Diesel Fuel… The Ins and Outs

Diesel Articles - General

A few weeks back I wrote an article on the major differences between gas and diesel vehicles entitled “Diesel 101.” This week, in my ongoing effort to educate you on diesel vehicles I will cover the topic of the fuel such vehicles use… Diesel Fuel! Read on.


Diesel Fuel Energy Density and Rating at the Pump

Petroleum Oil Refinery into Component FuelsWhen crude oil is processed at refineries, its separated into several different kinds of fuel. At U.S. refineries in 2012, a 42-gallon barrel of oil typically delivered 19 gallons of gasoline, 11 gallons of diesel fuel, and lesser quantities of jet fuel, home heating oil, kerosene, liquefied petroleum gas and other distillates. Diesel fuel produced for vehicle use is sometimes called “diesel oil” or No. 2 diesel. Kerosene, which is chemically similar but more highly refined and less viscous, may also be referred to as No. 1 diesel.

Diesel fuel is heavier and oilier than gasoline, and it supports compression ignition with an auto ignition temperature (the temperature at which a substance will burn without exposure to an external flame or spark) of just 410°F compared to 536°F for gasoline. Diesel fuel also has a higher energy density than gasoline, hence the increased fuel efficiency. One gallon of diesel contains approximately 17.6 more energy – 147,000 British Thermal Units (BTUs) compared to 125,000 BTUs for gasoline. This additional fuel energy is the main reason why diesel engines get better fuel economy than their gasoline counterparts.

Diesel fuel is normally sold in a single grade whose quality is identified by a cetane number. The cetane number stands for the fuel’s ability to ignite when sprayed into hot compressed air. Higher cetane diesel fuel typically provides smoother operation and a reduction in engine noise. On-road diesel fuel in the U.S. must have a minimum cetane number of 40, although most pump diesel is in the 42 to 45 range. “Premium” diesel fuels with higher cetane numbers and a variety of special additives are sold in some markets. Diesel fuel in California typically has a higher cetane number due to some unique fuel blending requirements in that state.

Ultra Low Sulfur Diesel

Diesel fuel has historically contained a lot of sulfur, which is a naturally occurring element in crude oil. Sulfur dioxide emissions in diesel exhaust have been a primary contributor to the engine’s reputation as a smelly powerplant, additionally sulfur emissions react with moisture in the atmosphere to form sulfuric acid rain.

As recently as 2006, the Low Sulfur Diesel (LSD) fuel sold for on-highway use contained up to 500 parts per million (ppm) of sulfur. Even at this level, the sulfur content in diesel fuel created harmful emissions and exhaust system deposits that prevented the use of advanced emission control equipment such as catalytic converters. To address these concerns and help meet increasingly stringent emission control requirements, Ultra Low Sulfur Diesel (ULSD) fuel containing less than 15 ppm of sulfur was introduced beginning in 2006. Today, all 2007 and later diesel engines in on-road vehicles must use ULSD and have instrument panel and fuel filler labeling expressing this requirement. Federal regulations also require the labeling of diesel pumps at gasoline stations to identify the type of fuel being dispensed. Use of anything other than ULSD in a 2007 or newer vehicle will reduce engine efficiency, fuel economy, and durability, and may prevent engine operation. In addition, fuels other than ULSD will damage advanced emission control systems and void the vehicle manufacturers warranty on any affected engine, fuel and exhaust system components.

Based on the fact that sulfur is a natural lubricant, one concern raised during the transition to ULSD fuels was that increased engine wear might result. Engine manufacturers responded by “case hardening” components that might be subject to greater wear, special motor oils have been developed for diesel engines that use ULSD and fuel providers now blend additional lubricity agents into diesel fuel. For those who operate older diesel engines and have concerns about the impact of using ULSD, aftermarket fuel additives are available to increase engine and fuel system protection. However, the use of diesel fuel additives should always be in accordance with the vehicle manufacturer’s recommendations.

Diesel Fuel Pricing

Through ‘04, the national average price of diesel fuel was usually comparable to, or slightly lower than that of regular grade gasoline. However, since ‘05 diesel fuel has been more expensive. The U.S. Energy Information Administration cites several reasons for this change; here are some of the key reasons for this price discrepancy:

  • High worldwide diesel fuel demand for both vehicle and industrial use. While U.S. gasoline consumption has declined 5 percent since 2004, diesel demand has increased 29 percent.
  • Limited diesel fuel refining capability. The U.S. refining infrastructure is optimized for gasoline production, transport and storage. Increasing diesel fuel output requires significant and expensive refinery upgrades. While domestic demand for diesel is up 29 percent, production has risen just 15 percent.
  • Ultra Low Sulfur Diesel fuel production required approximately $8 billion in refinery upgrades. This added an estimated 5 to 10 cents per gallon to the pump price of diesel fuel.
  • The Federal excise tax for on-highway diesel fuel of 24.4 cents per gallon is 6 cents per gallon higher the gasoline tax. The last increase was in the early ‘90s.
  • Diesel fuel sold for off-road use (farm tractors, construction equipment, etc.) is not subject to federal and state excise taxes and therefore costs less. To help prevent unauthorized use, these fuels are dyed red for easy identification. Anyone caught using off-road diesel fuel in an on-highway vehicle is subject to fines that can run into several thousands of dollars. In addition, diesel sold for off-road use can be either LSD (Low Sulfur Diesel) or ULSD Ultra Low Sulfur Diesel). The use of LSD in a 2007 or newer diesel engine certified for highway use, will damage expensive emission control components and void the factory vehicle warranty.

Diesel Fuel Availability

With the growing popularity of diesel pickups and the growing number of passenger cars with diesel engines, the number of gasoline stations that sell diesel fuel has increased, to between 40 and 50 percent of all stations now having diesel pumps. In the past, most diesel pumps were found at truck stops along major highways or in rural areas where farmers used diesel trucks and equipment. Today, it has become the “norm” for neighborhood gasoline stations to offer diesel, as well all grades of gasoline.

Locating a station that sells diesel fuel is nowhere near as challenging as it was 20 years ago. Modern Web tools and smart phone apps such as AAA Mobile make finding diesel retailers easy. In addition, the better fuel economy of most diesel-powered vehicles means more miles are available between fill ups to locate a station. But just to be safe, especially when traveling in unfamiliar or lightly populated areas, many seasoned diesel owners will start looking for an opportunity to fill up when the fuel level drops below half a tank.

Diesel Fuel Safety

In general, diesel fuel is safer to handle and store than gasoline. For example, the flash point of diesel (the lowest temperature at which it can form an ignitable mixture with oxygen) is 144°F. This is much higher than the -49°F flashpoint of gasoline, which is far more volatile. On the other hand, the chemical makeup of diesel fuel means it is far slower to evaporate than gasoline when spilled and it leaves an oily residue on the pavement that can be as slippery as ice. Diesel spills on roadways can present a significant safety hazard, particularly for bicycles and motorcycles. Proper cleanup of spilled diesel fuel requires the use of appropriate absorbents, solvents and/or detergents to remove the oily deposits.

That’s if for now, more diesel education in the coming weeks.

‘Til next time… Keep Rollin’


maxxTORQUE in Print

(Available through Amazon.com)

Follow us on Twitter