By Dan Watson
Ultra-low sulfur diesel, low-sulfur diesel, off-road full-sulfur diesel, number 1 or number 2
diesel, Cetane ratings for diesel and on and on it goes; what does it all mean and do I need to
use an additive to supplement my diesel fuel? Today’s diesel fuel is very different from diesel
of 30 years ago. Modern ultra-low sulfur diesel is required in all over the road diesel vehicles,
and failure to comply with this requirement can result in costly fines. In this article, I will
explain the sulfur content ratings and how the sulfur content affects the fuel system
components. I will also take a look at Cetane ratings and why Cetane levels are important. And
finally, I will explore using diesel fuel additives…how they work and whether you should
consider using an additive.
Sulfur is a natural component of crude oil that is not refined out when making diesel. Special
processes are used to remove sulfur, and these extra steps in finishing diesel add cost to the
finished product. Original diesel of 30 years ago was limited to 5,000 ppm sulfur in solution. In
the early 1990’s, low-sulfur diesel set the sulfur limit to 500 ppm sulfur in solution. In October
2006, the ultra-low sulfur diesel limit of 15 ppm in solution was stipulated. The 15 ppm sulfur
limit was primarily designed to prevent poisoning the catalytic convertor and adding to the
clogging of the diesel particulate filter. Certainly, the 15 ppm limit would reduce sulfur based
exhaust compounds such as sulfur dioxide and sulfur trioxides. These sulfur compounds are
directly related to acid rain, so reducing them is a good idea. So, there is no turning back on
the quest to limit exhaust emissions and consequently reducing sulfur content in diesel fuel; the
question is, at what cost to fuel system components and to diesel performance?
Since removing sulfur has a positive effect on emissions, why make such a big deal about the
resulting effect on the fuel system components? The process for removing sulfur to such a low
level of 15 ppm also strips out the compounds that provide diesel lubricity. Sulfur is not the actual lubricant but sulfur compounds are, and effectively removing sulfur from diesel results in a very poor lubricity rating for the fuel. Fuel pumps and injectors are lubricated by diesel and nothing else, so without sulfur providing the lubricity, these components are not lubricated. This is well known by the diesel manufacturers and the government, and standards for adding lubricants to the diesel have been established. It is a subject of discussion as to whether the levels of added lubricant are sufficient to provide for acceptable long-life expectations.
Cetane number or CN is a measure of a fuel’s ignition delay, the time period between the start
of injection and the first identifiable pressure increase during combustion of the fuel. In a
particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower
cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils. In short,
the higher the cetane number the more easily the fuel will combust in a compression setting
(such as a diesel engine). The characteristic diesel “knock” occurs when the first portion of fuel
that has been injected into the cylinder suddenly ignites after an initial delay. Minimizing this
delay results in less unburned fuel in the cylinder at the beginning and less intense knock.
Therefore, higher-cetane fuel usually causes an engine to run more smoothly and quietly. This
does not necessarily translate into greater efficiency, although it may in certain engines.
Diesel comes in two classifications, number 1 and number 2. Number 1 diesel is more highly
refined (more waxes are removed) and is more resistant to freezing (becoming slush) than
number 2 diesel. Contrary to many assumptions, number 1 diesel is not required to have a
higher cetane number than number 2 diesel. In many cases, number 1 diesel does have a
higher cetane, but it is achieved by adding a cetane enhancing additive.
Generally, diesel engines operate well with a CN from 40 to 55. Fuels with higher cetane
numbers have shorter ignition delays, providing more time for the fuel combustion process to
be completed. Hence, higher speed diesel engines operate more effectively with higher cetane
number fuels. In North America, most states adopt ASTM D975 as their diesel fuel standard
and the minimum cetane number is set at 40, with typical values in the 42-45 range.
Clearly, it is relatively straight forward to see that using an additive to raise cetane level would
be dependent on the quality of fuel available. If diesel with proper cetane levels is available,
then you might not get much improvement by using a cetane enhancing additive. Another
consideration would be the expected RPM of the diesel engine being used. If the engine is
turbo charged and runs at higher RPM, then the higher cetane will be useful is enhancing
Traditional Fuel Injector Pintal
High-Pressure Common-Rail Fuel Injector Pintal
When evaluating diesel additives for improving lubricity of ultra-low sulfur diesel, it is important
to look at other factors involved in fuel system and cylinder maintenance and performance.
Injector performance is a matter of design and quality coupled with cleanliness. A perfectly
clean injector operating at design pressure can be very effective in providing fully atomized fuel
to the cylinder for combustion. Allow deposits or lacquer to build up in areas of critical
clearances and you have the formula for trouble. Injectors fire at pressures between 10,000 psi
and 30,000 psi, depending on application and manufacturer. The slightest impairment can
cause diminished performance. The optimum performance and efficiency takes place when
the injector provides a 360 degree puff of fully atomized fuel to combust fully with no
unburned fuel. Any liquid drops will be partially burned and result in high carbon waste and
For years, the industry has concentrated on the injector tips as the critical point to maintain as
clean as possible. Any buildup of carbon (coking) at the tip would be immediately detrimental
to the desired 360 degree puff and would allow a droplet to be formed. Cleaning agents have
been developed to keep the tips clean and they have been effective. Today, with the extremely
high pressure injectors being smaller and having tighter clearances internal diesel injector
deposits (often abbreviated to “IDID”) have been found within the injector body itself, such as
at the armature group, on the piston and nozzle needle and inside the nozzle body. These
deposits can slow the response of the fuel injector, or cause sticking of moving internal parts,
which may result in loss of control of injection event timing, as well as of the quantity of fuel
delivered per injection.
In summary, you may or may not need a cetane boost additive, but you probably do need an
additive for enhanced lubricity and detergency. The current fuel injectors are really modern
marvels, and, as incredible as they are, they are more susceptible to being fouled by carbon and
lacquer than the old fashioned injectors. The ramifications of poorly performing injectors are
significant: poor performance, loss of fuel economy and clogged diesel particulate exhaust
filters (DPF). Of course, the exhaust system is another story all in itself and I will not go down
that path in this article. When looking for a good detergent additive, make sure you see some
mention of the internal diesel injector deposits (often abbreviated to “IDID”); if the additive
doesn’t address this problem, don’t use it. With the cost of today’s injectors running $600 to
$800, using an appropriate additive is a very cost-effective strategy.