Diesel, what the heck is it, anyway?

Diesel Fuel

The following information was posted to the VW-DIESEL mailing list by list member Brian Kmetz of MBCA Central Illinois in Chicago. where he works as a diesel fuel tester:

In the USA, all diesel fuel must meet the specifications set forth by the American Society for Testing and Materials (ASTM). In Canada it is the Canadian General Standards Board. They have web pages at www.astm.org and www.pwgsc.gc.ca. One can order documents for a fee. Diesel fuels are covered in documents ASTM D-975 and CGSB 3.6-M90 and 3.517-93. Their documents cannot be previewed. The CGSB lists the ASTM as a reference, they are that close. All the specifications for cetane, pour points, viscosity, flash point, BTU content, etc., are in these documents.
The most common question, Cetane? What is cetane? Cetane is to diesel fuel what octane is to gasoline. It is a measure of the fuel's ignition quality and performance. Cetane is actually a hydrocarbon chain, its real name is 1-hexadecane. It is written as C16H34, or a chain of 16 carbon atoms with 34 hydrogen atoms attached. All HC chains are also referred to as paraffins. Cetane is a hydrocarbon molecule that ignites very easily under compression, so it was assigned a rating of 100. All the hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. There is very little actual cetane in diesel fuel.
Most of the hydrocarbons in diesel fuel have similar ignition characteristics as cetane. Cetane is abbreviated as CN. A very loose way to think about cetane is if the fuel has a CN of 45, then the fuel will ignite 45% as well as 100% cetane. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage to keep raising the CN past 50. After that point the fuel's performance hits a plateau.
Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. The minimum CN at the pump is 'suppose' to be 45. The legal minimum cetane rating for #1 and #2 diesel is 40. Most diesel fuel leaves the refinery with a CN of around 42. The CN rating depends on the crude oil the fuel was refined from. It varies so much from tanker to tanker that a consistent CN rating is almost impossible. Distilling diesel is a crude process compared with making gasoline. Gasoline is more of a manufactured product with tighter standards so the octane rating is very consistent. So the CN rating at the pump can be anywhere from 42-46. That's why there is almost never a sticker on a diesel fuel pump for CN.
Premium diesel has additives to improve CN & lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersant, and other additives depending on geographical and seasonal needs. More biocides added in the south in summer, more anti-jell added in the north in winter. Most retailers who sell premium diesel will have little brochures called POPs (Point of Purchase) at the counter explaining what's in their fuel. Please don't ask the poor clerk behind the counter any technical questions after reading this post, all they know how to do is sell you beer, milk, cigarettes, lottery tickets, and take your money.
Texaco and Amoco are two big names who sell premium diesel in limited markets. Amoco mostly sells its Premier to specialized industrial and agricultural markets. I cannot get either in my area. Most fuel retailers buy additives or buy treated fuel. In the northern plain states, Koch is a well known marketer of premium diesel.
Because there are no legal standards for premium diesel yet, it is very hard to know if you are buying the good stuff. An ASTM task force has drafted standards for premium diesel. When the new specifications are accepted, information will have to be posted on the fuel pump. Retailers will no longer be allowed to label cheap blended diesel as 'premium'. They will have separate pumps with clear labels on both informing the customer what in being sold. The marketing and labeling will the same as with regular and premium gasoline. Retailers selling the real thing use this system now. Enforcement of all fuel standards is done on the state level in the USA.
Diesel fuel is an international commodity for industry with no brand name recognition. Because of this it's made as cheap as possible and is transported through most of North America by pipeline. At the area terminal you will see tankers with every oil company logo ever imagined all filling up with the same fuel. So don't get too picky about where you fill up. Shop for price from a large volume retailer so you have the freshest fuel.
The reformulation of diesel fuel in North America is due an international effort for lower emissions. Cleaner diesel emission laws are on the way. Diesel fuel is going to be reformulated into a cleaner fuel in general. Without getting too technical (this is very simplified and over generalized), diesel fuel for the most part is made up of two different hydrocarbon families: paraffin's and aromatics. The paraffin's have a naturally high cetane index, burn clean, but cause that annoying jell problem in winter. The aromatics have a naturally high lubricity, low cetane index, and cause a lot of diesel emissions and soot. Reformulated diesel will have a higher paraffin content, higher cetane number, and a much lower aromatic and sulfur content. It will also be more prone to jelling and have a lower lubricity. Big oil is working on improved additives.
The reason nothing has happened yet is because of infighting in the EPA on its new Tier II Emissions standards for gasoline and diesel. Ultra-clean technology for gasoline and diesel engines is almost ready to go but the refiners have to lower the sulfur level drastically in both fuels. Something should be formally set by the EPA by year 2000, with oil and auto industries whining and slowly complying shortly after.