Moped emissions 'more harmful than cars'

Emissions from mopeds are even worse than cars in some areas, according to a new report from the Paul Scherrer Institute, which claims that many of the benefits of the two-wheeled transport are offset by the detrimental environmental impact.

The new study suggests that, contrary to popular belief that cars and lorries are the worst polluters on the roads, it is actually mopeds, whose two-stroke engines are the main source of fine particles and other air contaminants in towns across the world.

An international research team at the Institute found that the key reasons for the high emissions are the combustion properties in two-stroke engines and the "overly lenient emission requirements" for small two-wheelers in many local authority areas.

The findings, published in the journal Nature Communications, show that although mopeds are small, low in consumption and city-friendly, the models with two-stroke engines are at the top of the list of air polluters in some towns, mainly in Asia, Africa and southern Europe. 

During their research, scientists used a custom smog chamber to measure the emission of organic aerosols and aromatic hydrocarbons from both mopeds and in standard driving cycles. 

They found that, even when standing still, mopeds with two-stroke engines emit amounts of gaseous substances known as arenes, which are several orders of magnitude higher than the limit values admissible in Europe and the USA. 

According to the study authors, waiting behind a two-stroke moped in traffic may, therefore, constitute a considerable risk to health. 

They state: "Basically, these are old, well-known problems which are typical for two-stroke engines like incomplete combustion, the high ratio of fuel to air in the fuel mixture or the need to add the lubricating oil directly to the fuel. Problems of this kind only occur to a minor degree if at all with four-stroke engines."

The researchers have calculated that in the Thai capital Bangkok, two-stroke mopeds generate as much as 60 per cent of emissions of primary organic aerosols, despite only accounting for ten per cent of fuel consumption in the city.

They concluded that the global concentration of specific air contaminants could be "considerably reduced" if two-stroke mopeds were gradually withdrawn from circulation, or at least replaced with four-stroke engines.