Patent Publication Number: US-6708481-B2

Title: Fuel injector for a liquid fuel burner

Description:
This application claims priority from U.S. provisional patent application, Ser. No. 60/365,657, filed Mar. 19, 2002, entitled “FUEL INJECTOR FOR A LIQUID FUEL BURNER,” which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to burner components, such as the burner components of an engine or other power source, and more particularly to a fuel injector for a liquid fuel burner. 
     BACKGROUND OF THE INVENTION 
     Burners, such as liquid fuel burners, may be used in a wide range of applications including power sources, heat sources, heating appliances and light sources. Typically, it is desirable to have a burner with properties such as high thermal efficiency and low emissions. One method to achieve low emissions is to mix a fuel with air before burning the fuel in the burner. Liquid hydrocarbons, such as kerosene and heating oil, need to first be evaporated before being mixed with air for burning. The evaporation of fuel in high power burners is traditionally achieved by atomizing the fuel into a fog of droplets that readily evaporate and mix with the combustion air. Liquid fuels are typically atomized by forcing the liquid fuel through a small hole with significant pressure. However, such an approach is typically limited to burner powers above 12 kW. Below this flow rate, good atomization requires impracticably small holes. Small oil heaters typically use wicks to evaporate the fuel and mix it with air. However, it is difficult to turn down a wick burner and it is therefore not a good choice for a burner for a power source such as an engine. 
     In addition to the limited low flow capabilities, another problem with typical fuel injectors is the coking of the fuel in the injector. Coking is the de-hydrogenation of the liquid hydrocarbon fuel that produces a tar that clogs the fuel injector ports. This is particularly a problem during shut down of a burner when the fuel flow is stopped while the burner is hot. The fuel left in the hot injector bakes and forms tar deposits. 
     As mentioned, a burner may be used in a power source, such as an engine. A burner for a thermal cycle engine, such as a Stirling cycle engine, should have a high thermal efficiency, low emissions, good cold starting capabilities and a large turndown ratio or wide dynamic range. Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker,  Stirling Engines,  Oxford University Press (1980), incorporated herein by reference. High thermal efficiency may be achieved by preheating the air that will be mixed with the fuel in the burner to approximately the Stirling heater head temperature. As discussed previously, low emissions may be achieved by mixing the fuel with the preheated air before burning the fuel in the burner. However, a burner for a thermal cycle engine should also be capable of being ignited and warmed-up with ambient temperature air. Therefore, the burner should be capable of good fuel/air mixing and flame stabilization over a wide range of air temperatures. In addition, the burner should be capable of good fuel/air mixing over a wide range of fuel flows. 
     SUMMARY OF THE INVENTION 
     In accordance with embodiments of the invention, a liquid fuel burner is provided for combusting a fuel-air mixture. The liquid fuel burner includes a fuel injector for injecting the fuel into the air in a throat of the burner so that the fuel and air mix to form the fuel-air mixture. The fuel injector has a fast acting valve to provide a pulsed flow of fuel and a nozzle coupled to the valve for receiving and atomizing the pulsed flow of fuel. The pulse of atomizing fuel is injected into the burner throat. The burner may include one or more air registers to direct air into the burner throat. The burner further includes a combustion chamber coupled to the throat of the burner for receiving and igniting the fuel air mixture using an igniter. A fuel controller coupled to the fuel supply and the fuel injector governs a rate of fuel delivery by controlling the duration of an opening period of the fast acting valve. 
     In one embodiment, the fuel controller governs the rate of fuel delivery by varying the frequency of the fast acting valve. Alternatively, the fuel controller may govern the rate of fuel delivery by varying a fuel pressure provided by a fuel pump coupled to the fuel supply. In a further embodiment, the fuel controller includes a pulse width modulated driver to control the frequency and duration of fast acting valve openings. The liquid fuel burner may further include a cooling loop coupled to the fuel supply and the fuel injector for cooling the fuel injector. The valve may be an automotive fuel injector designed for port fuel injection. The nozzle may be a pressure-atomizing oil burner nozzle. The fuel injector may be an automotive gasoline direct injection fuel injector. Alternatively, the fuel injector may be a diesel common rail injector. 
