PATENT ABSTRACT
A burner, in particular, for a motor vehicle supplementary or parking heater, for the burning of liquid fuel, in the presence of combustion air, has a combustion chamber ( 10 ), with a jet for the atomization of liquid fuel, arranged before said chamber. The jet has a Venturi tube ( 11 ) at the largest diameter end of the diffuser section thereof, opening into the combustion chamber ( 10 ), and the liquid fuel is introduced into the Venturi region which is at a low pressure when compared with the upstream end of the diffuser section ( 12 ), and through the inlet section ( 13 ) of which the air is pumped.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a binary burner for an auxiliary motor vehicle heater, for combustion of liquid fuel in the presence of combustion air, with a combustion chamber which is downstream of a nozzle for atomization of liquid fuel, and a Venturi nozzle for atomization of liquid fuel. 
     2. Description of Related Art 
     German Patent DE 41 18 538 C2 discloses an example of a burner of the initially mentioned type. This burner is based on a dual-fuel nozzle for atomization of fuel which has a relatively complicated structure. In particular, this nozzle consists of an essentially cylindrical mixing chamber, a feed pipe coaxial to its lengthwise axis for the liquid fuel, and a combustion air medium feed. The mixing chamber of the nozzle is closed by a nozzle insert for discharge of the atomized fuel/combustion air mixture and has a central nozzle hole and a conical inner bevel which converges in the flow direction. The combustion air feed consists of a feed pipe which tangentially penetrates the mixing chamber wall, dips into it and discharges at a greater height above the nozzle insert than the feed pipe for the liquid fuel. Due to this complex structure of the dual-fuel nozzle, for its proper operation for atomization of the liquid fuel, a considerable air overpressure is required which must be applied by a correspondingly powerful fan. 
     In conjunction with the gas burners, use of Venturi tubes as a mixing means is known. Furthermore, U.S. Pat. No. 4,396,372 discloses a burner system in which liquid fuel in the form of kerosene is delivered by means of a nozzle to a vaporizer provided with electrical heating means, with a Venturi-shaped inlet area. 
     SUMMARY OF THE INVENTION 
     One object of this invention is to devise a burner or a Venturi nozzle for atomization of the liquid fuel which does not require a high air overpressure for atomization of the liquid fuel and which operates reliably. 
     This object is achieved by a burner with nozzle having a Venturi tube or a Venturi nozzle with an inlet part which tapers in the flow direction of the combustion air and with a diffuser part which widens in the flow direction of the combustion air, the larger diameter end of the diffuser part discharging the liquid fuel into the combustion chamber, in the underpressure area in the transition area from the inlet part to the diffuser part, and by a Venturi nozzle that is axially divided into a fuel/combustion air discharge part and a fuel/combustion air supply part, the two Venturi nozzle parts being heat insulated relative to one another. 
     Accordingly, in accordance with the invention, the complex nozzle used for the binary burner is replaced by an arrangement with a Venturi tube or a Venturi nozzle, which to prevent coking and heat losses is made at least partially of a ceramic material. As a result of the pressure recovery of the Venturi tube or the Venturi nozzle it is not necessary to deliver air with a high overpressure so that the strong fan or additional fan which had been necessary in the past for this purpose can be omitted. The Venturi tube (or Venturi nozzle) used in accordance with the invention, moreover, has the advantage that economical production is possible. 
     Both the installation cost as well as the production costs are advantageously reduced if at least the front section of the Venturi diffuser part or the Venturi nozzle is made integral with the combustion chamber. 
     In the simplest case, the diffuser part of the Venturi tube or Venturi nozzle is formed with a uniform opening angle. However, according to one advantageous development, this Venturi tube or Venturi nozzle diffuser part can have sections of different opening angles, the section with the greatest opening angle bordering the combustion chamber. 
     Advantageously, the fuel is supplied to the Venturi tube or Venturi nozzle via a fuel feed tube with downstream end projecting into the Venturi tube. This downstream end of the fuel feed tube can discharge into a downstream dual-fuel nozzle or into the underpressure area of a second smaller Venturi tube which is operated in turn by the pressure drop of the Venturi tube or Venturi nozzle and ends in its underpressure region in order to achieve pre-atomization. Furthermore, the fuel feed tube is advantageously located within the Venturi tube or Venturi nozzle running coaxially to its lengthwise center. The downstream end can be located at different locations of the Venturi tube or Venturi nozzle, for example, in the inlet part or in the diffuser part of the Venturi tube or the Venturi nozzle or at its narrowest site between the inlet part and diffuser part. 
