Biogas vs Biodiesel for Vehicle Use

The Short Answer

If you drive a diesel car, pickup or truck: biodiesel blends are the relevant option. Check your manufacturer's approved blend level before using anything above the low blends sold as ordinary diesel. If you drive a gasoline vehicle: neither fuel applies directly. Biodiesel cannot be used in a gasoline engine, and biogas requires a dedicated natural gas fuel system. If you operate or are buying a CNG or LNG vehicle: upgraded biogas, sold as renewable natural gas (RNG), can be used the same way as fossil natural gas, without changes to the vehicle. If you are choosing a fleet strategy: the decision depends on engine configuration, the fueling infrastructure you can access, duty cycle, and which emissions or carbon goals you are trying to meet.

What Each Fuel Actually Is

Biodiesel

Biodiesel is a liquid fuel made by chemically processing vegetable oils, animal fats or used cooking oil. The result, technically called fatty acid methyl ester (FAME), has properties close enough to petroleum diesel to be used in compression-ignition (diesel) engines. In the US it is usually sold as a blend, labeled with a B number showing the percentage of biodiesel. B5 contains up to 5 percent biodiesel, B20 contains between 6 and 20 percent, and B100 is pure biodiesel. Low blends up to B5 are allowed within the standard specification for ordinary diesel fuel, so many drivers already use some biodiesel without noticing.

Biodiesel should not be confused with renewable diesel, sometimes sold as HVO. Renewable diesel is also made from fats and oils, but it is processed differently and is chemically very similar to petroleum diesel. It behaves differently from biodiesel in storage, cold weather and seal compatibility, so check which product a pump is actually selling.

Biogas and Renewable Natural Gas

Biogas is produced when bacteria break down organic matter without oxygen, a process called anaerobic digestion. Common sources include landfills, wastewater treatment plants, livestock manure digesters and food waste facilities. Raw biogas is mostly methane mixed with carbon dioxide, along with moisture and contaminants such as hydrogen sulfide. In that state it is not suitable as a vehicle fuel.

To fuel vehicles, biogas is upgraded by removing carbon dioxide and impurities until it is nearly pure methane. This product is called renewable natural gas or biomethane. It is interchangeable with fossil natural gas and can be compressed (CNG) or liquefied (LNG) for use in natural gas vehicles. Much RNG is injected into the existing gas pipeline network, so a CNG station may dispense RNG through contracts even when the gas physically delivered is a pipeline mix.

Engine and Vehicle Compatibility

Factor | Biodiesel | Biogas (as RNG); Physical form | Liquid, stored in a normal diesel tank | Gas, stored in high-pressure cylinders (CNG) or cryogenic tanks (LNG); Engine type | Diesel (compression ignition) | Natural gas engines, usually spark ignited; some dual-fuel diesel designs; Use in an unmodified diesel vehicle | Yes, at blend levels the manufacturer approves | No; Use in a gasoline vehicle | No | Only after a natural gas conversion or in a factory bi-fuel vehicle; Refueling | Standard diesel pumps or blend-specific pumps | CNG or LNG stations, or private fleet fueling; Typical users | Diesel car and truck owners, farm equipment, fleets | Transit buses, refuse trucks, regional freight and other fleets

The core difference is the fuel system. A diesel vehicle's fuel pump, high-pressure pump and injectors are designed to handle a liquid. A natural gas vehicle uses pressure-rated cylinders, a pressure regulator and gas injectors, and its engine configuration is set up for gaseous fuel. Converting a diesel engine to run on gas is a major engineering job, not a simple swap, and in the US aftermarket conversions must use certified systems to comply with emissions rules.

Owning a Diesel Vehicle on Biodiesel

The first step is checking what your manufacturer approves for your specific model, model year and engine. Approvals vary. Some diesel vehicles are approved for B20, others only for B5. Running a higher blend than approved can affect warranty coverage if a fuel-related failure occurs. The approval is usually stated in the owner's manual or on a label near the fuel filler.

