Is Biodiesel The Same As Diesel: Heavy-Duty Diesel Guide

Is Biodiesel The Same As Diesel?

No. Biodiesel is not the same as petroleum diesel. Petroleum diesel (ULSD) is a hydrocarbon distillate refined from crude oil meeting ASTM D975. Biodiesel is fatty acid methyl esters (FAME) produced from vegetable oils, animal fats, or used cooking oil meeting ASTM D6751. At the pump, most on-road diesel is a blend — typically B5 to B11 (5–11% biodiesel mixed with petroleum diesel). Pure biodiesel (B100) has different cold-flow properties, storage requirements, and OEM approval limits than straight ULSD. Modern heavy-duty trucks can run approved biodiesel blends, but biodiesel is a chemically distinct fuel, not interchangeable with petroleum diesel at all blend levels.

Fleet managers, owner-operators, and shop techs encounter biodiesel at truck stops labeled B5, B11, or B20 without always understanding what changes when the blend percentage rises. This guide covers the chemistry difference, specification standards, OEM approval limits, cold-weather behavior, aftertreatment impact, and diagnostic fault patterns tied to biodiesel use.

Chemistry: Petroleum Diesel vs Biodiesel

The molecular structure difference drives every operational distinction between the two fuels.

Petroleum diesel (ULSD) — A mixture of hydrocarbon chains (paraffins, naphthenes, aromatics) with carbon and hydrogen atoms only. Produced by crude oil distillation, hydrocracking, and hydrotreating. No oxygen atoms in the molecule. Energy density approximately 128,000 BTU per gallon.

Biodiesel (FAME) — Fatty acid methyl esters produced by reacting vegetable oil or animal fat with methanol and a catalyst (transesterification). The molecule contains an ester group with oxygen atoms bonded to carbon. Energy density approximately 118,000 BTU per gallon — roughly 8% lower than petroleum diesel.

Oxygen content — Biodiesel carries 10–11% oxygen by weight. This oxygen improves combustion completeness and reduces particulate emissions but lowers energy density per gallon. A truck running B100 will show slightly lower fuel economy than the same truck on straight ULSD.

Cetane number — Biodiesel typically tests 47–65 cetane, often higher than petroleum diesel (45–52 at the pump). Higher cetane improves cold-start combustion quality in biodiesel blends.

Sulfur — Both ULSD and ASTM D6751 biodiesel must meet low sulfur requirements. B100 from proper production typically contains less than 15 ppm sulfur. However, biodiesel from waste feedstocks with poor processing can carry contaminants that affect aftertreatment.

ASTM Standards: D975 vs D6751

Each fuel type has its own specification standard. Blended fuel at the pump must meet both when biodiesel is present.

ASTM D975 (petroleum diesel) — Covers sulfur (≤15 ppm for ULSD), cetane number (≥40), distillation range, lubricity, water and sediment, cloud point, and cold filter plugging point. Every gallon of on-road diesel must meet D975 regardless of biodiesel content.

ASTM D6751 (biodiesel) — Covers biodiesel blend stock (B100) before mixing with petroleum diesel. Key parameters include ester content (≥96.5%), glycerin limits, methanol residual, water content, oxidation stability, and cold soak filtration.

ASTM D7467 (blended fuel) — Covers finished B6 to B20 blends, specifying how B100 must be blended with D975 diesel to create a compliant pump product. Most truck stop "diesel" with biodiesel content falls under this standard.

B-label system — The "B" number indicates biodiesel percentage by volume. B5 is 5% biodiesel and 95% petroleum diesel. B11 is 11% biodiesel. B20 is 20% biodiesel. B100 is pure biodiesel.

OEM Approval Limits for Biodiesel Blends

Engine manufacturers set maximum biodiesel blend percentages. Exceeding the limit can void warranty coverage on fuel system and aftertreatment components.

Cummins — Approves B20 (20% biodiesel) for current on-highway engines when the biodiesel meets ASTM D6751. B5 is the general recommendation for normal operation. Check the specific engine owner's manual for your model year.

Detroit Diesel (DD13, DD15, DD16) — Approves B5 for all engines. B20 approved for 2007 and newer EPA07/EPA10/GHG14/GHG17 engines when biodiesel meets ASTM D6751. Verify in DDDL calibration and owner's manual.

PACCAR (MX-11, MX-13) — Approves B20 for current engines meeting ASTM D6751 specifications. B5 recommended for cold-weather operation below 20°F.

