Diesel Fuel 101

The development of the internal combustion engine began in the late eighteenth century. Slow but steady progress was made over the next hundred years. By 1892, Rudolf Diesel received a patent for a compression ignition reciprocating engine. But his original design, which used coal dust as the fuel, didn’t work.

Thirty-three years earlier, in 1859, crude oil had been discovered in Pennsylvania. The first product refined from crude was lamp oil (kerosene). Since only a fraction of the crude made good lamp oil, refiners had to figure out what to do with the rest of the barrel. Diesel, recognizing that the liquid petroleum by-products might be better engine fuels than coal dust, began to experiment with one of them. This fuel change, coupled with some mechanical design changes, resulted in a successful prototype engine in 1895. Today, both the engine and the fuel still bear his name.

The first commercial diesels were large engines operating at low speeds. They were used to power ships, trains, and industrial plants. By the 1930s, diesels also were powering trucks and buses. An effort in the late 30s to extend the engine’s use to passenger cars was interrupted by World War II. After the war, the automotive diesel became very popular in Europe, but hasn’t enjoyed comparable success in the United States.


When a cold diesel engine is started (cold start), the heat of compression is the only energy source available to heat the gas in the combustion chamber to a temperature that will initiate the spontaneous combustion of the fuel (about 750°F [400°C]). Since the walls of the combustion chamber are initially at ambient temperature rather than operating temperature, they are a significant heat sink rather than a heat source. And since cranking speed is slower than operating speed, compression is also slower, which allows more time for the compressed air to lose heat to the chamber walls. (A glow plug provides an additional source of heat in indirect-injection diesel engines.)

Even after the engine has started, the temperatures in the combustion chamber may still be too low to induce complete combustion of the injected fuel. The resulting unburned and partially burned fuel is exhausted as a mist of small droplets that is seen as white smoke (cold smoke). This situation normally lasts for less than a minute.  A fuel with a higher cetane number can reduce the problem by shortening the time during which unburned fuel is emitted to the atmosphere.

A fuel that combusts more readily will require less cranking to start an engine. Thus, if other conditions are equal, a higher cetane number fuel makes starting easier.




The cetane number of diesel fuel is an indicator of how readily and completely the fuel will burn in the combustion chamber.  The higher the cetane number, the faster the fuel will ignite and the more completely it will burn. These attributes are important because as the fuel burns faster and more completely, the engine experiences greater performance and produces fewer harmful emissions.

Diesel fuel in North America, with a typical cetane number of 40-42, generally carries a lower cetane number than diesel fuels from other regions of the world.

Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid pressure rise is responsible for the diesel knock that is very audible for some diesel engines.  Increasing the cetane number of the fuel can decrease the amount of knock by shortening the ignition delay. Less fuel has been injected by the time combustion begins and it has had less time to mix with air. As a result, the rapid pressure rise, along with the resulting sound wave, is smaller.
High-quality premium diesel fuels, like the products sold by Radio Oil Co., Inc. incorporate a dose of cetane improver.  The addition of this additive helps cold engines start quicker, significantly improves combustion efficiency, and reduces harmful emissions. In general, the engines just work better.

High cetane fuels may even provide more power. Diesel fuels with cetane improver added have higher Btu content than even a naturally high cetane fuel.  And higher Btu content means more energy per gallon and ultimately more power and better fuel economy. So, using our diesel fuel gives you the best of both worlds – higher cetane number and higher energy (Btu) content.  A high-cetane diesel fuel provides:

  • More complete combustion
  • Improved cold starts
  • Less engine noise and knocking
  • Reduced white smoke and warm-up time
  • Fewer misfires
  • Lower exhaust emissions: nitrogen oxide, hydro carbon, carbon monoxide, and sometimes particulate matter.


Fuel and/or crankcase lubricant can form deposits in t he nozzle area of injectors – the area exposed to high cylinder temperatures. The extent of deposit formation varies with engine design, fuel composition, lubricant composition, and operating conditions. Excessive deposits may upset the injector spray pattern which, in turn, may hinder the fuel-air mixing process. In some engines, this may result in decreased fuel economy and increased emissions.

Detergent additives, like those that are in our premium diesel fuels can clean up fuel injector deposits and keep injectors clean.  These additives are composed of chemicals that bond to existing deposits as well as elements of the fuel that are destined to become deposits and chemicals that dissolves in the fuel.  Thus, the additive can re-dissolve deposits that already have formed and reduce the opportunity for deposit precursors to form deposits.

Dispersant additives work in conjunction with the detergents.  They disperse the particulates that form as a result of the detergency, preventing them from clustering into aggregates large enough to plug fuel filters or injectors.


Lubricity additives are used to compensate for the poor lubricity of severely hydrotreated diesel fuels.  They contain a polar group that is attracted to metal surfaces, causing the additive to form a thin surface film. The film acts as a boundary lubricant when two metal surfaces come in contact.


Diesel fuel stabilizers help maintain fuel integrity over longer periods of time.  Poor fuel stability can cause sludge to build up, leading to plugged filters, increased engine and injector deposits, and fuel degradation.  These additives include antioxidants and metal deactivators.


De-Icing Additives  Free water in diesel fuel freezes at low temperatures. The resulting ice crystals can plug fuel lines or filters, blocking fuel flow. Low molecular weight alcohols or glycols can be added to diesel fuel to prevent ice formation. The alcohols/glycols preferentially dissolve in the free water, giving the resulting mixture a lower freezing point than that of pure water.

Low Temperature Operability Additives  These are additives that lower a diesel fuel’s pour point or cloud point, or improve its cold flow properties.  Most of these additives are polymers that interact with the wax crystals that form in diesel fuel when it is cooled below the cloud point.  The polymers lessen the effect of the wax crystals on fuel flow by modifying their size, shape, and/or degree of agglomeration.  To be effective, the additives must be blended into the fuel before any wax has formed, i.e., when the fuel is above its cloud point.  The most meaningful specification is the cold filter plugging point (CFPP) which is the lowest temperature that a specified volume of diesel fuel will pass through a standardized size filter in a specified amount of time.  These additives typically include pour point depressants and cold flow improvers.