Abstract:
An apparatus for fuel preparation ( 100 ) for a burner ( 18 ) for regenerating a particle filter ( 16 ) of an exhaust gas aftertreatment system comprising an exhaust gas passage ( 14 ) for discharging a hot exhaust gas flow from an internal combustion engine ( 12 ), in particular a compression-ignition internal combustion engine, is proposed. The apparatus for fuel preparation ( 100 ) has an atomizing device ( 110 ) for preparing the fuel, a combustion chamber ( 116 ), an ignition device ( 26 ) and a combustion air supply ( 24 ). The combustion air supply ( 24 ) provides a primary air feed line ( 112 ) for preparing the fuel and a secondary air feed line ( 114 ) for feeding combustion air in order to produce an ignitable fuel/air mixture.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The increasingly stringent legal requirements for protecting public health and the environment, which regulate the emissions of motor vehicles, represent a challenge for the development of engines and for the exhaust gas aftertreatment technologies. The reduction of particle emissions thus becomes particularly important and in this respect above all emissions of compression-ignition internal combustion engines, which have among other things a carcinogenic effect. Hence, many methods and devices have been developed to remove the soot particles that are present in exhaust gases. The use of particle filter systems for capturing pollutants present in particle form is well known. 
         [0002]    In order to carry out particle separation, exhaust gases are passed through a heat resistant filter medium. The particles captured by the filter medium, which especially have diameters in the range of 10 to 5000 nm, coat the filter and thus lead to an increase in the pressure loss across the filter. This makes it necessary to continuously or cyclically perform a regeneration of the filter medium. The regeneration generally takes place by means of a practically residue-free combustion of the accumulated soot particles using oxygen. Ignition temperatures of approximately 600E C are however required for particle combustion, said ignition temperatures being only seldom achieved in the exhaust gas of a compression-ignition internal combustion engine even under full load conditions. For this reason, it is known to provide a burner in order to obtain a corresponding increase in the exhaust gas temperature. 
         [0003]    A catalytic burner is known from the German patent publication DE 100 24 254, which is disposed between the internal combustion engine and the particle filter and increases the exhaust gas temperature without an ignitable fuel mixture having to be provided. The fuel injected downstream of the engine by the catalytic burner is converted at an oxidation catalyst while reaction heat is being released, wherein the exhaust gas temperature is raised to the level of a regeneration temperature. It is however required for the temperature of the exhaust gases to have already reached a certain value so that the catalytic reaction can take place at the oxidation catalyst. Below this temperature, the activity of the catalyst is so low that the injected fuel is not sufficiently converted. A regeneration of the particle filter is not reliably possible in each engine load range. Furthermore, an additional injection of fuel is required, which is only an option in compression-ignition internal combustion engines having common rail technology, wherein an extensive intervention into the engine management system is necessary. 
         [0004]    The use of a combustion device in the filter apparatus is likewise known for producing the required temperature in the exhaust gas flow. A combustion device is thus known which comprises a combustion chamber and a burner, wherein metered fuel and metered combustion air are mixed to form an ignitable fuel/air mixture. The fuel/air mixture formed in the combustion chamber is ignited by an ignition device. The hot combustion gases are mixed with the exhaust gases and raise the exhaust gas temperature to such an extent that the soot particles accumulated in the particle filter oxidize. In general, this process relates to a forced-air burner comprising a pressure-atomizing nozzle. 
         [0005]    The burner capacity required to sufficiently heat the exhaust gas flow is generally a function of the engine operation, wherein the burner capacity is adapted to the current operating point of the internal combustion engine. If the entire exhaust gas flow is brought to the required burn-off temperature, a high burner capacity is required which leads to a higher fuel consumption. A large quantity of air is thus required in order to achieve an ignitable fuel/air mixture; thus requiring a larger burner. The air supply, which substantially ensues from a compressed air reservoir or via an electrically driven air pump, thus puts a significant load on the on-board electrical system of the vehicle as a result of the relatively high power consumption of the associated electric motor. This can lead to malfunctions at low rotational speeds of the combustion engine. On the other hand, a correspondingly increasing flow velocity in the air feed line occurs as a result of the increased quantity of air, whereby an increased back pressure is produced. The disadvantage of this result is that an increased back pressure leads to an increase in the electrical power consumption. In the case of low burner capacity, the danger also exists that the preparation of the fuel is insufficient. When the fuel is insufficiently prepared or respectively atomized, uncombusted fuel can reach the particle filter and destroy the same. 
