Ultra low sulfur diesel, or ULSD, is a highly refined diesel fuel with extremely low sulphur content. In the United States, ULSD contains a maximum of 15 ppm sulphur, while UK road diesel is generally limited to 10 mg/kg sulphur under current fuel-quality requirements. Its lower sulphur level helps reduce sulphur-related emissions and protect modern emissions-control systems, including diesel particulate filters and catalytic equipment.
Diesel fuel has changed considerably over the past few decades. One of the most important developments has been the dramatic reduction of sulphur content, which has helped modern diesel engines use increasingly sophisticated emissions-control technology.
This is where ultra low sulfur diesel, commonly abbreviated to ULSD, comes in.
ULSD is a highly refined diesel fuel containing only a very small amount of sulphur compared with older diesel formulations. Lowering the sulphur level helps reduce sulphur-related exhaust emissions and allows technologies such as diesel particulate filters and catalytic after-treatment systems to operate effectively.
However, ULSD is not quite defined in exactly the same way everywhere. In the United States, ULSD generally means diesel containing no more than 15 parts per million of sulphur. In the UK, current road diesel requirements are even stricter, with a maximum sulphur content of 10 mg/kg under the applicable fuel-quality specification.
This guide explains what ultra-low sulphur diesel is, how it is produced, where it is used, its advantages and disadvantages, how it compares with older diesel fuels and what businesses should consider when purchasing and storing it.
What Is Ultra-Low Sulphur Diesel?
Ultra-low sulphur diesel is petroleum-derived diesel that has undergone additional refining to remove most of the naturally occurring sulphur compounds found in crude oil.
Traditional diesel once contained far higher levels of sulphur. When this fuel was burned, sulphur could contribute to sulphur dioxide and sulphate particulate emissions.
As governments tightened vehicle-emission requirements, fuel quality also had to improve.
Reducing sulphur became particularly important because modern diesel exhaust-treatment technologies can be adversely affected by high-sulphur fuel.
The US Environmental Protection Agency began phasing in a 15 ppm sulphur limit for road diesel in 2006, with ULSD becoming the required highway fuel after the transition period.
Britain has adopted even tighter road-fuel sulphur limits. Current UK diesel fuel specifications set the maximum sulphur content at 10 mg/kg.
Why Does Diesel Naturally Contain Sulphur?
Crude oil contains many different hydrocarbons as well as naturally occurring impurities.
Sulphur compounds are among those impurities.
The amount depends on the crude-oil source. Some crude oils naturally contain relatively little sulphur, while others contain much more.
During refining, crude oil is separated and processed into products such as:
- Petrol
- Diesel
- Kerosene
- Heating oil
- Jet fuel
- Lubricants
- Petrochemical feedstocks
Removing sulphur requires additional refining processes before diesel can meet modern specifications.
The result is cleaner diesel fuel containing only a tiny fraction of the sulphur found in older formulations.
How Is Ultra-Low Sulphur Diesel Produced?
ULSD is typically produced using a refinery process known as hydrotreating or hydrodesulphurisation.
During this process, diesel-range hydrocarbons are exposed to hydrogen under elevated temperature and pressure in the presence of a catalyst.
Hydrogen reacts with sulphur-containing compounds and allows the sulphur to be separated from the fuel.
The process can be simplified into several stages.
Diesel Feedstock Enters the Treatment Unit
A partially refined diesel stream is introduced into the hydro processing system.
Hydrogen Is Added
Hydrogen is mixed with the fuel under controlled conditions.
Sulphur Compounds React
The hydrogen reacts with sulphur-containing molecules.
Sulphur is converted principally into hydrogen sulphide, allowing it to be separated from the diesel stream.
Fuel Is Further Treated
The refined diesel is then processed and blended until it meets the required fuel specification.
Additives May Be Introduced
Further additives can be incorporated to help achieve required characteristics such as lubricity, cold-weather performance, stability and cleanliness.
The final product is still diesel, but with dramatically lower sulphur content.
Ultra Low Sulfur Diesel Specifications
The exact definition depends on the market in which the fuel is being sold.
For US road use, the EPA defines ULSD as diesel containing no more than 15 parts per million sulphur.
