Patent Publication Number: US-2007108039-A1

Title: Control of air flow for apparatus to produce reduction agents

Description:
The invention concerns a device to generate ammonia for the selective catalytic reduction of nitrogen oxides in the exhaust of an internal combustion engine with an air supply.  
      An appropriate secondary treatment of exhaust gases is required in connection with future legal requirements regarding the emissions of nitrogen oxides. The selective catalytic reduction may be used to reduce NOx emissions (NOx reduction) of internal combustion engines, specifically diesel engines, with generally predominantly clean exhaust, i.e. rich in oxygen. This process adds a defined amount of a selectively acting reduction agent to the exhaust. This may take the form of ammonia, which is added directly in gaseous form or which is obtained from a precursor solution, such as urea, or from a urea-water solution (HWL). The disadvantage of the use of HWL is that HWL is consumed during the operations of the combustion engine. The rate of use is about 4% of the fuel consumption. The supply of urea-water solutions must therefore be diffuse, such as in gasoline/fuel filling stations. It is also disadvantageous that HWL must be carried in the vehicle.  
      Thus, U.S. 2004/0168905 A1 proposes to generate ammonia from nitrogen oxide obtained from open air in a gas-discharge plasma and fuel-rich fuel-air mixture in a catalyst. This uses only inputs already carried in the vehicle or obtainable from open air. A sufficient reduction of NOx emission requires that the air input into the plasma generator be tightly controlled and that the gas flow containing ammonia into the exhaust also be tightly controlled. For good exhaust cleaning, these airflows should be within +/−5% of the intended value. Given the various operating level of the internal combustion engine, the exhaust pressure may vary between 0 and 400 mbar, where the high value is obtained under full load. Furthermore, the load changes cause pressure changes in periods of less than one second. The air inflow systems of the current state of the arts cannot fulfill these requirements.  
      It is the objective of this invention to create an air inflow system that facilitates a sufficiently precise and fast adjustment of air inflow of an ammonia-generating device.  
      The objective is achieved by adding a blower for a compression step for the air inflow that is independent of the speed of the internal combustion engine. This allows for an efficient production of reduction agents under varying operating conditions of the internal combustion engine and for adding the correct amount of reduction agent to the exhaust to be cleaned.  
      A particularly safe operation with components that have been proven in durability and reliability proposes to use a turbine, a positive displacement pump, or a rotary pump for the compression step.  
      If the rotation step is powered with electricity, the control can adjust to changing operating conditions particularly quickly and the air inflow can operate independently of the internal combustion engine. An electric motor that powers the compression step can be powered by direct current from the vehicle net and may be embodied as a standard direct current motor or an electronically commutated motor. The power supply may also use pulse-width modulation. If the air input is designed to include an electronic control unit with a temperature probe and/or a pressure probe and/or a flow meter, the control unit can identify the air flow at the output of the air inflow with precision and can adjust it to the current operating status of the internal combustion engine and modify the compression status accordingly. The individual components of the electronic control unit may be linked with a CAN bus (CAN=Controller Area Network) to each other and to other components of the control system of the internal combustion engine and they may include a self-diagnosis function.  
      A simplified structure of the vehicle electronic system incorporates the electronic control unit into the control system of the internal combustion engine and/or into the control system of the reduction agent supply.  
      If a check valve and/or a pressure regulating valve and/or an air flap are incorporated into the inflow system at the output of the compression stage, it is feasible to preclude the flow of exhaust into the reduction agent generator, when the air inflow system is turned off. The use of a pressure-regulating valve permits a purely mechanical control of the air inflow, which saves the costs of an electronic control.  
      If the air inflow system contains a compression stage with constant output pressure, the output pressure may be set to match the maximum requirement for the output pressure, such that the control of the compression step is reduced to turning it on and off. This simplifies the control system.  
      The useful life of the compression stage may be extended by combining the input of the compression stage with the output of the compression stage for air input into the internal combustion engine. In many operating situations of the internal combustion engine, the output of the compression stage for air input is sufficient and the compression stage in the air inflow supply system does not need to be operated. 
    