Practical points to know when using biodiesel blends:

Fuel filter: biodiesel acts as a mild solvent and can loosen deposits left by petroleum diesel in tanks and fuel lines. When switching to a higher blend, the fuel filter may clog sooner than usual at first, so carry a spare or plan an earlier change. Cold weather: biodiesel starts to gel at higher temperatures than petroleum diesel. Higher blends need more attention in winter. Suppliers usually adjust blends seasonally, but B100 in cold climates generally requires heated storage or blending back down. Water and storage: biodiesel attracts more moisture than petroleum diesel and degrades with long storage. This matters most for seasonal equipment or vehicles that sit unused for long periods. Seals and hoses: older vehicles may have rubber components that biodiesel can degrade, especially at high blends. Most modern vehicles use compatible materials, but inspect older fuel lines. Modern injection systems: common-rail diesels run very high fuel pressure through precise injectors and a high-pressure fuel pump. Fuel that is off-specification or degraded can cause deposits or wear, which is one reason manufacturers limit approved blend levels. Emissions systems: on engines that inject extra fuel to regenerate a diesel particulate filter, some fuel can reach the engine oil. Biodiesel evaporates less readily than petroleum diesel, which is one concern behind blend limits on some light-duty diesels. Follow the manufacturer's oil change guidance.

Buy blends from suppliers whose fuel meets the applicable ASTM specification. Fuel quality has a bigger effect on reliability than the blend percentage alone.

Owning or Operating a Vehicle on Renewable Natural Gas

If your vehicle already runs on CNG or LNG, switching to RNG requires no hardware changes. Once upgraded to pipeline quality, the fuel is chemically the same as fossil natural gas, so engine calibration and fuel trim (the adjustments the engine computer makes to keep the air-fuel mixture correct) behave the same way. The real considerations are about natural gas vehicles in general:

Vehicle availability: factory-built natural gas passenger cars are now scarce in the US market. Natural gas is mainly used in medium and heavy-duty fleet vehicles, so for most private drivers RNG is not a realistic option. Fueling access: CNG stations are far less common than diesel pumps, and many serve fleets. Range planning matters more than with a liquid fuel. Fuel storage and safety: CNG cylinders hold gas at high pressure and have inspection requirements and service lives set by their certification. Regular leak checks on fittings and lines are part of normal maintenance. Space and weight: gas cylinders are bulkier and heavier than a liquid fuel tank of similar energy content, which can reduce cargo or payload capacity. Maintenance: spark-ignited natural gas engines use spark plugs and ignition components that diesel engines lack, and they need their own service schedule.

Emissions Compared

Both fuels are promoted mainly for their lifecycle greenhouse gas benefits, meaning emissions counted from feedstock to tailpipe, not just what leaves the exhaust. Results depend heavily on the feedstock and how the fuel is produced, so no single number applies to every gallon or cubic foot.

Biodiesel generally lowers lifecycle carbon emissions compared with petroleum diesel. The benefit grows with the blend level and is usually larger for waste-based feedstocks such as used cooking oil. At the tailpipe, biodiesel tends to reduce particulate matter, carbon monoxide and unburned hydrocarbons. Effects on nitrogen oxides vary by engine, and some older engines show a small increase. On modern diesels with particulate filters and selective catalytic reduction, the aftertreatment system largely determines tailpipe emissions.

RNG can score very well on lifecycle carbon accounting, particularly when made from manure or landfill gas. Capturing methane that would otherwise escape to the atmosphere is credited as a benefit, and under some low-carbon fuel programs this can give certain RNG pathways a carbon intensity below zero. Natural gas engines also produce very little particulate matter. The main caveat is methane leakage. Methane is a potent greenhouse gas, so leaks during production, distribution or fueling reduce the climate benefit.

How to Choose by Situation

Private diesel owner: use biodiesel blends within your manufacturer's approved level. Biogas does not apply unless you change vehicles. Owner of an older diesel or one that sits for long periods: stay with low blends, watch the fuel filter, and avoid storing high blends for long. Driver in a cold climate: lower blends in winter are the safer choice unless your supplier provides winterized fuel. Fleet already running CNG or LNG: contracting for RNG is the simplest way to cut lifecycle carbon without touching the vehicles. Fleet with diesel trucks and no gas infrastructure: biodiesel blends, or renewable diesel, are usually the lower-disruption path. Moving to RNG means new vehicles and fueling access. Fleet planning new purchases with central depot fueling: RNG is worth evaluating for return-to-base duty cycles such as refuse, transit and regional delivery, where on-site fueling solves the station-access problem.

Bottom Line

Biodiesel and biogas both come from organic waste and crops, but they serve different engines. Biodiesel is a drop-in liquid for diesel vehicles within approved blend limits, which makes it the practical renewable option for most diesel owners. Biogas becomes a vehicle fuel only after it is upgraded to renewable natural gas, and it works only in natural gas vehicles, which in the US are mostly fleet trucks and buses. Start with the engine you have or plan to buy. That usually settles the choice before cost or emissions comparisons come into play.