UK road diesel requirements are tighter. Current regulations applying BS EN 590-related specifications set several important limits, including:
Property | UK Diesel Requirement |
Sulphur content | Maximum 10 mg/kg |
Cetane number | Minimum 51 |
Density at 15°C | Maximum 845 kg/m³ |
95% distillation recovery | Maximum 360°C |
Polycyclic aromatic hydrocarbons | Maximum 8% m/m |
FAME content | Maximum 7% v/v |
These values illustrate why ultra low sulfur diesel specifications involve considerably more than sulphur alone. Commercial diesel must satisfy several fuel-quality requirements relating to combustion performance, density, composition and compatibility.
Is UK Diesel the Same as American ULSD?
Broadly, they serve the same purpose, but the technical sulphur limits differ.
American ULSD permits up to:
15 ppm sulphur
Current UK road diesel generally permits up to:
10 mg/kg sulphur
For practical comparison, mg/kg and ppm are effectively equivalent at these concentrations, meaning UK road diesel typically has an even lower permitted sulphur level.
HMRC terminology also distinguishes between historic “ultra low sulphur diesel” and “sulphur free diesel”. HMRC guidance describes sulphur-free diesel as containing no more than 10 ppm sulphur.
So, when UK drivers fill a modern diesel vehicle at a normal filling station, they are generally buying fuel that meets a sulphur limit stricter than the American 15 ppm ULSD definition.
ULSD vs Older Low-Sulphur Diesel
The difference becomes clearer in a direct comparison.
Feature | Ultra-Low Sulphur Diesel | Older Low-Sulphur Diesel |
Sulphur content | Extremely low | Significantly higher |
US benchmark | Maximum 15 ppm | Historically up to 500 ppm |
UK modern road standard | Maximum 10 mg/kg | Older standards permitted higher levels |
DPF compatibility | Suitable | Potentially unsuitable |
Modern catalyst compatibility | Suitable | Can impair some systems |
Sulphur-related emissions | Much lower | Higher |
Availability for modern road use | Standard | Largely superseded |
Suitable for new diesel vehicles | Yes | Generally, no |
The EPA previously classified low-sulphur diesel as containing up to 500 ppm sulphur before introducing the 15 ppm ULSD requirement.
That represents a reduction of approximately 97% in the permitted sulphur level.
Why Was Sulphur Removed from Diesel?
Removing sulphur provides two major benefits.
The first is reducing sulphur-related pollutants.
The second, and arguably more important for modern vehicles, is enabling advanced exhaust after-treatment systems.
Older diesel engines could operate using fuel containing substantially more sulphur because they lacked many of today’s emissions-control components.
Modern engines may use:
- Diesel particulate filters
- Diesel oxidation catalysts
- Selective catalytic reduction
- NOx sensors
- Exhaust gas recirculation
- Complex engine-management systems
High sulphur concentrations can interfere with some of these technologies.
EPA guidance notes that particulate traps require low-sulphur fuel because higher sulphur levels can damage or impair emissions-control equipment, while diesel oxidation catalysts work more effectively with ULSD.
Environmental Benefits of ULSD
Ultra-low sulphur diesel is still a fossil fuel, so it should not be mistaken for a renewable or zero-carbon fuel.
Its environmental benefit relates primarily to cleaner combustion products and compatibility with modern exhaust treatment.
Reduced Sulphate Particulates
Sulphur in diesel contributes to the formation of sulphate particles during combustion.
Reducing fuel sulphur therefore reduces this source of particulate matter.
EPA technical guidance specifically notes that sulphate is a constituent of diesel particulate emissions and that lowering sulphur reduces these combustion by-products.
Lower Sulphur Dioxide Emissions
Less sulphur in the fuel means less sulphur available to form sulphur dioxide during combustion.
Better Emissions-Control Performance
One of ULSD’s greatest environmental contributions is indirect.
By allowing advanced exhaust-treatment systems to function properly, low-sulphur fuel enables vehicles to achieve significantly lower overall emissions.
Supports Modern Diesel Standards
Modern diesel vehicles are engineered as complete systems.
Fuel quality, engine calibration and exhaust treatment work together rather than independently.
Using suitable low-sulphur diesel therefore plays an important role in maintaining the emissions performance for which the vehicle was designed.
