Patent Publication Number: US-2010107604-A1

Title: Ammonia generator, vehicle and method for generating ammonia

Description:
The invention relates to an ammonia generator, in particular for mobile applications, as generically defined by the preamble to claim  1 ; a vehicle having such an ammonia generator as generically defined by the preamble to claim  5 ; and a method for generating ammonia as generically defined by the preamble to claim  11 . 
     PRIOR ART 
     In a vehicle of the type defined at the outset, for exhaust gas treatment of oxygen-rich gasoline and diesel exhaust gases from an internal combustion engine of the vehicle for eliminating nitrogen oxides, it is already known from German Patent Disclosure DE 101 45 808 A1 of the present Applicant to employ what is known as the SCR (Selective Catalytic Reduction) process, in which the nitrogen oxides (NO x ) in the exhaust gas are reduced in an SCR catalytic converter, after prior delivery of ammonia (NH 3 ) to nitrogen (N 2 ) with high selectivity. The ammonia needed for the SCR process is generated there, in an ammonia unit carried along on board the vehicle, for instance by the Haber-Bosch process, from nitrogen and hydrogen at a pressure of more than 100 bar. 
     It is also already known, from German Patent Disclosure DE 199 22 960 A1, to generate the ammonia required for reducing nitrogen oxides by the SCR process in an exhaust gas cleaning system of an internal combustion engine on board, in an ammonia generator catalytic converter from combustion exhaust systems of the internal combustion engine, whose cylinders are for that purpose acted upon at least intermittently or partially with a rich fuel-air mixture, so that the exhaust gas emitted by the internal combustion engine contains not only uncombusted hydrocarbons but also hydrogen and a certain quantity of nitrogen oxides. From these latter two ingredients, ammonia is then generated in the ammonia generator catalytic converter. However, generating ammonia from exhaust gases of the internal combustion engine has the disadvantage that the internal combustion engine cannot be adjusted optimally in terms of either its fuel consumption or the driving qualities of the vehicle. 
     ADVANTAGES OF THE INVENTION 
     The ammonia generator of the invention having the characteristics recited in claim  1 , the vehicle of the invention having the characteristics recited in claim  5 , and the method of the invention having the characteristics recited in claim  11  by comparison offer the advantages that the efficiency of ammonia generation, compared to generation from exhaust gases of an internal combustion engine serving as a drive motor for the vehicle, can be improved; that because of the decoupling of the ammonia generation and the operation of the internal combustion engine as a vehicle driving motor, an impairment in the driving qualities of the vehicle and an increase in fuel consumption can be avoided; and that the free-piston engine, which forms part of the ammonia generator, can furthermore be used in the vehicle as an “auxiliary power unit” or APU, preferably for generating hydraulically usable energy, for instance for a high-pressure injection system of the vehicle. 
     A preferred feature of the invention provides that the exhaust gas of the free-piston motor, for synthesizing the ammonia, is conducted through a catalytic converter, preferably a noble-metal/ammonia catalytic converter; beforehand, fuel is optionally added to the exhaust gas, in order to increase its content of uncombusted hydrocarbons, from which hydrogen is catalytically split off in the catalytic converter and used for the catalytic synthesis of ammonia. Moreover, with the aid of the uncombusted hydrocarbons, oxygen that is a hindrance to the ammonia synthesis taking place in the catalytic converter is reactively removed. 
     In a further preferred feature of the invention, the ammonia catalytic converter communicates with an exhaust system of the internal combustion engine that serves to drive the vehicle, specifically upstream of an SCR catalytic converter, in which the nitrogen oxides contained in the exhaust gas of the internal combustion engine and of the free-piston engine are reduced catalytically to nitrogen with high selectivity, using the previously synthesized ammonia. 
     As already indicated, the free-piston engine can not only be used as part of an ammonia generator for obtaining ammonia but can simultaneously and advantageously also be used as an “auxiliary power unit” (APU) of the vehicle, for instance to increase the pressure of a fuel injected into the internal combustion engine in order to supply the required high-pressure oil to a power steering system of the vehicle, or for hydraulic brake boosting in the case of heavy utility vehicles. 
     To increase the fuel pressure with the aid of the free-piston engine, the fuel is expediently delivered directly to a work cylinder of the free-piston engine, in a first pressure elevation stage, and then with the aid of a downstream pressure booster increased with further boosting stages to the desired pressure level of between 300 and 2000 bar. 
