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Primary NOx Reducing Measures

These are measures aimed at reducing the amount of NOx formed during combustion by optimizing engine parameters with respect to emission. Some of the methods are listed below –
A. Water addition:-
  1. Direct water injection:- Greater heat capacity is utilized to reduce high peak temperatures as the water evaporates immediately upon injection. Rapid evaporation of injected water also helps to create a homogeneous fuel-air mixture.
  2. Emulsified fuel or fuel-water emulsion:- It is favored by some manufacturer claiming a clear reduction in NOx emission at low cost with no significant design changes.
B. Altered fuel injection:-
  1. Delayed fuel injection:- Retarded fuel injection timing retards the combustion process. Nitrogen oxide formation occurs later and with lower concentration.
  2. Common rail control:- It has proven to be a very effective way in combating NOx reduction techniques.
  3. NOx optimized fuel spray pattern:- Different fuel nozzle types and model injection also has an influence.
C. Combustion air treatment:-
  1. Exhaust gas circulation:- This is one method of adding dilutants to intake air, reducing the burned gas temperature for any given mass of fuel and oxygen. But it is more practical for engines burning cleaner fuel such as low sulphur and low ash fuel.
  2. Humidifying the scavenge air:- Injection of very fine water after the turbocharger using special nozzles. The fine water droplets evaporate fast and further heat is introduced in the air cooler and humidifies the combustion air. The drawback is too much humidity can be harmful to the cylinder condition.
  3. Reducing the amount of scavenge air, and hence reducing the amount of excess oxygen available for conversion to nitrogen oxide.
  4. Water-cooled rest gas:- In an electronically controlled exhaust valve timing, it is quite easy to leave some of the exhaust gas in the cylinder. This obviously has a negative impact on engine performance, however, this can be dramatically reduced by cooling the rest gas with a water spray, in which case the rest gas accounts for some of the NOx reduction and the water spray for the rest.


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