Where ethanol serves gasoline engines, biodiesel serves diesel ones. It is made not by fermentation but by a chemical reaction that turns fats and oils into a fuel a compression-ignition engine can burn.
How biodiesel is made
Biodiesel is produced by transesterification. Vegetable oil, animal fat or used cooking oil is reacted with an alcohol — usually methanol — in the presence of a catalyst. The reaction converts the oil’s triglycerides into fatty-acid methyl esters (FAME), which is biodiesel, plus glycerin as a saleable by-product. The FAME is washed and dried to meet fuel specifications such as ASTM D6751.
Feedstocks
The dominant feedstocks are soybean oil in the United States and rapeseed (canola) oil in Europe, alongside palm oil, used cooking oil and animal tallow. Waste and lower-value oils are increasingly preferred because they carry a lower lifecycle carbon intensity and avoid some of the food-versus-fuel and land-use concerns attached to dedicated oil crops. See feedstocks for detail.
Biodiesel vs renewable diesel
An important distinction: biodiesel (FAME) is chemically different from petroleum diesel, which limits how much can be blended and gives it cold-weather and storage constraints. Renewable diesel (also called HVO) is made from the same oils and fats but by hydrotreating, producing a hydrocarbon chemically near-identical to petroleum diesel. Renewable diesel is a true drop-in fuel usable at high levels without the limits of FAME, and its production has grown rapidly as a result.
Properties and performance
Biodiesel has some genuine advantages as a diesel fuel. It is a strong lubricant, so even a small blend can improve the lubricity of modern low-sulphur diesel and protect fuel-system components. It has a high cetane number (good ignition quality), is biodegradable and far less toxic than petroleum diesel, and burns with reduced particulate and sulphur emissions. Its drawbacks are equally practical: it holds slightly less energy per gallon than petroleum diesel; it can degrade certain seals and hoses in older engines; it has a shorter storage life and can encourage microbial growth in tanks if neglected; and, most importantly, it gels in cold weather. These traits are why FAME is normally used in blends rather than neat, and why winter operation relies on lower blend levels and cold-flow additives.
Co-products and standards
Transesterification produces glycerin as a by-product — roughly a tenth of the output by volume — which is refined and sold into soap, cosmetics, pharmaceuticals and animal feed, and whose value affects plant economics. Finished biodiesel must meet a fuel standard before it can be sold: in the United States that is ASTM D6751, and in Europe EN 14214. These specifications govern properties such as cetane number, cold-flow behaviour, water content and oxidation stability, and meeting them consistently is a large part of what distinguishes fuel-grade biodiesel from raw transesterified oil.
Where biodiesel is used
Because low blends drop into existing diesel infrastructure, biodiesel is widely used in road freight, public-transit bus fleets, agricultural and construction machinery, marine engines and stationary generators — settings where a B5–B20 blend needs no engine modification. Municipal and fleet operators have often been early adopters, since centralised fuelling makes blend management straightforward. The European Union is the largest biodiesel producer and consumer, reflecting the prevalence of diesel cars there, while in the United States biomass-based diesel sits within the advanced category of the Renewable Fuel Standard.
Blending and use
Biodiesel blends are written “B” plus the percentage: B5 and B20 are common and work in most diesel engines, while B100 (pure biodiesel) needs a compatible engine and careful handling in cold weather. As with ethanol, demand is underpinned by the Renewable Fuel Standard (biomass-based diesel category) and by low-carbon fuel standards that reward low-carbon feedstocks, especially waste oils.