    
    
     
       DRAWINGS 
       The invention will be described in further detail below in terms of an exemplary embodiment in conjunction with the associated drawings. Shown are: 
         FIG. 1 , a schematic illustration of the functional principle of a free-piston engine; 
         FIG. 2 , a schematic illustration of a motor vehicle having an internal combustion engine and a free-piston engine, the latter forming part of an ammonia generator and a pressure generator. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENT 
     The free-piston engine  2  shown in  FIG. 1 , operating by the two-stroke process, has a cylinder  4  in a manner known per se, in which a free piston  8 , mounted on the end of a piston rod  6 , is freely movable. The free piston  8  subdivides a cylinder chamber  10 , enclosed by the cylinder  4 , into two chambers: a fresh-air chamber  12 , on the side toward the piston rod  6 , and a combustion chamber  16 , on the opposite end of the free piston  8 ; on one face end the combustion chamber is provided is provided with an injection nozzle  14 . When the free piston  8  is located in the vicinity of its “bottom” dead center, shown in  FIG. 1 , the two chambers  12  and  16  communicate with the ambient air via an air inlet  18  and with an exhaust gas line  22  via an exhaust  20 , and they communicate with one another on the two-stroke motor principle, for instance through lateral air slits  24  (only one of which is shown). 
     A further piston  26  with a smaller cross section is fixedly mounted on the piston rod  6  and is movable in a compression or restoration cylinder  28  that adjoins the cylinder  4  on the side of the fresh-air chamber  12 . The compression or restoration cylinder  28  is subdivided by the piston  26  into a compression chamber  30  and a restoration chamber  32 , of which the first can be made to communicate with a compression reservoir  38  via two bores  34 ,  36 . The bore  36  adjacent to the face end of the compression chamber  30  has a small diameter, and it communicates with the compression reservoir  38  via a check valve  40  and a regulating valve  42 , while the other bore  34  has a larger diameter and communicates directly with the compression reservoir  38 , but in the vicinity of the “bottom” dead center of the free piston  8  shown in  FIG. 1 , it is closed by the piston  26 . 
     From the compression or restoration cylinder  28 , the piston rod  6  extends onward into a work cylinder  44 , which forms part of a hydraulic circuit  46 , through which an incompressible hydraulic fluid flows and in which the pressure of the hydraulic fluid is to be increased by means of the free-piston engine  2 . The hydraulic circuit  46  includes a high-pressure working reservoir  48  on the high-pressure side of the work cylinder  44  and a low-pressure working reservoir  50  on the low-pressure side of the work cylinder  44 ; these reservoirs each communicate in such a way, via a respective check valve  52  and  54 , with a work chamber  56  enclosed by the work cylinder  44  that the hydraulic fluid is aspirated from the low-pressure working reservoir  50  into the work chamber  56  and from there is expelled under pressure into the high-pressure working reservoir  48 . The hydraulic circuit  46  further includes a consumer, communicating with the high-pressure working reservoir  48  or the high-pressure side of the work cylinder  44  via a high-pressure line  58 , and also includes a return line  60  from the consumer to the low-pressure working reservoir  50 . 
     In operation of the free-piston engine  2 , in the outset position shown in  FIG. 1  at “bottom” dead center of the free piston  8 , a fluid under pressure, stored in the compression reservoir  38 , flows slowly via the regulating valve  42  and the bore  36  into the compression chamber  30 , whereupon the piston  26  and thus also the piston rod  6  and the free piston  8 , at the onset of an “intake and compression stroke”, move gradually in the direction of the combustion chamber  16 . In this process, fresh air is first aspirated through the air inlet  18  into the fresh-air chamber  12 , and exhaust gas from the combustion chamber  16  is expelled through the exhaust  20  into the exhaust gas line  22 . As soon as the free piston  8  closes the air inlet  18 , the outlets toward the combustion chamber of the air slits  24 , and the exhaust  20 , the piston  26  opens the second bore  34 , and as a result the compression reservoir  38  is abruptly evacuated into the compression chamber  30 . As a result, the piston  26  and thus also the free piston  8  are accelerated in the direction of the injection nozzle  14  and rapidly compresses the air in the combustion chamber  16 , causing the air to heat up markedly. Shortly before the free piston  8  reaches its “top” dead center in the vicinity of the injection nozzle  14 , fuel is injected through this nozzle into the combustion chamber  16 . The fuel is atomized, partly evaporated, and ignites, causing the free piston  8 , during its ensuing “working stroke” moves back in the direction of the “bottom” dead center ( FIG. 1 ). Shortly before “bottom” dead center is reached, compressed air from the fresh-air chamber  12  is forced via the air slits  24  into the combustion chamber  16  and flushes out this chamber, and the exhaust gas escapes into the exhaust gas line  22  through the exhaust  20 . 
