The original diesel engine was designed to run on vegetable oil, and older-model diesel engines can be converted to run primarily on common fry oil rather than petroleum products. Clean cooking oil from the grocery store is considerably more expensive than petroleum diesel fuel, however, so most veggie oil fuel car owners run on used fry oil obtained free from restaurants. Running your car on veggie oil requires installation of a complex conversion kit that allows for fuel selection, and preparation of the fuel by filtering and heating to remove impurities that can otherwise destroy your car engine.
Obtain an older-model diesel-engine car. Choose a car as free of mechanical and structural defects as available options and your budget allow. Do not convert a new diesel car to run on used vegetable oil; their engines have lower tolerances for fuel variations and impurities.
Purchase a vegetable oil fuel conversion kit. Install it according to manufacturer's directions, or take your diesel car to an alternative-fuels garage and have its mechanics install the conversion kit for you.
Ask local restaurants for their used all-non-hydrogenated vegetable oil from their food-frying units. Arrange to leave a clean 55-gallon drum at the restaurant for their kitchen staff to fill as they need to dispose of oil. Pick 55-gallon drum up and replace it with an empty one when appropriate.
Set your filled 55-gallon drum next to an empty one with its lid removed. Set up the filter sock on its stand over the empty 55-gallon drum.
Insert hand-cranked pump intake into used vegetable oil. Attach tubing to other end, and run tubing into filter sock. Turn crank to run used vegetable oil into filter sock. When filter sock is filled, wait until it drains, then repeat until 55-gallon drum is filled with filtered vegetable oil.
Insert intake end of hand pump into filtered vegetable oil. Attach tubing to other end of pump and place open end of tubing in stock pot. Turn crank to pump filtered oil into stock pot.
Place stock pot on portable camp stove burner. Light burner and warm the oil on low heat for one to two hours to drive out water content imparted by food cooked in the oil. Let the oil cool and, using funnel, pour it into clean gas cans.
Fill your car's veggie fuel tank (usually located in the boot) with filtered, purified vegetable oil. Start the car on diesel fuel. When the engine has fully warmed up, flip the on-dash fuel selector switch (installed as part of your conversion kit) to vegetable oil. Drive normally until 10 miles before ending your car trip, then switch the on-dash fuel selector back to diesel fuel.