Common plastic is actually made from fossil fuels. It's possible to convert plastic into a liquid fuel. Heating the plastic makes a toxic flammable gas that is then condensed to a liquid fuel. The result is a liquid fuel burnable in an internal combustion engine, and this liquid fuel is sourced from what would otherwise be filling a landfill. Some companies are setting up commercial distilling operations to remove plastics from waste streams to create fuel.
CEDP 2012 Gakona's Plastic to Oil Machine - Part 1 - YouTube