soot and oil particles combust, but rather to be heated only in phases for the purpose of regeneration. Suitable times for this are for example operating states in which the turbocharger is running at only a low rotational speed, in order that soot and oil particles that may be released, un-combusted or only partially combusted, from the filter structure do not degrade the compressor of the turbocharger. Furthermore, at a low rotational speed of the turbocharger, the gas throughput through the filter is low, such that the heating power to be imparted is low. 
         [0010]    Advantageous refinements of the description can be gathered from the subclaims and the description. 
         [0011]    The description is explained in more detail below on the basis of the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further advantageous refinements of the description are disclosed in the dependent claims and in the following description of the figures: 
           [0013]      FIG. 1  shows a schematic illustration of an engine EGR system; and 
           [0014]      FIG. 2  shows a schematic illustration of a flow chart of a method to control exhaust-gas recirculation. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The  FIG. 1  shows a diagrammatic sketch of an internal combustion engine having turbocharging and having low-pressure exhaust-gas recirculation. Although the exemplary embodiment relates to a diesel engine, the description may however also be applied to other types of internal combustion engine. 
         [0016]    A schematically illustrated multi-cylinder diesel engine  2  has inlet ducts  4  and outlet ducts  6 . The outlet ducts  6  open out via a collector  8  into an exhaust line  10 , which opens out into a turbine  12  of a turbocharger  14 . The turbine  12  is coupled by means of a shaft  16  to a compressor  18  of the turbocharger  14 . The turbocharger  14  may be a turbocharger with fixed geometry (FGT) or a turbocharger with variable geometry (VGT). 
         [0017]    The outlet of the turbine  12  is adjoined by an exhaust section  20  in which are arranged, in this sequence, a diesel oxidation catalytic converter  22 , a diesel particle filter  24 , a control system for controlling the exhaust-gas back pressure, which control system comprises a throttle flap  26  and a bypass  28 , which leads past the throttle flap  26 , with an integrated valve, and a silencer  30 . 
         [0018]    A low-pressure EGR line  32  is connected to the exhaust section  20  downstream of the turbine  12  and upstream of the diesel oxidation catalytic converter  22 , which low-pressure EGR line  32  opens out via an EGR valve  34  into a fresh-air line  36  that conducts fresh air from an air filter  38  into the compressor  18  of the turbocharger  14 . 
         [0019]    The mixture of fresh air and recirculated exhaust gas that is compressed by the compressor  18  passes via an air inlet line  40  into a combined inlet air cooler and distributor  42 , where said mixture is cooled and distributed between the inlet ducts  4 . The inlet air cooler and distributor  42  comprises a bypass (not shown), with the inlet air mixture being conducted, as required, either through the inlet air cooler and distributor  42  or through the bypass and past the inlet air cooler and distributor  42 . 
         [0020]    A throttle flap  44  may also be provided in the inlet line  40  in order to close the inlet line  40  when the diesel engine  2  is shut down. 
         [0021]    The low-pressure EGR line  32  comprises a heatable particle filter  46  that is traversed by the recirculated exhaust gas. The particle filter  46  comprises an electric heater, for example in the form of grids, which are integrated into the filter matrix, composed of heating or glow wires  47 , by means of which any soot and oil particles in the recirculated exhaust gas are burned. 
         [0022]    The low-pressure EGR line  32  may also comprise, downstream of the particle filter  46  and upstream of the EGR valve  34 , a heat exchanger  48  that dissipates the heat contained in the exhaust gas to an arbitrary heat sink—such as for example the inlet air collector and cooler  40 . The heat exchanger  48  comprises a bypass (not shown), with the inlet air mixture being conducted selectively either through the heat exchanger  48  or through the bypass and past the heat exchanger  48 . 
         [0023]    Referring now to  FIG. 2 , a method to control EGR for an internal combustion engine is shown. Routine  200  begins at  202  where engine operating conditions are determined. Engine operating conditions are determined from sensors and actuators. In one example, routine  200  determines engine temperature, ambient temperature, the pressure drop across a particulate filter in the high pressure EGR loop, the pressure drop across a particulate filter in the exhaust system, time since engine start, engine load, engine torque demand, engine speed, and amount of air inducted to the engine. In other example embodiments, additional or fewer operating conditions may be determined based on specific objectives. 
