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why diesel engine need to add urea?
Mar 29, 2017

Why diesel engine need to add urea?

The main reason why diesel engine need to add urea is that urea can have reactions with oxynitride.

The urea can have two main effect in the engine. First, it can lower the emission standard. Second, it can bring the pollution under control. The pretreatment can effect the permanent and the later treatment can alleviate the symptoms. The emission of the engine are mainly NOX, particle PM and HC. The CO and HC of diesel engine are less than gasoline engine. Well, the PM are more than gasoline engine.

The emission standard of diesel engine and gasoline engine are:

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In the earlier 90s, a lot of techniques are assisted under electronic control and they have been developed rapidly. It can realize the purification performance through the improvement of combustor, injection characteristics and air inlet and air outlet. But whenever in Europe or China, the fourth stage has been a turn point. It is impossible to meet the standards by totally depending on the pretreatment. As the evidence of complete burn or not, the CO does not change its standards after several stages. So it can clearly improve that it is a hard thing to continue optimize the burn and solve the emission.

It has been noted earlier that the emission of diesel engine are mainly NOX and PM. The particle treatment relies on DPF, and NOX mainly depends on catalytic reduction. NOX are main emission.

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The NOX are produced under high temperature.

The temperature of gasoline engine are more higher than diesel engine, about 100-200. Well they are nearly same during later pretreatment. Whenever diesel engine or gasoline engine, the combustion process can be divided into ignition delay period, quickly burn period and later burn period.

In the later period, because of the emission of quickly burn, slow burning period and intermediate products, the oxygen concentration are descend. If the oxygen concentration are insufficient, the combustion of remaining fuel are in rapid deterioration. And the result is that the economy efficiency decrease and the particle emission increase, etc.

The method of using urea can be called Selective Catalytic Reduction(SCR). The SCR can be some amine substance, for example, NH3,NH4OH, (NH2)2CO, and etc.

During the temperature of 250-500, the equations are followings:

4NO+4NH3+O24N2+6H2O

6NO+4NH35N2+6H2O

2NO2+4NH3+O23N2+6H2O

6NO2+8NH37N2+12H2O

If the temperatures are low, the equations are as followings

7O2+4NH3 4NO2+6H2O

5O2+4NH3 4NO+6H2O

2O2+2NH3 N2O+3H2O

3O2+4NH3 2N2+6H2O

Above 200 temperatures, the equations are as followings

(NH22CO+H2O 2NH3+CO2

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