Natural gas has about the same emissions as China V standard gasoline, so in practice it is a cleaner energy than the poor quality gasoline and diesel used in many parts of the country, said Huang Fanqing, an analyst with consultancy IHS Automotive.

"But a common problem with converted cars is that they may lose efficiency and power when using gasoline," Huang said, adding that dual-fuel cars made by manufacturers have precise adjustments in the fuel system control and perform better than those converted by local shops.

Geely is among a few domestic carmakers that make gasoline-natural gas vehicles. Its spokesman Yang Xueliang said that the company has dual-fuel versions of the Emgrand EC 7 and Gleagle Yuanjing. The majority of sales are from corporate buyers such as taxi companies.

"These cars generally cost 6,000 or 7,000 yuan more than conventional versions," Yang said. "Production is based on orders and we don't have them at the showrooms."

It is unclear how many natural gas vehicles - including converted cars - are on the roads in China, but some estimates put the number around 1.5 million.

'Limited potential'

Analysts said that natural gas vehicles have only "limited potential" in China, especially in the consumer market, due to inadequate gas supplies and fueling stations.

They note that the vehicles are viable in locales close to gas fields or pipelines.

"It is unlikely natural gas vehicles would be used on a large scale in big cities like Beijing and Shanghai where the priority use of natural gas supplies is households," Zeng with LMC Automotive said.

According to a recent Associated Press report, natural gas is making inroads in the US as a transportation fuel, particularly for trucks, buses and taxis. Sales are also gaining ground in the consumer market, but the vehicles are still expected to account for less than 1 percent of the auto market in the next few years.

"At the end of the day, natural gas is still a conventional energy - it's a fossil fuel and can't be compared with electric cars where electricity could be generated by renewable sources like water, sunlight and wind," Zeng said.

hantianyang@chinadaily.com.cn