Low NOx Engine Design

• This is currently the primary abatement approach assumed, for example, by MARPOL Annex VI.
• The standards (not considered particularly demanding) are normally achieved by:
  • Careful design of combustion by the shape of the combustion chambers and auxiliary chambers.
  • Careful design of the gas flows into the cylinder to ensure appropriate mixing
  • Careful design of valve timing; and
  • Appropriate timing of fuel injection.

INDEPTH
The optimisation is complex; the general approach is to achieve mixing to avoid high peak combustion temperatures at flame fronts, which is where NOx forms most easily. The certification also places constraints on the changes and modifications that can be made to the engine, such as timing of injection or valves.

Certification is done using distillate fuels in a defined and controlled cycle. Once operating, however, different fuels and different operating practices tend to mean that NOx emissions are higher than the certified limits.