     In another embodiment, the liquid fuel burner may be used to provide heat to a thermal cycle engine having a heater head for heating a working fluid by conduction. The fuel flow rate may be controlled to maintain a desired heater head temperature. The fuel flow rate is varied by a controller that varies at least one parameter of a control signal to the fuel injector valve based on the desired heater head temperature and measured heater head temperature. The control signal has the following parameters: signal amplitude, frequency and duty cycle. 
     In another embodiment, the liquid fuel burner further includes a mixing chamber coupled to the fuel injector for mixing the injected fuel and a portion of the air from the air supply before entry into the throat of the burner. The mixing chamber may include a mesh metal surface to absorb and evaporate the fuel. The mixing chamber may have a plurality of openings through which the portion of air enters the mixing chamber. In one embodiment, the mixing chamber is a cylinder aligned with the fuel injector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a burner with an intermittent fuel injector in accordance with an embodiment of the invention; 
     FIG. 2 a  is a cross sectional view of the burner and intermittent fuel injector in accordance with an embodiment of the invention; 
     FIG. 2 b  is a cross sectional view of the fuel injector of FIG. 2 a  in accordance with an embodiment of the invention; and 
     FIG. 3 is a schematic diagram of a burner with a fuel injector and a mixing chamber in accordance with an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While the invention will be described generally with reference to a Stirling engine, it is to be understood that many engines, burners, and other machines may similarly benefit from various embodiments and improvements that are subjects of the present invention. 
     In accordance with embodiments of the present invention, a liquid fuel burner with a pulsed fuel injector is provided that provides substantially complete and clean combustion over large turn-down ratios, very low firing rates and good durability. The liquid fuel burner of the present invention may be used in Stirling engines, particularly small (&lt;3 kWe) Stirling engines, thereby expanding the range of operating fuels for such engines and improving the portability of small Stirling engine applications. A small liquid burner may have applications in other small continuously fired power sources such as fuel cells and brayton-cycle engines. In addition, the liquid fuel burner as disclosed may be used in other applications requiring a small liquid burner, for example, heating small spaces such as truck and boat cabins and small heating applications such as glass and ceramic kilns. 
     Referring to FIG. 1, a burner including an intermittent fuel injector, in accordance with preferred embodiments of the invention, is shown schematically and designated generally by numeral  101 . Burner  101  includes, among other components, an intermittent fuel injector  100  and a cooling loop  112 . The intermittent fuel injector  100  includes a valve  108  and a nozzle  110 . In a preferred embodiment, valve  108  is a fast solenoid valve. Intermittent injector  100  produces good atomization of a liquid fuel and low fuel flow rates by producing periodic sprays of fuel at a high fuel flow rate for brief periods of time. The instantaneous fuel flow rate and pressure created by valve  108  should be high enough to produce good atomization of the liquid fuel through the nozzle  110 . However, the duty cycle of the valve  108  is set low enough to achieve the desired average fuel flow rate during operation of the burner. A volume capacitance of a combustion chamber  128  of the burner  101  is used to damp out the pulsed injections. In an embodiment in which the burner is used to heat a working gas of a Stirling engine, the thermal mass of the Stirling heater head will damp out the fluctuating heat releases caused by the intermittent injection of fuel. 
     Burner  101  also includes a fuel supply system to provide fuel to the intermittent fuel injector  100 . Fuel flows from a fuel tank  102  to a pump  104  that produces the desired fuel pressure upstream of the valve  108 . The fuel pressure may also be controlled by a back-pressure regulator  106  coupled to the fuel tank  102  and the pump  104 . In a preferred embodiment, pump  104  is a positive displacement pump with a built-in pressure regulator and the back pressure regulator  106  is replaced with a fixed orifice or resistance. In one embodiment, the resistance and pump pressure are selected to provide enough cooling to keep the nozzle  110  below 150° C. The fuel pressure should be at least 25 psig in order to produce finely atomized fuel droplets at nozzle  110 . 
