Emissions Characterization from Different Technology Heavy-Duty Engines Retrofitted for CNG/Diesel Dual-Fuel Operation 2015-01-1085

This study was aimed at experimentally investigating the impact of diesel/natural gas (NG) dual-fuel retrofitting onto gaseous emissions emitted by i) legacy, model year (MY) 2005 heavy-duty engines with cooled EGR and no after-treatment system, and ii) a latest technology engine equipped with DPF and urea-SCR after-treatment systems that is compliant with 2010 US-EPA emissions standards. In particular, two different dual-fuel conversion kits were evaluated in this study with pure methane (CH₄) being used as surrogate for natural gas. Experiments were conducted on an engine dynamometer over a 13-mode steady-state test cycle as well as the transient FTP required for engine certification while gaseous emissions were sampled through a CVS system.

Tailpipe NO_x emissions were observed at a comparable level for diesel and diesel/CH₄ dual-fuel operation for the 2010 compliant engine downstream the SCR. However, a reduction of ∼26% in diesel exhaust fluid (DEF) consumption was measured for dual-fuel operation, corroborating with engine-out NO_x concentrations that were observed to be ∼32% lower as compared to diesel-only operation, indicating that the addition of natural gas significantly decreased NO_x emissions formed during the combustion process. On the other hand, dual-fuel operation of the non after-treatment equipped, legacy engines showed conflicting NO_x emissions results. While NO_x emissions were observed to reduce for one of the legacy engines by ∼25%, dual-fuel operation of the second legacy engine resulted in a 6.6% increase in brake-specific NO_x.

Carbon dioxide emissions (CO₂) were observed to decrease across all steady-state engine load modes and by 3 to 8% over the transient FTP cycle with no specific trend towards an engine technology. Methane addition to the post-2010 engine, however, resulted in the highest overall CO₂ reduction for the given amount of CH₄ injected. Additionally, this study was focusing on evaluating possible greenhouse gas benefits (i.e. tank to exhaust) from diesel/NG retrofitting as a function of engine technology. It was observed that despite the reduction in CO₂ emissions the global warming potential (GWP) benefits of dual-fuel operation were outweighed by a significant increase in methane-hydrocarbon (MHC) emissions, specifically by factors of 1.6 and 2.3-4.8 for the post-2010 and legacy engines, respectively.