Vegetable oil as fuel: what you should know before trying

Two rising fuel prices push many drivers to explore fueling alternatives for their vehicles. One idea circulating online is using vegetable oil in place of diesel. But is this a viable solution or just an urban myth that can lead to serious engine problems? In this article we analyze in detail what using vegetable oil as fuel entails, which technical modifications are required, and whether the investment is worthwhile.

Before diving into the technical details, it’s essential to understand that there is a major difference between biodiesel and pure vegetable oil. This distinction is crucial for understanding the process and its limitations.

Difference between biodiesel and vegetable oil

Biodiesel is an industrially refined fuel produced in specialized facilities that adhere to strict quality standards and environmental regulations. Usually derived from soy or other plant sources, biodiesel undergoes complex chemical processes of transesterification that transform it into a fuel compatible with modern diesel engines. Many diesel vehicles can run on biodiesel or biodiesel-diesel blends without major modifications.

Pure vegetable oil, on the other hand, is exactly what the name suggests—unrefined oil, bought from supermarkets or recovered from restaurants. There is no industrial refining process, it is not standardized, and it does not automatically comply with fuel regulations. Its use requires significant modifications to the fueling system and carries risks for the engine.

Why you can’t use vegetable oil directly in the tank

The main technical problem with vegetable oil is its high viscosity. Unlike conventional diesel or biodiesel, vegetable oil is much thicker and denser, especially at low temperatures. This characteristic creates several problems:

Atomization problems

Fuel injectors are designed to spray diesel into very fine droplets for complete and efficient combustion. Vegetable oil, being far more viscous, cannot be atomized correctly. Instead of a fine spray, you get larger droplets that do not mix uniformly with the air in the combustion chamber.

Incomplete combustion

The large droplets of oil do not burn completely during the combustion cycle. The result is residue buildup on the walls of the combustion chamber, on the piston, on the valves, and on other vital components. Over time, these deposits can clog injectors, wear piston rings, and cause major engine damage.

Problems at low temperatures

Vegetable oil can solidify at low temperatures. At room temperature many vegetable oils are already very viscous, and below freezing they can become nearly solid. This makes cold starts practically impossible and can completely block the fueling system.

Incompatibility with gasoline engines

Gasoline engines use spark ignition and are designed for volatile fuels. Vegetable oil cannot ignite by this method. Moreover, the fuel lines, pumps, and sensors in a gasoline engine’s intake system are not sized for the density and viscosity of vegetable oil.

Technical modifications required to use vegetable oil

To make vegetable oil work as a fuel, extensive modifications to the fueling system are necessary. These are not simple interventions and require substantial investment.

Two-reservoir system

The most common technical solution is a two-reservoir system:

• Main reservoir contains vegetable oil
• Secondary reservoir contains conventional diesel

The engine always starts on diesel from the secondary reservoir. This allows the engine to warm up to its operating temperature. Only after the engine reaches optimal temperature does the system automatically switch to vegetable oil from the main reservoir.

Before shutdown, the system switches back to diesel for a few minutes. This process flushes vegetable oil from the fuel lines and injectors, preventing deposits and blockages.

Fuel heating system

Vegetable oil must be heated to reduce viscosity to an acceptable level. The conversion includes:

• Heater coils in the vegetable oil tank
• Heat exchangers on the fuel lines
• Connections to the engine’s cooling system to use excess heat

These components ensure the oil reaches the injectors at about 70-80°C, where its viscosity is similar to diesel.

Expanded filtration system

Vegetable oil, especially recycled from restaurants, contains impurities that must be removed:

• Multiple mechanical filters of different pore sizes
• Water separators to remove moisture
• Final high-precision filters (5-10 microns) before the injectors

Any food particles, spices, or other contaminants can rapidly clog injectors and damage the fuel pump.

