If a diesel engine starts hard or smokes more, the diesel quality is poor, right? Probably not a bad guess or estimation. Or, could it have something to do with the time of the year, plugged injectors or high moisture content in the fuel? Maybe it does but, we don't really know unless we pull a sample and have the fuel tested. Normally this isn't necessary! Simply an understanding of diesel fuel characteristics can help us make more informed decisions about diesel fuel.
There are currently only guidelines sug- gesting what characteristics are desirable for a diesel to be considered a premium diesel. So the only way to know what type of fuel you're buying is to be informed about what characteristics are important in diesel fuel and why.
Diesel fuel is a distillate fuel that has a boiling point higher than gasoline, yet lower than that of lube oil base stocks. The best diesel fuels are "straight-run" stocks, derived from simple distillation of crude oil. However many commercial fuels contain a proportion of catalytically cracked material to extend the yield of usable fuel.
The important properties of diesel fuels are:
- volatility
- heating value
- ignition quality / cetane number
- viscosity
- low-temperature flow
- storage stability
- component compatibility
- sulfur content
The volatility of a diesel fuel has little influence on its engine performance, except as it affects exhaust-smoking tendencies. The distillation range of a diesel does not allow much flexibility in this regard because of the interrelationship and interdependent with other specification factors. The controlling factor is the ASTM D 86 Distillation 90% evaporated temperature. Because diesel fuels are classified as nonflammable for freight purposes, minimum flash point restrictions are imposed.
Fleet operators, railroad and shipping companies are concerned about fuel economy. They aim to use the fuel with the greatest heating value, provided that other specification requirements are met. The factors that influence heating value are density and mid-boiling point.
This factor influences ease of starting, duration of white smoking after start-up, drivability before warm-up, and intensity of diesel knock at idle. Recent studies have correlated ignition quality with all regulated emissions. As ignition delay is reduced, the combustion process starts earlier and emissions (primarily CO and HC) are reduced.
Ignition delay is measured by the Cetane Number test (ASTM D 613), which uses a single-cylinder, variable compres- sion ratio engine analogous to the Octane Number engine. The factor measured is ignition delay (rather than knock) at a fixed compression ratio, and it is compared with that of standard reference fuels consisting of blends of n-cetane and heptamethylnonane. The cetane content of the blend that matches the ignition delay of the test fuel is its Cetane Number (CN).
Few laboratories are equipped with cetane engines, so a number of correlations between Cetane Number and physical properties have been developed. Best known is the ASTM D 976 Calculated Cetane Index, which uses the density and mid-boiling point of the fuel.
Diesel engines vary widely in their cetane requirements, and there is no commonly recognized way to measure this value. In general, the lower an engines operating speed, the lower the CN of the fuel it can use. Large marine engines can tolerate fuels with Cn's as low as 20, while some manufacturers of high-speed passenger car diesel engines specify 55 CN fuel. Canadian railroads now purchase fuel to a 37 CN minimum specifications, but most truck manufacturers call for a 40 or 45 minimum.
Using cetane improvers can increase Cetane Number (but not Cetane Index). These additives, usually organic nitrates, boost CN by 2 to 7 numbers, depending on the dosage and the type of base stock used.
ViscosityViscosity influences the spray pattern when the fuel is injected into the cylinder. Low-speed marine engines can use higher viscosity fuels than high-speed road-transport engines, and still run without excessive smoking. Minimum viscosity limits are imposed to prevent the fuel from causing wear in the fuel injection pump.
Unlike gasoline's, which have freezing points well below even the most severe winter ambients, diesel fuels have pour points and cloud points well within the range of temperatures at which they might be used. This presents no problem on ships or even in railroad applications where heated storage can be arranged. However, for on and off-highway use, precautions must be taken to tailor the low-temperature properties of the fuel to the weather.
Seasonal blending to control cloud point (the temperature at which wax separates from the fuel) is the refiner's assurance against field problems. ASTM D 97 is the procedure used to determine a fuel's cloud and pour points. In the winter, there is an increasing tendency to use flow improvers, polymeric additives that modify the wax structure as it builds up during cooling.
