On exhaust emissions from petrol-fuelled passenger cars at low ambient temperatures: Dissertation

The study at hand deals with regulated and unregulated exhaust emissions from petrol-fuelled cars at low ambient temperatures with present-day or near-future exhaust after treatment systems. The subject has been investigated at VTT over a decade and this report compiles data from various sub-studies carried out between the years 1993 - 1997. Each one of them viewed different aspects of the phenomenon, like determining the low-temperature response of today's new cars employing three-way catalytic converters or assessing the long-term durability and the influence of vehicle mileage upon the low-temperature emissions performance. Within these studies, together more than 120 cars of model years from 1990 to 1997 have been tested. Most of them were normal, in-service vehicles with total mileages differing between only a few thousand kilometres for new cars up to 80,000 km or even more for the in-use vehicles. Both the US FTP75 and the European test cycle have been employed, and the ambient temperatures ranged from the baseline (+22 °C) down to ± 0 °C, -7 °C and in some cases even to -20 °C. The studies attested that new cars having today's advanced emissions control systems produced fairly low levels of emissions when tested in conditions designated in the regulations that are the basis of the current new-vehicle certification. However, this performance was not necessarily attained at ambient temperatures that were below the normative range. Fairly widespread response was recorded, and cars having almost equal emissions output at baseline could produce largely deviating outcomes in low-temperature conditions. On average, CO and HC emissions increased by a factor of five to 10, depending on the ambient temperature and vehicle type. However, emissions of NOX were largely unaffected. Apart from these regulated emissions, many unregulated species were also determined, either by using traditional sampling and chromatography methods or on-line, employing the latest FTIR technology. Overall, the levels of these emissions were also mostly elevated at subnormal temperatures. Total vehicle mileage seemed not to affect cold-start emissions (CO and HC) at low temperatures. Nor did the overall durability of the emission control system appear to be worse in cold-climate conditions typical for Finland. The deterioration of the emissions performance in the tested vehicles either closely followed the average trend defined by the normal, assigned deterioration factors or was even lesser. The conclusions of this report underline the necessity of a separate low-temperature test in order to really effectively curb real-world emissions. Standards at normal temperature are no more effective alone, but need to be accompanied with additional requirements for good performance also in conditions closer to the everyday use, which comprises many cold-starts even in low ambient temperature conditions.