On-road and laboratory emissions of NO, NO2, NH3, N2O and CH4 from late-model EU light utility vehicles: Comparison of diesel and CNG.

Exhaust emissions of eight Euro 6 light duty vehicles - two station wagons and six vans - half powered by diesel fuel and half by compressed natural gas (CNG) were examined using both chassis dynamometer and on-road testing. A portable on-board FTIR analyzer was used to measure concentrations of reactive nitrogen compounds - NO, NO2 and ammonia, of CO, formaldehyde, acetaldehyde and greenhouse gases CO2, methane and N2O. Exhaust flow was inferred from engine control unit data. Total emissions per cycle were compared and found to be in good agreement with laboratory measurements of NOX, CO and CO2 during dynamometer tests. On diesel engines, mean NOX emissions were 136-1070mg/km in the laboratory and 537-615mg/km on the road, in many cases nearly an order of magnitude higher compared to the numerical value of the Euro 6 limit. Mean N2O emissions were 3-19mg/km and were equivalent to several g/km CO2. The measurements suggest that NOX and N2O emissions from late-model European light utility vehicles with diesel engines are non-negligible and should be continuously assessed and scrutinized. High variances in NOX emissions among the tested diesel vehicles suggest that large number of vehicles should be tested to offer at least some insights about distribution of fleet emissions among vehicles. CNG engines exhibited relatively low emissions of NOX (12-186mg/km) and NH3 (10-24mg/km), while mean emissions of methane were 18-45mg/km, under 1g/km CO2 equivalent, and N2O, CO, formaldehyde and acetaldehyde were negligible. The combination of a relatively clean-burning fuel, modern engine technology and a three-way catalyst has resulted in relatively low emissions under the wide variety of operating conditions encountered during the tests. The on-board FTIR has proven to be a useful instrument capable of covering, with the exception of total hydrocarbons, essentially all gaseous pollutants of interest.