The NOx emission is very sensitive to the engine combustion
temperature which increases with the increasing engine speed
and load. It is generally agreed that the higher the cylinder temperature,
the higher the NOx emissions. However to monitor the cylinder
temperature would be highly impossible, and some researchers
have been trying to find the correlation between engine exhaust
temperature and the NOx emissions since the exhaust temperature
is a good indicator of engine combustion temperature. For example,
Al-Shemmeri and Oberweis found that the NOx increase was
directly proportionally to the engine exhaust temperature [31].
However, even though our experimental results show that the exhaust
temperature decreases with the increasing biodiesel blend
ratio (refer to Fig. 5), it can be seen that the correlations between
the NOx emission and biodiesel blend ratio at 10% (Fig. 12a), 50%
(Fig. 12b) and 100% (Fig. 12c) loads are very random. Nevertheless,
it is still worth noticing that the NOx emission increases significantly
with the increasing engine load for all the tested fuels as
shown in Fig. 13