The Premium Edge

Many truckers don’t want to pay the extra price for premium fuel, or rarely visit truck stops where it’s sold. Those who do buy premium believe it’s worth it for improved performance and reliability.

While the benefits of premium diesel are well known, it’s hard to gauge if those bonuses are worth the extra pennies per gallon. And what’s also tough is determining exactly how “premium” any given fuel is, and whether that fuel matches the requirements of your engine.

Although diesel’s specifications are regulated, gasoline specs are much more tightly regulated, says David Daniels, director of cold flow improvers and performance technologies with Octel-Starreon, a major diesel additive manufacturer. Diesel fuels can differ considerably, whether marketed as premium or otherwise. The American Society of Testing and Materials’ standards for any fuel include cetane number; distillation temperature (how hot the fuel must be to evaporate); viscosity; and ash, sulfur, water and sediment limits.

“Meeting the minimum legal requirements does not indicate that you have a high quality fuel,” Daniels says. “The specs used (ASTM D975) fall far short of all the physical properties needed to assure good fuel.”

So how can you match a fuel with your engine manufacturer’s recommended specs? “The manual that comes with the vehicle, OEM websites, dealers, etc., should have this information,” Daniels says.

Unfortunately, he notes, it is difficult to get information on the fuel right at the pump. In some states, fuel must meet enhanced specs to be labeled as premium. If you get a copy of those specs from the state, you can compare them with your engine’s recommendations. If the state’s premium specs are appropriate, Daniels says, you could just look for the premium designation on any pump in that state.

The ideal situation is to get detailed specs on a supplier’s regular and premium diesel. Compare those with your engine maker’s specs. Does it appear the premium will show truly enhanced performance? If so, you could save money on maintenance and fuel consumption, and it would be worth your while to buy consistently from that supplier.

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To evaluate a fuel’s specs, make sure its ratings fall below any maximum specification and above any minimum specification or, in a few cases, within a specified range. Examples from the Detroit Diesel standards:

CETANE NUMBER. Minimum 45.

CETANE INDEX. Minimum 40.

CLOUD POINT. It should be 10 degrees below the lowest ambient temperature. So if you expect 30 degrees, insist on a cloud point of 20 degrees or lower.

WATER CONTENT. Maximum 0.02 percent.

DUPONT PAD TEST. Minimum 70. This is a measure of fuel stability.

NET HEAT CONTENT. 128,500-130,900 BTU/gallon.

FILTER PLUGGING POINT. Regardless of engine make, if a fuel’s plugging point temperature is below the lowest outside temperature you expect, or if you have a fuel heater that will keep it above that plugging point, you’re okay.

ASTM D975 includes tables based on the “10 percent minimum temperature,” or the lowest temperature you would see 90 percent of the time. Diesel fuel must work down to that temperature without filter plugging to pass the standard and be legal to sell. But, if the temperature goes significantly below that, you will likely get plugging.

“We had one customer who experienced equipment shutdown in the cold,” says Jim Coley, a staff chemist with Shell Global Solutions U.S. “The engines couldn’t pump the fuel. They switched to premium and not only saved downtime, but also reduced engine maintenance costs. It sometimes is a matter of, ‘Pay me now or pay your mechanic later.'” In addition to the low temperature compatibility, Shell’s premium has “higher cetane, better corrosion resistance, and better lubricity,” Coley says.

Another concern with modern trucks is the effect of engine heat on fuel, says Cy Henry, a research fellow with Octel-Starreon. “Fuel residue in the tank can run at 180 degrees,” he says, and modern diesel fuel under such heat can easily form deposits. Deposits form when diesel is more unstable than normal and its composition changes too easily. Unfortunately, stability is not defined in the ASTM specification because no single lab test exists to measure it.

Engine manufacturers are quick to note other problems in defining and regulating premium diesel. “The ASTM standard is a consensus process,” says Ian MacMillan, Octel-Starreon director of research and development. “The OEMs are trying to push for more severe requirements for fuel, but they have not gotten all the ones they want.”

For example, ASTM D975 requires diesel to have a cetane rating of 40. Volvo’s minimum is 40, too, though Volvo engineer Ed Saxman qualifies that “fuel with a higher cetane value may be required for high altitude or cold weather operation.”

Detroit Diesel calls for a cetane of 45 for its Series 60 engines, though the company “cannot vouch for any benefit from premium fuel,” says Tom Friewald, vice president of marketing. Friewald points out that large fleets can buy custom-spec’ed diesel fuel in bulk, then test it to verify that it meets their specifications. “Once out on the road, there is no way for a trucker to know what is in the premium fuel he buys at the pump,” Friewald says.

