can we use plant oils for cars

[OPatrick]

No, you should not use raw, unmodified plant oils directly in modern diesel engines. While it's technically possible with significant vehicle modifications, doing so in an unmodified car will likely cause severe and expensive damage to the fuel system and engine. The viable alternative is biodiesel, which is a chemically processed fuel derived from plant or animal oils.

Straight vegetable oil (SVO) is too viscous, or thick, for modern fuel injection systems. This high viscosity leads to poor atomization, where the fuel doesn't form a fine mist for efficient combustion. The result is incomplete burning, which causes carbon buildup on injectors and cylinders, reduced engine power, and increased harmful emissions. Cold weather exacerbates these problems, as the oil can become so thick it clogs fuel filters and lines.

To be used safely, plant oils must be converted into biodiesel through a chemical process called transesterification. This process reacts the oils with an alcohol (like methanol) and a catalyst, creating a fuel with viscosity and combustion properties much closer to petroleum diesel. Biodiesel can be used in pure form (B100) or blended with petroleum diesel (e.g., B20, which is 20% biodiesel).

Using biodiesel in a compatible engine offers benefits like reduced carbon emissions and better lubricity. However, it's crucial to ensure the fuel meets quality standards like ASTM D6751 to avoid issues with fuel stability and material compatibility.

Consideration	Straight Vegetable Oil (SVO)	ASTM-Grade Biodiesel (B100)
Engine Compatibility	Requires extensive vehicle modification (dual-tank system, heated fuel lines).	Can be used in many modern diesel engines with no modifications (check manufacturer warranty).
Viscosity	Very high, leading to poor combustion and engine damage.	Similar to petroleum diesel, ensuring proper atomization.
Cold Weather Performance	Poor; can solidify and block fuel system.	Better than SVO, but can still gel; winter blends are available.
Fuel Stability	Prone to oxidation and can spoil, forming gums and solids.	Requires proper storage and use of stabilizers to maintain quality.
Emissions	Higher particulate matter and carbon buildup due to incomplete combustion.	Lower net carbon emissions compared to petroleum diesel.

[VonAlexandra]

As a mechanic, I've seen the aftermath. Folks pour used frying oil straight into their old Mercedes diesel, thinking they're saving money. A few months later, they're in my shop with a tow truck. The injectors are clogged solid with hardened carbon, and the fuel pump is shot. The repair bill is always thousands more than any "savings." It's just not worth it. If you're serious about alternative fuel, invest in a proper biodiesel conversion kit or stick with certified fuel from a reputable station.

[VonWilliam]

It's a fascinating idea with an important environmental angle, but the devil's in the details. The real sustainable path is biodiesel. The chemical process cleans up the oil, making it a proper fuel. The net carbon footprint is lower than fossil diesel, which is a huge plus. However, large-scale production raises concerns about land use and food supply if we're growing crops just for fuel. The most ethical approach is using waste oils, like from restaurants, but that supply is limited. It's a piece of the puzzle, not a magic bullet.

[VanEmma]

I looked into this for my farm truck. You can't just use the stuff from the grocery store. You have to process it into biodiesel, which is a project in itself. It involves handling chemicals like lye and methanol—safety gear is a must. You also need to test every batch to ensure it's properly reacted; get it wrong and you'll ruin your engine. For me, it's a hobby that saves a little cash. For most people, buying pre-made biodiesel from a trusted supplier is the only sane way to go.

[DeSienna]

From an engineering perspective, the core issue is molecular structure. Triglycerides in virgin plant oils are large, complex molecules with high viscosity. Transesterification breaks these down into smaller methyl esters (biodiesel) and glycerol. This reduction in molecular size is what drastically lowers viscosity and improves combustion characteristics. The process also addresses oxidation stability. Modern high-pressure common-rail fuel systems are exceptionally sensitive to fuel properties; even minor deviations from spec can lead to catastrophic injector failure. This isn't a backyard experiment for modern vehicles.