October 2016

Process Engineering and Optimization

Build a diesel fuel performance additive, the right way—Part 1

Many diesel fuel additive companies in North America supply different additive products formulated for diesel-powered equipment.

Pipenger, G. G., Amalgamated, Inc.

Many diesel fuel additive companies in North America supply different additive products formulated for diesel-powered equipment. Each of these vendors claim that their specific products are the best available to upgrade diesel fuel for the fuel producer, fuel supplier and final fuel consumer.

However, actual testing has often proved to the contrary. Nearly all diesel fuel additive suppliers make generic product claims that their additives will “Yield better fuel economy,” “Increase engine power,” “Reduce smoke,” “Increase fuel lubricity,” “Reduce system deposits,” “Lower maintenance costs,” “Save money,” etc. Unfortunately for the additive purchaser, there is no global “watchdog group” or oversight mechanism to monitor and verify diesel fuel additive product claims.

The burden of proof for any diesel fuel additive’s performance has become the sole responsibility of the additive product purchaser. While some fuel additive claims can be verified with laboratory testing, proving other claims can only be conducted in the field by treating the diesel fuel and then actually running the “additized” fuel in the buyer’s daily fleet operations. Unless diligent steps are taken to laboratory test the prospective product before the purchase is made, there is no guarantee that the chosen product will achieve the benefits claimed.

Diesel fuel additive testing begins with choosing a reputable American Society for Testing and Materials (ASTM) qualified laboratory with established experience in testing diesel fuel additives. This can be challenging for the diesel fuel additive buyer, but a good laboratory can be found if the right questions are asked.

The laboratory search should begin with a review of ASTM D-975, “Standard specification for diesel fuel oils.”1 This document lists the various physical parameters that must be met for all diesel fuels, whether in an on-highway or off-road application. Unfortunately, ASTM D-975 provides only the basic recommended physical fuel property boundaries, and does not specifically address performance upgrades achievable with diesel fuel additives.

However, many of the same laboratory tests described in ASTM D-975 are appropriate to precisely determine the enhancements achievable with a prospective diesel fuel additive.

This work is Part 1 of a series, and it will review the details of the diesel fuel physical properties (and additive enhancements) required to optimize summer diesel fuel performance in today’s common-rail, fuel-injection diesel engines. Part 2, to appear in the November issue, will address relevant information regarding diesel fuel additives for modern common-rail, fuel-injection engines during cold-weather operations.

Cetane improver additive

This additive, which is predominantly 2-ethyl-hexyl nitrate, is the main chemical additive component used to decrease the ignition delay time (raising the fuel’s engine cetane number [ECN]) and improve the combustibility of a diesel fuel. Improving combustibility by raising the ECN is important, as there is a finite amount of time (microseconds) between fuel injection and exhaust during the combustion cycle in each cylinder.

A diesel fuel ignited at the optimum time, before piston top-dead-center (TDC) of the compression stroke, will burn correctly and release the maximum Btu from each fuel droplet during the power stroke. The fuel’s heat energy is converted into usable power that pushes the piston down, resulting in the production of maximum horsepower (hp). This directly correlates into lower unburned hydrocarbon emissions and improved fuel economy.

Enhanced fuel combustion manifests as less-visible smoke emissions, decreased combustion chamber fuel-related deposits, fewer post-combustion exhaust gas recirculation (EGR) valve and diesel particulate filter (DPF) deposit cleanings, reduced driver low-power complaints and less vehicle downtime.

In North America, ASTM D-975 requires a minimum ECN of only 40. While some diesel fuel refiners produce diesel fuels well above this minimum, others supply diesel fuels only two or three numbers above 40. The average ECN ranges from 44 to 46, despite the fact that diesel equipment operators know that engines run more efficiently on fuels with a much higher ECN. Enhancing diesel fuels with a cetane improver additive treatment is often left to the purchaser.

Fig. 1. ASTM D-613 engine cetane test cell.
Fig. 1. ASTM D-613 engine cetane test cell.

In theory, any increase in a diesel fuel’s ECN should provide some improvement in engine operation. However, only an increase of 4 to 5 ECNs will actually be noticed by the driver. This is especially relevant with the increase of European diesel vehicles entering North American markets. These diesel engines are designed to operate best on the European-mandated 51-plus ECN diesel fuels, and indications are that the European mandate will increase to 55-plus in 2019 or 2020.

