Abstract:
A heater, especially for mobile applications, is provided which includes a burner for burning a liquid fuel, an atomizer nozzle located on the burner for atomizing the liquid fuel into extremely fine fuel droplets, and a fuel supply for feeding the liquid fuel to the atomizer nozzle. A heating means is provided on the atomizer nozzle with which the liquid fuel can be vaporized before and/or during its emergence from the atomizer nozzle.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a heater, especially for mobile applications, with a burner for burning a fossil liquid fuel, an atomizer nozzle which is located on the burner for atomizing the liquid fuel into extremely fine fuel droplets, and a fuel supply for feeding the liquid fuel to the atomizer nozzle. Furthermore the invention relates to a motor vehicle with such a heater and a process for operating a heater in which a liquid fossil fuel is supplied to an atomizer nozzle using a fuel supply.  
         [0003]     2. Description of Related Art  
         [0004]     In heaters for mobile applications, burners for burning liquid fossil fuels are used. These burners are differentiated with respect to the type of combustion into atomizer burners and vaporizer burners. The atomizer burners are divided into two groups, so-called low pressure and so-called high pressure atomizer burners. In high pressure atomizer burners, the liquid fuel is delivered from a fuel supply at high pressure to an atomizer nozzle on which the fuel, in a so-called spray, is broken into extremely fine droplets which easily vaporize due to their large specific surface and thus are easily ignited.  
         [0005]     Low pressure atomizer burners deliver the liquid fuel almost without pressure into a combustion air jet which has a high flow velocity and in which the fuel then forms a spray with a droplet size which is widely distributed. For combustion air supply, atomizer nozzles are used in which the combustion air is routed in a controlled manner around the injection point of the fuel. Due to the low combustion air pressure and fuel pressure in the range of &lt;100 mBar, in low pressure atomizer burners, the generated spray is reluctant to ignite, especially at low temperature, and in general must be preheated by the preheated burner components. This causes a relatively long preheating phase which slows down the starting process.  
         [0006]     Vaporizer burners are provided with a vaporizer body of tile or grating in which the liquid fuel is distributed on a comparatively large surface and is evaporated from it.  
         [0007]     The known atomizer and vaporizer burners are often provided with an ignition means, such as a high glow temperature or a spark, with which the liquid fuel is heated during the starting phase of the pertinent heater and afterwards is ignited.  
         [0008]     Furthermore means for preheating liquid fuels are known. However, with preheating, there is always the danger of gas bubble formation in the liquid fuel and thus a not inconsiderable disruption of combustion during the starting and operating phase of a burner.  
         [0009]     In motor vehicles in which the aforementioned heaters are used, currently and in the future, high legal requirements with respect to especially low emission of partially burned and unburned hydrocarbons as well as soot particles (for example, aerosols and smoke) must be satisfied. Furthermore, at rest for example evaporation of liquid fuel on the motor vehicle must necessarily be prevented.  
       SUMMARY OF THE INVENTION  
       [0010]     Therefore, it is one object of the present invention to provide a motor vehicle with a heater which also starts very promptly in the cold and at the same time satisfies current and future legal requirements with respect to emission of partially burned and unburned hydrocarbons, and if possible, even exceeds the legal requirements.  
         [0011]     This object, and other objects, are achieved by the present invention in providing a heater of the initially mentioned type, especially a mobile heater, in which on the atomizer nozzle, there is a heating means with which the liquid fuel can be vaporized before and/or during its emergence from the atomizer nozzle. Moreover, the objects are achieved with a process for operating a heater in which the fuel which is supplied or which is to be supplied to the atomizer nozzle is converted at least temporarily by a heating means into an essentially gaseous state, and with a vehicle which is provided with the aforementioned heater.  
         [0012]     In the fuel supply of the burner for a liquid fossil fuel, there is a heating means for heating the fuel which is flowing through and, for the most part, for converting the fuel completely from the liquid into the gaseous phase. This type of fuel preparation is fundamentally in contrast to the fuel preheating of known heating devices in which the formation of vapor or gas bubbles in liquid fuel is to be avoided.  
