by Paul Abelson, technical editor
We tend to think of diesel as just a fluid that comes out of the pump and goes into our tanks. We assume it’s just the way it came from the refinery, and the way it will be when we use it now, tomorrow or next week. We grow up driving cars before we ever get near our first trucks, and we all know about gasoline. It, like diesel, is a hydrocarbon (made of hydrogen and carbon atoms) fuel for an internal combustion engine. Gasoline is very stable for a long time, but if you’re storing something like a lawnmower or snow blower for a few months, you should add an oxidation inhibitor like Sta-Bil to the gasoline.
Diesel oxidizes too, but worse. It comes from the refinery in as pure a form as can be, but almost immediately it starts to change. Unlike gasoline, in which each company refines, stores, transports and often sells it, diesel is refined, stored and transported as a commodity. With a few notable exceptions marketed as “Premium,” most No. 2 and No. 1 diesel goes into a few common pipelines. It is mixed and mingled with other refiners’ diesel, and removed at some remote point. Let’s say ExxonMobil puts 100,000 gallons into the pipeline. Through accounting methods, the company is entitled to remove 100,000 gallons at the other end. But the 100,000 gallons removed may have been put in by ChevronTexaco or Conoco. When everyone takes out what they put in, the system balances.
The fuel goes into storage and then into distribution. In times of high demand, inventories may be reduced, or fuel may be “bought” from competitors. When demand is low, inventories are replenished.
Additives are used throughout the process, because diesel has properties that affect its condition too. It is a complex mixture of hydrocarbon molecules, not a single, precise substance. Some of these molecules break down under varying measures of heat. Some react with oxygen in the air, and form sludge within the stored fuel. Humidity from warmer daytime air condenses when the air cools at night. The water condensed from the air mixes with and is suspended in the fuel. Bacteria and fungus spores are in the air, and they, too, enter storage tanks. These organic substances draw nourishment from the diesel molecules and oxygen from the water. They multiply and grow colonies that appear as a slimly scum.
Additives alter some properties of diesel fuel. By contract with the pipelines, all refiners treat their diesel entering the pipeline with minimal levels of inhibitors — not to improve the fuel, but to protect the pumps, pipes and valves. Diesel also may be treated to bring it up to nationally accepted minimum standards developed by the American Society for Testing and Materials (ASTM). Standard D975-81, Diesel Fuel Oils, is recognized as the standard for diesel in the United States. The standards are also included in the TMC Recommended Practices Manual, RP-304B. Additives sold at truck stops, dealers and jobbers go beyond the basic treatments used to keep pipelines flowing and diesel at minimum specifications. They are helpful in bringing diesel to the TMC Preferred Grade specifications outlined in RP309A. TMC Preferred Grade is an improvement over the ASTM specs. It raises cetane number from 40 to 50, cuts allowable water and sediment in half, includes a zero tolerance for bacteria and fungus, cuts allowable carbon residue and specifies energy content of at least 138,000 BTU per gallon. The fuel’s distillation range, a predictor of how easily and cleanly the fuel will burn, is also included.
Many of these properties are achievable only with the use of additives. While TMC’s recommended diesel is usually available only to fleets buying in very large volumes, you can improve ASTM spec truck stop fuel with the national brand additives available today.
What do additives do?
We are most familiar with anti-gel additives, useful in winter to keep fuel flowing. Diesel components include at least 65 percent paraffinic molecules, either the wax itself or similarly structured molecules. As temperatures drop, these tend to separate from the hydrocarbon molecules and join together to form a wax structure. Ultimately, they turn diesel fuel into a jellylike mass. Before that happens, clumps of these joined wax molecules will block flow-through fuel filters.
Years ago, when the filters clogged, they had to be thrown away. Now, several brands of concentrated additives will dissolve the jellied mass in the filter itself. But the secret to making money is to keep moving, not to stop and repair problems. That’s why we use anti-gel additives with wax modifiers. These chemicals attach to the paraffinic molecules and inhibit their ability to cling together.
Water control is another important additive function. In early winter, when the temperature first falls below 32 degrees F, the freezing point, water in the lines stops more engines than gelled fuel. Later in the season, when additives have water under control, gelling becomes the main villain. There are two types of water control: emulsifiers and de-emulsifiers. Emulsifiers break water into tiny droplets too fine to be seen. They are suspended in the fuel and pass harmlessly through injectors instead of hitting the hot injector tip as a slug of water, then turning to steam with enough explosive force to destroy the injector tip.
De-emulsifiers do just the opposite. They alter the fuel so it will not hold water. Water collects at the bottom of the tank and must be removed periodically, but none gets into the fuel system. Any organic (bacteria and fungus) growth remains at the tank bottom.
Sulfur is a great lubricant. Diesel used to contain 3,000 to 5,000 parts per million. Today, we have only 500 ppm allowed, and almost all except 15 ppm will be removed in 2006. Something else is needed to lubricate fuel pumps and injectors. Many additives add the much-needed lubricity.
No matter how cleanly the newest engines burn, there is always some soot deposited on injectors. With the extra fine holes and high pressures of today’s fuel injectors, soot can quickly clog injector holes and alter fuel spray, reducing power and fuel mileage. Detergent additives keep injectors clean and prevent gum, varnish and carbon buildup in the engine.
Some additives inhibit or kill organic growth in the fuel. Biocides must be identified on the product’s label. Other additives can raise the cetane index of fuel, a measure of how easily the engine will start and run when cold.
I can remember when drivers used additives only in winter. It wasn’t that the fuels didn’t need them. Quite the contrary. But engines were tolerant of soot, and high-sulfur fuel was lubricant enough for the fuel systems of the day. Today, additives are almost a necessity year round.
Diesel fuel may be a living thing, but that doesn’t mean it must live in the wild. Good quality additives, used regularly, can help tame diesel so engines will perform better, longer.
Paul Abelson can be reached at firstname.lastname@example.org