Abstract:
A method and system for freeze protecting liquid NO x  reductants, preferably used in vehicle applications, wherein a liquid NO x  reductant, carried onboard a vehicle exposed to cold weather conditions, is heated by existing thermal energy generated by fuel compression. The heated fuel heats a potentially frozen reductant and the liquid reductant is supplied to an exhaust gas pipe in front of a catalyst for reducing NO x  on the surface of the catalyst and catalytically converted into environmentally safe nitrogen and water.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to a method and system for freeze protecting liquid NO x  reductants and, more specifically, to a method and system for freeze protecting liquid NO x  reductants used in vehicle applications. 
     2. Background Art 
     Diesel engines are typically more efficient than gasoline engines, with regard to fuel economy and emit less greenhouse gasses. However, diesel engines typically produce emissions containing higher concentrations of nitrogen oxides (NO x ) compared to gasoline engines fitted with three-way catalysts. Future pollution control regulations may necessitate reducing the amount of NO x  and particulate emissions produced by diesel engines. 
     One method that has been employed to remove NO x  from diesel exhaust utilizes selective catalytic reduction (SCR) of NO x  with a liquid nitrogen containing reductant, such as aqueous urea. NO x  and the liquid reductant are brought into contact with a selective catalyst and catalytically converted into environmentally safe nitrogen and water. When a liquid reductant is used, the liquid reductant is typically injected directly into the exhaust pipe in front of a catalyst to effect reduction of NO x  on the surface of the catalyst or in the catalyst itself. 
     One major disadvantage of liquid reductants is that freezing of the reductant may occur. The freezing temperature varies relative to the composition and concentration of the dissolved reductant. For example, solutions having a urea content of about 32.5% in water (eutectic), typically freeze at about 12° F. (−11° C.). As can be readily expected, liquid reductant freezing is particularly a problem for the use of diesel vehicles in cold-weather climates when a liquid reductant is employed to help meet emission standards for NO x . Also, expansion of the liquid reductant due to freezing can cause damage to the system components. 
     One approach to address the problem of supplying liquid reductant from a frozen reductant source uses heat to warm the liquid reductant above its freezing point. Heating methods have been developed using additional sources of energy, such as diesel fuel, to run a heater, or electrical supplemental heat to warm the liquid reductant, in cold weather conditions. Utilization of supplemental energy to warm the liquid reductant is disadvantageous because the supplemental energy requirement can result in an inefficient use of energy and decreased fuel economy. Supplemental heating by fuel sources is further disadvantageous since it requires a second fuel injection system thereby increasing costs and emissions. Another disadvantage is that the liquid reductant could be heated too much causing the liquid reductant to evaporate and therefore be ineffective. 
     It would be desirable to have a system that can employ liquid reductants for decreasing NO x  emissions in cold weather climates without experiencing at least some of the above-mentioned disadvantages. 
     SUMMARY OF INVENTION 
     The present invention relates to a method and system for heating liquid reductant above its freezing temperature by utilizing existing heat generated by an engine under operating conditions, to enable use of the liquid reductant in cold weather conditions to reduce emissions of NO x , in conjunction with a catalyst, without decreasing overall fuel economy or overheating the liquid reductant. 
     This invention relates, more specifically to a method for operating an exhaust gas purification system. The method comprises directing fuel, returning from a high pressure fuel injection system, wherein the fuel becomes heated, to a reductant source, transferring heat from the fuel to the reductant to liquefy frozen reductant, and supplying the liquid reductant to an exhaust pipe at a location in front of a catalyst for purification of exhaust gas. 
     In a preferred embodiment of the invention, a heat exchanger with a reservoir may be the source of reductant liquefied by heat supplied to the heat exchanger by a high pressure fuel injection system through a return fuel line. In another preferred embodiment, a urea supply line contained within a return fuel line may be the source of reductant rapidly liquefied by heated fuel returning from a high pressure fuel injection system through a return fuel line. 
