Experimental Investigation of Applying Raw Fuel Injection Technique for Reducing Methane in Aftertreatment of Diesel Dual Fuel Engines Operating under Medium Load Conditions 2011-01-2093

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.