Fuel oil is the heaviest form of commercial fuel. It has many uses, everything from heating, transportation, and marine fuels. Like all other fossil fuels, it is made from crude oil. Fuel oil is mainly for industrial use and is most commonly known as a fuel type called diesel. But is diesel fuel and fuel oils the same thing? No, they are not. Diesel fuel is only one type of fuel oil. Fuel oil is actually broken down into several categories. So what is the difference in diesel fuel and fuel oil? What are some of the other categories of fuel oils?
What Is Diesel Fuel
Eighteen-wheelers, heavy work trucks, and a few types of cars all have something in common: they all run on diesel fuel. Diesel fuel is heavier than gasoline but lighter than lube oil. It is a blend of petroleum compounds called middle distillates and might contain additional additives. Other types of middle distillates include kerosene and No. 2 Heating Oil.
Unlike gasoline, which is ignited in the engine by the spark plugs, diesel fuel operates under compression. In a diesel engine, diesel fuel is injected into a into the compressed, high-temperature air in the combustion chamber and ignites spontaneously. The air is compressed by a piston that travels up the cylinder. When the piston reaches the top of the cylinder, diesel fuel is injected into the cylinder. The friction of the compression causes the fuel to ignite.
All diesel fuel has a cetane number. This is the measure of the ease in which diesel fuel is ignited in your engine. The cetane number of diesel describes the fuel’s ability to smoothly start-up, warm-up period, and even combustion. Every type of diesel engine has a minimum cetane requirement. However, if the cetane number os higher than what is recommended, it does not necessarily improve the performance and efficiency of the diesel engine. Most manufacturers of diesel engines recommend diesel fuels with a cetane number of at least 40 and most fuels meet or exceed this requirement..
Diesel fuels vary depending on the distillery facility. This depends on the latitude and weather of the facility. Unlike gasoline, diesel fuel does not react well with cold. Heavier diesel fuels will thicken and congeal in cold weather which is why diesel engines are often left running when it cold. Otherwise the engine may not start.
Diesel fuel is not as volatile as gasoline, kerosene, or other types of fuels. Depending on the method of distilling, diesel fuel will have a flash point (the point of combustion) of anywhere from 120-degrees to 160-degrees. It does not burn as clean as gas, though, which is why diesel pumps and engines are often dirtier than their gasoline counterparts. This is also why you see smoke coming from the stacks of an eighteen-wheeler every time it changes gears.
There is a cleaner burning form of diesel fuel which is called biodiesel. Biodiesel is produced from vegetable oil or animal fat. It works due to the reaction of alcohol with fatty-acids to produce a mono-alkyl ester. The technology is still in the early stages but a blend of biodiesel and petroleum fuel can be used in diesel engines without any harm or modifications to the engine.
What Is Fuel Oil
Fuel oil is classified as any liquid fuel that is burned in a furnace or broiler to generate heat or is used in an engine to provide power. In a broad term, fuel oil is referred to as the heaviest form of commercial fuel. Fuel oils are created by distilling crude oil and removing different fractions to make the different types of fuel. It is comprised of a mixture of petroleum distillate hydrocarbons. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics.
Fuel oil is divided into two main types which are then broken down into six different categories or grade numbers. The two types are distillate oils and residual oils. Distillate oils are refined from crude oil. Residual oils are less common because they are so thick, they have to be heated with special equipment before use and they contain a high amount of pollutants such as sulfur.
The grades go from grade No. 1 through No. 6. depending on their boiling point composition and purpose. No.1 through 3 are distillate oils and No. 4 through 5 are residual oils. The higher the grade of fuel oils, the cheaper they are. For instance, No. 6 grade cannot be used in vehicles and small engines but it is the cheapest fuel available. The grades for fuel oils are as follows:
- No. 1: Similar to kerosene and is the fraction that boils off right after gasoline.
- No. 2: This is diesel fuel that trucks and some cars run on. It is also referred to as road diesel.
- No. 3: is a distillate fuel oil and is rarely used.
- No. 4: This fuel oil is often a blend of distillate and residual fuel oils but is sometimes just a heavy distillate. It can be classified as diesel, distillate or residual fuel oil.
- No. 5: A residual fuel oil which is heavier than the distillate oil. This is is a mixture of No. 6 (about 75-80%) with No. 2. Used in boilers, trains, and large ships.
- No. 6: A residual fuel oil also called Bunker C. This is the remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation. Used in boilers, trains, and large ships.