Evaluation of emission toxicity of urban bus engines: compressed natural gas and comparison with liquid fuels

Emissions from a spark-ignition (SI) heavy-duty (HD) urban bus engine with a three-way catalyst (TWC), fuelled with compressed natural gas (CNG), were chemically analyzed and tested for genotoxicity. The results were compared with those obtained in a previous study on an equivalent diesel engine, fuelled with diesel oil (D) and a blend of the same with 20% vegetable oil (B20). Experimental procedures were identical, so that emission levels of the CNG engine were exactly comparable to the ones of the diesel engine. The experimental design was focused on carcinogenic compounds and genotoxic activity of exhausts. The results obtained show that the SI CNG engine emissions, with respect to the diesel engine fuelled with D, were nearly fifty times lower for carcinogenic polycyclic aromatic hydrocarbons (PAHs), twenty times lower for formaldehyde, and more than thirty times lower for particulate matter (PM). A twenty to thirty fold reduction of genotoxic activity was estimated from tests performed. A very high reduction of nitrogen oxides (NOX) was also measured. The impact of diesel powered transport on urban air quality, and the potential benefits deriving from the use of CNG for public transport, are discussed. Keywords Natural gas emissions; diesel emissions; diesel exhaust mutagenicity; heavy duty engine; polycyclic aromatic compounds.