Ever wondered why a particular brand of Diesel fuel makes you engine Zoom wherein the other makes it fart black smoke ?? Ever wondered why your friends diesel car performs better than yours although you both use diesel from same oil company ??
After reading many discussion posts and threads. Taking many advises I finally decided to put into place this thread which will give insight and answers to questions above.
Read On............... Diesel Fuel Quality
Fuel quality is defined by the physical property specifications given in the ASTM Standard Specification for Diesel Fuel Oils, ASTM D-975 or D-4737. Carbon residue, ash and sulfur increase engine wear and deposit formation.
Premium diesel fuels have lower specifications for these properties. Additionally, premium diesel fuels are more stable in storage than standard fuels, so the premium fuel quality you purchase won't degrade over time.
THE ATSM Worldwide Standards
The
ASTM 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.
ASTM D 4737 is a newer method and this method calculates CI using a four variable equation based on diesel's low, mid and high boiling points as well as density.
The Calculated Cetane Index by Four Variable Equation is useful for estimating ASTM cetane number when a test engine is not available for determining this property directly and when cetane improver is not used.
This also provides a means for estimating the ASTM cetane number (Test Method D 613) of distillate fuels from density and distillation recovery temperature measurements.
This specification
(ASTM D 4737) is being use by most petroleum companies inIndia as a standard measure.
Specifying Diesel Fuel 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 ensure the best quality fuel for our diesel engines, we should follow the engine manufacturer's specifications for all these characteristics and I am afraid its not given out by manufacturers here
. 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 also very important. In colder weather, cloud point and low temperature 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 bracketed by those of two reference fuels. The cetane number of the sample fuel is determined by estimating between the two reference fuel points.
Because the
ASTM D-613 test is time consuming and expensive, calculated cetane index
(ASTM D-976 or D-4737) is often substituted for cetane number.
The calculated cetane index is derived from the fuel's density and boiling range. While useful for estimating the cetane number of distillate fuels, this technique can not be applied to fuels containing additives that raise cetane number. These additives do not change the fuel density or distillation profile, so they do not alter the calculated cetane index.
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.
Diesel fuels with cetane number lower than minimum engine requirements can cause rough engine operation.
They are more difficult to start, especially 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 Owners manual for each specific engine or vehicle, although I could not find it in my vehicle manual.
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 measures of fuel quality include 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 emperature filter plugging point are also important.
Cetane number does not measure any of these above characteristics.
Cetane Improvers / Ignition Accelerators
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 can be added to these blends. The lower cetane 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, typical alkyl nitrate additive treatments can increase cetane by about 3 to 5 numbers (1:1000 ratio).
With high natural cetane premium base fuels 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 should not be used (if you intend to).
Now for the Cetane Specification by Oil Companies- Normal diesel from all brands has a Cetane number between 46 min - 48 max.
- Premium diesel from IOC, IBP, HP, Shell etc. has a cetane number between 48 min - 52 max.
I am also aware that System D (which I use) does not help improve the Cetane number. I cannot say the same about other additives as I havent researched on them.
I was not able to find the exact Cetane number specification on HP diesel fuel on their website, but as long as the follow the standards, it should be same as others.
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