Consulting report

South Africa flagship on green mobility: Johannesburg Metrobus, Part II: Assessment of diesel dual-fuel engine bus

Zero-emission vehicles

This second paper featuring the Metrobus fleet in Johannesburg, South Africa presents an operational and environmental assessment of the Diesel Dual-Fuel (DDF) Program implemented in 2016 by focusing on fuel consumption and real-world emissions measured by portable emissions measurement systems (PEMS). DDF technology retrofits a diesel engine into a dual diesel and compressed natural gas (CNG) engine, and the authors find, on average, a 7% rate of substitution of CNG in place of more expensive diesel.

The PEMS tests reveal that, compared to the legacy Metrobus fleet, the DDF buses provide significant reductions in nitrogen oxide and carbon dioxide emissions. Although a 20-year analysis of global warming potential also shows that total greenhouse gas (GHG) emissions from the DDF buses are slightly lower than the legacy buses, excess methane emissions mean higher GHG emissions from the DDF buses than all other technology options available today. This suggests that DDF can be an effective intermediate technology toward soot-free, cleaner technology, but in order to deliver long-term reductions in the air pollution and GHG emissions of the fleet, Metrobus must move beyond Euro V DDF technology.