Cetane Numbers: How diesel fuel’s biggest unknown affects your truck
SPECIAL REPORT: What Is Cetane?
“Cetane Number.” It’s a rating in the diesel world that perks up our ears. We know it’s an important variable in the fuel that powers our rigs. But for most of us who drive diesel vehicles, the role cetane plays in their overall performance is as much of a mystery as the actual cetane number of the fuel being pumped.
A good starting point on the topic is to understand the difference between “cetane” and “cetane number” (CN). Cetane is a set of naturally occurring chemical compounds found in diesel fuel. The more volatile of the two is hexadecane, commonly referred to as “cetane,” which ignites easily under compression and has a CN of 100. Isocetane is its poor-burning bookend with a CN of 15.
Both are directly related to the amount of paraffin wax contained in the crude oil from which diesel is derived. In general terms, the higher the wax content of the crude oil, the greater the amount of natural “cetanes” in the base-run diesel from that batch.
Cetane Number“CN is a measure of ignition quality of diesel,” according to Dr. Arjun Prakash, a scientist with Shell Eco-Marathon Americas’ Fuels Technology Group. “The cetane number of a given diesel is obtained by matching its ignition quality to a reference blend of cetane (set at 100) and isocetane (set at 15), hence the name ‘cetane number.’ Higher-CN diesel fuels require less time to ignite inside an engine than those with a lower CN.”
Think of the CN for diesel as one would for the octane rating of gasoline—only opposite in meaning. That’s because the higher the octane rating of gasoline, the slower it ignites, which stops detonation, or pinging. A high CN means the fuel ignites faster, producing a longer, cleaner fuel burn than diesel with a lower CN.
A cleaner diesel fuel burn means lower emissions. That’s why the U.S. Environmental Protection Agency, the government’s air-quality police, set a minimum CN of 40 across the country, with the final CN left up to individual states’ EPA regulations. For example, the California Air Resources Board (CARB) currently mandates a minimum 53 CN, while Texas (TxLED) set its highway diesel CN lower, at a minimum of 48.
Diesel Is DieselOther U.S.-refined, non-biodiesel fuel can vary in CN from 40 to 60, and those numbers fluctuate depending on a lot of variables, from the initial refining process to the time it flows out of the nozzle and into our truck’s tank.
One reason for this is the comingling of fuels that do not have the same CN. For example, refined diesel is sometimes mixed in the pipeline that runs to the distribution center or in the massive holding tanks where it was stored before it was trucked to a filling station, with higher- or lower-rated fuel.
Comingling can also occur when the new diesel is added to the fuel station’s underground tanks, and its CN changes when it mixes with the remaining fuel in your truck’s tank.
The bad part about such cross-mixing with varying cetane numbers is there’s no regulation that requires the actual CN for fuel coming out of the nozzle to be posted, as is done with gasoline. All you know for sure is it’s at least the “minimum” as required by the EPA for your area. Hence, the adage that “diesel is diesel.”
“It’s a crapshoot as to what CN [rating] you are pumping into your truck,” says Jeff Kramer, VP of Sales in the Western U.S. for Power Service Products, a major supplier of diesel fuel additives. “If you live outside of California or Texas, one day you could be filling up with 40 CN, the next week it could be 50 CN from the same pump.”
Chemistry LessonComingling’s does have an effect on CN, but it doesn’t have the same impact on diesel-fuel quality as the refiner, according to Bob Yondola, the Business Development Manager and an expert in the refining process at Allegheny Petroleum Products Company, which is one of the largest manufacturers of lubricants and fuel additives in North America.
Yondola says the wide variation of the CN is primarily dependent on the refinery’s process and the crude it uses. “The cetane number in distillate (diesel fuel) is a combination of the crude oil used at the refinery and the process units available at the refinery. All refineries are not the same in configuration and contain different process units.”
“High normal paraffin-, isoparaffin-, and olefin-containing crudes generally produce higher cetane-distillated diesel fuel versus crude oils that are high in naphthalene and benzene,” Yondola says. “Therefore, ‘waxy’ crudes typically yield diesel with higher CN numbers.”
Profits Dictate CNThe catch-22 is this: Waxy crudes are expensive compared to less-desirable options, and like any business, oil companies want to maximize profit.
“The refinery wants to keep costs as low as possible, so it uses the least-expensive crude that can be refined to meet its customers’ needs,” says Jim Kudis, CEO of Allegheny Petroleum, which supplies refineries, distribution centers, and a multitude of aftermarket customers with 2-Ethylhexyl nitrate (EHN), the primary chemical used to bump up the CN of diesel.
A recent Chevron white paper says if the natural CN from a crude oil is too low, “some refiners use a cetane number improver, when the additive cost is less than the cost of processing, to increase the cetane number.”
The retailer at the end of the diesel distribution chain wants to buy the cheapest fuel it can to meet the minimum EPA requirements for its area. The name of the game is to save a penny here and a penny there per gallon—multiplied by hundreds of millions of gallons sold.
From the fuel retailer’s perspective, it doesn’t mind selling low-cetane diesel fuel instead of premium product, because doing so opens the door for selling off-the-shelf additives—such as Power Service Products’ Diesel Kleen+Cetane Boost—which not only raise CN, but improve lubricity, clean the injectors, and add protectants that improve the performance of higher-mileage engines.
