Installing a compressed natural gas kit converts a vehicle to using natural gas, a less expensive gasoline alternative, as fuel. As of 2015, CNG kits are gaining popularity because gasoline costs are high and few vehicles designed explicitly to run on natural gas are available to the public.
Compressed natural gas is mostly methane. It is highly pressurized and can be used in engines originally designed for liquid fossil fuels, such as gasoline or diesel.
A 2014 government-sponsored study by Stanford and MIT concluded that, while emissions from burning CNG are not as polluting as gasoline, the negative consequences of methane leaks from drilling for natural gas balance out whatever positive effect on climate change that CNG may have.