Estimates of the emission rates of nitrous oxide from light-duty vehicles using different chassis dynamometer test cycles

Abstract

Nitrous oxide (N2O) is an important greenhouse gas (GHG) that can be formed over a catalyst in the vehicle exhaust. In this study, a total of 60 vehicles ranging from non-catalyst to super ultra low-emission vehicles (SULEV) technologies were tested over the Federal Test Procedure (FTP), with a subset of vehicles also tested over the cold start ST01, the hot running ST01, and more aggressive cycles, such as the US06 and Modal Emissions Cycle (MEC01v7). The N2O emission rate was highest for the earliest catalyst technologies and declined for more advanced technology vehicles. Of the 60 test vehicles, nearly half (25) of the vehicles had FTP N2O emission rates below 10 mg mile−1, while the emission rate of the remaining vehicles varied significantly depending on the specific vehicle control technology, operating, cycle and sulfur content of the gasoline. Real-time data showed that N2O emissions are primarily formed during the early period of catalyst light-off, and decline significantly as the catalyst reaches its equilibrium temperature. Only cycles with a cold start component, such as the cold start ST01 and FTP, showed any substantial N2O emissions. N2O emissions for aggressive cycles, such as the US06 or MEC01v7, or cycles where the catalyst was always at operating temperature, were near detection limits. Increases in fuel sulfur from 30 to 330 ppmw were found to increase N2O emissions by almost 4 times over the FTP and US06, while increases from 5 to 150 ppmw in fuel sulfur increased N2O by between 3 and 8 times depending on the cycle. It is concluded that gasoline sulfur, control technology and start conditions must all be considered in estimating N2O emissions inventories.