         [0024]    At  204 , the routine judges whether or not to flow EGR. The decision to flow EGR may be based on the operating conditions determined at  202 . In one example, EGR is activated after the engine has been operating for a threshold amount of time and after engine coolant temperature reaches a threshold level. In addition, other conditions may be used to activate or enable the EGR system. For example, EGR may be enabled after engine load is greater than a threshold or after engine speed exceeds a threshold. Routine  200  then proceeds to  206  if EGR is activated. Otherwise, routine  200  proceeds to exit. 
         [0025]    At  206 , the EGR valve is controlled in response to engine operating conditions. In one example, the EGR valve position is related to engine speed and driver demand torque. The EGR valve positions may be stored in a table or function indexed by engine speed and driver demand torque. The EGR valve positions correspond to an empirically determined EGR flow rate. The EGR valve position may be controlled by a vacuum actuator or by a stepper motor, for example. 
         [0026]    At  208 , routine  200  judges whether or not to regenerate a particulate filter in the EGR loop. In one embodiment, routine  200  makes a decision based on the pressure drop across a particulate filter. In another embodiment, routine  200  may decide to regenerate the particulate filter in response to a model. For example, a soot accumulation model that estimates the amount of soot produced by an engine may be the basis for regenerating a particulate filter. If the estimated amount of soot exceeds a threshold, particulate filter regeneration is initiated. On the other hand, if a pressure across the particulate filter is determined from a sensor or an estimating model, particulate filter regeneration may be initiated after the observed or estimated pressure exceeds a threshold. 
         [0027]    In addition, other conditions may be included that determine when to regenerate the particulate filter. For example, filter regeneration may not proceed if engine temperature is above a threshold temperature or if engine temperature is below a threshold temperature. 
         [0028]    In one embodiment an electrically heated particulate filter is activated after EGR begins flowing in the EGR tube so that oxidized particulate matter may be oxidized and released from the filter and then flow back into the engine before being exhausted. Further, in one embodiment, the temperature of the particulate filter may be elevated by flowing EGR into the engine for a predetermined amount of time before the electrical heater is activated to heat the particulate filter. In other words, current is not supplied to the particulate filter heater until exhaust gases have flowed from the exhaust system to the intake system for a threshold amount of time or until the particulate filter reaches a threshold temperature. By elevating the particulate filter temperature with exhaust gases, it is possible to lower the thermal gradient that the filter is exposed to and therefore degradation of the particulate filter and particulate filter heater may be reduced. In one example, the rate that current is applied to the particulate filter heater may be related to the temperature of the particulate filter at a time when regeneration is requested. For example, as the temperature of the particulate filter increases, the amount of current supplied to the particulate filter over a period of time can be increased. If particulate filter regeneration is desired and conditions are met, routine  200  proceeds to  210 . Otherwise, routine  200  proceeds to exit. 
         [0029]    At  210 , current is ramped to the electrical particulate filter heater that is in the EGR loop. For example, current may be applied at a low level and increased over a period of time. In one example, the heater current is ramped when the engine is relatively cold. For example, if the engine is started at 20° C. the particulate filter heater current may be slowly ramped so that heater or particulate filter performance does not degrade. At higher temperatures, the particulate filter heater current may be ramped at a higher rate of current per second. Thus, under a first condition of a particulate filter heater current is ramped at a first rate of current, and under a second condition of a particulate filter heater current is ramped at a second rate. 
         [0030]    At  212 , routine  200  judges whether or not particulate filter regeneration is complete or if conditions for regeneration are no longer present. In one embodiment, regeneration is determined complete when the pressure difference across the particulate filter is less than a predetermined amount. If routine  200  judges that regeneration is complete, routine  200  proceeds to exit. Otherwise, routine continues to loop back. 
         [0031]    It will be appreciated that the configurations disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above systems can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. 
         [0032]    The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
         [0033]    The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.