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.
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.