     In the region of the compression cylinder  28 , during the working stroke, the compression reservoir  38  is first re-charged via both bores  34 ,  36  and then via the bore  36  and the check valve  40 , while in the region of the work cylinder  44 , hydraulic fluid is positively displaced by the piston rod  6  out of the work chamber  56  into the high-pressure working reservoir  48 , this hydraulic fluid having been aspirated into the work chamber  56  from the low-pressure working reservoir  50  during the preceding “intake and compression stroke”. 
     It has been found that by a suitable choice of the operating point of the free-piston engine  2 , among other provisions by means of suitable adjustment of the fuel-air ratio in the combustion chamber  16 , the exhaust gas produced in combustion can be utilized with good efficiency to obtain ammonia, since it is possible for free-piston engines to be optimized in a very targeted way at one or only a few operating points with regard to the generation of ammonia. 
       FIG. 2  schematically shows a motor vehicle  62 , with a driving motor embodied as an internal combustion engine  64 ; in this vehicle, the free-piston engine  2  serves on the one hand as a preliminary stage for generating ammonia as a reducing agent for the nitrogen oxides contained in the exhaust gas of the internal combustion engine  64 , and thus serves the purpose of reducing emissions, and on the other is used as an “auxiliary power unit” (APU), in order to furnish the pressure required for high-pressure fuel injection into the internal combustion engine  64 . 
     For generating ammonia, the exhaust gas from the combustion chamber  16  of the free-piston engine  2  is delivered through the exhaust gas line  22  to an oxidation catalytic converter embodied as a noble-metal/ammonia catalytic converter  66 . In the noble-metal/ammonia catalytic converter  66 , some of the hydrocarbons contained in the exhaust gas are catalytically broken down, splitting off hydrogen, and the hydrogen is used for the catalytic synthesis of ammonia from molecules contained in the exhaust gas that contain nitrogen, such as atmospheric nitrogen or nitrogen oxides. To increase the content of uncombusted hydrocarbons in the exhaust gas that are required for synthesizing ammonia, a postinjection of fuel into the exhaust gas can also be done upstream of the catalytic converter  66 , at  68 . 
     The outlet of the ammonia catalytic converter  66  communicates with an exhaust system  70  of the internal combustion engine  64 ; besides an oxidation catalytic converter  72  and optionally a particle filter (not shown), the exhaust system includes an SCR catalytic converter  74 , in which the nitrogen oxides, contained in the exhaust gas from the internal combustion engine  64  and in the exhaust gas of the free-piston engine  2 , are reduced with high selectivity to nitrogen, in order to reduce the nitrogen oxide emissions from the motor vehicle  62 , with the aid of the ammonia synthesized in the catalytic converter  66  and delivered to the exhaust system  70  upstream of the SCR catalytic converter  74 . 
     To furnish the fuel pressure required for the high-pressure fuel injection into the internal combustion engine  64 , the work cylinder  44  of the free-piston engine  2  is supplied with fuel as its hydraulic fluid, the pressure of the fuel being increased in the work cylinder  44 . To that end, the low-pressure side of the work cylinder  44  communicates directly with the fuel tank  76  of the motor vehicle  62 , and this tank contains the fuel both for operating the internal combustion engine  64  and for operating the free-piston engine  2  and corresponds to the low-pressure working reservoir  50  in  FIG. 1 . The high-pressure side of the work cylinder  44  communicates with a pressure booster  78  that includes a plurality of pressure booster stages (not shown), with the aid of which the fuel pressure, increased in the work cylinder  44 , can be boosted to a desired level of between 300 and 2000 bar, before the fuel from the pressure booster  78  is delivered to a fuel injection system  80  of the internal combustion engine  64 .