         [0013]     Furthermore, the present invention is actually in contrast to the atomizer nozzle which is provided on the burner of the present invention and its operation, since the atomizer nozzle can no longer and will no longer atomize the gaseous fuel which has been vaporized, but introduces it simply as a gas jet into the combustion space of the burner. But it has been shown by the present invention that the gas jet of vaporized or pregasified fuel entails special advantages with respect to the formation of a mixture with the supplied combustion air in certain operating states of a heater so that, although the function of the atomizer nozzle is cancelled, fuel and combustion air are thoroughly mixed especially uniformly. When using the heater of the present invention, therefore, the aforementioned difficulties of fuel preparation and mixture formation, as are typical especially for a cold start of a low pressure atomizer burner, are eliminated.  
         [0014]     Therefore, the present invention provides a much more prompt starting process and the burner of the present invention reaches the full load mode much more quickly. As a result of the greatly improved mixture formation in the cold combustion space during a cold start, emissions as well as fumes, odor, hydrocarbon and carbon monoxide are greatly reduced.  
         [0015]     Moreover, when using the heater of the present invention, the total demand for electrical energy for ignition of the burner compared to conventional burners with a glow plug is less, especially when the vaporization of the fuel takes place by electrical heating. The heating means used is deactivated shortly after starting and the heated burner again in its original function works as a low pressure atomizer. This energy saving is based on the fact that only a very limited section of the fuel supply is heated while in conventional burners also sheets and other components of the burner must be heated with the glow plug in order to reach the required vaporization and ignition temperature.  
         [0016]     Finally, the present invention leads to still another advantageous effect by the fuel which has been vaporized with the heating means producing a gas volume on the fuel supply which acts as a more effective damper against pressure pulsations, for example, of the metering pump of the fuel supply. Fuel feed therefore becomes more uniform, especially in the critical starting phase.  
         [0017]     In one advantageous development of the present invention, the burner is made as a Venturi burner with a Venturi nozzle, as a prefilming burner with a prefilming nozzle or as a rotary atomizer burner with a rotary atomizer nozzle which acts as an atomizer nozzle. In particular, Venturi nozzles have been used for a long time with great success in the area of heaters for mobile applications for atomization of fuels and especially diesel fuels. In combination with the process of the present invention, it has been shown that such a Venturi nozzle leads to complete and especially uniform mixture formation of fuel and combustion air when the fuel is introduced into it as a gaseous phase. This gaseous phase can be delivered, and this is especially important, by a fuel tube which is provided on the Venturi nozzle and which is also used for feed of the liquid phase or the liquid fuel. An additional fuel supply with a vaporizer body or the like is not necessary.  
         [0018]     The heating means of the present invention is advantageously provided with a heating element which is operated with electrical energy. An electrical heating element can be relatively easily triggered and controlled with the control device of known heaters. The heating element can be especially advantageously a heating element with a positive temperature coefficient (PTC element) with which self-regulation to a predetermined temperature can be formed. A PTC element can also assume other functions such as, for example, detection of gas bubbles in the liquid fuel. Moreover, advantageously, two heating elements can be connected in series, of which one has a comparative low and one has a comparative high temperature coefficient. In such a connection arrangement, prompt heating is accomplished with the element with the low temperature coefficient, while a predetermined ideal temperature is adjusted with the element with the high temperature coefficient. This is possible by, for example, using the self-regulation action of a positive and (relatively high) temperature coefficient or the overall resistance of the arrangement which is a measure of the temperature being kept at a predetermined value by controlling the supplied electrical power.  
         [0019]     The heating means of the present invention is furthermore advantageously made with a resistance wire as the heating element with which concerted feed of heat energy into the flowing fuel is possible.  
         [0020]     The heating means can also be made in the conventional manner as a ceramic glow plug, in which it must be watched that, with the glow plug of the present invention, liquid fuel is deliberately converted into the gaseous phase and then is supplied by the atomizer nozzle of the burner.  
         [0021]     The heating means can furthermore comprise a heating element which is arranged in the form of a spiral in the tube segment through which the liquid fuel is routed. With the spiral, on a small installation space, high energy input into the flowing fuel can be implemented since the heated surface of the spiral is relatively large.  
         [0022]     The spiral can be mounted centrally in a cylindrical tube section such that a certain gap remains between the outside of the spiral and the inside of the tube section. The liquid fuel is then supplied through the gap and heated. This spiral can be integrated especially comparatively easily into an existing burner and its fuel supply. So that the contact surface between the spiral and the liquid fuel is especially large, the spiral should be provided in the center with a through opening for passage of fuel.  
