Diesel Facts

octane-cetaneNot all Diesel is created equal.

Similarly to common petroleum based gasoline, there are many types and hybrid mixes of fuel and oil in diesel. Just as ethanol is added to gasoline and you can find different octane variations, diesel fuel has a cetane rating and types 1-D or 2-D, as well as bio and synthetic.

Petroleum based fuels are rated on their octane (OCT) or resistance to auto-ignition. Diesel fuel is rated by its cetane (CN), which indicates how easy it is to ignite and how fast it burns. The higher the cetane number, the more volatile the fuel.

Standard diesel fuel (sometimes called diesel oil) comes in two grades: Diesel #1 (1-D) and Diesel #2 (2-D). Most diesel vehicles use fuel with a rating of 40 to 55 and all diesel automakers specify Diesel #2 for normal driving conditions. Truckers use Diesel #2 to carry heavy loads for long distances at sustained speeds because it’s less volatile than Diesel #1 and provides greater fuel economy.

cetaneTypical CN Ratings
United States Petroleum-Based Ultra-Low-Sulfur Diesel- 40 (minimum)
European Diesel- 51 (minimum)
Biodiesel (B100)- 47 (minimum)
Biodiesel (B20)- 40 (minimum)
Synthetic Diesel (made from natural gas, coal, sewage and industrial waste, biomass, or from a reaction of CO2, H20, and electricity)- 60 to 80
Dimethyl Ether (DME)- 55 to 60

Diesel fuel is more efficient than gasoline because it contains 10 percent more energy per gallon than gasoline. It’s also safer than gasoline because its vapors don’t explode or ignite as easily as gasoline vapors.

diesel-gelledDiesel Thickness
Like any oil, diesel fuel gets thicker and cloudier at lower temperatures.

Under extreme conditions, it can become a gel and refuse to flow at all. Diesel #1 flows more easily than Diesel #2, so it’s more efficient at lower temperatures. The two types of oil can be blended, and most service stations offer diesel fuel blended for local weather conditions. It should be noted, however, that those blends can be imperfect and additives may be necessary.

Caution: Because emissions from conventional diesel fuel have been found to be extremely toxic to humans and other living things, until safer forms of this fuel are developed, be careful not to inhale the fumes while pumping it into your fuel tank. (The same goes for petroleum gasoline!)

When the exhaust from conventional diesel fuel was found to cause cancer, clean diesel engines were developed. Although thousands of conventional diesel fuel-burning vehicles are still on the road, public pressure and environmental organizations have prodded individual states and the federal government to enact legislation and fund replacement programs to take them out of use as quickly as possible.

Much stricter diesel fuel standards have had a huge impact on cleaning up diesel exhaust. By 2007, the sulfur content of diesel fuels was restricted to 15 ppm (parts per million) compared to the dirty diesel previously in use that averaged something like 550 ppm. This ultra low sulfur diesel (ULSD) is derived by extra refining of the same high-sulfur petroleum diesel oil as before, but biodiesel fuels derived from agricultural and waste products are becoming more popular in North America.

Biodiesel Fuel
Biodiesel fuels derived from agricultural materials have the potential to provide a clean-burning alternative to dwindling sources of petroleum. Rudolph Diesel (the diesel engine inventor) designed his first engine to run on peanut oil, and Henry Ford envisioned plant-based fuel as the primary fuel for transportation and partnered with Standard Oil to develop biofuel production and distribution. However, the only type of biodiesel fuel that can be used in vehicles in the United States and Canada without violating manufacturer’s warranties is B5, a blend of 5 percent biodiesel and 95 percent regular diesel. Most diesel engines run just fine on blends of up to 30 percent biodiesel.

For higher blends, the engine control unit’s (ECU) electronic fuel “mapping” system, which regulates timing, fuel/air mixture, and so on, has to be reprogrammed to perform efficiently. The reason is that, although there’s no mechanical difference between a diesel engine that runs on diesel oil and a biodiesel-burning engine, biodiesel has slightly different energy and burning characteristics.

Biodiesel vehicles in the U.S. have been modified by do-it-yourselfers and specialty shops so that they can use higher biodiesel blends and fuels made from a variety of substances. It’s possible to make biodiesel from most any crop-based oil, and the news is filled with stories about modified vehicles that run on biodiesel made from french-fry oil and other restaurant grease, fresh-pressed cottonseed oil, you name it. But some of these oils contain compounds that can eat through gaskets and may be prone to going rancid if stored too long. Also, because biodiesel is a better solvent than standard diesel fuel, it can remove deposits in the fuel lines. That sounds like a good thing, but those deposits may foul up fuel filters and fuel injectors as they move through the fuel system. As a result, federal standards for the chemical composition of biodiesel fuel must be in place before it’s available in widespread use and before automakers will permit its use under warranty in anything but very diluted amounts.

TIP: The diesel fuel sold at truck stops is often cheaper than at service stations and the fuel is fresher too. Freshness is important, because diesel fuel can easily become contaminated by the water vapor that condenses in fuel tanks, and although it’s rarely found in North America these days, really dirty fuel can contain fungus and other microbes that can clog filters and fuel injectors. If you find yourself at a station that seems suspicious, look for slimy stuff on the nozzle of the fuel pump. Try to fill up at a truck stop on a Saturday morning, when commercial trucking action is light. Weekday evenings are the worst times to buy because muscling a small vehicle into a crowd of big rigs isn’t easy!

Emergency Substitutes
Theoretically, diesel engines should be able to run on kerosene, certain airplane fuels, biodiesel in all blends between 5 percent and 100 percent, and even home heating oil, but the key word here is “theoretically.” Do not use these oils in your vehicle except in absolute emergencies. Standards of refining, filtering, and blending these oils differ widely, and they can ruin your engine, void your warranties, and create a whole lot of trouble for you. If you find yourself low on fuel in a remote area, look for trucking companies, food-processing plants, electric plants, hospitals, and farms. These places usually have diesel engines on the premises, and some good Samaritan may take pity on you and let you have some. If you absolutely can’t find a source of diesel fuel, as a last resort borrow some home heating oil or buy some Jet-A fuel at a local airport. Diesel mechanics consider these substitutes to be like rotgut whiskey — it will get you there, but it’s not the best stuff for your system! Drive on these fuels only long enough to get to the nearest source of proper fuel.

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