Understanding Ultra-Low Sulfur Diesel and Clean Diesel Technology

About one-third of the country’s transportation fleet runs on diesel engines.

But while it’s safe to say that diesel-powered vehicles are the minority compared to gasoline engines, they have more detrimental effects on the environment than the latter.

Diesel fuel contains more sulfur than gasoline. Because of this, diesel exhaust tends to produce toxic pollutants like particulate matter, nitrogen oxide, and hydrocarbons.

To combat the negative effects of diesel the Environmental Protection Agency (EPA) began regulating the use of ultra-low sulfur diesel.

What Is Ultra-Low Sulfur Diesel?

Ultra-low sulfur diesel fuel is a more refined version of regular diesel that contains less than 15 parts per million (ppm) of sulfur.

Regular diesel contains approximately 500 ppm of sulfur. Using ultra-low sulfur diesel can reduce sulfur content by as much as 97%.

The EPA mandated the use of ultra-low sulfur diesel in 2006. The mandate applies to all vehicles manufactured from 2007 onwards. The transition to this type of diesel fuel was completed in 2010.

Ultra-low sulfur diesel has a lighter color and fainter smell compared to regular diesel.

Paying the Price for Using Ultra-Low Sulfur Diesel

The purpose of switching to ultra-low sulfur diesel fuel is to reduce the emissions of sulfur oxides and particulate matter from heavy-duty engines and diesel-powered vehicles.

While using this diesel variant brings the population a step closer to reducing the harmful effects of vehicles on the environment, doing so comes with a hefty price tag.

Producing ultra-low sulfur diesel requires additional steps to the refining process, so you’ll have to pay more for this fuel.

The reduced sulfur content on this variant makes it less efficient than regular diesel. Using ultra-low sulfur diesel will require you to top up on fuel more frequently than you’d like.

Lastly, the reduced sulfur content in ultra-low sulfur diesel affects its lubricity. So certain engine parts can wear out faster than normal, leading to increased maintenance costs.

Ultra-Low Sulfur Diesel Specifications

Ultra-low sulfur diesel powers vehicles that were manufactured from 2007 onwards. But what about the older model years?

Old diesel engines will run fine on ultra-low sulfur diesel, but there will only be a small reduction in particulate matter. It’s also recommended to replace gaskets and seals when switching to low-sulfur diesel.

Meanwhile, newer vehicles tend to develop problems when injected with high-sulfur diesel fuel.

Refueling a new engine with high-sulfur diesel is a federal law violation and voids the engine warranty. The excess sulfur can plug the particulate trap, resulting in back pressure and potential engine damage.

How Catalysts deal with NOx and Sulfur

As a diesel engine converts fuel to energy, combustion chamber heat soars above 2,500 F (1371° C), which causes nitrogen and oxygen in the air part of the mix to bind together, forming various different oxides of nitrogen (NOx), which are stored in the diesel’s NOx catalytic converter.

To facilitate this NOx storage, the temperature of the catalyst must be maintained in the 575-750° F (300-400° C) window. There is an exhaust gas temperature sensor that keeps track of the temperature of the converter.

As a diesel engine converts fuel to energy, combustion chamber heat soars above 2,500 F (1371° C), which causes nitrogen and oxygen in the air part of the mix to bind together, forming various different oxides of nitrogen (NOx), which are stored in the diesel’s NOx catalytic converter.

– Richard McCuistian, ASE Certified Master Automobile Technician

Storage of NOx and Regeneration

The oxides of nitrogen (NO) and any oxygen (O₂) are both trapped by the barium oxide layer of the converter as barium nitrate.

There is also a NOx sensor downstream of the NOx catalyst that measures the oxides of nitrogen flowing out of the catalytic converter. When the NOx reading exceeds a threshold value, the NOx storage catalyst is then emptied in a process called regeneration because it makes the catalyst like new again by removing stored NOx. Think of this as you would be cleaning a filter rather than replacing it.

To empty the catalyst of stored NOx, the ECM delivers extra fuel so that the engine runs very rich for a short time. This drives the exhaust gas temperature up to the 930-1100° F (500-600° C) range. At these temperatures, the barium nitrate in the converter reacts with carbon monoxide (CO), converting it to nitrogen and harmless CO2, which is what plants breathe.

That’s why the grass is so green and fast-growing by so many highways; the grass thrives due to the CO2 exhaust from vehicles.

