The Importance of Cetane in Diesel Fuels
What is Cetane Number?
Cetane number is a measure of the ignition quality of a diesel fuel. It is often mistaken as a measure of fuel quality. Cetane number is actually a measure of a fuel's ignition delay. This is the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels.
Cetane number should not be considered alone when evaluating diesel fuel quality. API gravity, BTU content, distillation range, sulfur content, stability and flash point are all very important. In colder weather, cloud point and low temperatujre filter plugging point may be critical factors.
Determining Cetane Number
The optical method for determining cetane number is ASTM test D-613. This method requires the use of an industry standard test engine equipped with accepted instrumentation and operated under specific conditions. In this test, the engine compression ratio is varied for the test sample and reference fuels of known cetane number to obtain a fixed ignition delay. The compression ratio of the sample is bracked by those of two reference fuels. The cetane number of the sample fuel is determined by estimating between the two reference fuel points.
How Does Cetane Number Affect Engine Operation
There is no benefit to using a higher cetane number fuel than is specified by the engine's manufacturer. The ASTN Standard Specification for Diesel Fuel Oils (D-975) states. "The cetane number requirements depend on engine design, size, nature of speed and load variations, and on starting and atmospheric conditions. Increase in cetane number over values actually required does not materially improve engine performance. Accordingly, the cetane number specified should be as low as possible to insure maximum fuel availability." This quote underscores the importance of matching engine cetane requirements with fuel cetane number.
Diesel fuels with cetane number lower than minimum engine requirements can cause rough engine operation. They are more difficult to start, expecially in cold weather or at high altitudes. They accelerate lube oil sludge formation. Many low cetane fuels increase engine deposits resulting in more smoke, increased exhaust emissions and greater engine wear.
Using fuels which meet engine operating requirements will improve cold starting, reduce smoke during start-up, improve fuel economy, reduce exhaust emissions, improve engine durability and reduce noise and vibration. These engine fuel requirements are published in the operating manual for each specific engine or vehicle.
Overall fuel quality and performance depend on the ratio of parafinic and aromatic hydrocarbons, the presence of sulfur, water, bacteria and other contaminants, and the fuel's resistance to oxidation. The most important measure of fuel quality included API gravity, heat value (BTU content), distillation range and viscosity. Cleanliness and corrosion resistance are also important. For use in cold weather, cloud point and low temperature filter plugging point must receive serious consideration. Cetane number does not measure any of these characteristics.
Cetane Improvers / Ignition Accelerators
U.S. diesel fuels are blends of distillate fuels and cracked petroleum hydrocarbons. The cracked hydrocarbons are low cetane compounds, largely due to their aromatic content. To meet the cetane number demands of most diesel engines, cetane improvers must be added to these blends. The lower cetane cracked compounds are less responsive to these cetane improvers than the higher cetane paraffinic fuels.
Cetane improvers modify combustion in the engine. They encourage early and uniform ignition of the fuel. They discourage premature combustion and excessive rate of pressure increase in the combustion cycle. Depending on the amount of high versus low cetane components in the base fuel, typically alkyl nitrate additive treatments can increase cetane by about 3 to 5 numbers (1:1000 treatment ratio). With high natural cetane premium base buels (containing a high percentage of parafins) and a 1:500 treatment ratio, cetane may increase up to a maximum of about 7 numbers.
Most cetane improvers contain alkyl nitrates which break down readily to provide additional oxygen for better combustion. They also break down and oxidize fuel in storage. This generates organic particulates, water, and sludge - all of which degrade fuel quality. The result is often a fuel which no longer meets even minimum requirements. (*Because of these drawbacks, nitrate cetane improvers are not used in Fuel Magic.)
Fuel Magic is blended to improve oxidation stability while providing a cetane number increase of 2 to 3 numbers. Fuel Magic improves combustion while reducing oxidation and particulate formation, increasing storage stability, and enhancing fuel quality.
Do Cetane Improving Additives Really Improve Fuel Quality?
Fuel quality is defined by the physical property specifications given in the ASTM Standard Specification for Diesel Fuel Oils, ASTM D-975. Carbon residue, ash and sulfur increase engine wear and deposit formation. Premium diesel fuels should have lower specifications for these properties. Additionally, premium diesel fuels should be more stable in storage than standard fuels, so the premium fuel quality you purchase won't degrade over time. This is the area where nitrate-containing cetane improvers cause problems. (*Fuel Magic contains no alkyl nitrates.)
More fuel retailers are introducing premium diesel fuels, touting high cetane number as the sole benchmark of fuel quality. Contrary to this assumption, a cetane number which is too high may cause too short an ignition delay period. This changes the timing of the pressure peak, resulting in loss of power. When this happens, many of the performance problems associated with low cetane fuel will result. While the problems due to low cetane largely disappear after the engine warms up, with too high a cetane, these problems will persist even with a hot engine.
Specifying Essential Diesel Fuel Characteristics
Cetane number is an important measure of ignition quality, or cold-starting ability. API gravity is an excellent indicator of heat value, which translates into fuel economy and power. The distillation curve reflects the molecular weight distribution, with higher boiling fractions providing better lubrication, higher cetane - and more deposits. Sulfur content is directly related to corrosion, this needs to be as low as possible. Oxidation stability, water, and sediment content affect the storage life of the oil. For winter use, low cloud point and low temperature filter plugging point are critical to uninterrupted operation. To insure the best quality fuel for your diesel engines, follow the engine manufacturer's specifications for all these characteristics.
"Importance of Cetane" article courtesy of PMC Technologies Inc.
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