     Excess fuel flows through the cooling loop  112  to cool the intermittent fuel injector  100 , in particular nozzle  110 . The nozzle  110  is in contact with the heated combustion air provided by air supply  120 . The combustion air, in some applications, may be heated to temperatures as high as 700° C. The flow of excess fuel cools the injector as it passes through the cooling loop  112  before returning to the fuel tank  102 . Cooling the injector serves to cool the intermittent fuel injector  100  so as to avoid coking of any fuel left in the small passages of nozzle  110 . The fuel injected by fuel injector  100  is mixed with combustion air from swirlers  122  in throat  124  to form a fuel-air mixture. The fuel-air mixture then flows through the throat  124  and is ignited by an igniter (not shown) in the combustion chamber  128  to form a recirculting flame. In a preferred embodiment, the igniter is a spark plug. In an alternative embodiment, the igniter may be a glow plug. 
     As mentioned above, the average fuel flow rate is determined primarily by the duty cycle of the valve  108  and the fuel pressure. The frequency of the valve  108  may also impact the fuel flow rate. In addition, the frequency of the valve  108  has a marked effect on creating a self-sustaining flame in the combustion chamber  128 . Below a given frequency, a constant ignition source is required to ignite the pulses of fuel injected from the intermittent fuel injector  100  into the combustion chamber  128 . The minimum frequency of the valve  108  required to create a self-sustaining flame depends on several parameters including the volume of the combustion chamber  128 , the flame speed of the fuel-air mixture, the duty cycle of the valve  108  and the spray characteristics of the intermittent fuel injector  100 . For the embodiment in FIGS. 2 a  and  2   b  as discussed below, the self-sustaining frequency is preferably 32 Hz. 
     Valve  108  is controlled by a fuel-controller  130  that is coupled to the intermittent fuel injector  100  and the pump  104 . Fuel controller  130  varies one or more parameters of the fuel injector  100  or pump  104  to control the fuel flow rate through the valve  108  and the nozzle  110 . In one embodiment, where the liquid fuel burner is used in a thermal cycle engine (such as a Stirling engine) having a heater head, the fuel controller  130  controls the fuel flow rate to minimize the error between a desired heater head temperature  134  and a measured heater head temperature  132 . Both the desired heater head temperature  134  and the measured heater head temperature  132  are inputs to the fuel controller  130 . Fuel controller  130  may vary one or more of the following parameters of the fuel injector  100 : duration of opening of valve  108 , frequency of opening of valve  108  and amplitude of a control signal sent to the fuel injector to drive valve  108 . The fuel controller may, for example, include a pulse width modulated drive to provide a pulse width modulated control signal to the valve  208 . In a preferred embodiment, the operating ranges of frequency for valve  108  are 5 to 90 Hz with duty cycles from 2% to 100%. In addition, fuel controller  130  may vary the fuel pressure generated by the pump  104  to control the fuel flow rate. 
     Nozzle  110  atomizes the fuel by forcing it through a small opening. As mentioned above, methods of atomizing are well known in the art. In a preferred embodiment, the nozzle  110  is the smallest fuel pressure-atomizing nozzle generally available. In order to assure good atomization and therefore good emissions, it is necessary to allow only high or zero fuel flow rates through the nozzle  110 . Therefore, it is important to avoid low fuel flow rates through the nozzle  110  as the valve  108  opens or just after it closes. Preferably, the fuel flow rate through valve  108  resembles a square wave control signal used by the controller  130  to drive valve  108 . In order to avoid low fuel flows, a stiff fluid system is created that has a minimal amount of compliance between the pump  104 , the regulator  106  and the nozzle  110 . It is particularly important that the hydraulic system between the valve  108  and the nozzle  110  has minimal compliance. Compliance between the valve  108  and nozzle  110  is caused primarily by trapped gases between the valve  108  and the nozzle  110 . Accordingly, nozzle  110  is placed as close as possible to the valve  108  to minimize the volume of fuel between the two elements. In addition, gases should be bled out of the space between the valve  108  and nozzle  110  during assembly of the injector  100 . The open space should be arranged so that any remaining air is swept out of the volume between the valve  110  and the nozzle  108  as fuel flows from the valve to the nozzle. 