Ford 6.7 Power Stroke — Approves B20 for 2011 and newer engines. B5 recommended for winter operation in cold climates.

GM 6.6 Duramax — Approves B20 for 2017 and newer L5P engines. Earlier models may be limited to B5 — check the owner's manual.

Ram 6.7 Cummins — Approves B20 for current model years when biodiesel meets ASTM D6751.

Cold-Weather Behavior: Where Biodiesel Differs Most

Cold-flow properties are the biggest operational difference between biodiesel and petroleum diesel. This is where fleets experience real problems.

Cloud point — The temperature at which wax crystals begin forming in fuel. B100 cloud point ranges from 35°F to 50°F depending on feedstock (soybean oil gels earlier than tallow-based biodiesel). ULSD #2 cloud point is typically 10°F to 20°F in winter blends.

Cold filter plugging point (CFPP) — The temperature at which wax crystals clog the fuel filter. Biodiesel raises CFPP proportionally to blend percentage. A B20 blend in a truck spec'd for #2 ULSD winter fuel can gel the filter at temperatures where straight ULSD still flows.

Winter blending practice — Fuel terminals reduce biodiesel percentage in winter months. Midwest terminals may drop from B11 summer blend to B5 or B2 in December through February. Southern terminals may run B5 year-round.

Gelling symptoms — Truck cranks but won't start, or starts and dies. White smoke on attempted start. Fuel filter housing shows waxy crystals when opened. Scan tool may show SPN 157 (fuel rail pressure low) or SPN 97 (water in fuel — sometimes confused with gel).

Prevention — Use winter-blended #1/#2 ULSD from branded terminals in cold months. Add approved cold-flow improver if operating in B20 territory during cold snaps. Park indoors or use block heaters to keep fuel warm. Replace fuel filters before winter season.

Before: Fleet runs B20 year-round from a single terminal in Minnesota — four trucks gel and strand in January at -5°F with SPN 157 and no-start conditions. Tow bills exceed $2,000. After: Fleet switches to B5 winter blend from a branded terminal with documented cold-flow specs — zero gel events for the rest of the season.

Storage, Shelf Life, and Contamination Differences

Biodiesel degrades differently than petroleum diesel in storage tanks and vehicle saddle tanks.

Oxidation stability — Biodiesel oxidizes faster than petroleum diesel, forming acids and polymer deposits. ASTM D6751 requires minimum oxidation stability of 3 hours (Enzymatic method). Petroleum diesel is more stable in long-term storage.

Water absorption — Biodiesel absorbs water more readily than petroleum diesel. Water promotes microbial growth (bacteria and fungi) that form black slime in tanks and fuel filters. This is the most common storage-related failure with higher biodiesel blends.

Rubber and seal compatibility — Older trucks (pre-2007) may have fuel system seals and hoses not rated for biodiesel. B20 can swell or degrade natural rubber components. Post-2007 trucks use biodiesel-compatible materials.

Shelf life — Petroleum ULSD stores 6–12 months with proper tank maintenance. B100 should be used within 3–6 months. B20 blends follow a middle ground — use within 6 months and maintain water drains.

Tank maintenance — Drain water from saddle tanks monthly. Use biocide treatment in fleet storage tanks running B11 or higher. Inspect fuel filters for black slime during PM services.

Aftertreatment Impact and Fault Code Patterns

Biodiesel affects emissions and aftertreatment differently than petroleum diesel — mostly in positive ways for particulate, but with caveats for storage-related contamination.

Particulate reduction — Biodiesel's oxygen content improves combustion completeness, reducing PM emissions. Trucks running B20 may see slightly fewer DPF regen events under normal operation.

NOx interaction — Some studies show B20 can slightly increase NOx emissions depending on engine calibration. SCR systems compensate automatically, but a poorly maintained SCR on B20 fuel may show SPN 4364 (SCR efficiency low) sooner than on straight ULSD.

DEF consumption — No direct increase in DEF usage from biodiesel blends within OEM-approved limits. DEF dosing is controlled by NOx sensor feedback, not fuel type.

Contamination-driven faults — The real aftertreatment risk from biodiesel is not the fuel itself but water and microbial contamination in stored biodiesel. Slime-clogged fuel filters cause low rail pressure (SPN 157), incomplete combustion, and downstream SCR efficiency faults (SPN 4364) that look like catalyst failure but trace back to fuel quality.

