Funny, look at this:
http://www.free-definition.com/Cetane.html
Cetane or CN is to diesel fuel what octane is to gasoline. It is a measure of the fuel's combustion quality.
Cetane is actually an alkane, 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.
So, the cetane number measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components.
Cetane number is a measure of how readily the fuel burns. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.
There is very little actual cetane in diesel fuel. 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. 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.
Im not sure I buy the stuff in bold.
After reading the free-definition.com/cetane definition and RogueTDI’s comments about the lines in bold, I think a few things need to be explained here.
I wrote that article back in March of 1999. The original article was one of several I posted on a Mercedes Benz email list. From there it was cross posted to several other lists including a VW IDI email list, and several diesel pick-up truck lists. Parts of this article have been posted on a few websites and everyone seems to edit and add to it as they please, including
www.free-definition.com. One article made it to the Turbo-Diesel Register. The editor of the TDR actually paid me for it. So technically I’m a professional writer! There is no copyright on this stuff so in a way I’m proud it was written well enough that it is still being read, although in constantly altered form.
The original context was to be just technical enough to get the points across and not bore and lose the non-technical readers. The technical fundamentals are still correct but some of the information is a bit dated now. Back in 1999 there were such things as diesel fuel with high cetane numbers in the 50 to 60 range. However, these were experimental fuels that only existed in fuel testing labs and not available to the public. Fast-forward five years and we are in a whole different world now. So by today’s standards, I am in complete agreement with RogueTDI’s statement. When viewed in the context of today’s high tech engines and fuels, the bold text is incorrect.
New diesel engine technology CAN take advantage of very high cetane fuels. And with that I have to add that it is going to take a few years for this high cetane diesel fuel to be widely available. So now you have some bold text that is correct.
You can read the articles I wrote at:
http://www.mercedesmailinglist.com/
Click the archives page and type cetane in the search engine. Here is a direct link to the archives page. All the articles I wrote about diesel fuel and cetane were written in 1999.
http://www.mercedes.cx/archives/
Here is the original unedited article as I posted it on March 31, 1999. Do not use the email address in the article to contact me, it is invalid. Use the email address in my profile here at TDIClub.com.
Premium Diesel (long)
(Brian.P.Kmetz@ucm.com)
Wed, 31 Mar 1999 13:56:13 -060
Now we have a thread that I can talk about! Part of my job is to extract the BTUs through oxidation (burn) from mass quantities of methane and fuel oils so I have a clue how some of this stuff works.
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.,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 paraffin's. 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.
All 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. I buy it when I travel into northern Wisconsin. Because there are no legal standards for premium diesel yet, it is very hard to know if you are buying the good stuff. I have good news. 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. That's about the best advice I can give. 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 as I type this.
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.
When I learn more, I'll post more.
Brian Kmetz
brian.p.kmetz@ucm.com
MBCA Central Illinois
Chicago VW Org.
85 MB 300D-T
97 VW Passat TDI