Main Benefits of Ultra-Low Sulphur Diesel
Compatibility With Modern Diesel Engines
ULSD is designed for modern road vehicles and diesel equipment requiring low-sulphur fuel.
Reduced Sulphur-Related Exhaust Emissions
Lower sulphur means fewer sulphur-derived combustion products.
Protection of Diesel Particulate Filters
DPFs depend on appropriate fuel and engine operation.
Higher-sulphur diesel can interfere with emissions after-treatment, making low-sulphur fuel essential for many modern vehicles.
Improved Catalyst Effectiveness
Diesel oxidation catalysts and other after-treatment technologies can operate more effectively when exposed to very low sulphur levels.
Widespread Availability
For road vehicles in Britain, low-sulphur diesel is the norm rather than a specialist fuel.
Drivers do not usually need to search specifically for ULSD at ordinary UK forecourts.
Suitable for Commercial Applications
ULSD is widely used across commercial transport, logistics, construction and other diesel-dependent industries.
Are There Any Disadvantages of ULSD?
Yes. Removing sulphur solved important emissions problems, but the refining process introduced some considerations of its own.
Reduced Natural Lubricity
Hydrotreating removes sulphur compounds but can also remove some naturally occurring compounds that contribute to fuel lubricity.
Lubricity is important because diesel fuel helps lubricate components within high-pressure fuel systems.
Modern diesel specifications therefore include lubricity requirements, and fuel suppliers use appropriate refining methods and additives to ensure compliant fuel protects diesel equipment.
Higher Refining Complexity
Producing extremely low-sulphur fuel requires more intensive refinery processing than producing older high-sulphur diesel.
This increases production complexity and hydrogen demand.
It Remains a Fossil Fuel
ULSD is cleaner with regard to sulphur, but it still originates primarily from crude oil.
Burning it therefore produces carbon dioxide.
Businesses seeking significant lifecycle carbon reductions may consider alternatives such as HVO where suitable.
Storage Still Requires Care
Low sulphur does not make diesel immune to contamination.
Stored fuel can still experience problems involving:
- Water
- Sediment
- Microbial growth
- Oxidation
- Biodiesel-related stability issues
- Tank corrosion
Good fuel-management practices remain essential.
Does ULSD Have Less Lubricity Than Traditional Diesel?
Raw ULSD coming directly from intensive hydrotreatment can have reduced natural lubricity compared with older diesel.
However, this does not mean properly specified retail ULSD is automatically harmful to modern engines.
Fuel standards include lubricity requirements, and refiners or suppliers can incorporate suitable lubricity-improving additives.
Modern engines are designed around the fuel specification available in their market.
Problems are more likely to arise from fuel that is:
- Off specification
- Contaminated
- Incorrectly blended
- Poorly stored
- Unsuitable for the engine
Rather than simply being caused by the fact that the fuel is ULSD.
What Vehicles Use Ultra-Low Sulphur Diesel?
ULSD is used across a wide range of diesel-powered applications.
Passenger Cars
Modern diesel cars require low-sulphur road diesel to protect their exhaust-treatment systems.
Commercial Vans
Delivery vehicles, service vans and other light commercial vehicles commonly use modern diesel formulations.
Heavy Goods Vehicles
HGV fleets rely heavily on ULSD alongside systems such as SCR, DPFs and AdBlue.
Buses and Coaches
Passenger transport vehicles using diesel engines also depend on suitable low-sulphur fuel.
Construction Equipment
Excavators, telehandlers, loaders and generators may require low-sulphur fuel depending on engine type and emissions standard.
Agricultural Machinery
Modern tractors, combines and other machinery increasingly use sophisticated diesel engines and after-treatment systems.
Industrial Equipment
Diesel generators, pumps and specialist industrial machinery may also use ULSD where the manufacturer’s specification requires it.
ULSD and Diesel Particulate Filters
A Diesel Particulate Filter captures soot particles from diesel exhaust.
Periodically, accumulated soot must be burned off through a process known as regeneration.
Using appropriate low-sulphur fuel is important because sulphur contributes to ash and sulphate formation and can interfere with emissions-control performance.
A DPF should therefore be viewed as part of a complete system involving:
- Correct diesel fuel
- Suitable engine oil
- Proper engine operation
- Exhaust temperature
- Sensors
- Regeneration strategy
Using the wrong fuel can potentially undermine that system.
