By Rober L. Leisenring, Jr., Texaco Additives International (Many thanks to BKmetz for sending this to me from the July/August edition of "The Star")

Diesel engine reliability, durability, and performance are affected by fuel quality. Since engine performance may be enhanced if fuel properties exceed today's minimum standard for diesel fuel (ASTM D975), it is prudent to use fuels that exceed those minimums. Still, you need to know what you're getting and what you're not getting. You also need to know whether your individual needs call for fuel that exceeds the minimum specifications. My previous article focused on premium diesel fuel. This one concentrates on fuel properties that impact the operation of your diesel engine.

The need remains for reliable information on the low temperature operability of diesel fuel; otherwise, the usual penalty is a call for a tow truck. Attention to diesel fuel's low temperature properties is essential for any diesel owner. For ambient air temperatures above 20 degrees F, most diesel engine builders recommend ASTM D975 Grade No. 2-D diesel fuel.

For extremely low temperatures, diesel engine builders recommend No. 1-D diesel (kerosene), which usually has a lower cloud point and a lower pour point than No. 2-D. Cloud point is the temperature at which a haze or cloud of wax crystals first appears in the fuel when it is cooled under test conditions. Pour point is the lowest temperature at which a sample of diesel fuel will flow when cooled under test conditions.

At very low temperatures, such as -15 and 20 degrees F, "winterized" No. 2-D diesel is recommended. This is No. 2-D that has been modified for lower temperatures, usually by blending No. 1-D with No. 2-D, but additives can also be used.

The final No. 1-D/No. 2-D blend will start to gel at a lower temperature because of a lower cloud point. This lower cloud point, rather than pour point, improves the low temperature operability of the fuel blend. Pour point, on the other hand, is routinely used for quality control and to indicate the fuel's storage and handling limits. Pour point has often been used as an indicator of a diesel fuel's low temperature vehicle operability, but this is neither reliable nor predictable.

At low temperatures, wax crystals can start to form in diesel fuel. The fuel starts to look hazy, not the pristine clear that we are used to. In a vehicle's fuel system these wax crystals can collect on fuel filters andplug them, causing engine stumbling or stalling. The temperature at which this occurs is called the low temperature operability limit of the fuel and vehicle. Both fuel system design and fuel properties are important factors in determining this minimum temperature for acceptable operation.

While winterizing No. 2-D fuel by adding No. 1-D improves low temperature performance, economic and performance penalties are associated with doing so. No. 1-D diesel (kerosene) has an energy content (heat of combustion) less than that of No.2-D. The energy content of diesel fuel is the amount of energy stored in a gallon. Measured in British thermal units (Btu) per gallon, diesel fuel's energy content is related to its hydrocarbon mixture.

The terms "gross" and "net" are used with the term, "heat of combustion." The gross heating value includes the latent heat of water from combustion that condenses in the test procedure. In an engine, the water is exhausted as vapor, so the "net" heat of combustion is most important.

In some engines, diesel fuel with a higher energy content can provide higher power and better fuel economy. Likewise, diesel fuel with less energy provides less power and reduced fuel economy. This is pertinent during the winter because of "winter blending," discussed earlier. When No. 1-D (a lower gelling fuel) is mixed with No. 2-D, the resulting blend typically has a lower overall Btu content compared to a straight, unwinterized No. 2-D. This lower Btu content, which hurts fuel economy, is directly proportional to the amount of No. 1-D in the blend.

As Figure 1 shows, it is possible for fuel economy penalties to reach 8.7 percent as various amounts of No.1-D are blended into No. 2-D. This blending has the same negative effect on engine power, which can be reduced as much as 9.2 percent as various amounts are blended (see Figure 2). Simply, No.1-D (kerosene) can cost more than No. 2-D. This extra cost and the loss of fuel economy can significantly raise operating cost. Figure 3 combines the cost penalty for reduced fuel economy and the additional cost for No. 1-D fuel.

Yes. Additive treatment is an alternative for improving low temperature operability of diesel fuel. Such additives change the size and shape of wax crystals that form as diesel fuel is cooled below its cloud point. Wax crystals still form on the filter, but now they have been modified to allow fuel to pass through the wax and the filter into the engine. Without additives you have a solid block of wax in the filter. With additives this wax block has microscopic passageways for fuel to continue to flow.

Numerous concerns surround the use of winter diesel fuel additives. The chemistry of such additives varies, and not all performance additives are compatible with low temperature operability additives. Some additives negate the benefits of others, resulting in poorer fuel handling and performance. Proper testing by the additive supplier is an important factor when selecting an additive.

How will you know if the additives work? The obvious answer is that if your car stops, so did the additive. As ridiculous as this may sound, it happens. Therefore, a consumer needs to know how an additive's properties were measured and its benefits substantiated. For consumers, these facts may prove very difficult to get. Feet managers who buy additives in bulk may have better luck obtaining this information because they may deal directly with a salesperson familiar with the product.

