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What is Diesel Fuel?

By Eric Tallberg
Updated May 23, 2024
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Diesel fuel is a petroleum-based fuel that's used to power many types of vehicles and boats. It's made of a blend of crude oil components called hydrocarbons. The components for making this fuel are refined out of crude oil, usually by fractionate distillation. Though it's often used for similar purposes to those of gasoline, it burns differently, and so needs a different type of engine to work. Additionally, although diesel is heavier and less volatile than gasoline, it is often more efficient, especially with heavy loads.

Characteristics

This fuel is relatively dense and oily, leading some people to mistakenly call it diesel oil. It is composed of a blend of different types of hydrocarbons, including paraffins, naphthenes, olefins, and aromatics. Different types of diesel fuel have different blend ratios, depending on what the fuel will be used for, the temperature of the area in which it will be used, and regional governmental regulations. One of the main components addressed by governments is sulfur, which can lead to very harmful emissions when the fuel is used. In modern times, much of the diesel sold in the US and EU is Ultra-Low Sulfur Diesel (ULSD), which has most of the sulfur removed.

Uses

This fuel has a wide range of uses, including in private vehicles, public transportation vehicles, and 18 wheelers and other large delivery trucks. Many off road vehicles also run on diesel, as do many boats. Its efficiency makes it popular for use in farm and military vehicles as well. Diesel is used to power machinery as well: a lot of industrial and construction equipment runs on it. Also, it's commonly used to power generators.

Production

Diesel is usually produced by distillation in oil refineries. Different hydrocarbons of crude oil vaporize at different temperatures — those used in making diesel usually vaporize at between 482 and 662° F (250 and 350°C). When the oil is heated to that temperature in a device called a fractional distillate column, the specific hydrocarbon chains are vaporized and are pulled out for use. This can also be done with the use of chemicals instead of a fractional distillate column. After the components are isolated, they're blended together in specific ratios to make different types of diesel fuel.

Compared to Gasoline

Diesel fuel is about 18% heavier than gasoline and consists mainly of hydrocarbons that range from C10 to C24, meaning 10 to 24 carbon atoms with various configurations of hydrogen atoms attached to the carbon atoms. Gasoline, on the other hand, is usually in the C7 to C11 range. The higher the number of carbon atoms, the heavier the product. Due to this, diesel often solidifies when it gets cold, while gasoline is basically unaffected by colder temperatures.

Though diesel fuel is heavier than gas, it is lighter than other petroleum-based products such as crankcase oil and lubricating oil. With a flash point of 120-degrees to 160-degrees, depending on the method of distillation, it is not as volatile as either kerosene or gasoline. Gas, however, burns considerably cleaner than it. Despite this, diesel has a higher energy density than gasoline, which means that it can be used to get better mileage in a vehicle.

The way these two fuels work in an engine is different as well. While a gasoline and oxygen mixture is ignited in the engine cylinder by a spark from the spark plug, diesel fuel is ignited by compression. The piston in the engine travels up the cylinder, compressing the air. At the top of the stroke, atomized fuel is injected into the cylinder, and ignites in the heat created from the friction of this compression.

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Discussion Comments

By kylee07drg — On Oct 28, 2012

@OeKc05 – Diesel engine fuel has more particles with less filtering, and this is part of the reason. The other reason is that you can hear the diesel igniting within the engine.

You have probably noticed that older trucks are often louder than newer ones. My dad has an older model diesel truck that will wake the neighbors when he drives it, but my brother has one that is so much quieter because of changes that have been made to the design.

If someone has a loud diesel truck, they can quiet it down some by buying an oil that is made to reduce engine noise. I've seen this stuff in auto parts stores.

By OeKc05 — On Oct 27, 2012

Why is it that pickup trucks that run on diesel are so much louder than those that run on unleaded gasoline? Does it have to do with the fuel or the size of the vehicle?

By JackWhack — On Oct 27, 2012

@shell4life – It's possible if you drive a motorcycle, a really old car, or a truck. They don't have that plate under the gas cap that makes only small nozzles fit, so you could easily get distracted and put diesel into your tank.

However, if this did happen, your engine would not start. This happened to my sister once, and she had to get her husband to drain all the fuel out and put regular fuel in, but even after that, it took awhile for it to crank.

It finally did crank, and there was no permanent damage. So, you won't kill your car if you do put diesel into it. You will cause yourself a lot of trouble and waste a lot of time fixing the problem, though.

By shell4life — On Oct 26, 2012

@claire24 – I don't think this is even possible. I once got confused and tried to put the diesel nozzle in my tank, but it wouldn't fit. It was way too big, so I recognized what was going on right away.

By claire24 — On Mar 25, 2011

I am always nervous when I pull up to combination gasoline and diesel fuel pumps. I'm always afraid I'm going to accidentally grab the wrong nozzle and put diesel in my car, which is most definitely made to use only gasoline.

What would happen to my car if this actually happened? Would diesel fuel ruin a car engine that is made for gasoline? And is there a way to get the diesel out if you've already put it in?

By calpat — On Mar 24, 2011

I had a van that could run on a fuel that is made out of corn. I thought it was amazing! It was cheaper than regular gasoline, and so much better for the environment.

I had no idea that alternative diesel fuels were being made too. I think it's great!

By reader888 — On Mar 21, 2011

Wow, biodiesel fuel sounds great! It seems to be much better for the environment, and I imagine it is also much better for the people who have to breathe the air where a lot of diesel vehicles are present.

I hope engine technology catches up quickly. Hopefully biodiesel fuel will be able to be used for all diesel vehicles someday soon.

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