However, tailpipe-out NOx emissions are higher from diesels reflecting difficulties in reducing NOx to N2 in the highly oxidizing environment of diesel exhaust, which typically contains approximately 8% O2.
Engine out NOx levels are lower for diesel engines than their stoichiometric gasoline counterparts reflecting the somewhat lower combustion temperatures in the diesel engine. Road transport is the most important cause of urban NOx emissions worldwide contributing to more than 50 % of the NOx. NO is a colorless and odorless gas, while NO2 is a reddish brown gas with pungent odor. It is progressively converted to NO2 in atmospheric air. NOx are referred as nitrogen oxide (NO) and nitrogen dioxide (NO2). Increasing the temperature of combustion increases the amount of NOx as much as triple for almost every 100 oC increase.
This is when the flame temperature is the highest. Most of the emitted NOx is formed early in the combustion process, when the piston is still near the top of its stroke. The amount of produced NOx is a function of the maximum temperature in the cylinder, oxygen concentrations, and residence time. So the major influences of the formation of NOx are the temperature and concentration of oxygen in the combustion. However, high temperatures above 1,600 oC in the cylinders cause the nitrogen to react with oxygen and generate NOx emissions. Usually, in the diesel engine combustion process, the nitrogen in the air does not react with oxygen in the combustion chamber and it is emitted identically out of the engine. a number of harmful products are also generated during combustion. Bur for many reasons as: the air– fuel ratio, ignition timing, turbulence in the combustion chamber, combustion form, air–fuel concentration, combustion temperature, etc. For perect thermodynamic equilibrium, the complete combustion of diesel fuel generates only CO2 and H2O. The four main pollutant emissions from diesel engines are carbon monoxide-CO, hydrocarbons-HC, particulate matter-PM and nitrogen oxides-NOx. Diesel engines are one of the largest contributors to environmental pollution caused by exhaust emissions.
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