No, you should not put straight, unmodified cooking oil directly into a diesel car's fuel tank. While diesel engines and some vegetable oils share similar combustion properties, using cooking oil without proper preparation and vehicle modifications can lead to severe and expensive damage to your fuel system and engine. The core issue lies in viscosity and chemical stability.
Standard diesel fuel is refined to a specific viscosity, allowing it to flow easily and be atomized by the fuel injectors. Used cooking oil is significantly more viscous (thicker) and can clog fuel filters, injectors, and fuel lines. Furthermore, unlike purified biodiesel, straight vegetable oil can oxidize, leading to sludge and deposits in the engine. In cold weather, it can solidify, completely blocking the fuel system.
For long-term use, a vehicle requires a two-tank conversion system that heats the vegetable oil to reduce its viscosity before it reaches the engine. However, the only safe and recommended way to use a renewable fuel is Biodiesel, which is a chemically processed form of vegetable oil that meets strict fuel standards (ASTM D6751). Many modern diesel engines can safely run on blends like B5 (5% biodiesel) or B20 (20% biodiesel), but you should always consult your owner's manual first.
| Fuel Type | Viscosity (mm²/s at 40°C) | Cetane Number (Ignition Quality) | Potential for Engine Deposits | Cold Weather Performance |
|---|
| Petro-Diesel | 2.0 - 4.5 | 40 - 55 | Low | Good (with additives) |
| Straight Vegetable Oil | 30 - 40 | 35 - 40 | Very High | Poor (solidifies) |
| Biodiesel (B100) | 4.0 - 6.0 | 48 - 65 | Moderate (higher than diesel) | Fair (can gel) |
| B20 Blend | ~3.0 | ~50 | Slightly Higher | Good (similar to diesel) |
Attempting to use cooking oil as a cheap fuel alternative is a high-risk gamble that will likely result in repair costs far exceeding any savings on fuel.
