Towards the effort of using natural gas as an alternative fuel
for a diesel engine, the concept of Diesel Dual Fuel (DDF) engine
has been shown as a strong candidate. Typically, DDF's
engine-out emission species such as soot and nitrogen oxides are
decreased while carbon monoxide and hydrocarbons are increased. The
aftertreatment system is required in order to reduce these
pollutant emissions from DDF engines. Additionally, DDF engine
exhaust has a wide temperature span and is rich in oxygen, which
makes HC emissions, especially methane (CH₄), difficult to treat.
Until now, it is widely accepted that the key parameter influencing
methane oxidation in a catalytic converter is high exhaust
temperature. However, a comprehensive understanding of what
variables in real DDF engine exhausts most influencing a catalytic
converter performance are yet to be explored.
In the current study, a technique, called raw fuel injection, is
selected for heating up exhaust temperature via Diesel Oxidation
Catalyst (DOC) sufficient to treat methane. The benefits of using
this technique can be seen in the lower power consumption with
minimal complexity and costs in many diesel aftertreatment
operations. The system is installed into the tailpipe of a
four-cylinder turbocharged diesel engine which is converted into a
DDF engine operating under premixed natural gas and direct diesel
injections. The injector introduces vaporized diesel fuel into the
exhaust system in front of the first DOC. The second DOC is
installed downstream of the first DOC in order to treat methane.
Engine speed, lambda, engine-out exhaust temperature, and raw fuel
injection amounts are varied to investigate influences of raw fuel
injection characteristics on reducing methane.
Results showed that the exhaust temperature of 230°C was
sufficiently high for oxidizing diesel fuel in DOC. Once this
condition was met, the key important parameter on reducing methane
is not only fuel injection amount (resulting in exhaust temperature
variations), but also engine-out's lambda (oxygen concentration
in the exhaust) and exhaust species compositions. Fuel injection
amounts could be optimized with engine-out's exhaust
temperature and CH₄ conversion efficiency. The modified exhaust
characteristics via raw fuel injection demonstrated improvements in
reducing methane emissions from DDF engines.