Summary
ULSD in off-road operations has the same pro’s and con’s as seen in on-road, but mostly technical and operational issues. Putting actual numbers to benefits is tough, as the numbers depend on what they are compared to originally - including actual engines, fuels, oils, additives, etc. Locomotives operating in Northern climates may use a heavier mix of No.1 diesel, similar to Jet fuel, so that confuses the issues.
Analysis
To an outsider, the diesel fuel marketplace may look simple. In reality, No.2 distillate fuel is made up of on-highway diesel (over half), home heating oil (over 10%), and then equal amounts of fuels (appx. 5%) of rail, marine, farm, commercial, industrial, off-highway and the rest. Some distillates are more closely related than others. Then there is No.1 distillate - and now renewable and Bio-diesel in the mix. Characteristics of each can vary by the type of crude oil used, which is a boon for the additive industries. States regulate fuel specifications and they may vary, but they mostly try to follow the same standards.
The bottom line is that comparing the benefits depends on the basis of what is being compared, so generalizations come with caveats. Additionally, there are different issues with low speed diesels like in locomotives versus high speed engines in trucking versus higher speed diesels in autos versus high load factors (like in farming). I’ll take a stab at it anyway!
As we know, US sulfur level mandates have been changing and are different for that used for home heating and aviation (<3,000-5,000 PPM) versus that now being sold in Low Sulfur Diesel (LSD) off-road (<500 PPM) and trucking (<15PPM). Each have different ASTM specs, as does the rest of the world with their own nomenclature.
An oversimplified look at what sulfur in fuel does follows - starting with lowering NOx generation in engines. To get NOx down, the manufacturers use exhaust recirculation which results in engines running hotter. Hotter running engines increase soot (hydrocarbons) which results in the utilization of diesel particulate filters (DPF’s) to trap and burn off with the selective catalyst reduction (SCR) technology. Factors affecting the life of an SCR/DPF are fuel quality and engine oil.
Sulfur plugs the DPF at a rate of 25X+ with 500PPM v. 15PPM diesel, so therein lies the need for 15PPM ULSD with DPF/SCR's. With higher levels of sulfated ash, phosphorus, sulfur and zinc older oils, these can also result in plugged DPF’s - therefore the new oils (and additives). Again from this is a brief look, one can hopefully see that ULSD is mostly a technology enabler.
ULSD by itself in older engines lowers soot / hydrocarbons by up to 10% depending on a number of items. The noted 13% reduction in hydrocarbons in the article can be within the margins, albeit on the high side. Arguments about lower CO and NOx can be made, but the amounts if any are debatable - again depending on the reference point. Cetane is generally higher with ULSD. The other positives have to do with price and availability of USLD versus LSD, as this will be an opportunity to smooth out the current swings being experienced.
The negatives are noteworthy, but at least they are tackling it now instead of later. I may be missing something, but using ULSD in older engines with older oil will not be a negative as reported. That is our experience on the trucking side so far. If one uses LSD with the new oils, there may be deterioration – but new oils aren't needed here in locomotives yet.
The main negatives however have to due with fuel lubricity, seal leakage, water in the fuel, bacterial growth, loosening deposits in tanks, fuel gelling, higher fuel usage and a bigger carbon footprint (again an argument). The process used to remove sulfur also removes the natural lubricants in the fuel of oxygen / nitrogen compounds, aromatics, hydrocarbons and sulfur. This results in higher fuel systems wear / premature failures, so additives (or Biodiesel) helps. As aromatics lower with sulfur removal, the elastomer quality of the seals change – they shrink, dry out, crack and leak. Replacing seals fixes the problem and the new ones adapt to the new fuel. Water forms more readily with ULSD, allowing for more biological growths (which sulfur tended to kill). USLD also can act more as a solvent, loosening deposits in fuel tanks. ULSD has a higher cold pour point (more prone to gelling) due to aromatic removal. Water, loose deposits and geling will show up as plugged fuel filters, but additives help here, too. Finally, the energy content is a couple percentage points less with ULSD, resulting in slightly increased fuel consumption (and the increased carbon footprint argument).
If some of this sounds like Biodiesel introduction issues, there are many similarities. To those using some Biodiesel today, ULSD will be more of a cake walk! To those who plan for these issues, they will not be show-stoppers, but quite annoying.
