The use of biomass fuels for vehicles has been a focus of
attention all over the world in terms of prevention of global
warming, effective utilization of resources and local
revitalization. For the purpose of beneficial use of unused biomass
resources, the movement of the use of bioethanol and biodiesel made
from them has spread in Japan.
In Japan, biodiesel is mainly made from waste cooking oil
collected by local communities or governments, and in terms of
local production for local consumption, it is used as neat fuel
(100% biofuel) or mixed with diesel fuel in high concentration for
the vehicles. On the other hand, extremely low emission level must
be kept for not only gasoline vehicles but also diesel vehicles in
the post new long-term regulation implemented from 2009 in Japan.
It is necessary for diesel vehicles to equip an advanced type of
aftertreatment such as Urea-Selective Catalytic Reduction (SCR)
system or lean NOx trap (LNT) catalyst system in order to comply
with this regulation.
In this study, engine bench tests were conducted to understand
the emission characteristics in the use of high concentration of
biodiesel for an engine system with the urea-SCR system which is
expected to be equipped for a lot of heavy-duty vehicles in the
near future.
The results indicated that NOx emission in biodiesel operation
increased compared with that in conventional diesel operation under
Japanese JE05 mode test. This is because of the NOx emission
increasing in the engine out and the NOx reduction efficiency
decreasing in the urea-SCR system. Especially, B100 (Neat
biodiesel) increased NOx emission over the New Long-Term regulation
limit, even though this engine system complied with the new
long-term regulation level enough. This was mainly affected by the
decrease of NOx reduction efficiency in urea-SCR system due to the
decrease of NO₂/NOx ratio at the inlet of urea-SCR.
The factor of the decrease in NO₂/NOx ratio was considered to be
the decrease in NO₂ concentration derived from the reduction of
engine-out NO₂ emission and the deterioration of oxidizability of
diesel oxide catalyst (DOC). As for the deterioration of
oxidizability of DOC, it was thought to be due to the decrease in
exhaust gas temperature, catalyst poisoning and reduction action by
SOF adhered in DOC. Therefore, it was necessary to improve
oxidizability of DOC in order to decrease NOx emissions.