It is important to note that an ECN increase of 4 to 5 numbers is needed to effectively measure and document a significant fuel economy improvement. It is vital to test a prospective diesel fuel additive with the buyer’s diesel fuel in an engine test cell under ASTM D-613 test2 procedures, “Standard test method for cetane number of diesel fuel oil,” to verify the additive’s potential ECN increase (FIG. 1).

Detergent additive

This predominantly amine-based chemistry is the major additive component used to clean existing fuel-related deposits and prevent them from reoccurring in the fuel delivery system. Detergent additives also play an important role in diesel fuel combustion and power production, as any deposits in fuel pumps or fuel injectors will negatively affect the spray pattern produced in the engine cylinders during each injection cycle.

Fig. 2. Peugeot XUD9 fuel injector test apparatus and test injector tip photos.
Fig. 2. Peugeot XUD9 fuel injector test apparatus and test injector tip photos.

If the fuel spray pattern droplets are not uniform, or the amount of injected fuel is impeded or limited due to internal or external deposits on even one fuel injector, then optimum combustion and maximum power production are impossible (FIG. 2).

This condition will produce increased smoke, more unburned hydrocarbon emissions and reduced engine hp for all throttle settings, leading to increased downtime.

Unburned diesel fuel related to poor fuel detergency will increase deposits in the engine combustion chamber, escalate deposits in post-combustion areas (exhaust valves, EGR valves, DPF, etc.), increase fuel dilution in the crankcase oil and shorten the normal engine maintenance overhaul period. These situations increase costs and decrease the useful life of diesel-powered equipment.

An ASTM test procedure or easy rating method to determine the detergent content in diesel fuel or diesel fuel additive does not exist, nor does an easy methodology to determine a detergent’s effectiveness in keeping fuel injectors free of deposits and operating properly.

The only recognized means of testing diesel fuel detergency is the costly and time-consuming Coordinating European Council (CEC) F-98-08 (S), “Direct injection, common-rail diesel engine nozzle coking test.”3 Diesel fuel additive suppliers should be required to “certify” that their particular additive product treated at the recommended treat rate will achieve a DW-10 pass rating.

This diesel engine injector test is conducted over a 72-hour cycle period, alternating high-speed/load and low-speed/load to determine the power loss resulting from fuel injector deposits. The test uses a European Peugeot 4-cylinder, 2.0 liter, direct-injection turbocharged light-duty, common-rail engine with a maximum injector pressure of 1,600 bars.

The CEC F-98-08 test runs for a specified time using a base diesel fuel treated with 1 ppm of zinc (to increase injector deposits). As the fuel injector deposits increase, fuel flow through the injector and power production decrease. The same diesel fuel treated with detergent additive is tested under the same conditions and compared with the non-additized fuel. A power loss of less than 2% during the test with detergent-treated fuel is considered a DW-10 pass.

Lubricity additive

This diesel fuel additive component (non-acid synthetic type) is important, as the diesel engine fuel delivery system is lubricated by the diesel fuel itself. If the diesel fuel’s lubrication value is inadequate, then the fuel pumps and injectors will not operate properly, leading to increased wear.

Diesel engine fuel injectors are designed to operate with extremely high injection pressures (35,000 psi and more) to better atomize each fuel droplet. Fuel is injected multiple times through extremely fine injector tip holes during each injection cycle in the excessively high temperature environment of each engine cylinder.

If the fuel does not provide proper lubrication to the system, then the fuel injectors will “stick,” causing a chatter-like noise, and the required fuel will not be injected. The net effect will be incomplete combustion, poor power production at all engine power levels and reduced fuel efficiency. Ultimately, without adequate fuel lubrication, the engine will seize.

Diesel fuel additive buyers should undertake their own laboratory testing for diesel fuel lubricity value enhancement using ASTM D-6079-11, “Standard test method for evaluating lubricity of diesel fuels by the high-frequency reciprocating rig (HFRR).”4 This procedure requires only 90 minutes of laboratory time, and can be accomplished with a small amount of diesel fuel and additive to verify the lubricity enhancement claim.

Fig. 3. Two-place HFRR lubricity test apparatus.
Fig. 3. Two-place HFRR lubricity test apparatus.

The HFRR test method (FIG. 3) measures the wear scar produced on a small metal ball reciprocated against a polished disc, which is immersed in the subject diesel fuel. The simulation provides the expected internal wear in fuel injectors and pumps in the diesel fuel delivery system related to the fuel lubrication value. Improved fuel lubricity results in a decreased HFRR wear-scar measurement, and a correlation to the improved lubrication of the fuel delivery system.

