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Tuesday, September 30, 2014

Lube Notes: Diesel Fuel Additives

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 of 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 performance.
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 deposits.
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 $600.00 to $800.00, using an appropriate additive is a very cost effective strategy.

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