#2 Diesel fuel is the fuel of choice for transportation and industrial transport vehicles. And well it should be. Diesel engines last longer than spark-ignition engines do. Diesel fuel isn't as highly refined as gasoline and doesn't have quite the flammability and refinery additization requirements as the more volatile gasoline. The advantages of diesel over gasoline fit perfectly into the needs of the commercial users that drive national and international economies around the world.
The most common problems seen with diesel fuel use tend to be either operational or during storage. Fuel mileage maximization, control of water build-up in stored fuel, improving vehicle cold-starting and elimination of rough running - all are desired areas of improvement for the typical diesel fuel user, both large and small.
Fuel mileage directly impacts the budget of every large diesel fuel user and the financial bottom line of the small one. Proper vehicle maintenance help maximize fuel mileage, as does keeping the diesel engine clean and free of deposits. This means cleaning and removing deposits in the combustion chamber, valves and injectors of diesel engines that can accumulate these fuel-sinks over the 500,000 to 1 million miles of their usable life. We call them “fuel-sinks” because one way these deposits retard fuel mileage is by absorbing some of the atomized fuel in the combustion chamber. Any fuel caught in this way cannot contribute to the machine producing maximum work for a given unit of fuel. Gradually restoring the engine back to its original condition can recapture lost fuel mileage and performance for the diesel user.
Diesel fuel is stored for longer periods of time than gasoline/ethanol is. This practice inevitably leads to water accumulation in storage tanks. Even the best fuel maintenance programs used in industry can’t stop this phenomenon (because it’s due to changes in temperature between day and night and resulting condensation). So diesel users have to control the water after the fact.
Water doesn’t cause the same kind of problems in diesel as it does in ethanol (phase separation). But controlling the buildup of water in diesel fuel is crucial for all diesel users large and small, both for minimizing water-related oxidative degradation of the stored fuel over time, the minimizing of possible injector blowouts (if water in the fuel tank gets sucked up into the injectors) and for the prevention of microbial growth in stored fuels.
Oxidative degeneration means that water contributes chemically to the chain of chemical reactions that form in diesel fuel and cause its quality to be destroyed over time. Good diesel fuel can gradually turn brown and see its quality degraded by the formation of “sludge” and deposits that form on the bottom of storage tanks (because sludge contains the heavier parts of diesel fuel that break out of the diesel blend and sink due to their weight).
Microbial growth in diesel is almost a universal problem and will happen to almost every large diesel user at some point. Microbes grow wherever there is water buildup available to sustain their growth. Poor fuel performance and slimy clogged filters are the initial symptoms. Left unchecked, microbial colonies will destroy fuel quality and cause expensive corrosion inside storage tanks due to the acids produced by their biological processes. Many large diesel users have had to spend thousands of dollars to fix a microbial problem left unchecked. Small-time diesel users who use diesel fuel quickly don’t have to worry about this issue nearly as much, as long as they’re getting their diesel fuel from a supplier who keeps their own fuel supplies in check.
These terms may not mean a lot to small diesel users but they know what happens when these two fuel properties are off. Lubricity means how well the fuel lubricates, and cold flow means how well the diesel fuel behaves in cold weather and whether it gels or not.
The advent of ultra-low sulfur diesel in 2006 also brought other fuel problems to the forefront - lubricity and cold flow properties. Since the 1990s, EPA mandates have slashed the amount of sulfur in diesel fuel to less than 1% of its former level – currently 15 parts per million (ppm), formerly 5,000 ppm. This sulfur reduction helps the environment and helps curb sulfur-based urban air pollution (caused in large part by diesel traffic from large vehicles like trucks and buses).
As with many things in life, this comes with tradeoffs. The down side to stripping all the sulfur out is you get a diesel fuel with lower lubricity (the ability of the fuel to provide lubrication) and poorer cold-flow properties.
In the past, the sulfur in the fuel helped to lubricate parts like fuel injectors and fuel pumps. Hydrotreating the diesel fuel at the refinery takes out the sulfur and some of the complex organic molecules in the fuel that performed this function. The “ultra low sulfur diesel” fuel now does not lubricate injectors and fuel pumps nearly as well, raising concerns about the life of these critical parts.
Ultra-low sulfur diesel also gels at higher temperatures at higher temperatures than before. Gelled fuel can shut an engine down by plugging the fuel filter up with gelled wax that has fallen out of the cold diesel fuel. Some users have gotten around this gelling problem in the past by mixing kerosene into their #2 diesel. But this runs them afoul of the sulfur regulations (raising potential for big fines if they are caught). All of this spells bad news for trucks and fleets up north in the winter.
Up till now, this sulfur issue has been thought to be the problem for on-road truckers and vehicles only. But those days are coming to an end. The sulfur exemption for non-road diesel fuels (like those used in marinas and generators) is now coming to an end. Very soon all #2 diesel fuel for every diesel application will need to be ultra low-sulfur.