Numerical Investigation of NO
2
Formation Mechanism in H
2
-Diesel Dual-Fuel Engine
2012-01-0655
The nitrogen dioxide (NO₂) emissions of compression ignition
diesel engines are usually relatively small, especially when
operated at medium and high loads. Recent experimental
investigations have suggested that adding hydrogen (H₂) into the
intake air of a diesel engine leads to a substantial increase in
NO₂ emissions. The increase in NO₂ fraction in the total
NOx is more pronounced at lower engine load than at
medium- and high-load operation, especially when a small amount of
H₂ is added. However, the chemistry causing the increased NO₂
formation in H₂-diesel dual-fuel engines has not been fully
explored.
In the present work, kinetics of NO and NO₂ formation in a
H₂-diesel dual-fuel engine are investigated using a CFD model
integrated with a reduced hydrocarbon oxidation chemistry and an
oxides of nitrogen (NOx) formation mechanism. A low-load
and a medium-load operating condition are selected for numerical
simulations. The experimental trends of NOx emissions
are reproduced with the numerical model. The effect of in-cylinder
chemical and thermal conditions on the formation of nitric oxide
(NO) and NO₂ is studied through a set of numerical simulations. It
is found that the evolution of in-cylinder HO₂ radicals and
in-cylinder mixture temperature are both responsible for the
observed trends in NO₂ emissions. The presence of increased levels
of HO₂ and reduced temperature of the combustion products provide a
favorable environment for conversion of NO to NO₂. Although the HO₂
radicals necessary for this conversion are produced mainly during
the mixing controlled diffusion combustion of H₂ and diesel, the
conversion of NO to NO₂ is mainly observed after the completion of
the main combustion process.