     Furthermore, it is advantageously provided that the combustion chamber have at least one additional inlet for secondary air. This secondary combustion air inlet is preferably located in the plane of the combustion chamber in which the Venturi tube or Venturi nozzle discharges. 
     The invention is explained below by way of example using the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows half of a lengthwise section of the combustion chamber and Venturi tube according to one embodiment of the dual-fuel burner, 
     FIG. 2 shows a diagram of the axial pressure variation in the Venturi tube shown in FIG. 1, 
     FIG. 3 shows one variant to FIG. 1 with a small Venturi tube instead of a nozzle for fuel atomization, and 
     FIG. 4 shows a Venturi tube with an axial separation site in the area of the diffuser. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The dual-fuel burner in accordance with the invention comprises a combustion chamber  10  of a preferably cylindrical or cuboidal shape. A Venturi tube  11  is connected to the fuel/combustion air inlet side of the combustion chamber  10 . The Venturi tube  11  which forms the Venturi nozzle is formed preferably, at least in the connection area, integrally with the combustion chamber  10 . 
     The Venturi tube  11 , in the conventional manner, has a diffuser part  12  and an inlet part  13 . The diffuser part  12  has a longer axial extension than the inlet part  13  and preferably has a conical shape, the end with the largest diameter of the conical diffuser part  12  being connected to the fuel/combustion air inlet of the combustion chamber  10 . The inlet part  13  which, likewise, has a conical shape or at least one inlet radii with an optionally connected cylindrical part, with a section of greatest diameter pointing away from the combustion chamber  10 , adjoins the end of the diffuser part  12  which is smallest in diameter. 
     The Venturi tube  11  and the combustion chamber  10  are preferably on a common lengthwise central axis  14 . The fuel feed tube  15 , with a dual-fuel nozzle  16  being connected to its downstream end, is flush with this lengthwise central axis. The outlet opening of the dual-fuel nozzle  16  lies at the narrowest point between the diffuser part  12  and the inlet part  13  of the Venturi tube  11 , i.e., in an area at a lower pressure relative to the combustion chamber  10 , as illustrated in FIG.  2 . 
     Instead of the dual-fuel nozzle  16 , according to the version shown in FIG. 3, there can also be a small Venturi tube  19  at the same location which is operated by the pressure drop of the Venturi tube  11  and which ends in its underpressure area in order to achieve pre-atomization. In this case, the fuel feed tube  15  discharges in the smaller Venturi tube  19  by which air is likewise delivered. 
     Combustion air is fed into the inlet part  13  of the Venturi tube  11  by means of a fan (not shown) and which provides combustion air with only a small overpressure. The combustion air is delivered along the arrow  17  to the Venturi tube  11 . 
     The axial pressure characteristic in the Venturi tube  11  is shown in FIG. 2, in a diagram with the length ε of the Venturi tube  11  plotted on its x axis, and the pressure differential Δp in the Venturi tube  11  plotted on its y axis. Accordingly, the combustion air at the inlet point into the inlet part  13  of the Venturi tube  11  has a low overpressure which is achieved by the fan which delivers the combustion air. With flow through the inlet part  13 , due to the narrowing of the cross section the speed increases, at the same time the combustion air pressure drops, and in the transition to the subsequent diffuser part  12 , reaches a minimum value. This minimum absolute pressure corresponds to a maximum underpressure compared to the combustion chamber pressure level. This underpressure decreases downstream in the continuation of the diffuser part  12  so that the combustion air entering the combustion chamber  10  is roughly at the combustion chamber pressure. 
     The nozzle  16  has at least one radial hole  18  which discharges into the axial fuel delivery hole of the nozzle  16 . Via this radial hole  18 , air is introduced into the delivery path of the liquid fuel so that in the nozzle  16  swirling of the combustion air and liquid fuel occurs. This mixture then emerges atomized from the outlet opening of the nozzle  16  and mixes in the diffuser part  12  with the combustion air which is taken in along the arrow  17  and which is provided to the inlet part  13  by the fan with low pressure. 
     Moreover, preferably secondary combustion air is supplied to the combustion chamber  10 . For this purpose, at the point at which the diffuser part  12  of the Venturi tube passes into the combustion chamber  10 , distributed around the periphery, secondary air openings  20  are formed via which secondary air is fed into the combustion chamber  10 . There can also be secondary air openings alternatively or additionally on the jacket of the combustion chamber  10 . FIG. 4 schematically shows, in a lengthwise section, the rear part of the combustion chamber of a burner equipped with an embodiment of a Venturi nozzle of the invention for an auxiliary motor vehicle heater. 