Specialized injectors

Standard injectors may not cope with the pressure and different characteristics of vegetable oil. Many conversions require:

• Injectors with larger orifices to compensate for viscosity
• Higher injection pressure for better atomization
• Corrosion-resistant materials suitable for vegetable oils

Enhanced glow plugs

Glow plugs are essential for starting a diesel engine when cold. For use with vegetable oil, they must:

• Generate more heat
• Remain active longer
• Be resistant to carbon deposits

Process of preparing vegetable oil

Even with all technical modifications, vegetable oil must be properly prepared before use:

For fresh oil from commercial sources

1. Quality check – use only food-grade oil
2. Preventive filtration – even new oil can contain impurities
3. Viscosity testing – some oils are more suitable than others

For used restaurant oil

1. Decanting – let the oil settle to separate large particles
2. Primary filtration – remove chunks of food and large solids
3. Water separation – remove water resulting from cooking
4. Fine filtration – pass through 50, 25, then 10 micron filters
5. Testing – check for absence of water and particulates
6. Proper storage – in clean containers, protected from light

Legal and environmental aspects

Using vegetable oil as fuel raises several legal considerations:

Tax regulations

In many countries, including Romania, fuels are subject to excise duties. Using an untaxed fuel can constitute tax evasion. It is necessary to check local legislation and obtain any required authorizations.

Vehicle homologation

Major modifications to the fueling system may require re-homologation of the vehicle. A modified vehicle without approval may:

• No longer be insurable
• Fail the periodic vehicle inspection (ITP)
• Attract fines or confiscation of the registration certificate

Pollutant emissions

Although vegetable oil is theoretically considered more eco-friendly, its incomplete combustion can produce:

• Fine particulates in greater quantities
• Emissions of aldehydes and acrolein
• Distinct smell and visible smoke

These emissions can exceed environmental norms and cause the vehicle to fail pollution standards.

Cost-benefit analysis

Let’s assess whether the conversion is economically viable:

Initial costs

• Professional conversion kit: 5,000–15,000 RON
• Specialized installation: 2,000–5,000 RON
• Additional reservoir: 1,000–3,000 RON
• Total investment: approximately 8,000–23,000 RON

Fuel costs

• Fresh vegetable oil: 15–25 RON/L
• Used vegetable oil: 0–10 RON/L (if constant free sources exist)
• Diesel: about 8 RON/L (2024)
• Filters and additional maintenance: 500–1,000 RON/year

Payback calculation

Assuming:

• Average consumption: 7 L/100 km
• Annual distance: 20,000 km
• Annual consumption: 1,400 L

With diesel: 1,400 × 8 = 11,200 RON/year

With free used oil: 1,400 × 3 (filtration + diesel start) = 4,200 RON/year + 800 RON maintenance = 5,000 RON/year

Annual savings: 6,200 RON

Payback period: 8,000 RON ÷ 6,200 RON ≈ 1.3 years (in the most optimistic scenario)

This calculation assumes:

• A constant supply of free used oil
• No major mechanical issues
• You perform all maintenance and filtration yourself
• No additional depreciation of the vehicle

Risks and downsides

Besides the initial costs, there are several significant risks:

Mechanical risks

• Damage to the fuel pump – a very expensive component (5,000–15,000 RON)
• Injector clogging – new injectors: 2,000–8,000 RON
• Premature engine wear – carbon deposits, wear of piston rings
• DPF issues – the diesel particulate filter may clog faster
• EGR failure – exhaust gas recirculation valve can deteriorate

Practical risks

• Warranty void – manufacturers do not cover damage from non-homologated fuels
• ** resale difficulties** – modified vehicles are harder to sell
• Workshop challenges – shops may refuse to work on modified engines
• Inconsistent supply – used oil sources may disappear

Legal risks

• Fines for modified vehicle
• Insurance problems in case of accident
• Tax compliance issues

Conclusions

While using vegetable oil as a fuel is technically feasible, in practice it is a complex solution requiring:

• A substantial initial investment (8,000–23,000 RON)
• Advanced technical knowledge for maintenance and fuel preparation
• Significant time for filtration and upkeep
• Constant access to vegetable oil at favorable prices
• Acceptance of mechanical and legal risks

Our recommendation is to avoid this conversion, unless:

1. You have access to large quantities of free vegetable oil
2. You have advanced mechanical knowledge and can perform maintenance yourself
3. The vehicle is dedicated solely to this purpose and you don’t care about resale value
4. You have checked and comply with all local legal requirements

For the average driver, more practical alternatives to reduce fuel costs include:

• Economical and anticipatory driving
• Regular maintenance for optimal efficiency
• Using apps to find the cheapest fuel stations
• Considering a more economical vehicle or hybrid/electric options
• Car-sharing or public transport for short trips

The investment required for a conversion could be better used to purchase a newer, more efficient vehicle that offers long-term savings without the risks and complications associated with vegetable-oil use.