A numerical study was performed to compare the formation of
nitric oxide (NO) and nitrogen dioxide (NO₂), collectively termed
NOx, resulting from biodiesel and diesel combustion in an internal
combustion engine. It has been shown that biodiesel tends to
increase NOx compared to diesel, and to-date, there is no widely
accepted explanation. Many factors can lead to increased NOx
formation and it was of interest to determine if fuel chemistry
plays a significant role. Therefore, in order to isolate the fuel
chemistry from mixing processes typical in a compression ignition
engine, sprays were not considered in the present
investigation.
The current study compares the NOx formation of surrogates for
biodiesel (as represented by methyl butanoate and n-heptane) and
diesel (n-heptane) under completely homogeneous conditions.
Combustion of each fuel was simulated using the Senkin code for
both an adiabatic, constant volume reactor, and an adiabatic,
single-zone HCCI engine model. The fuel chemistry is represented
using an updated version of a mechanism that combines reduced
mechanisms for methyl butanoate and n-heptane. NOx chemistry is
predicted using a 19-step model that includes species and reactions
for both thermal and prompt NOx.
It was found that the biodiesel surrogate can cause a NOx
increase when compared to diesel surrogate, but the relative
increase was small (≺3%) for most equivalence ratios. The
differences in initial temperatures required to match ignition time
make it difficult to definitively link the NOx increase to the
oxygen in the fuel under these conditions. The largest NOx increase
(26%) was seen at near-stoichiometric conditions. However, it was
found that the fuel-bound oxygen in biodiesel did not increase NOx
to the extent that the same amount of oxygen would create if it
were available in the surrounding air. While the presence of O₂ in
the biodiesel surrogate does slightly impact NOx formation, it does
not appear to be a dominant factor for HCCI engines, where mixture
conditions are well below stoichiometric. In conventional diesel
combustion, where equivalence ratios are often above
stoichiometric, these results suggest that the fuel chemistry can
play a role in the observed NOx increase.