Transit Bus Emission Study: Comparison of Emissions from Diesel and Natural Gas Buses

Nils-Olof Nylund, Kimmo Erkkilä, Maija Lappi, Markku Ikonen

    Research output: Book/ReportReport


    This study on the emission performance of both diesel and CNG buses was linked to a comprehensive national program on bus emissions carried out by VTT Processes in Finland. For emission testing of buses VTT is using a 2.5 meter dia. dynamic chassis dynamometer and a full flow constant volume sampler (CVS) system. VTT has been granted accreditation for its emission and fuel consumption measurements of heavy-duty vehicles.

    For the CNG section of the study, seven European vehicles, three diesel buses and four natural gas buses were evaluated for dynamic emission performance. All vehicles were model year 2002...2004 vehicles in prime condition. The diesel buses represented Euro 3 technology with electronically controlled injection. The exhaust after-treatment options were without exhaust after-treatment, with oxidation catalyst, and with continuously regenerating trap (CRT) (particle filter) particulate filter installed by the original equipment manufacturer (OEM). The diesel fuel used was ultra-low sulphur diesel. All CNG vehicles were equipped with catalysts, and the emission certification ranged from Euro 3 to enhanced environmentally friendly vehicle (EEV).

    For the evaluation of emissions, two dynamic duty cycles were used, the European Braunschweig and the US Orange County cycle. The list of emission components evaluated is comprehensive, including regulated emissions, unregulated gaseous components, chemical composition of particles, and even particle number and mass size distributions.

    The results demonstrate that regarding particle mass and number emissions, the CNG vehicles, on average, are equivalent to CRT filter equipped diesel vehicles. The particle matter (PM) emissions of both CRT diesel and CNG vehicles were some two orders of magnitude lower compared with the baseline diesel engine. No abnormity could be found regarding the numbers of nanoparticles emitted from CNG vehicles. The formaldehyde emission of the catalyst equipped CNG vehicles was low, as well as the emission of polyaromatic hydrocarbons (PAH) components. The genotoxicity of CNG emissions was extremely low, determined by the Ames mutagenicity tests and calculated as a reference value per unit of driven distance. As for NOx emissions, CNG vehicles provide similar or superior emission performance, depending on the emission certification class.

    The results for the unregulated emissions from this study are in conflict with some US studies showing high toxicity for natural gas exhaust. One explanation is that US natural gas vehicles normally are not equipped with catalysts, whereas all European manufacturers use exhaust after-treatment and sophisticated fuel injection on heavy-duty natural gas vehicles.
    Original languageEnglish
    PublisherVTT Technical Research Centre of Finland
    Number of pages63
    Publication statusPublished - 2004
    MoE publication typeD4 Published development or research report or study

    Publication series

    SeriesVTT Research Report


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