Bottom Line
Fuel additives
To add or not to add? that is the question

by Paul Abelson

Going through some old files, I came across notes for a presentation I gave to The Maintenance Council (TMC) back in September 1989. The subject was "Fuel Additives: Fact or Fiction." Back then, as today, there were a few major national brands of additives: Power Service, Howes, FPPF and Silco. Nalco Chemical and Stanadyne were in the business, but they sold more to fleets than to owner-operators. There were, however, hundreds of local brands. Some were good, but many were harmful. They contained inexpensive solvents that would attack fuel systems, hoses and seals. The bad ones were so bad that in 1984, TMC developed an advisory, now RP312, "Qualifying Questions to Minimize the Potential for Negative Side Effects from an Aftermarket Diesel Fuel Additive Package." These local home-brews helped give fuel additives the names "Snake Oil," or "Mouse Milk," and caused many fleets to prohibit drivers from using any over-the-counter products. 

Since those days, diesel fuel has improved only slightly. The federal government mandated lower levels of allowable sulfur, and limited the percentage of aromatics (more on that later) in fuel. The Engine Manufacturers' Association and TMC each developed a set of specifications for diesel fuel, but neither has been generally accepted yet. Diesel is still sold as meeting specifications set by the American Society for Testing and Materials (ASTM).

What is diesel fuel?

Before we examine additives, lets look at what diesel fuel is and how we get it. Number two diesel is a middle distillate fuel. Crude oil (petroleum from the ground) is heated, and various components evaporate at increasingly higher temperatures. First to go is butane, the lighter-than-air gas used in cigarette lighters. Last are the heaviest, the road tars used to make asphalt paving. In between are gasoline, jet fuel, heating oil, lubricating oil, bunker fuel that ships use and, of course, diesel fuel. The fuel used in trucks is a mixture of different types of molecules of hydrogen and carbon. Different arrangements of these atoms produce hydrocarbons with differing properties. Ignition occurs when there is sufficient heat to start a reaction between the hydrocarbon fuel and the oxygen in the air. Aromatics are molecules with rings of atoms linked together. If you remember the benzine ring from high school chemistry, you can picture how these molecules are structured.

Aromatics are molecules more difficult to ignite, and they may not burn as completely as paraffinic molecules. Their advantages are that they are plentiful in the petroleum mixture (although with limits on aromatics, that may be a disadvantage) and they tend to resist waxing that plugs fuel systems.

Paraffinics are straight-line molecules. To visualize the differences, picture aromatics as a string of hexagons linked together, while paraffinics are a string of "plus signs" all in a row. Paraffinics ignite easier and burn more completely, but they cling together in cold weather forming the wax matrix we call gelling.

Diesel fuel cannot burn in liquid form. It must vaporize into its gaseous state. This is accomplished by injecting the fuel through spray nozzles at high pressure. The smaller the nozzles and the higher the pressure, the finer the spray will be and the more readily the fuel will vaporize. When more fuel vaporizes, combustion is more complete, so less soot will form inside the cylinders and on the injector nozzles. Soot is the residue of carbon, partially burned and unburned fuel.

Sulfur is found naturally in crude oil. When I gave my presentation, diesel fuel could have had as much as 5 percent sulfur, although most domestic fuel had less than 3 percent. Sulfur is a slippery substance, and it helps lubricate fuel pumps and injectors. It also forms sulfuric acid when it burns, and it is a catalyst for the formation of particulate matter (one of the exhaust emissions being regulated). Today, sulfur is limited to 500 parts per million (ppm) or one-twentieth of 1 percent. In order to meet future emissions regulations, it will probably be limited to 15 ppm. The upside is cleaner burning fuel, but the downside, in addition to higher refining cost, is loss of lubricity. This can be partially offset by improvements in engine materials and precision manufacturing, but it may still present problems in the future unless lubricity of the fuel is increased.

Diesel has other properties that affect its performance. It can deteriorate over time, when exposed to moisture and oxygen. New diesel is light in color. Older diesel is darker, due to the deterioration that forms sludge. Diesel can hold water, which condenses on tank walls as they cool. Water in the fuel causes a number of problems. A tiny water droplet can blow the tip off an injector if it is suddenly turned into super hot steam. It can freeze in fuel lines and block flow. And it promotes the growth of bacteria that can grow on the interface between the fuel and water. Bacteria form the slime that can be found blocking fuel filters.

The main problems with diesel fuel are:

  • difficulty getting it to start burning
  • difficulty getting it to burn completely
  • fuel waxing and gelling
  • reduced lubrication in the fuel system
  • soot clogging injector nozzles
  • water in fuel
  • bacterial growth¬†

The role of fuel additives

Fuel additives (also called conditioners, supplements or treatments) are designed to address these problems.

