Modern Diesel Fuel

Issue 10 and Volume 18.

By Christian P. Koop

Ever wonder about the quality of the diesel fuel you are putting into your emergency response vehicle’s (ERV) fuel tank? If not, you should be. The quality and ingredients used to formulate modern diesel fuel and how it is stored and transported can adversely affect a fuel delivery system’s life, emissions, and even fuel economy. The main purpose of this article is to give a brief history of diesel fuel, some of its main components, their purpose, and some of the most important issues surrounding diesel fuel today. Additionally, I want to make those unfamiliar with diesel aware of what they can do to test the diesel fuel they are using in their ERVs and what they can do to improve it. It may not be up to the standards diesel engine manufacturers require for their engines.


Before discussing diesel fuel, I need to give credit to the inventor of the diesel engine, Rudolf Christian Karl Diesel. Diesel was a German refrigeration engineer born in Paris, France, in 1858. He received a patent for his invention in 1892. Interestingly, his first fuel of choice for his compression ignition engine was coal dust. However, he had problems injecting the coal dust into the cylinder. After an explosion destroyed his first engine, he began testing the use of vegetable oils as another fuel source. Eventually, he was able to successfully use peanut oil; however, he continued to experiment with other possible fuel sources. Finally, he found what eventually would be known as diesel fuel, a stable byproduct of the petroleum (crude oil) refinement or distillation process. Other fuels derived from petroleum through this process include bunker oil (fuel for large ships), gasoline (petrol), jet fuel (kerosene, paraffin), mineral spirits, and heating oil (very similar to diesel).

Diesel fuel is also referred to as fuel oil and has a wide boiling point range between 320°F and 690°F. Keep in mind that petroleum contains a large number of hydrocarbons and other components that are used to manufacture many commercial products-not just fuels. Diesel died in 1913 at the relatively young age of 55. However, by this time, his engine had been granted many patents. When his main patent expired in 1907, other companies such as Mercedes Benz and Peugeot began developing their own engines. By 1936, Mercedes showed the first nonexperimental diesel-engine-powered passenger car at the Berlin Fair.

The Environmental Protection Agency (EPA) actually began regulating emission standards for on-highway and transit compression ignition engines in 1974. Over the years, it gradually tightened the standards on hydrocarbons (HC), carbon monoxide (CO), particulate matter (PM) or soot, and nitrogen oxide (NOx) emissions. However, it was not until 1985 that a restriction on NOx was issued, and it began limiting PM for the first time in 1988. This is why the heavy duty diesel engine manufacturers began producing electronic controls for their fuel injection systems in the mid 1980s. More precise control over timing and fuel injection means better combustion, which equates to less PM and cleaner air. This cleaned up the diesel engine emissions considerably, but stricter (EPA) regulations to lower PM and to reduce NOx emissions even further were on the horizon.

In 1993, the EPA issued a new standard for diesel fuel, reducing the sulfur content to 500 parts per million (ppm), named low-sulfur diesel (LSD). In 1997, the EPA issued a new standard for the 2004 model year with major changes to reduce NOx and PM even further for model years 2007 and 2010. These changes would require reformulating diesel fuel to reduce the sulfur content even further. Beginning in 2006, it dropped the sulfur content even lower to 15 ppm and called it ultra-low-sulfur diesel (ULSD).

Sulfur in diesel is linked to acid rain, causes health problems, and can also lead to acid formation inside the engine. There are different types of PM in our atmosphere. However, PM from diesel engine exhaust comprises extremely small particles that can cause serious health issues including brain damage and cancer. NOx, which is created inside the cylinder during very high combustion temperatures as nitrogen and oxygen react, is a major ozone pollutant. It is partially responsible for smog and reacts with moisture to create nitric acid vapor, which can cause or aggravate several respiratory diseases such as emphysema and bronchitis.

In 2004, exhaust gas recirculation (EGR) valves began to appear on heavy duty diesel engines to reduce NOx emissions. They perform the same function in a diesel engine as they do in a gasoline engine. They do this by recirculation of spent or inert exhaust gases that help to lower combustion-chamber temperatures and in doing so lower NOx emissions. Diesel motor oils had to be reformulated to disperse the higher levels of soot going into the oil because of the EGR system.

Diesel Fuel Formulation

The formulation of diesel fuel required by the EPA for the purpose of cleaning up the emissions of atmospheric pollutants has undergone many changes during the past 20 years. You can somewhat compare it to the changes gasoline formulation underwent beginning in 1973 when lead was being removed to pave the way for the introduction of catalytic converters. If leaded gasoline is introduced into an engine equipped with a catalytic convertor, it will severely damage it. Sulfur, which is naturally occurring in diesel, improves lubricity when combined with nickel. However, it is not compatible with the diesel particulate filters (DPFs) that became mandatory on all medium and heavy duty diesels in the United States beginning with the 2007 model year.

