Modern Diesel Fuel


Today’s diesel fuel is manufactured to be cleaner and produce lower emissions than before. The chemical components and additives in the fuel have been regulated to stringent levels. Most of the changes were mandated to curb pollution. To comply with these new regulations, petroleum refiners must employ new production methods that change certain characteristics of the fuel.

Energy is produced by your diesel engine when the chemicals that make up fuel are burned. The released energy is captured by the mechanisms, propelling the vehicle. Basic low sulfur diesel fuels are essentially similar. Chemical additive packages mixed with the fuels are what differentiate brands. Fuel quality can be measured several ways. Cetane is a property that determines how easy the fuel will combust in the engine. High cetane fuel burns easier and, therefore, is more desirable than low cetane. There are two ways to measure Cetane; cetane number and cetane index.

BTUs (British Thermal Units) is a measure of the energy stored in fuel. A percentage of the fuel is made of wax (paraffin) molecules. The more wax in the fuel, the higher the BTU value. However, too much wax in fuel causes cold temperature operating problems, so refiners must control its content as temperatures change. Other families of chemicals called aromatics and olefins are also present in diesel fuel. Almost all of the fuel is either paraffin, olefin or aromatic hydrocarbon (carbon and hydrogen) molecules. Aromatics and olefins are harder to burn, and produce smoke. They offer advantages in that they remain a liquid in cold temperatures, an important consideration in winter! Diesel fuels are blended to produce a final product that offers the best possible balance of properties for the season and location where they will be sold.

Fuel may contain impurities, such as sulfur. These impurities may produce unwanted combustion by-products or harm the engine’s components. Cetane number is a direct measurement of the ease with which a fuel sample combusts in a test engine. The cetane index is a four variable calculation that estimates the cetane number. The minimum specification for both is 40. As refiners attempt to increase the cetane of fuel, the result provides several effects:

  • Higher average cetane.
  • Less smoke and NOX emissions.
  • More wax, on average, than before.
  • Older fuel additives have less effect on fuel performance.
  • Lighter fuels have poorer cold flow properties.
  • Fuels become less stable.
  • More water than previous fuels.

Shortly after the new low sulfur (LS) fuels were introduced, an outbreak of fuel pump failures spread throughout the market. These failures were blamed, initially, on the inability of LS fuel to lubricate important components in the system. Indeed, rotary fuel pumps, common in pickups, delivery trucks, and agricultural equipment, are fully fuel lubricated and are at risk of premature wear. But the majority of the problems in large engines related to seal failures, and once the seals were replaced, the problems ceased to occur.

LS fuel is unstable in storage. If stored several months, varnishes and gums may appear which could lead to excessive smoke. When varnishes and gums plug fuel filters and/or injectors it can alter the pressure and/or result in poor fuel atomization into the combustion chamber, which leads to incomplete burning of the fuel. This unburned fuel leaves with the exhaust in the form of black smoke.

Water contamination is a very common problem. The ASTM D-975 diesel fuel specification limits the water content to 500 parts per million (ppm) or 0.05%. Water can form ice that might block fuel filters or lines in cold weather. Chemical manufacturers began producing additives that prevent cold weather problems resulting from ice and wax. It is important to choose the right type of fuel for the vehicle’s operating conditions. Penray additives will effectively combat these problems. Some of the fuel properties that aftermarket products modify are shown below in Chart 1.

Chemical Result
Cetane improver An ignition delay improver useful for smaller or older engines.
In necessary concentrations, fairly expensive to use. In most
instances it improves power and may significantly reduce smoke.
Stabilizer Preserves the quality and color of fuel in storage. Especially useful
for emergency stand-by or generator-set operators.
Lubricant Protects fuel system components from premature wear resulting
from the compromised lubricity of some LS diesel fuels.
Smoke Reducer Reduces particulate emissions.
Biocide Biocides prevent and kill bacteria and fungus. These chemicals may
not mix well with other additives.
Fuel Injector Detergent Dirty injectors cause power complaints due to poor atomization
of fuel and incomplete burning. This unburned fuel is visible as
excessive smoke. Power and fuel economy are compromised.
Detergents can prevent or even clean up dirty injectors to restore
optimum performance.
Cold Flow Improver (CFI) These cost effective winter operation enhancers permit the fuel to
flow at low temperatures. Do not use alcohols – they do not work
on wax.