         [0006]    The German patent publication DE 10 2009 051 327 A1 describes a method for controlling the temperature of combustion air of an oil burner. The feed of combustion air to a mixing zone of the burner can result as a function of an operating mode by means of a first and/or a first and a second feed pathway, wherein heat in each case is supplied to the combustion air. 
         [0007]    Atomizing burners for liquid fuels are likewise known from the German patent publication DE 38 26 446 A1, wherein heavy fuel oil comprising a primary air flow is fed in a nozzle tube, which is surrounded by a casing tube, over which secondary air is fed. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the invention, an apparatus for fuel preparation for a burner for regenerating a particle filter of an exhaust gas aftertreatment system of an internal combustion engine comprising a combustion air supply is proposed, wherein the combustion air supply provides a primary air feed line for preparing the fuel and comprises a secondary air feed line that supplies combustion air in order to produce an ignitable fuel/air mixture. 
         [0009]    In a particle filter of an exhaust gas aftertreatment system based on known filter bodies, for example: filter channels, filter meshes or filter pockets from ceramic or sintered metallic material, particles accumulate over time. The particle accumulations can be pronounced to such an extent that the exhaust gas flow through the exhaust gas aftertreatment system is limited or even blocked. This leads to an increased back pressure and finally to a decrease in performance, increased exhaust gas temperatures and to an increase in fuel consumption. For that reason, the particle filter is either continuously or periodically regenerated with hot air during operation. 
         [0010]    According to the invention, the temperature required in the exhaust gas flow for regenerating the particle filter is produced by a combustion device, in particular a burner comprising a combustion chamber and an atomizing device, wherein the combustion air is supplied to the burner via a primary and a secondary air feed line. A primary air mass flow, which can be supplied largely independently of the burner capacity, is made available to the fuel preparation at the atomizing device via a primary air feed line. In the case of increased burner capacity and the ensuing increased air requirement, the air supply takes place aside from the primary air feed line further via the secondary air feed line, wherein the back pressure can be held substantially constant at a low level at the atomizing device. 
         [0011]    According to the invention, a primary air mass flow is supplied via the primary air feed line to the atomizing device, which finely atomizes the injected fuel. An aerosol which is ignitable is obtained immediately downstream of a nozzle by an intensive mixing of the fine fuel mist with the combustion air. When the air volume requirement is increased, a secondary air mass flow is conveyed via the secondary air feed line. The additional air feed line facilitates a setting of a suitable fuel injection ratio, or fuel/air ratio, can influence the injection direction and produces a correct size of the fuel droplets. In addition, the back pressure can be held low and therefore a mechanical and electrical power requirement, for example of an air pump, can be held substantially low. 
         [0012]    In a preferred embodiment, the secondary air feed line opens out directly into the combustion chamber, the fine aerosol consisting of fuel mist and combustion air, which is produced by the atomizing device, mixing thereby with the secondary air mass flow. This arrangement can be very easily configured so that the costs of this apparatus can be kept low. In addition, the back pressure in this arrangement remains at a low level because no throttling point is provided in the secondary air feed line. 
         [0013]    In an alternative embodiment of the apparatus for fuel preparation for a burner in an exhaust gas aftertreatment system, the secondary air mass flow supplied by the secondary air feed line enters immediately into the atomizing device; and therefore a support of the atomization of the injected fuel is obtained. The secondary air mass flow of the atomizing device can thus be supplied in a manner which allows the atomization of the injected fuel to be influenced and substantially improved with regard to the size of the fuel droplets that is achieved and the homogenization of the fuel/air mixture. Furthermore, influence can be exerted over the flame produced at the burner, for example by firstly subjecting the emerging flame to a swirl and if need be secondly to a constriction; and in so doing, a flame stability is achieved even when the air ratios continue to deviate from the target value. By means of the secondary air mass flow, contamination at the atomizing device, in particular at the nozzle, can be held to a minimum so that fine misting nozzles can also be used in the atomizing device. The nozzle can therefore be cleaned and cooled down by means of the air feed line. 