     FIG. 2 a  is a cross-sectional view of a burner with an intermittent fuel injector in accordance with an embodiment of the invention. Fuel injector  240  includes a fuel valve  208  such as an automotive fuel injector, typical of those found in multi-point fuel injection engines. Nozzle  210  may be a modified 0.5 gph, 60 degree, hollow cone oil nozzle. Preheated air flows through passages  230  to swirler  232 . An automotive spark plug  242  may be used to ignite the fuel-air mixture. 
     Other known injector technologies may be adapted as the valve  108  (as shown in FIG. 1) or as both the valve  108  and the nozzle  110  (as shown in FIG.  1 ). These include injectors used for gasoline direct injection (GDI) and electronically controlled common rail diesel injectors. Both GDI and common rail injectors include an electronically controlled fast valve and a pressure atomizer nozzle. 
     FIG. 2 b  is a cross-sectional view of the fuel injector  240  of FIG. 2 a  in accordance with an embodiment of the invention. Nozzle  210  has a center-body  220  that is modified to work with a secondary oil filter  222 , typical of lower fuel-flow rate nozzles. This construction allows the tip of the fuel valve  208  to be mounted very close to the oil nozzle  210 . Preferably, in one embodiment, the volume between the valve  208  and nozzle  210  is approximately 2 cm  3 . 
     An embodiment of the cooling loop  112  (as shown in FIG. 1) is also shown in FIG. 2 b . The excess flow of fuel that bypasses fuel valve  208  of the fuel injector  240  flows into a cooling block  248  (also shown in FIG. 2 a ) via a first port  250 . The fuel then flows around a fuel injector mount  212  through passages  252  before exiting through a second port (not shown). The cooling loop maintains the fuel flowing through nozzle  210  at a temperature below the decomposition temperature of the fuel. 
     FIG. 3 is a schematic diagram of a burner with a fuel injector and a mixing chamber in accordance with an alternative embodiment of the invention. A mixing volume  150  is provided to premix the injected fuel  148  and a portion of the combustion air  144  to form a rich fuel-air mixture. The rich fuel-air mixture is then mixed with the remaining portion of the combustion air  140  in the throat  124  and the combustion chamber  128  of the burner. In alternative embodiments, all the combustion air will flow through the mixing volume  150 . The mixing volume  150  is also used to damp out the fuel pulses provided by the intermittent fuel injector  100 . Preferably, the mixing volume  150  is made sufficiently large to produce a steady flow of fuel and air at an exit  152  of the mixing volume. 
     The combustion air supplied from the air supply  120  is directed through a restriction  142  to produce a primary air supply  144  and a secondary air supply  140 . The primary air  144  then flows into the manifold entirely around the mixing chamber  150 . The walls  146  of the mixing volume  150  are constructed of perforated material, preferably metal, to allow the primary air  144  to enter the mixing volume  150  and mix with the injected fuel  148 . The perforated walls  146  have a lining of mesh or wire screen that serves two purposes. Firstly, fuel droplets that contact the wall  146  will be absorbed by the lining. Secondly, the lining encourages evaporation of the fuel from the heat of the incoming primary air  144 . The evaporation is very important during operation of the burner with the preheated air. The secondary air flows through the swirler  122  and enters the burner throat  124 . The secondary air then mixes with the richer fuel-air mixture from the mixing volume  150  to form a lean fuel-air mixture in the throat  124  of the burner. The lean fuel-air mixture then enters the combustion chamber  128  where it is ignited by an igniter (not shown). 
     During start up operation, the intermittent fuel injector  100  operates at a high firing rate depending only on the nozzle  110  for atomization. Once the metal walls  146  of the mixing volume  150  are heated, the fuel flow may be reduced so that some of the fuel evaporating off the walls  146  smoothes out the pulses of fuel through the exit  152  of the mixing volume  150 . In addition, as mentioned above, the mixing volume  150  itself damps the fuel pulses from the intermittent fuel injector  100 . 
     All of the systems and methods described herein may be applied in other applications besides the Stirling or other thermal cycle engine in terms of which the invention has been described. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.