Cetane and HorsepowerDiesel’s CN has a definite effect on performance. However, it’s not in a way you might expect. The major diesel-engine manufacturers calibrate powerplants based on running 40 CN diesel, according to GM Senior Fuels Engineer Shailesh Lopes.
“Traditionally, low CN fuel primarily impacted the cold-temperature performance of the engine, such as no-starts, misfires, torque fluctuations, or noise and vibrations. With the modern common-rail engine, the manufacturers have made changes that make the new engines less sensitive to low cetane’s impact on cold-flow performance,” says Lopes.
Lopes also says even though today’s diesel engines are calibrated to run on 40 CN fuel, using 53 CN or higher fuel can still provide measurable benefits for “improved combustion stability and engine-out HC/CO emissions” even after the engine is at operating temperatures.
“Running low CN fuel results in late burn/retarded timing that produces black smoke on acceleration or blue-white smoke when a cold engine is idling,” adds Oregon Fuel Injection’s Mark Gotchall. “Increasing the CN allows the engine to burn cleaner, which results in fewer regeneration events on DPF-equipped vehicles.”
It’s Not “Magic Liquid”Here’s the bad news: Regardless of “cetane-booster” product claims, the CN of the fuel we use doesn’t have any significant impact on torque, horsepower, or fuel economy according to Lopes and diesel engineers at other major diesel-engine manufacturers. Boosting CN from 45 to 53, for example, isn’t going to change horsepower or fuel economy.
“For the present engine technology, higher or lower cetane number can potentially impact engine horsepower, torque, and fuel mileage only during the start-up phase in cold winter conditions,” Lopes says. “But it’s not expected to have a significant impact on these performance parameters once the engine warms up.”
In short, CN has everything to do with reducing emissions and nothing to do with going faster or delivering better fuel economy.
Take Your MedicineIf raising the CN number doesn’t improve power or increase mpg, then why go through the process of using additives and cetane boosters? Using these products is recommended because diesel-fuel additives are a lot like medicine—use them as prescribed by an expert who diagnoses the ailment from which your rig’s engine is suffering.
“This goes back to using a multifunctional additive that boosts CN, cleans injectors, and provides the necessary lubricity to protect fuel pumps and injectors instead of only chasing a cetane number,” Kramer says.
“EHN will boost operating performance by improving combustion when the engine is cold. But what diesel owners should be more concerned with is the bigger picture: the overall health of their trucks’ fuel system,” he adds.
“When you use a diesel-fuel additive consistently, you know the fuel now has the necessary amount of detergent, cetane, and lubricity that may not be present in sufficient quantities for a particular diesel engine in the fuel coming from the dispenser.”
Additives’ BenefitsWe realize that up to this point in our report, the information we’ve given you may burst a few performance bubbles some of you have when it comes to pouring a bottle of cetane boost in the tank. No, it doesn’t suddenly improve your truck’s quarter-mile or grade-climbing ability.
The benefits such additives bring to the table are improved cold-weather starts, reduced combustion noise when the engine is cold, and reduced harmful emissions when the engine reaches full operating temperature.
Dosing the tank at every fill-up with fuel additives that have CN improvers as part of the mix is a way of ensuring the diesel engine in your truck works as smoothly as it can when cold—and that it runs cleaner and has a longer life than it would without such a fuel treatment.
So don’t sweat the cetane number when you’re filling up your rig. It is what it is. But do think seriously about dosing the tank with a cetane-boosting additive that works like a multivitamin for the engine’s long-term health.
Recommended DosageIt’s important to note cetane-boosting additives should only be used as recommended; more is not necessarily better. Too much 2-Ethylhexyl nitrate, the common chemical that boosts CN, can lead to altered fuel density, according to the experts we spoke with.
Read the additive’s label of contents and dose your truck’s fuel tank accordingly. For example, if a gallon of “Additive A” treats 400 gallons of fuel, then .32 ounce treats 1 gallon. (A U.S. liquid gallon equals 128 fluid ounces.) If you add 20 gallons of fuel, that equates to 6.4 ounces of that particular additive.
Usually, the greater the volume a measure of the additive treats, the more concentrated it is. For instance, if a gallon of “Additive A” treats 400 gallons, and a gallon of “Additive B” treats 1,000 gallons, the latter is 2.5 times more concentrated—all other ingredients being equal. That helps make a fair price comparison.
It’s also important to know low CN fuels (40-45) respond less to boosters than those with high CN (48-53). Hence, the reason “cetane boosters” usually show a point spread instead of a single number gain. The sweet spot in CN for today’s trucks is to be in the low-to-mid-50s.
“Fuels with lower CN need more ppm of cetane improvers like EHN to register a change. For example, a 42-CN fuel will have a smaller upward point change than a 50 CN fuel when using the recommended dosage,” explains Jessica Crabtree Kattner, Director of Technical Services at Power Service Products.
“While the CN may change more when EHN is added to a higher-cetane base fuel, there will be a more noticeable improvement in cold-engine performance with a low-cetane-based fuel when treated with cetane improver,” Kattner adds.