The next step is to add hydrogen into the mix. Early in 2007 a station will begin offering Hythane (hydrogen + methane, the main component of natural gas). Initially they will be using a mix of 10 percent hydrogen/90 percent CNG as well as offering straight hydrogen for experimental vehicles that can operate on it. The hydrogen blend will reduce NOx and particulate emissions by about 50 percent. All the CNG equipped vehicles are able to operate on the 10 percent hydrogen mix without further modification. Higher percentage mixes require some additional modification, and vehicles equipped to handle up to 30 percent hydrogen will start entering service in 2007. The hydrogen is four times the cost of CNG but has three times the specific energy, so the improved mileage should mitigate most of the cost.
[Source: Hindustan Times, tip from Manu Sharma ]