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Here is some information I received from a discussion with one of the major oil refiners/distributors a while back:
Mark L. Gaubatz
David Rubinoff wrote:
> Yep that's my question...Texaco advertises PREMIUM DIESEL
None (see prior post).
> Is there any kind of "octane-type" rating differentiation?
Diesel uses a "cetane" rating. For the most part, you don't have to worry about it if you're buying fresh fuel from a station that has a lot of diesel traffic.
One of the local mini-chains where I live advertises their cetane ratings and the "additional power" that their diesel fuel provides. I have found no basis for this advertising claim in real life use (the "additional power" must be a laboratory exercise), and their fuel performs no better on average than from their competitors in the area.
What has made a difference, however, is the ability to purchase fuel from some card-lock facilities - the fuel is fresh and the price is right.
Mark L. Gaubatz
With all of the recent talk about using Kerosene as a motor fuel and the differences between grades of Diesel Fuel, I thought I would pass along to the group my short course on refined petroleum. If anybody wants more specific information about any matter that I touch on, please contact me privately by e-mail and I'll be glad to go into further detail.
In the hierarchy of refined petroleum products from highest to lowest (from a gaseous state, then liquid, to solid) are: natural gas; "wet" natural gas; high-octane aviation gasoline; automotive gasoline; finished kerosene; ; home heating oil; diesel fuel; industrial fuel oil; finished lubricating oils; waxes and paraffins; gas oil; coke and finally asphalt. Also moving from highest to lowest, the viscosity, or stiffness, of the refined product increases. For example, at room temperature, automotive gasoline flows much more freely than finished lubricating oils.
Diesel fuel lies in the middle of the refined petroleum hierarchy and is considered one of the middle distallates -- slightly heavier than kerosene and slightly lighter than industrial (bunker) fuel oil. Like automotive gasoline, diesel fuel is refined into several sub-catagories or grades. From highest to lowest viscosity are Number 1 Diesel Fuel (1-D), Number 2 Diesel Fuel (2-D) and Number 4 Fuel Diesel (4-D). There used to be a Number 3 Diesel Fuel (3-D), but it is no longer refined.
Number 4 Fuel Diesel Fuel is slightly lighter than industrial fuel oil and is used in low and medium speed engines that operate at a constant or near-constant speed, such as stationary powerplants or railroad locomotives. Even though Number 4 Fuel Diesel Fuel has an ignition quality similiar to Numbers 1 and 2 Diesel Fuel, it is too thick to work well in a truck engine where the load on the engine is constantly changing and requires varying amounts of fuel to be injected into the cylinders.
Just above Diesel fuel in the middle distallate catagory is Kerosene. Like Number 4 Fuel Diesel Fuel, Kerosene has an iqnition quality similar to Numbers 1 and 2 Diesel Fuel. But unlike Number 4 Fuel Diesel Fuel, which is too thick, Kerosene is too thin to work well as an engine fuel. The thickness of the diesel fuel itself acts as a lubricant to prevent wear of the engine's fuel injectors. This lubricating quailty of diesel fuel is why some Oldtimers still refer to it as "Diesel Oil." Adding a common lubricant to Kerosene usually decreases its ignition quality.
Numbers 1 and 2 Diesel Fuel are the primary fuel for mobile diesel engine applications. Number 1 Diesel Fuel is commonly labeled at the pump as "Premium Diesel" or with a Cetane number of 44 or 45. It is not as thick as Number 2 Diesel Fuel and for this reason is the choice for motorists during the cold winter months. The disadvantage of Number 1 Diesel Fuel is that it does not have the lubricating qualities associated with Number 2 Diesel Fuel. While Number 2 Diesel Fuel has a higher lubricating quality than Number 1 Diesel, its thickness can cause rough starting in a cold engine and rough-running in cold weather. Number 2 Diesel Fuel is usually labeled at the pump with a Cetane number of 40.
Home Heating Oil is closest to Number 2 Diesel Fuel in ignition quality and lubricating ability. But before anybody rushes to put this non-road taxed fuel in thier truck, consider this: refiners don't intend Home Heating Oil to be used in an internal combustion engine and the furnace fuel that is sitting in your basement tank may or may not have the smoke surpressants, ignition accelerators and biocides to kill fungi and bacteria that we generally assume to be present in the Diesel Fuel at the pump.
> The one that remains is, what exactly > in the Cetane number? I know that Octane is a measure of a gasoline's > resistance to pre-combustion. Is the Cetane number a similar type of scale?
Cetane basically plays the same role in diesel engine technology that the octane number plays in the conventional gasoline engine, in that Octane and Cetane numbers are measurements used to express the ability of each to reduce engine knocking. Engine knocking ( a rattling, or pinging sound), indicates abnormal combustion, or an octane/cetane rating that is below the requirement of the engine at that moment. In a gasoline engine, the spark that initiates the combustion should fan out from the spark plug and travel evenly across the combustion chamber. If the end of the air/fuel mix ignites before the flame front, the sudden jump in pressure in the cylinder creates a knocking sound. In the diesel engine, the fuel ignites in the presence of hot air, utilizing Cetane, (an autoignited fuel - no spark), which is a rapidly igniting hydrocarbon that provides ignition. The hot gasses generated, move through the cylinder, and the knock is generally a result of nonuniformity of that pressure. And, although gasoline and diesel are different chemicals: Gasoline being a mixture of iso-octane and heptane, and diesel being methylnaphthalene, each can be viewed similarly in terms of the Cetane and Octane numbers and the measurement that they reflect.
Octane and cetane are both measurements of ignition performance, but they represent completely opposite performance characteristics.
To put it another way, octane represents the fuel's ability to resist pre-ignition. In diesels, just the opposite is desired. The fuel should readily ignite, and cetane is a measurment of that characteristic. In both cases, the higher the number, the better the performance. Generally, diesel cetane falls around low to mid 40s. Premium diesel should be 50+ cetane.
As a side note, the stanadyne additive claims to improve cetane by up to 5 points. This is why they claim improved mileage.
The Cetane Number you see on the diesel pump at the service station, is an indication of the ignition quality of the diesel fuel. Ignition quality is the ability of the fuel to ignite and burn. The higher the Cetane Number, the faster the fuel burns, or the shorter the time between the entry of the fuel into the cylinder until it ignites.
Cetane itself is actually a clear liquid hydrocarbon which ignites very very easily. Using the Cetane as a test fuel it is compared with diesel fuel to determine the ignition quality of the diesel fuel. The Cetane rating scale ranges from 1 to 100, with Cetane itself at the top of the scale at 100.
As I mentioned in my earlier post to the group, Number 1 Diesel Fuel has a Cetane Number of 44 or 45 and Number 2 Diesel Fuel has a Cetane Number of 40. Whenever you see a pump labeled "Premium Diesel," they're usually talking about Number 1 Diesel Fuel. Since Number 1 Diesel Fuel has a higher Cetane Number, it will ignite easier and is good for use in cold weather.
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