Varying emissions from buses using biodiesel in Madrid, Spain.

Lopéz et al. (2009) compared two types of emissions after-treatments on urban buses in Madrid.  The first treatment utilizes selective catalytic reduction (SCR) combined with urea, and the second uses exhaust gas recirculation (EGR) with a particulate filter.  The effects on greenhouse gas emissions of these two treatments were studied on buses using diesel fuel, fuel that is 20% biodiesel (B20), and 100% biodiesel fuel (B100).  Reductions in carbon monoxide (CO), carbon dioxide (CO2), unburned hydrocarbon (THC), nitrogen oxides (NOx), and particulate matter (PM) emissions, as well as fuel consumption varied according to treatment technology and fuel type. Jenny Ward
Lopéz, José María, and Felipe Jiménez, Francisco Aparicio, and Nuria Flores. 2009. On-road emissions from urban buses with SCR + Urea and EGR + DPF systems using diesel and biodiesel. Transportation Research Part D: Transport and Environment. 14, 1–5.

The authors used a driving cycle, designed by the Madrid Municipal Transit Company, which was developed for fuel economy and emission testing with on-board equipment.  The measurement device used was the Horiba OBS 2200, which collects data under real driving conditions.  Particulate matter amounts were measured using laser technology.  Measurements for each fuel type were obtained from five test runs within the driving cycle.
Lopéz et al. observed that between the two after-treatment technologies, SCR produced greater reductions in CO2 and NOx emissions, while EGR performed better according to CO and PM emissions reductions.  More importantly, varying trends were seen when diesel and biodiesel powered buses were compared.  Both the B20 and B100 fuels caused greater NOx and CO2 emissions and consumed more fuel than regular diesel buses, but the biodiesel buses did have a greater reduction in particulate matter emissions.  Despite increases in some greenhouse gas emissions, the authors recommend that biodiesel still be used as an alternative fuel source because “it is non-fossil, biodegradable, CO2 neutral and its combustion is sulphur oxide free”.

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