Abstract
Natural gas powered vehicles (NGVs) represent a promising approach to reduce vehicle CO2 and pollutant emissions. These vehicles use compressed natural gas (CNG) as engine fuel, profiting from its considerably lower carbon content per unit energy to save vehicle CO2 emissions. Another benefit of NGVs is that they can rely on approved powertrain and exhaust aftertreatment solutions that generally demand only minor adaptations to CNG. Finally, the environmental impact of NGVs regarding greenhouse gas emissions can be greatly reduced when employing methane gas from renewable sources.
Therefore, an experimental investigation on a chassis dynamometer test bench has been carried out with a sample of 13 in-use NGVs to determine their emission performance regarding CO2 and both regulated and unregulated gaseous pollutants. The vehicle sample consisted of 12 Euro-4 and 1 Euro-5 NGVs including original equipment manufacturer (OEM) completions and both OEM and external retrofits. Test runs with the statutory driving cycle for Europe NEDC have been performed as well as the European real-world driving cycles CADC and IUFC15. The latter two are based on car driving behaviour studies and reflect representative urban, rural and motorway driving.
The results obtained show an acceptable pollutant emission performance of the considered NGVs, having only 2 vehicles failing statutory hydrocarbon (HC) emission limit compliance. But emissions of HC and nitrogen oxides (NOx) are remarkable in real-world urban hot driving, indicating that the reduction of NOx in the catalytic converter employing HC as the oxidant may not occur entirely because HC emissions of NGVs mainly consist of methane, a powerful greenhouse gas that has a less pronounced catalytic oxidation activity. Besides, pronounced emissions of HC and also ammonia together with low emissions of NOx in high load and dynamic real-world rural and motorway driving indicate occasional fuel-rich combustion. The often low-end engine control systems used for NGVs until now and the fact that lambda sensors are cross-sensitive to methane are assumed to be responsible for this observation. In contrast, neither cold start nor hot emissions of carbon monoxide (CO) and non-methane hydrocarbons (NMHC) are critical. A comparison to sample emissions of 26 Euro-4 gasoline vehicles confirms these findings and also highlights benefits in CO2 emissions for the considered NGVs of around 21% for the Euro-4 sample and 33% for the Euro-5 vehicle.
It can be concluded that modern NGVs demonstrate their CO2 emission reduction potential compared to gasoline vehicles under real-world driving conditions with mostly improved pollutant emission levels. However, certain fuel-specific developments in the fields of engine control and exhaust aftertreatment are still to be undertaken to achieve best possible pollutant emission performance of NGVs.
Keywords: compressed natural gas, vehicle exhaust emissions, CO2, pollutant, real-world driving