Straight from the NGPSA
NGL (natural gas liquids)
Natural gas liquids are those hydrocarbons liquefied at the surface in field facilities or in gas processing plants. Natural gas liquids include ethane, propane, butanes, and natural gasoline.
LNG (liquefied natural gas)
The light hydrocarbon portion of natural gas, predominately methane, which has been liquefied. LNG is natural gas in a liquid form. Its temperature is usually less than -100 Degrees F.
No, using straight NGL's in a car won't work because of all the molecules lighter than butane will just boil away. In the old days, we would "weather" the NGL letting those butanes and lighter evaporate. The remaing natural gasoline had about 75 octane, what a ride in your junker car with 8 to 1 compression ratios. To stop the car after driving, you got out, open the hood and put a piece of wood over the air intake because the engine was dieseling.
In the eary 80's, the tax man figuired it out and the back taxes on the fuel used was reason enough to stop this practice.