         [0023]     The pregasification of the liquid fuel of the present invention should be used especially before and/or during the starting phase of the heater. As has already been mentioned above, a much more prompt starting sequence can be implemented and the burner reaches the full load mode much more quickly. In addition, the present invention leads to a considerable reduction in emissions especially during the starting phases.  
         [0024]     The type of fuel supply of the present invention with a vapor phase and a subsequent liquid phase can also be used to produce a type of “lightning start”. In this start, the fuel supply during the starting phase, for example directly on the atomizer nozzle, is first blocked and only opened after a predetermined amount of liquid fuel has been converted into the essentially gaseous state with the heating means. The gas volume can be produced for example in the atomizer nozzle upstream of an electrically heated check valve. In this case, the gas enters the combustion chamber of the heater only after the opening of the valve. The preheated and pent-up gas volume which has been vaporized in this way is ignited in the combustion chamber for example using a conventional glow plug and then, as a result of the comparatively large amount of energy which is stored in the gas volume, leads to prompt and comprehensive heating of the heater burner. Heating of the burner is therefore supplied from the chemical energy of the fuel and not for example from the electrical energy of a conventional glow plug.  
         [0025]     The conversion of the liquid fuel into the gaseous phase, which takes place at least partially or in sections, can also be advantageously used during the heating phase of the heater. With the present invention, specifically small output stages of the heater, which actually no longer make available enough energy for reliable maintenance of combustion, can be stabilized by “support heating” or by supporting vaporization of the liquid fuel. Moreover, the procedure of the present invention can be used to break down deposits on or in the burner. Thus, for example, the position of the flame on the burner can be changed at least temporarily by delivering the prevaporized fuel. Moreover, the combustion air/fuel ratio can be changed in the direction to an elevated flame temperature. In this way, temperatures over 600° C. can be deliberately reached; they lead for example to a breakdown of carbon deposits which have formed in the combustion of fuel from the liquid phase on the burner or on its heat exchanger.  
         [0026]     In addition to the aforementioned applications of the present invention, the liquid fuel can also be converted into the essentially gaseous state during or after a stop phase of the heater. This procedure can be a good idea since the interruption of combustion or a flameout during combustion of a liquid fuel spray entails comparatively numerous unburned fuel droplets and a corresponding emission of pollutants. If conversely with the atomizer nozzle, a gas jet is briefly produced and the combustion is interrupted during it, there is comparatively little fuel on the atomizer nozzle and it is still finely distributed. The little gaseous fuel therefore burns with comparatively low emissions.  
         [0027]     The aforementioned emission reduction during a stop phase can be further increased by first opening the fuel supply during the stop phase and by it only being blocked after a predetermined amount of liquid fuel has been converted into the essentially gaseous state with the heating means. By blocking the fuel supply, further vaporization of the liquid fuel is reliably prevented. With the gas which has remained in the blocked fuel line, a pressure equalization volume is created which especially prevents the liquid fuel from being compressed by the blocking means in the fuel line as a result of a temperature-induced pressure rise. Moreover, with the gas volume which has remained in the fuel supply, the foundation for a speedy restart of the burner is created as a result of the prevaporization of the liquid fuel.  
         [0028]     In the partial gasification of the fuel which is otherwise supplied in liquid form, the liquid fuel with the heating means per unit of time is supplied with so much energy that the energy density on the surface of the heating means remains under a predetermined critical limit (the Leidenfrost phenomenon of film boiling). In this way, the energy can be efficiently delivered into the fuel and chemical decomposition processes in the fuel can be avoided. Alternatively or in addition, the temperature of the heated fuel should be within a predetermined temperature interval. This temperature interval is dependent on the type of the fuel and for diesel is, for example, roughly 170° C. to 400° C. Chemical decomposition of the fuel can be avoided in this way.  
