Two-stroke vehicles are the worst polluters

Six per cent of bikes and 70 per cent of three wheelers run on two-stroke engines in India

By Vivek Chattopadhyay
Published: Thursday 15 May 2014

Waiting in traffic behind a two-stroke scooter, at junctions and while the scooter is idling, can be highly deleterious to health

Two-stroke scooters and mopeds are a significant source of vehicular pollution. According to a study by researchers of Paul Scherrer Institute in Switzerland, low-powered vehicles are a major source of toxic air pollutants such as particulate matters, secondary organic aerosols and aromatic hydrocarbons. These vehicles form a small fraction of the total fleet of vehicles in many towns of Asia, Africa and southern Europe, yet they emit more air contaminants than cars and trucks.

The researchers measured the emissions from different category of vehicles and from different regions and found that the exhaust from two-stroke scooter was as high as 300,000 micrograms per cubic metre (or 146 parts per million by volume). Explaining the severity of the amount of emissions, the researchers say, “Occupational safety recommends that workers wear special breathing equipment when exposed to benzene (a type of aromatic hydrocarbon) at levels exceeding 1 ppm for 15 minutes. Waiting in traffic behind a two-stroke scooter, at junctions and while the scooter is idling, may therefore be highly deleterious to health.”

Read more on air pollution in India

The study, published in Nature Communications on May 13, 2014, has also quantified secondary organic aerosol production from two-stroke scooters. The exhaust gases from two-stroke vehicles also form potentially toxic ‘reactive oxygen species’, it notes.

To understand whether banning two-strokes can have any impact on pollution levels, the researchers conducted field studies in two cities of China—Guangzhou and Dongguan. In Guangzhou, the concentration of aromatic hydrocarbons or arenes in the air reduced by over 80 per cent in 2005 following a ban on two-stroke mopeds. On the other hand, Dongguan struggles to bring down the concentration of arenes despite strict traffic restrictions in place. The researchers thus note that the concentration of specific air contaminants in southern European towns could be considerably reduced if two-stroke mopeds are phased out. They also recommend introducing tighter norms and use of alternative technology. Four-stroke engines, although not completely harmless, would be better than two-strokes, they say.

India needs to pulls up its socks

In India, the share of two-stroke engines is declining in two-wheelers. The market share of two-strokes in India is about 6 per cent of the total two-wheelers sales. But 70 per cent of all petrol three wheelers are powered by two-stroke engines in the country, which is a concern.

Indian policy with regards to lubricating oil has evolved only in the last decade. Though in July 1998, the Supreme Court intervened and asked refueling stations to dispense pre-mixed lubricants and petrol through meters, it is being done only in 15 large cities.

So the need of the hour is to tighten emission norms. At present two and three wheelers adhere to Bharat Stage III emission standard. But India must move to the next stage of Bharat Stage IV at the earliest possible. But the government has not yet decided when to introduce BS IV standards. Besides, India needs to set separate target for hydrocarbon (HC) and nitrogen oxides (NOx), which will ensure better control over the pollutants. At present, the norms require combined standards of HC+NOx. Combining the norms gives cushion to both two and four stroke vehicles; two strokes are known to emit higher HC and four strokes are known for higher NOx.

Some cities in India have restricted registration of two stroke three wheelers. For instance, Delhi banned registration of two-stroke auto rickshaws in the national capital territory from May 1, 2002. In Kolkata, the high court directed to phase-out all two-stroke three wheelers and to replace them with four-stroke three wheelers running on clean fuel. Should such bans be extended to other cities?

Report: Motorized two-wheelers in Indian cities: a case study of the city of Pune

Report: Managing two and three-wheelers in Asia

Report: Proposals for emission controls on petrol two-stroke engine driven two- and three-wheelers and other issue

Document: Kolkata cracks whip on two-stroke three-wheelers

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