Diagnostic approach — When SCR or fuel codes appear after a fuel source change, pull a fuel sample before replacing catalyst or injectors. Use Cummins INSITE or Noregon JPRO to read the full active and inactive fault list. Check our fault code lookup for SPN 157, SPN 4364, and SPN 97 diagnostic paths.

Before: Shop replaces SCR catalyst on a DD15 after SPN 4364 — fault returns in 3 days because the saddle tank has microbial slime from 6-month-old B20 that was never treated with biocide. After: Shop drains and cleans the tank, replaces fuel filters, treats with biocide, runs two tanks of fresh B5 ULSD — SPN 4364 clears after a forced regen without catalyst replacement.

Can Your Truck Run Biodiesel? Decision Guide

Check the owner's manual — OEM biodiesel approval is model-year and engine-specific. Never assume B20 approval without verifying.

Know your pump blend — Truck stops in the Midwest often sell B11 without prominent labeling. Coastal and southern stops typically sell B5. Ask the terminal or check the pump decal.

Adjust for climate — Run B5 or lower in winter regardless of maximum OEM approval. Cold-flow failure strands trucks faster than any emissions benefit justifies.

Maintain tanks and filters — Higher biodiesel blends demand more frequent water draining, biocide treatment, and fuel filter replacement.

Do not mix homebrew — Biodiesel produced without ASTM D6751 certification may contain excess methanol, glycerin, and free fatty acids that damage injectors and seals. Only use commercial biodiesel from licensed producers.

Warranty documentation — If a fuel-related failure occurs under warranty, the dealer will test fuel for biodiesel content and ASTM compliance. Non-compliant fuel voids warranty coverage on fuel system and aftertreatment repairs.

Read more fuel and aftertreatment guides on our blog. Contact us for fleet fuel policy recommendations and biodiesel-related derate diagnosis.

FAQ

Is biodiesel the same as regular diesel?

No. Regular diesel (ULSD) is a petroleum hydrocarbon meeting ASTM D975. Biodiesel is fatty acid methyl esters from plant oils or animal fats meeting ASTM D6751. Most pump diesel is a blend of both (B5 to B20).

Can I run B100 in my diesel truck?

Most OEMs do not approve B100 for on-highway engines. B100 has cold-flow, storage, and seal compatibility issues that make it unsuitable for modern common-rail trucks. Fleet and agricultural equipment may use B100 in specific applications with engine modifications.

Does biodiesel damage diesel engines?

ASTM D6751-compliant biodiesel within OEM-approved blend limits (typically B20 maximum) does not damage modern engines designed for biodiesel-compatible materials. Non-compliant biodiesel with excess methanol, glycerin, or poor cold-flow properties can damage injectors, seals, and fuel pumps.

Is biodiesel better for the environment than petroleum diesel?

Biodiesel reduces lifecycle greenhouse gas emissions and particulate matter compared to petroleum diesel. However, feedstock sourcing (soybean farming, land use) affects the net environmental calculation. From an engine perspective, the operational difference is cold-flow and storage behavior.

Does biodiesel lower fuel economy?

Yes, slightly. B100 has about 8% lower energy density than petroleum diesel. A B20 blend typically reduces fuel economy by 1–2% compared to straight ULSD. Most fleets do not notice this at B5 blend levels.

Can biodiesel void my engine warranty?

Using biodiesel above the OEM-approved blend percentage or using non-ASTM D6751 fuel can void warranty coverage on fuel system and aftertreatment components. Stay within the owner's manual limit and buy from reputable terminals.

How do I know what biodiesel blend I am buying?

Check the pump label — most states require B-label disclosure above B5. Ask the truck stop or fleet fuel supplier for the current blend percentage. Midwest terminals often run B11 in summer and B5 in winter.

Does biodiesel cause fuel filter plugging?

Biodiesel can dissolve petroleum deposits from tank walls, which clogs filters during the first few tanks after switching to a higher blend. This is a one-time cleaning effect, not ongoing filter failure. Replace filters after the transition period.

What biodiesel blend do most truck stops sell?

B5 is the most common default blend nationwide. B11 is common in the Midwest. B20 is available at some fleet terminals and progressive truck stops but is not the default at most locations.

Should I add anything to biodiesel blends?

Use approved cold-flow improver in winter, biocide in storage tanks, and maintain regular water draining. Do not add petroleum diesel to B20 to "thin it" — buy the correct winter blend from the terminal instead.