The fuel and additive industries use two tests to evaluate low temperature operability of diesel fuel with additives: the Cold Filter Plugging Point test (CFPP) and the Low Temperature Flow Test (LTFT). Both attempt to predict a minimum temperature at which the vehicle will have acceptable performance.

The CFPP test, used successfully in Europe for years, is a quick-cool test. A fuel sample is rapidly cooled (by 40 degrees C per hour). The CFPP test temperature is the temperature of the sample when 20 ml of the fuel first fails to pass through a wire mesh in less than 60 seconds.

In the LTFT, developed in North America, the fuel sample is cooled more slowly, at one degree C per hour, to match the behavior of fuel exposed to overnight temperatures. In some vehicle and engine configurations, LTFT has a better correlation with field tests compared to that of CFPP testing, but CFPP testing correlates well with other vehicles. Because of the extremely slow cooling rate, which requires 12 to 24 hours, the LTFT method is impractical for routine testing.

If you can acquire CFPP or LTFT results, scrutinize them. Do they sound believable? If in doubt, look for a supplier who conforms to the National Council on Weights & Measures NCWM) Premium Diesel Fuel requirements. There is a low temperature operability specification, so chances are good that you will get the low temperature performance promised by such a supplier. The NCWM spent a lot of time reviewing data before making the low temperature recommendation as part of its definition of Premium Diesel.

Today, fuel injectors make an engine run at peak performance. Today's cars and engines last longer. It is not unusual for a big-block engine in a truck to have 700,000, 800,000, or even 900,000 miles before overhaul. Mercedes-Benz diesel engines typically exceed 200,000 miles.

Fuel injectors (see photo) are the heart of a diesel engine. These precision components meter fuel to high accuracy. Correct engine behavior depends on the injector doing its job properly; otherwise, there will be repercussions: noise, smoke, emissions, poor performance.

Proper injector functioning is essential for peak performance, so it is beneficial to keep the nozzles operating at their peak performance. Buildup of carbon on the injector can disrupt the spray pattern of the fuel being injected into the cylinder. This can lead to incomplete combustion, which can cause increased black smoke, decreased power, or poor fuel economy.

The tip of the fuel injector lives in a very harsh environment. It is in direct contact with the combustion process, both in direct and pre-chamber (indirect) injection engines. Products of combustion are deposited on the tip and can significantly alter the injector's operation.

For the pre-chamber engines in Mercedes-Benz cars, combustion products can partially block progressive delivery of fuel during various loads of the engine, so combustion can become violent and disorganized. With pre-chamber engines, coking is inevitable due to the type of injectors used. Coking involves carbon-type deposits that remain after exposure to hot combustion gases. The amount of coking on the injector tip and in spray holes depends on fuel quality. Excessive coking cannot be tolerated if you expect peak performance.

The solution to injector fouling is a detergent additive. High doses can clean an already-coked nozzle, while smaller doses maintain clean operation. The ability of an injector to stay clean will be more important as emission regulations, performance requirements, and customer expectations put pressure on engine manufacturers to squeeze every bit of performance out of the engine.

Detergent additives to control injection nozzle deposits became an important part of a premium diesel fuel when the Engine Manufacturers Association endorsed a detergent requirement in its FQP-1A document. The test method, L-10 Injector Depositing Test, was developed by Cummins to evaluate fuel and fuel additive effectiveness in reducing deposits on direct-injection nozzles typically found in heavy-duty engines.

The test utilizes a Cummins 1988 turbocharged L40 engine with Pressure Time (PT) injectors. The engine is cycled at 15-sec intervals between closed rack and partial rack for a total of 125 hours. At the end of the test, the injectors are flow-tested to determine the percent of fuel flow lost. Plungers in the injector bodies are removed and visually rated for deposits using the Coordinated Research Council (CRC) scale. Typically a "dirty" fuel, with no additives, will have CRC ratings over 20; a fuel with additives will have ratings below 10.

Cetane number is one of the most widely-known parameters of diesel fuel. Awareness doesn't always mean understanding, and there is a danger that cetane number can be confused with cetane index. A brief explanation is in order.

A diesel engine is a compression-ignition engine. The fuel is ignited by the high-temperature, high-pressure air created in the cylinder as the piston nears the end of the compression stroke. Fuel in gasoline engines is ignited by a spark plug.

The cetane number is a measure of the ease with which diesel fuel is ignited during the compression stroke. The number is determined using a specified laboratory test engine. The cetane index, on the other hand, is calculated using an equation involving the API gravity (density) and the distillation curve of the fuel. Consequently the cetane index cannot be increased and improved by cetane-improving additives because the equation doesn't account for the amount of cetane-improving additive in the fuel. If there is nowhere to put the additive into the equation, there is no way to change the cetane index except by changing API gravity or distillation.