     The cylindrical combustion chamber  110  is connected downstream of the combustion air supply chamber  111  into which combustion air is supplied by means which are not shown, typically by means of a fan. The combustion chamber  110 , running coaxially to its lengthwise central axis L which is shown by the dot-dash line, has a connection opening  122  for connection of the downstream end of the Venturi nozzle  112  which is located completely within the combustion air supply chamber  111 . Around this connection opening  122  is a ring of smaller openings  123  for the passage of secondary combustion air into the combustion chamber  110 . 
     The Venturi nozzle  112 , in the downstream area, comprises a conical diffuser  113  which discharges into the combustion chamber  110  and which tapers in the upstream direction. In the upstream direction, the Venturi nozzle  112  comprises a conical inlet part  114  which has a diameter variation which is opposite the diffuser  113 , i.e., conically tapers in the direction toward the diffuser  113 . The outside end of the inlet part  114  adjoins a cylinder part  115  which discharges into the combustion air supply chamber  111 . The diffuser  113  and the inlet part  114  are connected via a cylinder part  116  which is small in diameter and into which a fuel feed tube  117  discharges. The fuel feed tube  117  runs coaxially to the lengthwise central axis of the Venturi nozzle  112  and is routed out of the combustion air supply chamber  111  at an angle outside of this nozzle. 
     In this arrangement of the combustion air supply chamber  111  and the Venturi nozzle  112 , liquid fuel is taken into the combustion air by the underpressure which prevails in the area of the narrowest point (in the area of the cylinder part  116 ) and atomized. The combustion air supplied to the inlet part  114  from the combustion air supply chamber  111 , together with the atomized fuel from the Venturi nozzle  112 , is delivered into the combustion chamber  110 . The mist of fuel and combustion air is ignited in the combustion chamber  110  and burned as it is additionally mixed with additional secondary combustion air from the openings  123 . For this purpose, an ignition means  124  projects into the combustion chamber  110  and its end extends into the area of the exit of the conical diffuser  113  of the Venturi nozzle  112 . 
     According to the invention, the Venturi nozzle  112  is axially divided into two parts, specifically into a discharge part  118  which borders the combustion chamber  10 , and a supply part  119  which is located upstream of this discharge part  118 . The axial division of the Venturi nozzle  112  is made in the area of its diffuser  113  so that the supply part  119  is roughly twice as long as the discharge part  18 . 
     In the preferred embodiment shown in FIG. 4, the discharge part  118  and the supply part  119  are separated from one another by an annular gap  120  with a width which is typically between 0.1 and 0.8 mm. Preferably, the gap width is chosen to be roughly 0.3 mm. In the axial direction, the annular gap  120  is bordered by the facing annular end faces of the discharge part  118  and the feed part  119  and radially by a ring seal  121  which seals the annular gap  120  and thus the conical diffuser  113  relative to the outside. The material of the ring seal  121  is preferably a heat-insulating material such as, for example, a ceramic. The Venturi nozzle  112  is made of metal, but preferably is also at least partially made of a ceramic. The material for the two Venturi nozzle parts  118 ,  119  can be the same. However, preferably, the material of the discharge part  118  has a lower thermal conductivity than the material of the supply part  119  in order to transfer as little heat as possible to the annular gap  120  between the two Venturi nozzle parts  118 ,  119 . 
     Due to the heat-insulated division of the Venturi nozzle  112  of the invention, it has a cold and a hot part. The upstream cold supply part  119  is typically exposed to temperatures below 180° C. in operation due to the division of the Venturi nozzle into two parts so that cracking of the fuel cannot occur in this nozzle part  119 . On the other hand, the flame-side hot discharge part  118  of the Venturi nozzle  112  is typically exposed to temperatures above 500° C. so that liquid fuel striking its inside wall from the fuel feed tube  117  vaporizes without leaving crack residues. Thus, it is ensured that the Venturi nozzle  112  is not clogged with residues, and thus, its efficiency is not adversely affected. 
     The ring seal  121  can fill the entire annular gap  120 . Alternatively, it is also conceivable for the ring seal  121  to be completely omitted. For a very narrow annular gap  120 , there is almost no leakage of fuel-air mixture to the outside. Small leaks would be supplied to the combustion chamber with the secondary combustion air and burned there. 
     As a result of the low pollutant discharge which can be achieved with a burner equipped with the Venturi nozzle in accordance with the invention, a longer service life of the burner, and thus of the heater, and less environmnental impact are ensured. Finally, the starting behavior is optimum by the use of the Venturi nozzles according to the invention.