    
      The invention is described in more detail in the following by reference to the embodiment examples depicted in the figures. They show:  
       FIG. 1 a  diagram of the pressure in the exhaust channel in a driving cycle,  
       FIG. 2 a  speed diagram based on US06 Supplemental Federal Test Procedure,  
       FIG. 3  an internal combustion engine with an ammonia generating system,  
       FIG. 4  the ammonia generating system with air suction behind an intercooler,  
       FIG. 5  the ammonia generating system with a mechanical pressure control. 
    
    
       FIG. 1  shows a pressure diagram  30  for the exhaust pressure in an exhaust manifold or the exhaust pipe of an internal combustion engine during an actual driving cycle. The pressure axis  31  and the first time axis  32  with second intervals show the pressure upstream of a particle filter  34  and the pressure downstream of a particle filter  35 . Furthermore, the rpm axis  36  shows the motor rpm  33 . The motor rpm  33  vary during the drive between 750 rpm and 4500 rpm. The pressure upstream of particle filter  34  is as much as 500 mbar higher than the ambient air pressure at high motor speeds, where the pressure may vary as much as 100 mbar within one second.  
       FIG. 2  shows the speed diagram for a high-load cycle  40  under US06 Supplemental Federal Test Procedure for aggressive highway driving. The speed axis  41  identifies the driving speed  42  in miles per hour by reference to a second time axis  43 . There are several fast changes of speed  42 , which lead to comparable variations of the operating conditions and are shown in  FIG. 1  for motor rpm  33  and thus also for the pressure upstream of particle filter  34 .  
       FIG. 3  shows an internal combustion engine  20  with an exhaust pipe  25  and an exhaust system  26  and a SCR catalyst  28  (SCR=Selective Catalytic Reduction) for the reduction of nitrogen oxides. Exhaust system  26  is connected to a generator  24  of a reduction agent, which is supplied with air by way of air supply  10 . The input of air supply  10  connects an air intake  22  with an air filter  21 , which also supplies air to the internal combustion engine  20 . Air intake  22  feeds air into a compression stage  11 , from where air is fed through check valve  13  and pressure pipe  23  to the generator  24  of a reduction agent. The compression stage may also be embodied as a piston pump powered by an electric motor or as a rotary pump. In this invention, the power supply is independent of the speed of internal combustion engine  20 . Pressure pipe  23  includes a temperature probe  15  and a pressure probe  16 , which are connected to control unit  12  by means of the signal feed  18 . Control unit  12  also controls the compression stage  11  and is connected with a CAN Bus controller  19 . During operations, control  12  controls compression stage  11  such that it generates enough airflow in pressure pipe  23  that the down-stream generator  24  of a reduction agent produces the proper amount of the reduction agent required in the current operation of the internal combustion engine to clean the exhaust. The airflow is monitored by checking the temperature with temperature probe  15  and the pressure with pressure probe  16 . The check valve  13  prevents any exhaust from flowing back through exhaust system  26  into air supply  10 , when the generator  24  of a reduction agent is idle.  
       FIG. 4  shows air supply  10 , where the airflow is monitored by a flow meter  17 . This flow meter is connected by means of signal feed  18  to control  12 , which adjusts the speed of compression stage  11  as needed. The inflow of compression stage  11  is taken from air supply  22  from a turbocharger with charge-air cooling  27 . This unit supplies air at pressures between 100 and 1300 mbar pressure. Thus, the compressed air may suffice at many operating levels of internal combustion engine  20 , and control  12  may turn off power to compression stage  11 . In such cases, control  12  uses an air baffle  14  to modify the pressure in pressure pipe  23 .  
       FIG. 5  shows an embodiment of air supply  10 , where control unit  12  merely turns compression stage  11  on and off, but does not modify the pressure value. The pressure is adjusted mechanically by check valve  13 , which is designed for an opening pressure close to the maximum system pressure. This balances pressure variations. An alternative embodiment designs the compression stage such that its output pressure in pressure pipe  23  is close to the maximum system pressure, which would eliminate the need for an electronic component of control  12  as well as a pressure-related function of check valve  13 .