When injected into the combustion chamber of the cylinder, fuel must quickly mix with air then ignite with no other ignition source. The time between the beginning of fuel injection and the start of combustion is called "ignition delay." Higher cetane number fuels result in shorter ignition delay, providing improved combustion, lower combustion noise, easier cold starting, faster warm-up, less white smoke, and, in many engines, reduction of some emissions. Society of Automotive Engineers publications have reported better fuel economy and increased power as a result of increasing the cetane number with additives.

When a diesel engine was run with naturally high-cetane fuel instead of the naturally low-cetane fuel improved to the same cetane number with additives, Texaco's research demonstrated an average 4.6 percent decrease in power, with an average 4.2 percent increase in fuel consumption.

The disadvantage of natural high cetane fuel compared to "additive-improved" cetane fuel is that the former is generally less dense. This lower density, as with winterized fuel, means lower volumetric energy content, so fuel economy and power per volume of fuel is reduced. Since fuel is bought by volume, this is a direct economic loss to the customer.

The NCWM analyzed data for more than 300 diesel fuel samples. Their average cetane number was approximately 44. This is probably representative of all U.S. diesel fuels. An increase of three points in cetane number above average (an increase of seven points above the minimum ASTM specification of 40) will provide added performance in some engines.

The Engine Manufacturers Association, the American Trucking Association, the American Automobile Manufacturers Association, and the recently-announced automotive manufacturers' World-Wide Fuel Charter all stress that the cetane number for premium diesel should be well above the national average. They feel that diesel engines operate better on fuels with cetane numbers above 50.

When stability is mentioned, most of us think of storage stability. That is, as fuel ages it can become unstable and form insoluble particulates that accumulate and eventually end up on the fuel filter. Diesel fuel is increasingly used as a coolant for high-pressure fuel injection systems, which can thermally stress the fuel. Sometimes this can cause the fuel to degrade and form insoluble materials, which can restrict fuel flow through filters and injection systems. Three tests are routinely used to evaluate a fuel's stability, ASTM D 2274, the Octel F21-90 minute test, and the Octel F21-180 minute test. ASTM D 2274 is an accelerated oxidation stability test. Oxygen is bubbled through a fuel sample for 16 hours, after which the fuel is filtered to collect any insoluble materials. Fuels that have insoluble materials of less than 15 mg/liter are usually deemed stable.

Both Octel tests are thermal stability tests, the difference between the two being the amount of time the fuel is thermally stressed. A sample is subjected to a 302 Degree F bath for either 90 or 180 min then filtered to collect insoluble materials, the result being measured by light reflectance. ASTM is developing the 180-min test as a standard because it works better than the 90-min test.

Pipeline companies consider fuel with a reflectance of greater than 70 percent to be good regular diesel fuel, whereas NCWM considers fuel with 80-percent reflectance to be a step above regular diesel fuel.

Microorganisms (bugs, fungi) in diesel fuel are a constant concern for users. Prevention is important in fuel storage facilities. These bugs grow wherever water meets fuel, feeding at the interface but living in the water. Bug growth can form a gel like substance that can clog fuel lines and filters.

The best way to prevent the problem is to keep water out of your fuel system, but this is not easy. A recent study showed many storage tanks tilted in the opposite direction than the owners thought they were, so water was accumulating without the owner's knowledge-a perfect breeding spot for bugs.

Once bug contamination of a storage tank occurs, cleanup can be difficult and expensive. For severe contamination-a large interfacial mass-physical removal of the bugs' debris is important. Depending on the problem's severity, manual cleaning of the tank may be required to remove debris and corrosion (byproducts of microorganism growth). If a biocide is added to the fuel before cleaning, the break-up of the microbial mass increases debris. In less severe contamination, with little debris, it may only be necessary to filter the fuel to remove it, but this may require frequent filter changes.

Back On Your Own

I hope this helps you to become an educated consumer. Although several organizations watch the fuel industry, the ultimate responsibility for assuring that you get what you pay for rests with you. Dealing with a trustworthy supplier and reading your owners manual make that easier. One day there may just be a standard for all critical properties of diesel fuel.

Roger L. Leisenring, Jr. is a Technologist for the Research & Development Department of Texaco Additives Inter-national (TAI), a major worldwide marketer of fuel additive packages and components, in Beacon, N.Y. Before joining TAI, he had more than 18 years experience in diesel and gasoline research and development with Texaco's Fuels Technology Department. He chairs the ASTM diesel fuel section and SAF's diesel fuel section and is co-chairman of the NCWM's Premium Diesel Task Force. He can be reached at 914/838-7336 or fueladditives@texaco.com.

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