         [0029]     To vaporize the liquid fuel, the heating means of the present invention can be preheated advantageously before feed of the liquid fuel during a time interval of roughly 2 to 10 seconds, especially from roughly 2 to 6 seconds, and most preferably roughly 5 seconds. This preheating time ensures that the heating means makes available the required amount of energy for prompt and comprehensive vaporization of a predetermined amount of liquid fuel. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0030]      FIG. 1  shows a partially cutaway schematic of one embodiment of the heater of the present invention; and  
         [0031]      FIG. 2  shows a cutaway view of one embodiment of an atomizer nozzle of the heater of the present invention as shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]      FIG. 1  shows a heater  10  which contains as the important component a burner  12  for burning a fuel/combustion air mixture. The heater  10  furthermore comprises an annular channel fan  14  with a fan motor  16  with which combustion air  20  is taken in via an air inlet connection  18  and is blown on the pressure side into the combustion air collecting space  22 . Part of the combustion air which is made available in the combustion air collecting space  22  is delivered as primary air by an atomizer nozzle  24 , which in this case is made as a Venturi nozzle, into the combustion chamber  26 . The remaining part of the combustion air in the combustion air collecting space  22  is delivered as secondary air by secondary air holes  28  into the combustion chamber  26 . Division of the combustion air into primary air and secondary air is useful in order to make available an ignitable mixture at the discharge of the atomizer nozzle  24  which is detailed below.  
         [0033]     The atomizer nozzle  24  comprises an inlet zone  30  and a diffusor  32 , with which the Venturi effect is produced. Within the atomizer nozzle  24 , there is a fuel needle  34  which is supplied via a fuel line  36  with liquid fossil fuel, especially with diesel fuel. In the inlet zone  30 , the primary air with a high flow velocity is made available and, moreover, the liquid fuel from the fuel needle  34  is discharged almost without pressure. The emerging fuel is pulled into filaments.  
         [0034]     As the combustion air continues to flow through the diffusor  32 , the fuel/combustion air velocity is first drastically increased, by which the aforementioned fuel filaments are broken into droplets.  
         [0035]     The flow velocity is again subsequently radically reduced as a result of the widening of the diffusor  32 , by which pressure is recovered and formation of a pilot flame is supported. The pilot flame is ignited with a glow plug  38  which is located upstream of the outlet of the diffusor  32 .  
         [0036]     In the combustion chamber  26 , there is a baffle plate  40  on which the fuel/combustion air mixture emerging from the atomizer nozzle  24  is radially displaced to the outside and thoroughly mixed. The area between the atomizer nozzle  24  and the baffle plate  40  is thus used as a mixing zone  42 , while the area downstream of the baffle plate  40  is used as a reaction zone  44 . As the combustion tube  46 , which surrounds the baffle plate  40  continues, the fuel/combustion air mixture is completely burned and routed out of the heater  10  through a part which routes exhaust gas. The heat energy which is released by the exhaust gas heats water  48  which is entering the heater  10  as a heat transfer medium on the exhaust-carrying parts and heats a heat exchanger which is made on it so that hot water  50  emerges from the heater  10 . In an embodiment which is not shown, air is used as the heat transfer medium instead of the water.  
         [0037]      FIG. 2  details the atomizer nozzle  24 . The atomizer nozzle  24  comprises a nozzle body  52  on which, by means of a through-opening, an air inlet area  54  and an air exit area  56  are formed. The air inlet area  54  and the air exit area  56  are connected to one another via the flow path  58 . This flow path  58  is divided into the aforementioned inlet zone  30  and the diffusor  32 . The nozzle body  52  is furthermore used as a holder for the glow plug  38 .  
         [0038]     In the center of the inlet zone  30  along the lengthwise axis of the flow path  58 , the fuel needle  34  is made in the form of a pipe or tube. In the fuel needle  34  on the end area facing away from the nozzle body  52 , a widening is formed within which there is a heating element  60 . The heating element  60  is made as a resistance wire spiral  62  of a material with a positive tmp coefficient.  
         [0039]     With the heating element  60 , the fuel which has been delivered by the fuel needle  34  if necessary is converted into the gaseous state so that it emerges as a gas jet from the fuel needle  34  and travels into the flow path  58 . In this way, during various operating phases of the heater  10 , the aforementioned advantages of the present invention can be achieved.  
         [0040]     The spiral  62  is arranged in the tube of the fuel needle  34  such that between the outside of the spiral  62  and the inside of the fuel needle  34 , a comparatively thin gap  64  is made available by which the fuel is delivered and in doing so heated.  
         [0041]     Outside of the gap  64 , a through-opening  66  as a flow path is also available to the liquid fuel. This opening is relieved within the spiral  62  in the lengthwise direction of the fuel needle  34 .  
         [0042]     In conclusion, it should be noted that all the features which are named in the application documents and especially in the dependent claims, in spite of the formal reference made to one or more certain claims, are to acquire independent protection also individually or in any combination.