Sulfur Regeneration

That brings us to the issue with sulfur (S) in the fuel, which turns out to be a problem for the NOx storage catalytic converter. Why? Well, it’s because the NOx catalyst stores sulfur as well as oxides of nitrogen, and that stored sulfur reduces the catalyst’s storage capacity for oxides of nitrogen.

The ECM calculates the need for sulfur regeneration using the signal from the NOx sensor.

Furthermore, sulfur is not removed from the catalyst during regeneration/removal of the nitrogen oxides, thus, there is a separate regeneration process to deal with sulfur.

During regeneration of sulfur, the engine runs on a rich mixture. The NOx catalytic converter reaches a temperature window of 1100-1200° F (600-650° C), which releases sulfur and allows it to flow out of the exhaust.

“Premium” and “Excellium” branded fuels contain sulfur to make them more suitable for vehicles with NOx storage catalytic converters.

Other Advancements in Clean Diesel Technology

Aside from ultra-low sulfur diesel, there are other developments in diesel technology that aim to reduce harmful exhaust emissions, including biodiesel and e-diesel fuel.

Biodiesel

Biodiesel is a renewable fuel from vegetable oils, animal fats, or even restaurant greases. This biodegradable fuel is known as mono-alkyl esters of long-chain fatty acids. It’s essentially pure fuel that’s blended with diesel fuel.

Biodiesel isn’t any different from regular diesel in terms of some features.

It helps vehicles maintain similar horsepower, torque, and fuel economy with the added benefit of costing less when purchased in bulk. It also helps lubricate engine components and has the same maintenance requirements as regular diesel.

But because biodiesel has a higher cetane number than regular diesel, it can also improve the engine’s overall performance.

Biodiesel blends are denoted as “BXX,” with “XX” representing the percentage of biodiesel in the mixture. Biodiesel is denoted as “B100” in its pure form. Keep in mind, however, that using pure biodiesel might require certain engine modifications.

Meanwhile, biodiesel blends such as “B20” indicates that the mixture contains 20% biodiesel and 80% petroleum diesel.

Biodiesel generally costs 30 to 40 cents more per gallon than regular diesel.

E-Diesel

E-Diesel, also known as “diesohol,” is standard No. 2 diesel with up to 15% of ethanol content.

The ethanol in e-diesel lowers its cetane number, so using this fuel type can cause a delay between injection and ignition.

A cetane-enhancing additive is thrown into the e-diesel mix, which can either be ethylhexylnitrate or ditertbutyl peroxide.

Using e-diesel can reduce particulate matter emissions by as much as 40%, carbon monoxide by 20%, and oxides of nitrogen by 5%.

Ultra-Low Sulfur Diesel FAQs

Get straight answers for some of the most frequently asked questions about ultra-low sulfur diesel.

How Long Will Ultra-Low Sulfur Diesel Last?

Ultra-low sulfur diesel can last anywhere between six and 12 months if kept around 70℉. It can last up to more than a year if kept below 70℉.

Can You Mix Additives to Ultra-Low Sulfur Diesel?

Yes. Because ultra-low sulfur diesel has a lower lubricity than regular diesel, it can cause certain engine components to wear out faster.

Using a lubricity additive for ultra-low sulfur diesel can help reduce engine wear scarring of various engine parts.

Can You Mix Ultra-Low Sulfur Diesel With Regular Diesel?

No. Mixing the two can result in problems like tank corrosion.

What Makes Diesel Go Bad?

Water or condensation, heat, bacteria buildup, fungus growth, and contact with zinc or copper can make diesel go bad.

The Bottom Line

Today, ultra-low sulfur diesel is the standard type of diesel used in almost all modern vehicles with diesel engines. The use of ultra-low sulfur diesel is mandated by the Environmental Protection Agency, which means that failure to do so can result in a federal offense.

There are, however, some specifications for using this type of diesel fuel.

Older engines are still allowed to use high-sulfur diesel, but they can switch to the environment-friendly variant if they make certain engine modifications like replacing gaskets and seals.

But while this type can significantly reduce the amount of sulfur in the atmosphere by as much as 97%, it does have drawbacks.

Ultra-low sulfur diesel tends to cost more than regular diesel because of the refining process. It’s also less efficient than regular diesel and can cause engine components to wear out faster than normal because of its low lubricity.