The good news is that a diesel engine is already designed to accept biodiesel fuels, so it does not have to be completely overhauled to become "green." The inventor of the diesel engine, Rudolf Diesel, originally envisioned peanut oil as the engine's fuel source, but a usable petroleum-based waste oil became more commercially available. Modern diesel engines still continue to run primarily on petroleum diesel, but a number of owners are now converting to alternative biodiesel fuels such as corn-based E-85 and used vegetable oils collected from restaurants and food processing plants. Biodiesel conversion kits are available for diesel car owners, with an average installation cost approaching $1000 dollars US Dollars (USD).

One of the main challenges with converting a diesel engine to accept biodiesel fuel is the nature of the fuel itself. Food-grade vegetable oils have a tendency to thicken or solidify as the air temperature drops. To be useful as a biodiesel fuel, the vegetable oil or other biodiesel must be heated. This heating is accomplished by installing a second fuel tank in the car's trunk and running lines from the radiator to provide radiant heat. Petroleum or mixed petroleum/vegetable biodiesel fuel is stored in another tank, since it does not need to be heated.

The biodiesel conversion kit also includes a thermostat and fuel tank switch placed near the driver's seat. When the vegetable oil has reached an acceptable temperature, the switch is activated and the heated oil is drawn through a second fuel line into the engine block. At this point the car is being completely operated on organic biodiesel fuel. The standard petroleum fuel line is shut off until the switch is reversed.

Because used vegetable oil may contain a number of food particles and other contaminants, a biodiesel conversion kit also contains a special filter mounted near the engine block. This filtering process ensures that only heated vegetable oil reaches the engine, reducing the risk of clogged fuel lines and contamination of the engine block. Biodiesel fuel blends containing both petroleum and organic materials have already been filtered, so that fuel is not sent through the filter. Biodiesel blends also use oils which have been processed to remain liquefied regardless of air temperature.

Converting a standard diesel engine to biodiesel is mostly a question of providing a second fuel tank and fuel line to deliver the organic fuel to the engine. If a stable form of vegetable oil with a lower point of solidification can be developed, the need to heat the biodiesel tank may be eliminated in the future. It is important to understand that the conversion process only works on diesel engines, not the gas-powered internal combustion engines found on most cars on the road today.