Low temperature urea injection method

Abstract

The invention relates to a method for injecting urea in an exhaust line of an engine ( 1 ), the urea containing ammonia which is used for chemically reducing, during a selective catalytic reduction reaction, or SCR reaction, the nitrogen oxides produced by the engine, the injection being performed upstream of a catalyst ( 3 ) in which the reaction takes place, the process comprising the following steps: the temperature in the exhaust line of the engine is measured ( 7 ); an amount of urea to be injected is determined ( 5 ), from a known relation between the temperature and the amount of urea to be injected for all possible operating temperature values; a determined urea amount is injected ( 6 ) in the engine exhaust line.

1 . Method of injecting urea into an exhaust line of an engine, the urea containing ammonia to be used in a selective catalytic reduction reaction, or SCR reaction, to chemically reduce nitrogen oxides discharged by the engine, the injection taking place upstream of a catalyst in which the reaction takes place, the method comprising the following steps:
 measuring the temperature in the engine exhaust line,   determining, based on this temperature, a maximum quantity of urea that can be broken down into ammonia and thus be injected into the engine exhaust line with no risk of polymerization, and   injecting the quantity of urea thus determined into the engine exhaust line.   
   
   
       2 . Method according to  claim 1 , comprising the step of measuring the quantity of nitrogen oxides entering the catalyst, and using this measurement to determine the quantity of urea to inject. 
   
   
       3 . Method according to  claim 1 , comprising the step of measuring the quantity of ammonia exiting the catalyst, and using this measurement to determine the quantity of urea to inject. 
   
   
       4 . Method according to  claim 1 , comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject. 
   
   
       5 . Method according to  claim 1 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       6 . System for injecting urea into an engine exhaust line, the urea containing ammonia to be used in a selective catalytic reduction reaction, or SCR reaction, to chemically reduce nitrogen oxides discharged by the engine, the system comprising:
 a catalyst which is the site of the reduction reaction,   a device for measuring the temperature of the gases in the exhaust line, and   a device for calculating, based on a measured temperature, the maximum quantity of urea that can be broken down into ammonia and thus be injected into the engine exhaust line with no risk of polymerization, and   a urea tank and a urea injector located upstream of the catalyst.   
   
   
       7 . Method according to  claim 2 , comprising the step of measuring the quantity of ammonia exiting the catalyst, and using this measurement to determine the quantity of urea to inject. 
   
   
       8 . Method according to  claim 2 , comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject. 
   
   
       9 . Method according to  claim 3 , comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject. 
   
   
       10 . Method according to  claim 7 , comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject. 
   
   
       11 . Method according to  claim 2 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       12 . Method according to  claim 3 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       13 . Method according to  claim 4 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       14 . Method according to  claim 7 , comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject. 
   
   
       15 . Method according to  claim 8 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       16 . Method according to  claim 9 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 
   
   
       17 . Method according to  claim 10 , wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.