ULSD and SCR Systems
Selective Catalytic Reduction is another major emissions-control technology.
SCR primarily targets nitrogen oxides rather than sulphur.
Many SCR-equipped vehicles inject AdBlue into the exhaust, allowing chemical reactions within the catalyst to convert NOx into nitrogen and water.
Low-sulphur diesel supports the overall durability and effectiveness of these after-treatment systems.
The EPA notes that its modern heavy-duty emissions standards rely on technologies including SCR and that durability depends on using diesel meeting ULSD requirements.
Ultra-Low Sulphur Diesel vs HVO
ULSD and HVO can both be used in diesel applications, but they are fundamentally different products.
Feature | ULSD | HVO |
Primary source | Crude oil | Renewable oils and waste fats |
Sulphur content | Extremely low | Typically, extremely low/negligible |
Renewable | No | Yes, depending on feedstock |
Lifecycle carbon reduction | Limited | Potentially substantial |
Engine compatibility | Standard diesel engines | Many approved diesel engines |
UK availability | Very high | Growing |
Typical price | Usually, lower | Usually, higher |
Main purpose | Conventional diesel fuel with low sulphur | Renewable diesel alternative |
ULSD addresses sulphur-related pollution but does not solve the wider issue of fossil carbon emissions.
HVO is therefore increasingly considered by businesses seeking to reduce lifecycle greenhouse gas emissions while retaining compatible diesel assets.
Is ULSD the Same as Red Diesel?
Not exactly.
“Red diesel” refers primarily to rebated gas oil containing fiscal markers and dye, whereas ULSD describes fuel according to sulphur content.
Historically, different classes of gas oil could have different sulphur limits.
UK Government information has noted that red diesel intended for certain non-road mobile machinery can have a maximum sulphur content of 10 ppm under BS 2869 Class B2, while gas oil intended for other applications such as heating may fall under different specifications.
Therefore, colour, tax treatment, legal usage and sulphur specification are separate issues.
Can Older Diesel Engines Use ULSD?
In many cases, yes.
ULSD itself can generally be burned in older diesel engines.
However, very old engines may require additional consideration because their fuel systems were designed around older diesel formulations.
Potential issues can include:
- Seal condition
- Fuel-system deposits
- Lubricity requirements
- Previously accumulated contamination
- Fuel-pump wear
The transition from high-sulphur to ultra-low-sulphur fuel occurred many years ago, so most vehicles still operating on UK roads have long been exposed to modern fuel.
Owners of vintage, historic or specialist diesel equipment should nevertheless follow manufacturer or specialist advice.
How Should ULSD Be Stored?
Bulk diesel should be managed carefully whether it contains high or low sulphur.
Good storage practices include:
Keep Water Out
Water can enter tanks through condensation, damaged seals or poor delivery practices.
Water encourages microbial contamination and corrosion.
Inspect Tanks Regularly
Tanks should be checked for:
- Rust
- Sediment
- Sludge
- Water
- Leaks
- Damaged seals
Rotate Fuel
Long-term storage increases the risk of fuel degradation.
Use older stock before newer deliveries where practical.
Use Appropriate Filtration
Filtration helps prevent contaminants reaching engines and fuel-system components.
Monitor Commercial Fuel Stocks
Businesses using substantial quantities should monitor tank levels so that they can arrange deliveries before reaching critically low reserves.
For organisations managing fleets, generators or construction sites, a fuel supplier such as 123 Oil can form part of a planned bulk-fuel management strategy rather than relying on emergency purchases.
ULSD for Generators
Diesel generators are another important ULSD application.
Unlike road vehicles, backup generators may sit unused for weeks or months.
This places greater emphasis on storage quality.
Fuel in standby systems should be monitored for:
- Water contamination
- Microbial growth
- Oxidation
- Sediment
- Filter blockage
- Tank condition
Simply purchasing high-quality ULSD does not guarantee that it will remain in perfect condition indefinitely.
Fuel management is particularly important for hospitals, data centres, industrial facilities and other organisations where generator reliability is critical.
Does ULSD Improve Fuel Economy?
