The problem here is not the fuel type, it's the engine. The study found that there were high emissions of methane coming from the 2-strokes because as much as a third of the CNG is sometimes improperly burned. Methane is a greenhouse gas and has a global warming potential significantly higher than CO2. In addition, unburned lubricating oil in these engines produces substantial high particulate matter emissions.
The study also tested 4-stroke engines, and these had significantly lowered emissions across the board. In fact, according to the researchers, it would have been better for the environment – and cheaper overall – to simply upgrade the 2-stroke engined rickshaws to the cleaner gas-burning four-stroke models. Well, now we know.
[Source: Science Daily | Image: Carol Mitchell – C.C. License 2.0]