We most often think of additives in winter to prevent gelling. Chemicals called wax modifiers attach themselves to the paraffin molecules and block them from joining together as they normally would. While they can't prevent all gelling, they can reduce the gel point by as much as 30 degrees F, depending on the makeup of the fuel. That could lower the CFPP (cold-filter-plug-point) by about 15 to 20 degrees.

Fuel conditioners or additives must also control water so it doesn't freeze to block fuel flow. There are two types of additives to control water: emulsifiers and de-emulsifiers. Emulsifiers break the water into microscopic droplets and prevent the droplets from joining together. The water is suspended in, and carried along with, the fuel. The volume of droplets is not great enough to block lines or damage injectors. FPPF is a typical emulsifier.

De-emulsifiers work the opposite way. They modify the diesel so it cannot hold water. The water, being heavier than the fuel, drops out and settles at the bottom of the tank. It cannot travel through the system to do damage, unless its level builds to where it is up to the level of the fuel pick-up tube. When using a de-emulsifier, such as Howes, it's a good idea to check the water level in your tanks with water-detecting paste on a stick. If there's an inch or less, no problem, but more than that, you should siphon the water out or drain it.

To prevent soot deposits from building on injectors and altering the spray pattern, good additives have detergents. They prevent the vicious cycle of soot forming, depositing in the spray holes, blocking the spray so the pattern is less efficient, which causes incomplete combustion that creates more soot, which blocks injectors even more, continuing the cycle. By keeping things clean, detergents maintain factory-engineered spray patterns. Often, detergents can clean-up badly clogged injectors, but this takes time and rarely gets them completely clean. That's why it's advisable to use a detergent additive year round.

In the days before electronic engines and low sulfur, higher paraffin fuel, cetane improvers were important. Cetane, actually a paraffinic molecule, is the measure of diesel fuel's ability to ignite, just as octane, a molecule in gasoline, is a measure of its ability to resist pre-ignition, known as "knock." Since the newer fuels have more parafinnic molecules, which ignite more easily, cetane improvers are not as important as they once were. The same is true of chemicals that support combustion.

Lubricity improvers, on the other hand, have become more important and when ultra-low sulfur diesel comes along, they will be even more so. Since diesel fuel has no added lubricity as delivered, fuel conditioners with improvers can help prolong fuel system life. In fairness to the engine builders, though, their engines are designed to work with today's "dry" fuel, and should be when the next generation of diesel arrives.

The final problem, bacterial growth, has traditionally been addressed with biocides. In recent years, the EPA has developed strict rules regarding their use, and many fuel additive suppliers no longer incorporate biocides. Pure biocides can still be found, but they should be used very carefully, following directions precisely. You may need a biocide if your fuel tanks smell like rancid butter or if your fuel filter has smelly sludge on it. For clean systems, stabilizers like those in Penray's Fire Prep will prevent the formation of bacteria colonies - the slimy scum sometimes found in tanks and on filters.

People still ask me if fuel additives are necessary in this age of electronic engines and low-sulfur fuel. My opinion is that with fuel injection pressures around 30,000 psi and injector nozzles more finely machined than ever before, a well-formulated additive package is even more important in keeping engines operating at peak designed efficiency for all the years we expect our engines to last.

I'm also asked, "Which one is best?" That depends on where you operate, how you run, what fuel you use, and how you maintain your truck. The national brands are all good. To make an informed decision, read the labels carefully. Decide what you need done, and select the product that will best meet your needs. Once you've selected a product, stick with it. If it's not available wherever you run, either keep a supply with you or switch to a brand you can find easily. Don't keep switching among brands. You may discover some chemical incompatibilities that produce unwanted results.

Fuel conditioners can help your maintenance program when used properly, but they can't work miracles. The best additive may not fix the problems from a bad load of fuel, so be careful what you buy. What may seem like a bargain may cost far more than your apparent savings.

Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.


The diesel you buy

Although a vehicle fuel, diesel is shipped and formulated quite differently from gasoline. There are far fewer diesel terminals, and most retailers and truckstops tend to buy "pipeline fuel." Refiners all place diesel into the diesel pipeline, and they all draw diesel out, but what each takes out has probably been refined by another company. That means the brand you buy may not be from the company that refined the fuel. For that reason, diesel is generic. This is different from gasoline, with its advertised properties. Gasoline is a differentiated product, not a generic one. The one exception is Amoco's premium diesel, which is made to EMA preferred spec's. Being generic, pipeline diesel has only enough anti-corrosion and other additives to protect the pipelines, not your engine.

There are, however, differences in how pipeline diesel is handled, shipped and stored. Housekeeping is important. If tanks aren't designed to prevent rainwater from entering during delivery, or if truckstops don't check tanks for water and bacterial growth regularly, they may be breeding bad fuel.