Among the gaseous pollutants emitted by the diesel engine, NOx are the most significant. In this respect, the diesel engine is not very much behind the gasoline engine. NOx being most hazardous, the limit is set to 350 ppm in many countries. In many diesel engines, NOx varies from a few hundreds to 1000 ppm.
The mechanism of formation of NOx is the same as discussed in the petrol engine. The conditions which create the highest local temperature (2000 K) and have sufficient O2 give the highest NOx concentration in diesel engine too.
The pre-combustion chamber diesel engines produce less NOx than a direct injection engines because of low peak temperature. High fuel-air ratio (rich mixture), an additional fuel tends to cool the charge, and localised peak temperature falls and reduces the NOx emission.
In addition, injection pattern, injection period, cetane number of the fuel, viscosity, and rate of burning also affect NOx formation significantly. The effect of load on NOx emission rates for a four-stroke normally aspirated engine and four-stroke turbo-charged engines are shown in Fig. 11.26. The NOx emission for turbocharged engine is considerably high compared with a normally aspirated engine at all load conditions.
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