     This invention also relates to a system for operating an exhaust gas purification system. The system comprises a source of fuel, a first source of liquid reductant, and an exhaust pipe for discharging exhaust gas from the vehicle. The system further comprises a second source of liquid reductant that is disposed between the first source of liquid reductant and the exhaust pipe, a high pressure fuel injection system disposed between the fuel source and the second liquid reductant source, a first conduit fluidly connecting the fuel source with the high pressure fuel injection system, a second conduit fluidly connecting the high pressure injection system with the fuel source, and a third conduit fluidly connecting the first liquid reductant source with the exhaust pipe. The system further comprises a first high pressure fuel pump to deliver fuel from the fuel source through the high pressure fuel injection system, past the second liquid reductant source, returning to the fuel source. The compression of the fuel in the high pressure fuel injection system heats the fuel. The system further comprises a second pump to deliver liquid reductant from the second liquid reductant source to the exhaust pipe. 
     The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a schematic diagram of a first embodiment of the invention; and 
     FIG. 2 illustrates a schematic diagram of a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, the general arrangement of a fuel and liquid reductant delivery system  10  of a preferred embodiment of the present invention is illustrated. While the system  10  of the present invention may be used with any engine capable of creating NO x  emissions, the present invention is particularly well-suited for use with internal combustion engines with high pressure fuel injection systems. 
     As shown in FIG. 1, the system  10  comprises a diesel fuel tank  12  that holds diesel fuel  14 . In the illustrated embodiment, a reductant tank  16 , that holds liquid reductant  20 , is positioned within the diesel fuel tank  12 . At engine startup, the temperature of the diesel fuel  14  ranges from ambient temperature to fuel tank  12  operating temperature, depending on the length of time the engine has been off. Ambient temperature is typically between about −30° C. and about 40° C. The diesel fuel tank  14  operating temperature is typically between ambient temperature and about 60° C. Positioning the reductant tank  16  within the diesel fuel tank  12  provides, additional, natural insulation for the reductant in the reductant tank  16 , by the diesel tank  12  itself and also from the diesel fuel  14  within the tank  12 . While placement of the reductant tank  16  within the diesel fuel tank  12  is a preferred location for the reductant tank  16 , alternative locations for the reductant tank  16  within a vehicle may also be used, such as, but not limited to, locations capable of being exposed to heat during vehicle operation or locations that provide relatively high levels of protection from direct exposure to the external environment. Examples of alternative protected locations can be found in the engine compartment, as well as in the passenger compartment, provided that the temperature at these locations remains about at, and more preferably below, the liquid reductant decomposition or boiling temperature. 
     The system  10  further includes a high pressure fuel injection system  22  and a diesel fuel supply line  24  extending between and fluidly connecting the high pressure fuel injection system  22  and the diesel fuel tank  12 . A switching valve  28  with a temperature switch  30  is disposed between the diesel fuel tank  12  and the high pressure fuel injection system  22 . 
     A high pressure fuel pump  32  is preferably disposed between the high pressure fuel injection system  22  and the diesel fuel tank  12 . A preferred high pressure fuel injection system  22  is a common rail injection system, but the high pressure fuel injection system  22  may be any high pressure fuel injection system known to those skilled in the art. The high pressure fuel pump  32  pumps the diesel fuel  14  through the system  10 . 
     In a first embodiment, the system  10  further includes a heat exchanger  40  fluidly connected to the high pressure fuel injection system  22  by a fuel injection system return line  42 . An injector return line  50  is disposed between the high pressure fuel injection system  22  and the fuel injection system return line  42 . The heat exchanger  40  is also fluidly connected to diesel fuel tank  12  by a diesel fuel return line  44 . The heat exchanger  40  is also fluidly connected to the switching valve  28  by a diesel fuel recirculation return line  46 . A check valve  48  is disposed between the heat exchanger  40  and the diesel fuel tank  12  to help selectively direct the flow of diesel fuel  14  from the heat exchanger  40  to the diesel fuel tank  12  or the switching valve  28 . 
     The system  10  further includes a reductant supply line  52  for fluidly connecting the reductant tank  16  to the heat exchanger  40 . A reductant reservoir  54 , schematically illustrated in FIG. 1, is disposed within the heat exchanger  40 . The heat exchanger  40  and an exhaust gas pipe  56  are connected by a reductant dispensing line  58 . An injection device  60  is disposed at the end of the reductant dispensing line  58  to direct liquid reductant  20  into the exhaust gas pipe  56 . A reductant pump  64  is disposed between the heat exchanger  40  and the injection device  60 . Alternatively, the reductant pump  64  could be disposed within the heat exchanger  40 . The reductant pump  64  directs (i.e., pumps) the liquid reductant  20  to the injection device  60 . 
