No, you should not put straight vegetable oil directly into a modern diesel car's fuel tank. While diesel engines can technically run on vegetable oil due to the similar combustion principles, using it unprepared can cause severe and expensive damage to your fuel system. The primary issue is viscosity; straight vegetable oil is much thicker than standard diesel fuel.
This high viscosity can clog fuel filters, injectors, and fuel lines, especially in colder weather where the oil can become gelatinous. Modern diesel engines have extremely precise, high-pressure fuel injection systems (like common-rail systems) that are engineered for the specific properties of ultra-low-sulfur diesel. Vegetable oil does not have the same lubricity or cetane rating (a measure of combustibility) as diesel, leading to poor combustion, carbon buildup, and potential failure of fuel pumps and injectors.
| Fuel Type | Approximate Viscosity (at 40°C) | Cetane Rating | Key Risk in Modern Engines |
|---|
| Standard Diesel | 2-4.5 mm²/s | 40-55 | Designed for optimal performance |
| Biodiesel (B100) | 4-6 mm²/s | 48-65 | May degrade certain seals over time |
| Straight Vegetable Oil | 30-40 mm²/s | 35-40 | High risk of clogging and injector damage |
The correct and safe alternative is biodiesel, which is a chemically processed fuel made from vegetable oil or animal fats. This process, called transesterification, reduces the viscosity to a level compatible with diesel engines. Biodiesel is often blended with petroleum diesel (e.g., B5, B20). If you're determined to use vegetable oil, it requires significant vehicle modifications, including a dual-tank system that heats the oil to reduce its viscosity before it reaches the engine. For the vast majority of drivers, sticking with manufacturer-recommended fuels is the only safe choice.
