|Emissions ratios from two-wheelers and cars in the Swiss fleet. A number >1 indicates more emissions from the two-wheelers. Click to enlarge.|
Motorcycles collectively emit 16 times more hydrocarbons, three times more carbon monoxide and a “disproportionately high” amount of other air pollutants compared to passenger cars in the Swiss fleet, according to a Swiss study to be published in the Jan. 1 issue of the American Chemical Society’s journal Environmental Science & Technology.
The study, by Ana-Marija Vasic and Martin Weilenmann of the Swiss Federal Laboratories for Materials Testing and Research, found both two- and four-cycle motorcycle engines emitted significantly more of these pollutants than automobile engines.
The researchers measured exhaust emissions of CO, HC, NOx, and CO2 from eight two-wheelers and compared them to previous measurements from 17 gasoline-powered Euro-3 compliant passenger cars performed at EMPA with the aim of ascertaining their relevance.
Several comparisons show that the powered two-wheelers on the market in 2001 produced significantly higher emissions of all pollutants except CO2 than gasoline-powered passenger cars from the same sales period. Whether in a direct comparison of mean unit emissions (in g/km), mean yearly emissions (in kg/vehicle/year), or fleet emissions (in tons/year) [calculated for the Swiss vehicle fleet], the two-wheelers’ HC and CO emissions were all, and often significantly, higher. In addition, the NOx contribution of the motorcycle fleet is roughly one-fifth that of the car fleet and is thus not negligible.
Motorcycles aren’t a primary means of transport in most developed countries, the authors note. As a consequence, they say, the importance of [motorcycle] emissions has been underestimated in legislation, giving manufacturers little motivation to improve aftertreatment systems.
Even though the motorcycle fleet is small in comparison with the car fleet, and logs lower yearly mileage per vehicle, their contribution to traffic emissions has become disproportionately high.
Present-day aftertreatment technologies for motorcycles are not as efficient as those for cars. Until recently, for instance, US emission standards for highway motorcycles hadn’t been updated in 25 years.
That regulatory situation is about to change, but more attention is required, according to the authors.
Even if they account for a comparatively small number of vehicles, motorcycles’ impact on traffic emissions cannot be overlooked. Directive 2002/51/EC of the European Parliament and Council is a step in the right direction. With the introduction in 2006 of new emissions limits which are intended to correspond to Euro 3 gasoline cars, and with checking procedures for the correct operation of emission control systems, motorcycle emissions are expected to decrease.
However, the fact that more than half of the two wheelers [in the research] failed the statutory test is indicative of the need for periodical inspection and maintenance.
With regard to this study, the introduction of similar regulations as for passenger cars such as checking the durability of the aftertreatment system and periodic testing of exhaust gases should be considered. It would therefore be expedient to repeat this study two to three years after introduction of the new rules.
Comparison of Real-World Emissions from Two-Wheelers and Passenger Cars; Ana-Marija Vasic and Martin Weilenmann; Environ. Sci. Technol., ASAP Article 10.1021/es0481023 S0013-936X(04)08102-7