     Under operation conditions, diesel fuel  14  from the diesel tank  12  is supplied through the diesel fuel supply line  24  past the switching valve  28  and into the high pressure fuel injection system  22  by operation of the high pressure fuel pump  32 . Diesel fuel  14  warming occurs due to compression of the diesel fuel  14  by the high pressure fuel pump  32 . The temperature of the diesel fuel  14  remains below the boiling temperature of the diesel fuel  14 , i.e., typically below 70° C. The typical maximum operating temperature for the high pressure fuel injection system  22  is about 60° C. The diesel fuel  14  begins to warm within a few seconds of the vehicle startup, depending on the diesel fuel  14  flow rate. 
     From the high pressure fuel injection system  22 , diesel fuel  14  can be injected into the combustion chamber of the engine (not shown) or supplied to the heat exchanger  40  through the fuel injection system return line  42 . This invention utilizes existing heat generated by the compression of the diesel fuel  14  to supply heat to the reductant  20 . In an embodiment of the invention, as illustrated in FIG. 1, the heat exchanger  40  is incorporated into the diesel fuel return pathway between the fuel injection system return line  42  and the diesel fuel return line  44  to enable use of the existing heat to warm the reductant  20  by thermal transfer of heat from the diesel fuel  14  in the heat exchanger  40 . The injector return line  50  circulates fuel from the injectors to the fuel injection system return line  42 . 
     The reductant  20  is supplied to the heat exchanger  40  from the reductant tank  16  through the reductant supply line  52  by operation of the reductant pump  64 . In the heat exchanger  40 , the warmed fuel  14  comes into contact with the reductant reservoir  54  and thermal energy is transferred from the heated diesel fuel  14  to the reductant  20  in the reservoir  54  to liquefy frozen reductant  20  if the temperature of the reductant  20  drops below its respective freezing point and to maintain a supply of liquid reductant  20  for exhaust gas reduction. The heat exchanger  40  allows relatively rapid warming of the reductant  20  using existing thermal energy from the compressed diesel fuel  14  without causing the reductant  20  to reach excessive temperatures that would vaporize or decompose reductant  20 . When the reductant  20  is a water-based solution, the uppermost temperature allowed for the solution to remain a liquid is the decomposition or hydrolysis temperature. Preferably, because the temperature of the compressed fuel remains below 90° C., the heating of the reductant  20  with diesel fuel  14  in the heat exchanger  40  does not cause the reductant  20  to exceed its decomposition or hydrolysis temperature. 
     The warmed reductant  20  is supplied from the heat exchanger  40  through the reductant supply line  58  to the injection device  60 . The liquid reductant  20  is injected through the injection device  60  directly into the exhaust gas pipe  56  in front of a catalyst reduction unit (not shown). The injection device  60  can be any suitable device capable of controlling flow of the reductant  20  from the reductant dispensing line  58  into the exhaust gas pipe  56 . The exhaust gas  62  passes through the exhaust gas pipe  56  to a catalyst reduction unit (not shown) where the reductant  20  reduces the NO x  on the surface of the catalyst to form environmentally safe nitrogen and water. 
     Diesel fuel  14  used to heat the reductant  20  in the heat exchanger  40  continues circulating, at about ambient pressure conditions and above fuel cloud point and below the boiling temperature, from the heat exchanger through the diesel fuel return line  44  to the diesel fuel tank  12  and/or the diesel fuel  14  recirculates through the diesel fuel recirculation return line  46  to the switching valve  28  back to the high pressure fuel injection system  22  by operation of the high pressure fuel pump  32 . The check valve  48  provides resistance to help selectively control the diesel fuel  14  flow to the switching valve  28  when the temperature of the diesel fuel is low, as determined by the temperature switch  30 , and to control the diesel fuel flow to the diesel fuel tank  12  when diesel fuel temperature is sufficiently high. The switching point may be set, preferably at about 0° C.-50° C. for this embodiment of the invention. 
     Referring now to FIG. 2, a second embodiment of the present invention is shown. The second embodiment has many components that are substantially the same as corresponding components of the first embodiment. This is indicated by the use of the same reference numbers in FIG. 2 as were used in FIG.  1 . 
