Biodiesel as an Alternative Diesel Fuel: Properties and Vehicle Uses
The Short Answer
Biodiesel is a renewable fuel made from vegetable oils, animal fats or recycled cooking grease. It is used mainly as a substitute for, or additive to, petroleum diesel in compression-ignition (diesel) engines. Most of it is burned as a blend with conventional diesel rather than on its own. Common uses include on-road diesel cars and trucks, commercial fleets, buses, agricultural and construction equipment, generators, and heating oil.
One qualification changes the answer for every owner: whether a vehicle can use a given blend depends on what its manufacturer approves for that engine, model year and market. Low-level blends are widely accepted. Higher blends and pure biodiesel are approved only for some engines and may raise warranty, cold-weather and maintenance concerns. Biodiesel is not a fuel for gasoline engines.
What Biodiesel Is
Chemically, biodiesel is a mix of fatty acid methyl esters, often shortened to FAME. It is produced by transesterification, a process that reacts fats or oils with an alcohol, usually methanol, in the presence of a catalyst. The process yields biodiesel and glycerin as a co-product. Refining the oil this way lowers its viscosity, or thickness, close enough to diesel's that it can flow through a diesel fuel system and atomize properly in the cylinder.
Biodiesel is often confused with two other fuels:
Straight vegetable oil (SVO): Unprocessed oil that has not gone through transesterification. It is much thicker than biodiesel and is not a drop-in diesel fuel. Engines generally need conversion to run on it. Renewable diesel (sometimes labeled HVO, for hydrotreated vegetable oil): Made from similar feedstocks but by a different refining process. The result is a hydrocarbon that is chemically very similar to petroleum diesel. It is not biodiesel, and its compatibility and handling characteristics differ.
Blend Labels and Fuel Standards
Biodiesel blends are named with a "B" followed by the percentage of biodiesel in the fuel. B5 is 5 percent biodiesel and 95 percent petroleum diesel. B20 is 20 percent biodiesel. B100 is pure biodiesel. In the US, ASTM International publishes the consensus fuel standards:
ASTM D6751 covers pure biodiesel (B100) intended for blending. ASTM D975, the conventional diesel specification, permits up to 5 percent biodiesel. Fuel sold as ordinary diesel may therefore already contain some biodiesel. ASTM D7467 covers blends from B6 to B20.
Fuel that meets the relevant standard is a basic expectation. Many manufacturer approvals and fuel-related warranty terms assume it.
Key Properties Compared With Petroleum Diesel
The table below summarizes general, widely documented differences. The actual values vary with the feedstock, how the fuel was processed, the blend level and storage conditions. No specific test measurements were available for this article, so the comparisons are directional only.
Property | Biodiesel tendency | Practical meaning; Source | Renewable fats and oils | Can be produced domestically from crops and waste fats; Lubricity | Higher | Even low blends can improve lubrication of fuel pumps and injectors in ultra-low-sulfur diesel; Cetane number | Typically higher | Ignition quality is generally good; Energy content | Somewhat lower per gallon | Fuel economy can be slightly lower, most noticeably at high blends; Cold-flow behavior | Gels or clouds at warmer temperatures | High blends need winter management; Solvent effect | Loosens existing fuel-system deposits | Fuel filters may clog early after switching; Oxidation stability | Lower | Shorter recommended storage life; Water affinity | Higher | More risk of water contamination and microbial growth in tanks; Material compatibility | Can degrade some older seals, hoses and certain metals | Older fuel systems may need inspection or parts replacement; Toxicity and biodegradability | Lower toxicity, biodegrades more readily | Lower environmental risk from spills
Where Biodiesel Is Used
Passenger diesel vehicles and pickups: usually as B5 or B20, within the manufacturer's approved limit. Fleets and transit buses: an established way for operators to add renewable fuel without changing vehicles, using approved blends. Heavy-duty trucks: blended fuel sold at the pump or supplied in bulk. Agricultural, construction and other off-road equipment: diesel machinery that can often run on blends, subject to equipment-maker approval. Marine engines and stationary generators: diesel engines where lower toxicity and biodegradability can be attractive. Home and building heating: biodiesel blended into heating oil is commonly marketed as Bioheat. Fuel additive: small amounts are added to ultra-low-sulfur diesel mainly to restore lubricity.
Vehicle Compatibility: What to Check
Before using any blend above what is already in standard pump diesel, check these points:
Owner's manual or manufacturer fuel guidance: this is the authoritative source for the maximum approved blend on a specific engine, model year and market. Warranty terms: damage blamed on fuel outside the approved specification may not be covered. Emissions hardware: on some modern diesels with diesel particulate filters, manufacturers have cited concerns that higher blends can dilute the engine oil during filter regeneration. This is one reason some engines are limited to lower blends. Fuel system age and materials: older vehicles may have seals or hoses that do not tolerate biodiesel well. Climate: in cold regions, ask the supplier whether the fuel is winter-blended. Treat B100 with particular caution.
Ownership Considerations
After a first switch to a higher blend, the solvent effect can release deposits that have built up in the tank and lines. Owners and fleet managers often plan for an early fuel filter change. Because biodiesel oxidizes and absorbs water more readily than petroleum diesel, it suits vehicles and equipment that use fuel regularly better than machines that sit unused for long periods with a full tank. For equipment stored long term, follow the manufacturer's or fuel supplier's storage guidance.
Emissions and Environmental Profile
Burning biodiesel generally lowers tailpipe emissions of particulate matter, unburned hydrocarbons and carbon monoxide compared with petroleum diesel. The reduction grows with the blend level. Nitrogen oxide (NOx) emissions can rise slightly in some engines and conditions, although modern aftertreatment systems manage NOx regardless of fuel. Lifecycle greenhouse-gas benefits come from the renewable origin of the feedstock. The size of that benefit depends heavily on what the fuel is made from and how it is produced. Waste fats and oils generally score better than crops grown specifically for fuel.
Information Gaps
No confirmed data on specific vehicles, prices, availability, emissions results or test measurements was available for this article, so none is given. Figures such as fuel-economy change, cloud point or emissions reduction vary by feedstock and engine. Look them up in manufacturer documentation, fuel supplier specifications or published test reports for the exact fuel and vehicle in question.
Bottom Line
Biodiesel is a practical renewable alternative and supplement to petroleum diesel. Its main role is as a blended fuel in existing diesel vehicles and equipment, and in heating oil. It offers good lubricity and lower emissions of several pollutants. Its weak points are cold-weather performance, storage stability and compatibility with some materials and engines. For any specific vehicle, the manufacturer's approved blend level determines what is appropriate to use.
