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About Diesel Fuel

By Bill King 12/8/2000

With all of the recent talk about using Kerosene (number 1 diesel) 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.  

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 paraffin's; 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 distillates -- slightly heavier than kerosene and slightly lighter than industrial (bunker) fuel oil.  Like automotive gasoline, diesel fuel is refined into several sub-categories 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 power plants or railroad locomotives. Even though Number 4 Fuel Diesel Fuel has an ignition quality similar 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 distillate category is Kerosene.  Like Number 4 Fuel Diesel Fuel, Kerosene has an ignition 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 quality of diesel fuel is why some Old-timers 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 their 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 suppressants, ignition accelerators and biocides to kill fungi and bacteria that we generally assume to be present in the Diesel Fuel at the pump.


#2 diesel should have a cetane rating between 40-55. If I remember correctly, nothing lower then 45 should be run in most common diesel engines, lest it's non-turbo. The higher the cetane rating the better, especially on cold days, as it will make starting an easier process. In the winter, look for the highest number you can find. Also ensure you are using a good additive (Power Service Grey bottle, I have personally had good luck with..it not only has the antigel/anti wax agent, but it also provides for good lubrication of the injector pump, and thus should also keep the rest of the system 'good to go').

Dump Motor Oil or ATF in your Diesel ?

by Peter Hipson 11/10/2003

There is *no* advantage to adding motor oil, or transmission fluid to diesel fuel. Period.

There are disadvantages to doing this however, including damaged injector pumps (especially with turbo diesel electronic pumps), clogged filters, etc.

The manufacturer of our injector pumps strongly cautions against using either when their pumps are used.

There are many additives that are approved, and are in fact *much* cheaper than either motor oil, or transmission fluid. As an added bonus, these additives really work!

Most diesel fuels and engines don't need additives, but if you feel the urge to pour something else in the tank besides diesel fule, use a product that is *specifically designed for the purpose*.

Kerosene is added to diesel fuel by some suppliers, though in small quantitites. Kerosene has virtually no lubrication qualities--adding more (and an essientially unknown amount, since you don't know if or how much has already been added by the supplier!) is a sure way to cut the life of your injector pump. Kerosene is routinely added to home heating oil, in large quantities. The furnace doesn't know, or care. The furnace oil pump does not have the same clearances (they are more crude, greater clearances, lower pressure...) and the kerosene won't hurt them. Most will (and often do) run on straight kerosene--here in NH, if the oil tank is outside, the mix will be either 50/50 or straight kerosene. Kerosene doesn't have the same heat values either, you won't get the same amount of power from a gallon of kerosene as from heating oil, or diesel fuel.

#2 diesel fuel is basically #2 heating oil, with the exception that rather than adding kerosene the supplier will add an anti-gel additive. Costs more, but then it sells for more. In the summer, there is virtually no difference, other than legal issues. (again well debated).

Bottom line:

A wise owner will add nothing to the fuel tank that is not designed for the purpose. People who pour whatever they have at hand (motor oil, atf, gasoline additives (yes, some do!)) simply add to the profits that repair shops (such as mine) make. It is a big buck job to replace the injector pump, you dno't want to do it any more than necessary.

The 6.5s injector pump is good for at least 100K miles if not abused. Most owners get many more miles than that, however.