Banned in Delhi for a decade, smog-spewing combustion engine–powered rickshaws are fading away in India and in many other countries, thanks not only to inroads by minivans, but also to  improved rickshaw motor designs coupled with laws to mothball dirtier models. In January, for example, Jakarta officials seized 30 unlicensed rickshaws.

Because such transport often lacks catalytic converters and is poorly maintained, lightweight two-stroke gasoline-powered three-wheelers (also known as tuk-tuks and tricycles) cough up roughly 13 times more lung-damaging particulates than other engine types. Such soot kills across Asia—both ending and shortening the lives of those most exposed.

But three-wheelers are a valuable cog in the sustainable transit chain: Provided older makes go green, according to a new analysis from Embarq, the transit unit of the World Resources Institute, an environmental group. Their merits include affordability, maneuverability in snarled cities—after accidents they often serve as ambulances—and accessibility for the disabled, elderly and women. Most important, their role thwarting auto use as "last mile" feeders by ferrying multiple passengers helps improve the planet's health by curbing greenhouse gas emissions.

View a slide show of rickshaws today and cleaner alternatives

For example, because rickshaws have less mass and smaller engines, their CO2 emissions are about one third those of private cars, the latter of which consequently contribute as much as 90 percent of India's total urban road passenger transport emissions. Moreover, "Since [rickshaws] wear out the road much less and use less materials in construction and operation, their contribution to global warming will be much less than that of a heavier car," explains Dinesh Mohan, a transportation expert at the Indian Institute of Technology Delhi.

Today, rickshaws make up 20 percent of urban trips in India but comprise less than 11 percent of all vehicles because they are shared by multiple riders. So what can be done to make the rickshaw cleaner? Several things, as it turns out.

Cleaning the air
Because air quality in half of India's cities suffers from particulate pollution 1.5 times global standards, a shift to greener rickshaws could help dent emissions there sizably. Despite nationwide car ownership predicted to soar 40-fold by 2050, rickshaw production in India doubled between 2003 and 2010 thanks to growing affluence and urbanization.

This growth suggests that the rickshaw could also play a similar emissions-cutting role elsewhere with better engine and communications technologies like Global Positioning System and cell-phones. Suburban Delhi's Radio Tuk Tuk, for example, is popular for its "dial-a-rickshaw" four-stroke fleet. So are electric tuk-tuks and pedicabs in Bangkok and Kathmandu.

"Regulatory reforms, technology, finance and operational improvements can help entrepreneurs scale up, so that cleaner rickshaws meet their full environmental and economic potential," says Embarq's Akshay Mani, the report's lead author. Embarq's rickshaw framework argues for "avoid-shift-improve" principles: avoid unnecessary trips, shift to more sustainable modes and improve performance in all modes—through policy, services and technology.

"If you want cleaner [three-wheelers], you have to introduce new technologies and eliminate polluting rickshaws," explains Sophie Punte, executive director of the Clean Air Initiative for Asian Cities, from Manila.

But scrapping older models isn't easy. Today, five million rickshaws ply India's streets with roughly 80 percent of these iconic open-air taxis still boasting two-stroke engines in midsize cities such as Pune, Rajkot and Surat; many of the rest either run on two-stroke compressed natural gas (CNG) engines or the larger and cleaner-burning four-stroke gasoline or CNG engines.

Here's how a two-stroke engine works: In the first stroke, a mixture of gas, lubricating oil and air in the cylinder is compressed by the upward movement of the piston, which ignites when the spark plug fires. In the second stroke, the ensuing combustion pushes the piston down. Exhaust gases next escape through an open port cleared by the downward moving piston as a result of the combustive pressure. As the piston presses down it turns the crankshaft to power the wheels. The resulting vacuum opens what is known as a reed valve, which lets in the next charge of air, fuel and oil from the carburetor. With the next upward compression stroke, the spark plug fires again, and the cycle repeats anew.

Because two-stroke engines are mechanically simpler, and thus cheaper to operate and repair, they are popular. But they are also dirty: unburned fuel and oil escapes through the exhaust port as soot, whereas the four-stroke engine more thoroughly burns the gas, which is not mixed with oil.

India's industry, for its part, is attempting to diversify the technology available for this transportation alternative. Rickshaw giant Bajaj Auto just announced its first four-wheeled rickshaw-like vehicle. It emits just 60 grams of CO2 per kilometer due to its 400-kilogram weight for low-speed city use, versus 150 grams and 1,300 kilograms for many highway-bound sedans, according to Bajaj. The four-stroke RE60 will also incorporate fuel-injection technology that could deliver 20 percent better fuel efficiency.

"Eliminating two-stroke models will prevent dumping them elsewhere or resale to poorer drivers," Punte says. But simple permit bans for older rickshaws often backfire, as many rickety two-stroke workhorses slip through on a thriving permit black market.

City leadership

To eliminate legacy rickshaws, local leadership is paramount. In 2005 in Ahmedabad officials fretted over India's seventh-largest city's air quality as nearly 300 new vehicles joined its roads daily, including rickshaws. So they mandated CNG rickshaw engine retrofits, installed CNG pumps citywide and facilitated bank loans for prospective rickshaw buyers. Result? In 2009 the city had dropped to the 66th most polluted in the nation, down from fourth in 2005. Today all its 115,000 rickshaws and 1,650 city buses use CNG—and other cities are emulating this CNG example.

And when laws don't work, personal appeals can. Consider the midsize city of San Fernando in the Philippines. In 2001 two-stroke models were 71 percent of 1,600 city-permitted rickshaws—some as much as 30 years old—and each spewing soot emissions on par with 10 jeeps. To encourage a four-stroke shift, Mayor Mary Jane Ortega offered free medical checkups, loans for green upgrades and maintenance seminars, with permit renewals linked to timely repayment. Although efforts were phased, drivers quickly traded up. Today all city rickshaws are four-stroke.

Tackling emissions from rickshaws are also part of global efforts to combat climate change. In February a U.S.-led coalition proposed limiting short-living pollutants like soot and methane because they offer a quick way to restrain global warming. The effort has earmarked funding for simple developing country actions such as adding filters to diesel engines and converting them to run on cleaner CNG fuel. All told, such efforts to restrain methane and soot emissions could help hold back global average temperature increases by more than 0.5 degree Celsius this century and improve public health. In essence, the effort would help other countries catch up to the U.S., where two-stroke engine lawnmowers were banned roughly a decade ago.