         [0014]    The atomizing device can, for example, have a nozzle tube, which is surrounded by a casing tube. Lateral openings are provided on the casing tube for introducing the secondary air mass flow into the formed annular space. A certain alignment of the openings can ensure that via the secondary air mass flow the flame is subjected to a swirl and/or a constriction, wherein a stabilization of the flame is achieved. A constriction of the air mass flows introduced produces a high flow velocity. A high flow velocity of the primary air mass flow or a correspondingly large difference in velocity with respect to the fuel flow can be used to atomize the fuel. A high difference in velocity between the primary mass flow and the secondary mass flow promotes the aerosol formation, the difference in velocity being especially great if both air mass flows are subjected to a swirl having a different orientation. 
         [0015]    The metering of the combustion air is generally controlled via a control device. For example, the air supply occurs via a magnetic valve from a compressed air reservoir or from an electrically driven air pump, for example via a rotary vane pump, claw pump or an eccentric rotor pump. In an alternative embodiment of the apparatus according to the invention, a pressure regulating valve is disposed in at least one of the air feed lines, preferably in the secondary air feed line. The pressure regulating valve in the air feed line can be embodied as a simple mechanical pressure valve or as a controllable pressure regulating valve. The option therefore exists to feed the combustion air, starting at a pressure in the air feed line to be determined, via the primary and the secondary air feed line to the burner. In so doing, the back pressure, which rises when the air volume is large, is limited. The atomization of the injected fuel can thereby take place under optimized conditions and the stability of the flame is achieved in a simple and cost effective manner. 
         [0016]    During a start-up phase of the burner, in which only a small volume of air is required, the prevailing pressure lies below the opening pressure of the pressure regulating valve that is disposed in at least one air feed line. The air volume is only provided via one air feed line of the apparatus for fuel preparation. As the power output of the internal combustion engine increases, the power requirement of the burner increases; and therefore the required air volume rises, the back pressure increases, and the air volume is supplied via the primary and the secondary air feed line after the opening pressure of the at least one pressure regulating valve has been exceeded. 
         [0017]    The switching of the air supply from one to two air feed lines can be arbitrarily controlled, wherein the conditions at a low burner capacity as well as at a high burner capacity can be taken into account. 
         [0018]    Provision is made in a further embodiment of the apparatus for fuel preparation for a burner for the fuel to be injected to be mixed with the air, in particular with the primary air mass flow supplied via the primary air feed line, upstream of the atomizing device and for this mixture to be delivered to the combustion chamber. For example, the fuel mass flow and the air mass flow are brought together at a junction of the lines, for example in the form of a T-piece. The combustible fuel/air mixture generated in this way enters premixed into the atomizing device. In addition, a reliable ignition of the fuel/air mixture is assured because the mixture is not generated in the combustion chamber but outside of the same. In so doing, an ignitable mixture having a specified air/fuel ratio can be set. In this embodiment, it is furthermore possible to combine several necessary components, for example: metering valves, check valves and/or pressure regulating valves, into one component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Exemplary embodiments of the invention are depicted in the drawings and explained in detail in the following description. 
           [0020]    In the drawings: 
           [0021]      FIG. 1  shows an exhaust gas treatment system according to the prior art; 
           [0022]      FIG. 2  shows an exemplary embodiment of an apparatus according to the invention for fuel preparation for a burner comprising a secondary air inlet into a combustion chamber; 
           [0023]      FIG. 3  shows and exemplary embodiment of an apparatus according to the invention for fuel preparation comprising a secondary air inlet into a mixing zone; 
           [0024]      FIG. 4  shows an alternative exemplary embodiment of the apparatus according to the invention for fuel preparation for a burner. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    An exhaust gas aftertreatment system  10  according to the prior art is depicted in  FIG. 1  in a strongly schematized representation. 
         [0026]    Exhaust gases from an internal combustion engine  12  arrive at a particle filter  16  via an exhaust gas duct  14  after passing through a burner  18  connected in between, wherein the burner  18  and the particle filter  16  are disposed consecutively in a housing  20  in the direction of flow of the exhaust gases. Soot particles, which are carried out with the exhaust gas, accumulate in the particle filter  16 . Depending upon the selected mode of operation, the fuel/air mixture generated by the feed of fuel  22  and air  24  is ignited at the burner  18  via an ignition device  26 . The hot gases exiting the burner  18  mix with the exhaust gases in an antechamber  28  of the particle filter  16  causing an increase in the exhaust gas temperature such that the soot particles accumulated in the particle filter  16  oxidize. The regeneration of the particle filter  16  occurs as a rule during the operation of the internal combustion engine  12 , wherein the residual oxygen in the exhaust gases can be used for the operation of the burner  18 . The combustion air is delivered via an electrically operated air pump  30  which is driven by an electric motor that is not depicted. The electric motor is connected to an on-board battery, which is charged by the alternator (not depicted). 