ULSD was developed primarily to reduce sulphur content rather than improve fuel economy.
Any difference in miles per gallon compared with historic diesel formulations depends on several fuel and engine characteristics.
Real-world diesel consumption is far more strongly influenced by:
- Engine efficiency
- Vehicle weight
- Driving speed
- Tyre pressure
- Load
- Idling
- Route
- Traffic
- Weather
- Maintenance
Businesses seeking to reduce diesel expenditure should therefore focus on complete fleet efficiency rather than expecting low sulphur alone to significantly improve consumption.
Common Misconceptions About ULSD
“ULSD Is a Renewable Fuel”
It is not.
Conventional ULSD is still petroleum-derived diesel.
“Low Sulphur Means Zero Emissions”
No diesel combustion is emission-free.
ULSD primarily reduces sulphur-related pollution and allows modern emissions-control technologies to operate effectively.
“All Countries Use the Same ULSD Limit”
They do not.
The US definition commonly uses 15 ppm, whereas modern UK road diesel is limited to 10 mg/kg sulphur.
“ULSD Damages Every Older Diesel Engine”
This is an oversimplification.
Modern commercial ULSD is formulated to satisfy fuel-quality requirements, including lubricity requirements.
“Sulphur Is the Only Important Diesel Specification”
Far from it.
Cetane number, density, distillation, biodiesel content, lubricity, cold-flow characteristics and contamination can all affect diesel performance.
Why Fuel Quality Matters for Modern Diesel Engines
Modern common-rail diesel engines operate at extremely high fuel pressures and use highly precise injectors.
Their fuel systems are much less tolerant of poor-quality fuel than many older mechanical diesel engines.
The quality of diesel can influence:
- Injector performance
- Fuel-pump durability
- Combustion quality
- Starting
- Emissions
- DPF performance
- Reliability
This is why buying from reputable suppliers and maintaining storage equipment correctly can be just as important as meeting the nominal sulphur limit.
Final Thoughts
Ultra low sulfur diesel was a major step forward in diesel-fuel development.
By dramatically reducing sulphur levels compared with older diesel, ULSD helped reduce sulphur-related emissions and made increasingly advanced exhaust-treatment technologies possible.
In the United States, ULSD is commonly defined as diesel containing no more than 15 ppm sulphur. UK requirements are even tighter for road diesel, with current regulations limiting sulphur to 10 mg/kg.
However, sulphur content is only one part of diesel quality.
Modern fuel must also satisfy requirements relating to cetane number, density, composition, biodiesel content and other characteristics to deliver reliable performance.
ULSD remains widely used in cars, vans, HGVs, buses, generators, construction equipment and industrial machinery. Its compatibility with modern diesel emissions systems makes it essential for today’s diesel fleet.
Its biggest limitation is that it remains primarily a fossil-derived fuel. While reducing sulphur improves local air-pollution performance and protects exhaust after-treatment, it does not eliminate carbon dioxide emissions.
For businesses, the best approach is therefore to choose diesel that meets the correct specification, store it properly, monitor fuel quality and assess alternative fuels where broader carbon reduction is required.
Frequently Asked Questions
ULSD stands for Ultra-Low Sulphur Diesel. It describes highly refined diesel containing very low levels of sulphur compared with older diesel fuels.
The limit depends on location. US ULSD contains no more than 15 ppm sulphur, while current UK road diesel specifications permit a maximum of 10 mg/kg sulphur.
Yes. ULSD is the standard type of road diesel used by modern diesel vehicles. Older or specialist machinery should still be operated according to the manufacturer's specified fuel requirements.
ULSD reduces sulphur-related emissions and allows advanced exhaust-treatment systems to operate effectively. However, conventional ULSD remains a fossil fuel and therefore still produces carbon dioxide when burned.
Modern UK road diesel already has an extremely low sulphur content, so fuel purchased at normal forecourts generally meets a maximum sulphur limit of 10 mg/kg.
Properly specified commercial ULSD is designed to meet fuel-quality and lubricity requirements. Problems are more likely to occur when diesel is contaminated, poorly stored or does not meet the required specification.
Older low-sulphur diesel could contain significantly more sulphur. In the US, low-sulphur diesel historically allowed up to 500 ppm, while modern ULSD is limited to 15 ppm.