These additives keep wax crystals small, so they can pass through the fine pores of fuel filters en route to the injector pump. With suitable base stocks, flow improvers can keep an engine running at temperatures as much as 10°C below the fuels cloud point.
In addition, pour point depressants are sometimes used in diesel fuel to ensure that it can be dispensed at low temperatures. Tests such as the Cold Filter Plugging Point (CFPP), IP 309, are used to assess the potential of a diesel fuel to have cold-weather field problems.
In storage, diesel fuels are attacked by atmospheric oxygen, which can cause deposits of varnish, and for marine fuels containing residual components, asphaltic material. Antioxidants and dispersants are added to prevent such problems, while copper metal deactivators reduce the catalytic effects of screens and other parts.
In the presence of water, bacterial action can cause a build-up of slime in the storage system, leading to filter plugging. Biocides are added to inhibit bacterial growth.
In cold-weather areas, there is the risk of static electric charges building up during high-rate dispensing of distillate fuels. Refiners include antistatic additives in diesel blends to prevent explosions.
Diesel fuels are injected into the engine through precision pumps and fine injector nozzles. Dirt and water contamination must be avoided to protect these critical components. Specifications include tight limits on water and sediment, but some fuel marketers also install final filters at the nozzles of service station pumps to protect against dirt picked up in the distribution system.
Depending on the crude source, diesel fuels contain various amounts of sulfur oxides on combustion. These can cause high rates of engine wear and rapid depletion of engine oil additives. Engine manufacturers often relate oil change intervals to the fuel sulfur content.
Deposit build-up in engines is influenced by fuel quality. Fuels that leave a heavy carbon residue and contain excessive mounts of high boiling point materials are prone to cause engine deposits. Therefore, limits are placed on carbon residue and ASTM D 86 90% evaporated temperature.
Sulfur ContentSulfur content is the first diesel fuel property to be widely controlled by legislation aimed at limiting exhaust emissions. Sulfur is present in all crude oils and refined products. During combustion, sulfur compounds burn to form acidic by-products, SO2 and SO3, which form sulfates in the exhaust gas stream. Sulfates are part of a diesel engine's particulate emissions; therefore, controlling fuel sulfur level reduces the level of sulfate pollutants.
All diesel fuels contain small amounts of water. Hydro-carbon type and distribution, and bulk temperature control the amount that a fuel can hold. As temperature decreases, the amount of water dissolved in the fuel will also decrease and may lead to a water layer forming on the bottom of the storage vessel. Draining the tank regularly will minimize this layer and prevent bacterial contamination and the inadvertent pumping of water into the fuel system. Excessive water in a fuel system can cause corrosion, filter plugging and icing (in the winter).
The properties that affect our prices the most are cetane and sulfur content. The cetane of the Total PD we used to have usually ran about 52 cetane. The Premier Diesel (APD) we have now runs between 49-50. The #2 high sulfur and low sulfur fuels in Michigan range from 40-47 cetane. Needless to say, the price varies equally.
Our high-sulfur fuels today are lower in sulfur than our low sulfur-fuels were 8 years ago. The EPA considers anything less than .005% sulfur content to be a "low sulfur" and anything over .005% to be "high sulfur" fuel. High sulfur fuels can only be used in "off-road" situations. Low sulfur fuel must be used for on road purposes.
There are a number of diesel fuels available here in Michigan. We have Marathon, Mobil, BP/Amoco, Sunoco, Citgo and Canadian high sulfur. If you buy in transport quantities we can provide you with any of these products. If you buy from our plants, I am currently stocking Mobil and Sunoco low sulfur and BP/Amoco premier diesel. If you don't want #2 or premier, we offer the Sunoco with our own additive package. This provides a 4-5 cetane boost, a detergent package and an injector cleaner. This PD will cost about 3 cents more than LS #2 and the premier will cost about 6-7 cents per gallon more than LS #2.
My goal is and always has been to offer quality products. Price is important but I will not let it diminish the importance of quality. My hope is to inform you to what there is on the market and allow you to make an educated decision on what products you want to use in your operation.