“The key issues with commercial diesel fuels are the lack of important definitions, like lubricity, in the industry specification (ASTM D975) and the inconsistency of the fuel delivered,” says Jerry Wang, a senior technical adviser at Cummins. For a comprehensive premium diesel, Cummins supports a standard published by the Engine Manufacturers Association, FQP-1A. “With a properly defined premium diesel, the customer can expect the fuel to meet OEMs’ fuel recommendations more consistently, leading to better life and performance of their equipment,” he says. “Unfortunately, many premium diesel fuels contain only a portion of the functionalities as defined in FQP-1A. These partial premium diesels may not provide long-term cost benefits.”

Mack Trucks engineers note that a fuel can legally be called “premium” and still fall below customers’ expectations. The National Council of Weights and Measures allows a fuel supplier to label a pump premium when the fuel meets only two of six defined premium fuel properties. However, the Engine Manufacturers Association and the Technology and Maintenance Council of the American Trucking Associations recommend that states not use this approach for defining premium diesel.

Caterpillar engineers say their engines using a fuel with a cetane rating above the company’s minimum standard of 40 will be easier to start, will produce less white smoke and will run quieter, though it’s up to the user to determine if those qualities are worth the extra price for premium fuel.

“Because there is no standard for what premium diesel must contain, the price differential between regular and premium varies a great deal,” says Fred Kirshner, one of the principals of the Petro Stopping Center in Bordentown, N.J. “It’s priced according to what is actually added.” MacMillan says premium usually costs 1 to 5 cents per gallon more.

Even with an unpredictable price spread, you can make an informed fuel choice if you know your engine’s specs, the climate where you operate, current prices and how different grades of fuel affect your cost per mile.


True premium fuel should not be confused with blended fuel or No. 1 diesel fuel. No. 2 is similar to furnace oil (with diesel additives), while No. 1 is essentially kerosene, a lighter hydrocarbon with much less paraffin or wax, and less energy content. Like premium, No. 1 also has additives.

Blended fuel is a mix of No. 1 and No. 2 appropriate for preventing waxing and filter clogging at the stated temperature. Straight No. 1 offers maximum protection against waxing, but poorer fuel economy than No. 2 or even blends.

Premium fuel consists of the heavier hydrocarbons of No. 2, but with additives designed to help operations in many ways, including preventing waxing and filter clogging in the cold. That’s why it’s easy to confuse premium with blended fuels.

Ian MacMillan and David Daniels, executives with diesel additive maker Octel-Starreon, list these typical characteristics of quality premium diesel fuel:

ENHANCED COLD-FLOW IMPROVERS. Because diesel contains a lot of paraffin, or wax, it can quickly clog fuel filters if it gets too cold. MacMillan says that fuel filters are getting finer to handle increasingly tight injector tolerances. Because of the tiny filter passages, “Low temperature operability is increasingly a challenge,” he says. While truckers frequently use their own additives to improve cold flow, it’s important to note that quality premium fuels may offer an extra margin of safety for extreme cold.

HIGHER DETERGENCY. Detergents can reduce fuel system wear and repairs and enhance fuel economy. “Engine deposits reduce the efficiency of a diesel by interfering with fuel spray patterns,” Daniels says. “Dirty injectors will limit power by restricting flow, increasing fuel consumption, and giving rise to higher smoke levels in all equipment.” The smoke comes from the way deposits distort the spray pattern and keep the fuel from mixing with enough air.

HIGHER CETANE. The cetane rating measures how easily the fuel ignites. Combustion enhanced by a high cetane rating can help an engine run more efficiently and last longer.

“Fuels with low cetane numbers are the cause of hard starting, rough operation, noise and increased smoke opacity,” Daniels says.

While the American Society of Testing and Materials D975 standard puts the minimum cetane rating at 40, Octel-Starreon’s experts believe ratings closer to 43 give optimum performance.

Exotic additives can bump up the cetane rating. According to Octel-Starreon’s experts and Chevron’s website, the most common one is ethylhexyl nitrate (EHN), which can boost the rating two to four points or more. Such an additive is often included in a quality premium diesel.

GREATER LUBRICITY. Being thick and oily, diesel lubricates the high-pressure injection system. “Shortened life of engine components, such as fuel injection pumps and unit injectors, can usually be attributed to a lack of fuel lubricity,” Daniels says. Lubricity affects fuel economy, too, if there is not enough to protect injectors.

CORROSION INHIBITORS. These additives help because injector and other fuel system parts can rust. Retarding corrosion can reduce fuel system wear and enhance fuel economy.

GENERAL CLEANLINESS. Premium diesel has less water and sediment, elements that can reduce filter life or plug filters, which leads to fuel starvation. Water can promote corrosion and microbial growth. Microscopic droplets of water that make it into the injector can vaporize like small explosions, damaging fuel injectors.

THERMAL STABILITY. Quality fuels contain additives that help them resist the degradation that comes with normal engine temperatures. “Modern diesel engines put today’s fuel under considerable heat stress,” Daniels says. “The fuel can quickly darken and form black particulate materials that will make them hazy and create gum residues.”


The Engine Manufacturers Association

The American Society of Testing and Materials