       Embodiment Variants 
       [0027]    An exemplary embodiment of an apparatus according to the invention for fuel preparation  100  for a burner  18  is depicted in  FIG. 2 . 
         [0028]    The apparatus for fuel preparation  100  comprises an atomizing device  110 , with which injected fuel is prepared in a suitable manner, in particular is atomized. Fuel is delivered to the atomizing device  110  via the feed  22 , wherein a magnetic valve or a fuel delivery pump is, for example, provided for metering the fuel, said magnetic valve or fuel delivery pump being driven by a controllable electric motor (not depicted). The fuel can be extracted from a fuel reservoir of the internal combustion engine  12  via an intake line, wherein the delivery rate can be changed by varying the rotational speed of the electric motor. According to the invention, combustion air is supplied to the burner  18  via a primary air feed line  112  and a secondary air feed line  114 . The metering of the air feed takes place, for example, via a magnetic valve or via an electrically operated air pump (not depicted). The fuel which was fed is finely atomized at the atomizing device  110 , which, for example, is designed as a swirl burner comprising a tangential fuel and/or air feed, an ignitable fuel/air mixture being thereby formed. The atomizing device  110  of the burner  18  can, for example, be constructed as an annular nozzle, the fuel sweeping along the annular atomizing tongue thereof and being finely atomized by a rotating air flow. The aerosol produced, which enters into a combustion chamber  116 , is normally ignited by an ignition device  26 . The use of ignition electrodes, pencil-type glow plugs or a glow unit is known. Besides the combustion air supplied as the primary air mass flow via the primary air feed line  112 , a required additional quantity of compression air is inventively delivered during an increase in burner capacity directly into the combustion chamber  116  via the secondary air feed line  114  pursuant to the exemplary embodiment depicted in  FIG. 2 . 
         [0029]    A pressure regulating valve  118  is disposed in the secondary air feed line  114 . Said pressure regulating valve  118  can be embodied as a simple mechanical pressure regulating valve, which unblocks the secondary air feed line at a specific opening pressure. The pressure regulating valve  118  can alternatively be embodied as a selectable pressure regulating valve, with which the opening pressure can, for example, be varied via a control device. A modulatable apparatus is thus available to supply the burner  18  with combustion air. Particularly in a start-up phase of the internal combustion engine  12 , the secondary air feed line  114  can be held closed, a primary air mass flow of the atomization of the fuel being available via the primary air feed line  112 . Thus, the atomization of the fuel can be optimized with regard to the air volume available and the design of the atomizing device  110 . 
         [0030]    In  FIG. 3 , an alternative embodiment of the apparatus for fuel preparation  100  of a burner  18  is depicted. In this case, the secondary air feed line does not open out directly in the combustion chamber  116  but in the atomizing device  110 . The atomizing device  110  comprises, for example, a central nozzle tube, in which fuel and a primary air mass flow are flowing. In addition, a casing tube which has openings surrounds the central nozzle tube, the secondary mass flow being fed through said openings. The alignment of the openings can thereby be such that said secondary mass flow is subjected to a swirl. The flame generated in the process is thereby stabilized and the direction thereof is determined. An air flow for cleaning and cooling the nozzle can also then be maintained using said atomizing device  110  if the fuel feed and the primary air feed is interrupted. The service life of such an atomizing device  110  is thereby increased. 
         [0031]    A further exemplary embodiment of the apparatus for fuel preparation  100  of a burner  18  is depicted in  FIG. 4 . The fuel supply  22  flows into the primary air feed line  112 , fuel being introduced into said primary air feed line  112  via a suitable device. In this way, the fuel supply  22  opens into an intake line of the air pump, for example, a rotary vane pump. The fuel is profoundly mixed with the air and the aerosol produced is supplied to the atomizing device  110  by means of a feed line  120 . When needed, further combustion air can be delivered via the secondary air feed line  114  across a pressure regulating valve  118 , said combustion air mixing with the aerosol which was produced and being ignited as an ignitable fuel/air mixture in the combustion chamber. It is furthermore shown in  FIG. 4  that the fuel is supplied in metered fashion, for example, by means of a controllable fuel pump or by means of a fuel metering valve  122 . A check valve is provided in the primary air feed line  112 . According to the invention, the several valves, which include a pressure regulating valve, a metering valve and a check valve, can be combined into one component.