F2010A035
Potential of Natural Gas Powered Vehicles in Reducing CO2 and Pollutant Emissions under Real-World Driving Conditions
The need to reduce CO2 in parallel to pollutant emissions caused by individual mobility has become a predominant task in the development efforts of vehicle manufacturers. As a result, varied powertrain concepts aiming at meeting these requirements are subsequently entering the market. One promising short-term approach is to use compressed natural gas (CNG) as engine fuel. Its lower carbon content per unit energy allows reducing CO2 emissions significantly. Another advantage represents the fact that reliable and approved technical solutions for energy conversion and exhaust after-treatment can be employed with minor specific adaptations to this fuel. Further, the possibility of employing methane gas from renewable sources is feasible and established for these vehicles without considerable additional technical constraints. Therefore, an experimental investigation on a roller test bench has been carried out with such vehicles to determine their emission performance of CO2 and both regulated and unregulated gaseous pollutants. The vehicle sample consisted of 13 in-use CNG vehicles of certification category Euro-4 (E4) including original equipment manufacturer (OEM) models 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 demonstrate that the vehicles considered feature an acceptable emission performance, having only 2 vehicles failing statutory total-hydrocarbon (THC) emission limit compliance. Emissions of the latter are also considerable during real-world hot driving. There, emissions of nitrogen oxides (NOx) are also mostly more pronounced indicating that the reduction of NOx employing THC as the oxidant in the catalytic converter may not occur entirely. This circumstance can be attributed to the fact that THC emissions mainly consist of methane, a powerful greenhouse gas that does not easily oxidize catalytically. Besides, the presence of ammonia (NH3) in the exhaust, which typically appears in slightly rich operation, also indicates that the fuel management of these cars does not operate optimally in some occasions. The until now often used low-end engine control systems and the fact that lambda sensors are cross-sensitive to methane might be responsible for that. In contrast, neither cold start nor hot emissions of carbon monoxide (CO) are critical. A comparison to sample emissions of 26 petrol (P) vehicles of the same certification category Euro-4 confirms these findings and also highlights benefits in CO2 emissions of the CNG vehicle sample of around 20%. Additionally, the CNG sample generally shows less variation in single emissions of CO2 and pollutants. This observation is linked to the smaller sample distribution of displacement, power and mass of the CNG sample. It can be concluded that CNG powered vehicles confirm their CO2 emission reduction potential under real-world driving conditions with a comparably good pollutant emission performance compared to petrol vehicles. However, some fuel-specific development in the field of engine control and exhaust after-treatment is still to be undertaken to achieve an overall good operation of such vehicles.
This abstract is supplemented by a PDF, which can be viewed here.
Session: Consumption and Emission Reduction