Abstract
Keywords: emission trends, emission development, legislative tests, real-world emissions, light vehicles
Within the last decade, vehicles for passenger transportation purposes have been continuously demanded to improve their impact on the environment caused by their operational discharge of noxious substances. These requirements mainly focused on passenger cars, as they were recognized as one of the main contributors to air pollution. The actions implemented in this regard resulted in valuably improved powertrain systems and elaborated solutions for exhaust after-treatment systems that allow operation with considerably lowered environmental impact. Emission monitoring on vehicles out of the traffic generally confirms this evolution, but also shows some discrepancies between certification and real-world emission behaviour.
In fact, a disagreement between test bench emission measurements employing the statutory cycles for vehicle certification and real-world cycles based on driving behaviour studies is recorded. Real-world vehicle emissions of subsequent certification categories improve clearly less pronounced and mostly exceed statutory emission levels. The improvements detected in the statutory test are therefore not equally reflected in real-world vehicle use. Besides, the emission limit utilization increases for most pollutants.
The current powertrain technologies also show some specific drawbacks: cold-start emissions of gasoline cars still represent the main hurdle to reach cleanest operation. CO2-reducing concepts like lean-burn combustion lead to increased formation of nitrogen oxides that cannot be handled with a three-way catalyst anymore. Implementing pollutant-specific after-treatment thus becomes necessary with the challenge to gain maximum operational efficiency, especially when based on a regenerative trap concept. In addition, particle discharge resulting from gasoline direct fuel injection also gains relevance.
Both these two process-related concerns count for state-of-the-art diesel propulsion technologies as well. There, however, regenerating particle traps are already an inevitable after-treatment technology to meet its present and future emission requirements. But some of the regeneration procedures and their frequencies of appearance valuably influence overall discharge of other pollutants. These cars also exhibit a rising share of NO2 within NOx in the exhaust as confirmed by roadside NO2 immision trends that, apart from its toxicity to humans, supports ozone formation, both alarming aspects regarding urban air quality.
The present paper addresses to the evolution and present status of the environmental impact of individual mobility as described above basing on respective test bench data derived from real-world vehicle samples of different certification classes up to Euro 4. It is evident that further technical efforts to improve conventional propulsion and after-treatment systems are needed to achieve near pollutant-free individual mobility and technological progress to gain CO2-neutral powertrain solutions.