     The system  110  illustrates a general arrangement of a fuel and liquid reductant delivery system of a preferred embodiment of the present invention. Similar to the embodiment of the invention illustrated in FIG. 1, the system  110  comprises a diesel fuel tank  12  that holds diesel fuel  14 . The system  110  further includes a high pressure fuel injection system  22  and a diesel fuel supply line  24  extending between and fluidly connecting the high pressure fuel injection system  22  and the diesel fuel tank  12 . A switching valve  28  with a temperature switch  30  is disposed between the diesel fuel tank  12  and the high pressure fuel injection system  22 . A high pressure fuel pump  32  is preferably disposed between the high pressure fuel injection system  22  and the diesel fuel tank  12 . 
     The system  110  further comprises a fuel injection return line  142  that fluidly connects the high pressure fuel injection system  22  and the diesel fuel tank  12 . Similar to system  10 , an injector fuel return line  50  is disposed between the fuel injection system  22  and the fuel injection system return line  142 . The fuel injection system return line  142  is also fluidly connected to the switching valve  28  by diesel fuel recirculation line  146 . 
     The system  110  further includes a reductant supply line  152  for fluidly connecting the reductant tank  16  to an injection device  60 . The reductant supply line  152  is positioned within, or coaxial with, the high pressure fuel injection system return line  142 . A reductant pump  164  within the reductant supply line  152  pumps the liquid reductant  20  through the reductant supply line  152  to the injection device  60 . The injection device  60  is disposed at the end of the reductant dispensing line  58  to direct liquid reductant  20  into the exhaust gas pipe  56 . 
     Under operation conditions for the system  110 , the thermal energy from the heated returning diesel fuel  12  is transferred to the reductant supply line  152  and the reductant contained therein to relatively rapidly liquefy frozen reductant  20  if the temperature of the reductant  20  drops below its respective freezing point and to maintain a supply of liquid reductant  20  for exhaust gas reduction. The system  110  provides a large surface area of contact between the fuel injection return line  142  containing heated return diesel fuel  12  and the reductant supply line  152 , resulting in relatively rapid liquefaction of frozen reductant for faster introduction of the liquid reductant into the exhaust gas. The system  110  also provides greater protection against liquid reductant freezing through additional insulation from ambient temperatures. 
     In a preferred embodiment of the invention, the liquid NO x  reductant  20  is an aqueous solution of urea. Aqueous urea solutions and hydrolysis products formed therefrom may be used as a source of ammonia to effect reduction of the NO x . Aqueous solutions of urea may be employed up to the solubility limit of the urea. Typically, the urea solution will contain from about 2 to about 65% reagent based on the weight of the solution, more preferably from about 5% to about 45% urea by weight. Most preferably, the concentration for mobile uses is about 32.5% urea by weight which exhibits the lowest freeze point without precipitation of urea. 
     While aqueous urea solutions and the hydrolysis products formed therefrom are preferred for NO x  reduction, alternative commercial solutions of hydrolysis products, and combinations thereof, may be used to supply a liquid reductant to effect reduction of NO x  on the surface of the catalyst. Commercial solutions of liquid reductants include, but are not limited to, solutions containing: ammonium carbonate, ammonium bicarbonate, ammonium carbamate, ammonium cyanate, ammonium salts of organic acids, including formic and acetic acid, and cyanuric acid. 
     In an embodiment of the invention, in which the liquid reductant may potentially freeze due to long-term exposure to extremely cold conditions, elastic materials with expansion coefficients higher than that of the liquid reductant can be used to form certain components of the system  10  and  110 , such as, the reductant supply lines  52  and  152 , to prevent the reductant-containing components against damage from bursting. Reductant-containing components include, but are not limited to, the reductant tank  16 , the reductant supply line  52  and  152 , the heat exchanger  40 , the reductant reservoir  54 , the reductant dispensing line  58 , and the injection device  60 . The expansion behavior of aqueous urea solutions are similar to the behavior of water. The thermal coefficient of expansion for ice is about 50×10 −6 /K. Suitable elastic materials include, but are not limited to, any organic, inorganic, metallic materials or mixtures or combinations of those which are suited or achieve the prescribed target. Especially preferred are polymeric materials that have a thermal expansion coefficient higher than that of the aqueous urea solution and also are chemically compatible with aqueous urea. These materials include, but are not limited to, polyethylene, polypropylene, nylon, and Teflon. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.