You may have wondered when you see a display of fuel additives, "If I use good diesel fuel, do I really need those?"

To answer, we need to know a bit about what diesel fuel is and how it gets from the refinery to your engine.

Diesel 101

Diesel is a middle distillate mixture of hydrocarbon compounds, as are jet fuel, home heating oil and kerosene. Of products made from petroleum, butane and methane are the lightest, then come the middle distillates, like the diesel fuel you use for your truck. The heaviest products include the tar used to pave blacktop roads. All start as crude petroleum.

As soon as diesel is refined, it is exposed to air and begins to oxidize. The airborne bacteria and fungus spores that animals are immune to thrive in diesel when exposed to moisture. Air contains humidity that condenses in fuel storage tanks. The water allows the organic matter to breed and form colonies that can clog filters and gum-up the injection system.

The longer it takes fuel to get from the refinery to your engine, the worse it can get. By contract with the pipeline companies that transport diesel in bulk, all fuel is treated with additives, but only to protect the pipeline, not to improve the diesel. That's because all companies use the same pipelines. Pumping locations that sell large volumes usually have the highest quality diesel. Because the fuel is continuously re-supplied, the diesel doesn't have time to deteriorate.

By contrast, fuelers who sell only low cost fuel generally have purchased surplus fuel that has been in storage too long.

Winter Woes

Winter brings its own set of problems, from slow starting to ice and gelling. Standards for all diesel call for at least 40 cetane, a measure of how easily an engine will start.

Cold weather affects the fuel at its most basic level. Some of the waxy molecules in diesel precipitate and join together as temperatures drop. They form a matrix that binds the fuel into a gel-like mass. Even before the fuel gels in the tank, the wax will plug fuel filters, preventing flow and literally stopping the engine cold.

Even the cleanest engines create soot when diesel burns. Soot can collect inside the engine, especially on injector nozzles. The build-up alters spray patterns, causing more incomplete combustion in a cycle that causes more and more injector clogging.

What You Need

Fuel additives counter these common problems. Not all have the same balance of ingredients, so each acts differently. It's up to you to read labels and determine which products will be best for your needs. Requirements vary geographically, and results will vary according to dosage. Read the instructions on each container, and use the right amount for your specific needs.

Winter additives are best known for their anti-gel properties. Many contain wax modifiers, chemicals that attach to the paraffin in the fuel and slow the gelling process. They can keep diesel flowing at temperatures as much as 20 degrees F colder than untreated fuel. Cetane improvers in winter additives can raise the index number by five or more points. They help your engine start more quickly when temperatures fall.

Additives control water. Some keep water suspended as tiny droplets that pass harmlessly through the fuel system. Others alter diesel so it cannot hold water. Any free water drops to the bottom of the tank, and should be drained or siphoned out regularly. By controlling water, you'll also control most organic growth.

Some additives have biocides that actively kill organic growth. When using biocides, read and follow label directions precisely. Biocides used to treat badly contaminated tanks are highly toxic.

Almost all additives have detergents that cut through carbon build-up and restore injector spray to design specifications. The detergents also dissolve any varnish or gum build-up in combustion chambers.

You can also look for additives that contain lubricants, which replace lubricity lost when sulfur is removed from diesel. Look for this if you're concerned about friction and wear.

Bottom Line

Poor fuel can be improved using additives year-round. Good fuel can be made to perform even better. Your engine needs to have water controlled, lubricity improved, starting to be quicker and organics to be controlled all the time. It needs soot prevention all year. Additives are most important in winter's cold, but they can improve engine performance in any season.