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 fuels such as used vegetable oils collected from restaurants and food processing plants. Biodiesel conversion kits are available for diesel car owners, although they can be somewhat expensive.
When most people think of converting their car to biodiesel, what they are really considering is how to utilize used vegetables oils as a fuel. Straight vegetable oil (SVO) itself is not biodiesel. Modifications to the engine must be made before a car can accept SVO. True biodiesel is processed before it goes into the car, and is added to the gas tank like regular fuel.
One of the main challenges with converting a diesel engine to accept SVO 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 fuel, the vegetable oil 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/biodiesel fuel is stored in another tank, since it does not need to be heated.
The SVO 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 fuel. The standard petroleum fuel line is shut off until the switch is reversed.
Used vegetable oil may contain a number of food particles and other contaminants, so a SVO conversion kit also contains a special filter mounted near the engine block. This filtering process ensures that only heated vegetable oil reaches the engine, which reduces 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.
There are also kits available that will convert used vegetable oil to biodiesel fuel. These are usually free-standing systems that can be housed in a garage or other outbuilding. The process of converting vegetable oils to biodiesel takes a number of steps, and can be dangerous if the proper safety precautions are not followed. Fuel created using such kits can be used by a diesel car just like regular diesel fuel.
Converting a standard diesel engine to SVO 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 tank may be eliminated in the future. It is important to understand that SVO and biodiesel work in diesel engines, not the gas-powered internal combustion engines found in many cars.