You can draw lines of comparison between the damage to a catalytic converter from leaded gasoline and the damage to a DPF by diesel fuel with sulfur. The main difference is that the damage is caused for different reasons. Removing sulfur from diesel is good for the environment and human health; however, it is believed to have created problems in the tight tolerances of fuel injection systems. And, it has been known to cause certain fuel pumps to fail and cause seals to shrink and leak. Other studies do not blame the removal of sulfur for the leaking seals but rather on old and previously damaged worn seals. The introduction of ULSD in 2006 caused many concerns over the lubricity issue. The industry responded by using additives at the fuel terminal level to increase lubricity to adequate levels.

Cetane Rating

One of the most vital aspects of diesel, as far as performance and its ability to burn cleanly, is its cetane level or rating. Cetane is basically the ability or quality of diesel to combust quickly when it is injected into the combustion chamber of a compression ignition engine. The higher the cetane rating number, the easier the fuel will combust when injected into the combustion chamber and the cleaner it will burn and emit less PM. Cetane plays a similar roll in a diesel engine as octane does in a gasoline engine but for opposing reasons.

The higher the octane rating for gasoline, the more evenly and slower it burns when it is ignited in the combustion chamber and the less likely it is to preignite. However, unlike gasoline which has to have its octane rating posted on the pump by law, diesel fuel does not. Cetane levels are known to be very inconsistent from location to location. It is very rare to see a cetane level for diesel fuel posted on a pump, and the only way to really know for sure is to have your fuel tested by a reputable laboratory.

Cetane Testing

There are several different methods to determine cetane levels in diesel fuel. The quality of the fuel plays a very important role in the amount of PM generated during combustion. In 1994, the United States required a cetane level of 38. This went up to 40 in 2000, and the cetane for ULSD is currently supposed to fall between 47 and 48. However, most engine manufacturers would like to see a minimum of 50. In Europe, the minimum cetane requirement is 51; California currently requires 53; and premium diesel, if you can find it, is 60. There are various other important components that a lab can analyze and measure such as sulfur, lubricity, cold-flow properties, and density. However, cetane is the factor most linked to performance and how easily a diesel engine will start, particularly in cold climates.


The quality of diesel fuel can vary greatly from location to location and is not as tightly regulated as gasoline. Because of this, there are a number of companies that produce additives to raise cetane levels, improve lubricity, and improve performance. As most of us know, the quality of additives can also vary greatly, and they don’t always do what they advertise. However, there are a number of them on the market today that are legitimate products that will raise cetane levels and improve performance. Many are recommended by the manufacturers. For example, Ford Motor Company has one fuel additive it recommends for its Power Stroke diesel engine that I have personally seen quiet the operation of the engine and improve performance significantly. Another large company that has a line of diesel fuel additives is Certified Labs in Texas, which also has completed very extensive testing and research documenting diesel fuel chemical properties and the value of additives. These additives not only will improve performance but can also reduce emissions, keep EGR valves from sticking, and even improve mileage.

Many large heavy duty over-the-road fleets use these products successfully and have documented savings that indicate improvements in fuel mileage, in extending the life of fuel injection systems and fuel filters, and in reducing equipment downtime. Another major issue with diesel fuel is that it can hold water and create an environment that allows fungal and bacterial growth. This can lead to plugging fuel filters, scoring expensive fuel injectors and injection pumps, and fuel lines freezing in northern climates. There are also additives on the market that contain antimicrobial agents and diesel fuel biocides designed to successfully treat these issues without dumping the fuel.

Looking Ahead

The significant changes and transformation of diesel fuel formulation to be compatible with the latest diesel emissions technologies have been significant and have reduced emission levels tremendously. This technology is helping to clean the environment and helps keep us healthy. However, it has also reduced the high levels of reliability and durability we, in the ERV service side, were accustomed to with the diesel engines of yesteryear. Because of the reduced reliability and the obvious critical need for ERV reliability, this subject is highly debated. If you are interested in more comprehensive information about diesel fuel, Certified Labs has a 128-page document you may access at

For those of you who are interested in knowing the cetane level and diesel fuel quality of what you use in your fleet, I recommend you have it tested with an accredited lab. If your fuel is not up to standards and you want to improve it with additives, do your homework and test the additive. It pays to document properly and test the additive in sister trucks that operate under the same service conditions for comparison to prove or disprove a product.

All diesel fuel is not the same quality, nor are the additives. Diesel fuel costs are higher than gasoline today because of increased demand for it beginning in 2004 and changes to its formulation required by the EPA to help clean the environment. Many transit fleets and other intercity fleets have been converting to compressed natural gas (CNG) and liquid propane (LP) for their fuel sources to reduce their oil dependency and clean emissions even more. I see LP- and CNG-powered ERVs in the not-too- distant future. For those of you who are in the service and repair side of ERVs, it might not be a bad idea to start learning about these systems and be prepared for even more change.

CHRISTIAN P. KOOP is the fleet manager for the Miami-Dade (FL) Fire Department. He has been involved in the repair and maintenance of autos, heavy equipment, and emergency response vehicles for the past 35 years. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/heavy truck technician and master EVT apparatus and ambulance technician. He is a member of the board of directors of EVTCC and FAEVT and a technical committee member for NFPA 1071, Standard for Emergency Vehicle Technician Professional Qualifications.