Autogas or LPG is liquefied petroleum gas (LPG) used as a fuel in internal combustion engines in vehicles , as well as in stationary applications such as generators .
It is a mixture of propane and butane .
Autogas is widely used as a „green” fuel because its use reduces CO-2 emissions in the exhaust gas by about 15% compared to petrol . One litre of petrol produces 2.3 kg of CO-2 when burned, while an equivalent amount of autogas (1.33 litres due to the lower density of autogas) burns only 2 kg of CO-2 . CO emissions are 30% lower compared to petrol and NOx emissions 50% lower. Its octane number (MON/RON) ranges from 90 to 110 and its energy content ( higher heating value -HHV) ranges from 25.5 megajoules per litre (for pure propane) to 28.7 megajoules per litre (for pure butane) depending on the actual composition of the fuel.
Autogas is the third most popular automotive fuel in the world. Approximately 16 million of the 600 million passenger cars run on this fuel, representing just under 3% of the total market share. Around half of all autogas-powered passenger vehicles are in the five largest markets (in descending order): Turkey, South Korea, Poland, Italy and Australia.

via Wikipedia