While ASTM D-975 shows a maximum HFRR wear scar of 520 µm, the EU Engine Manufacturers Association recommends a 460 µm maximum rating for fuel lubricity. European engine manufacturers have lowered their recommended HFRR fuel lubricity for initial engine break-in to less than 400 µm of wear scar. The US is experiencing a significant influx of European diesel-powered equipment, all manufactured with common-rail fuel injection systems, and new diesel engines being built in the US are incorporating common-rail fuel injection.

Stability additive

This chemical component protects diesel fuel and ensures optimum engine performance. As an organic product, diesel fuel degrades, oxidizes and breaks down from the time it is refined until it is consumed. Determining diesel fuel’s rate of oxidation (instability) through laboratory testing defines the extent of degradation that will occur between its manufacture and its use. This rating is important because, as a diesel fuel oxidizes (degrades), it generates fine, free carbon particulates that are abrasive and often collect internally in fuel injectors.

Unstable fuels also manifest in the fuel system as varnishes that coat and cause scoring of the moving parts. Varnishes cause sticking of the injectors, preventing the delivery of the proper amount of fuel to the engine and potentially stopping it entirely. Unstable diesel fuels will also create sludge materials that collect and build up in low areas of the fuel storage and delivery system (i.e., tanks and lines). The longer an unstable fuel is used, the more degradation byproducts will be formed.

Fig. 4. Dupont F-21 stability chart, pad readings.
Fig. 4. Dupont F-21 stability chart, pad readings.

A measurement of diesel fuel thermal stability was determined in a petroleum laboratory using the ASTM D-6468-08 test, “Standard test method for high-temperature stability of middle distillate fuels.”5

FIG. 4 shows that test pads can be visually rated to define the free carbon created during the test, or analyzed using a laboratory light reflectometer. The more carbon that is created, the less light will be reflected (lower reflectometer result). Diesel fuels treated with a “good stability additive” will have test pad ratings of 2 or less, and a light reflectometer result greater than 90%.

A quality stabilizing additive can be used to dramatically slow diesel fuel degradation, but it cannot completely stop the natural degradation process. Diesel fuel users must have the base (incoming) diesel fuel tested for stability to determine the state of degradation before treating with a stabilizing additive product.

Fig. 5. Diesel fuel storage tank pump/meter assembly and island filter with fuel corrosion.
Fig. 5. Diesel fuel storage tank pump/meter assembly and island filter with fuel corrosion.

The prospective stability additive should be tested at the same time and in the same base (incoming) diesel fuel to determine whether the stability additive will, in fact, stabilize the diesel fuel. It should be noted that other performance additive products can negatively affect the fuel’s oxidation rate (stability). Therefore, the purchaser should have fuel treated with any other additives and then stability tested. If necessary, extra stability additive should be added to counteract the negative effects of the other additives.

Corrosion inhibitor additive

This compound should be added to diesel fuels because:

  • All diesel fuels naturally contain water, which is corrosive
  • All water contains dissolved salt, which will create deposit buildup in fuel injectors.

While the diesel fuel water content may be small (typically 40 ppm–100 ppm), that amount of moisture is more than enough to cause rust and corrosion in the fuel delivery system. Although the salt content in the water may seem extremely small (typically a few ppb), the dissolved salt particles continually circulate throughout the fuel delivery system during operation.

Fig. 6. A diesel engine fuel injector pintle  after operation without a corrosion inhibitor  (A, left); and a similar pintle (B) without corrosion resulting from fuel treated with  an effective corrosion inhibitor additive.
Fig. 6. A diesel engine fuel injector pintle after operation without a corrosion inhibitor (A, left); and a similar pintle (B) without corrosion resulting from fuel treated with an effective corrosion inhibitor additive.

Since most diesel engines return 75%–80% of the fuel to the vehicle fuel tank as “return fuel,” the same salt particles can flow through the injector more than 500 times during the consumption of one tank of diesel fuel. This provides ample opportunity for the salt particles to form a deposit buildup inside the fuel injector.
FIG. 5 illustrates the corrosive effects on the fuel delivery system in a diesel fueling facility (after six months of continual use). Similar corrosive effects can occur in fuel injectors and pumps.

A NACE spindle corrosion test (ASTM D-655)6 should be conducted to determine the effectiveness of the corrosion inhibitor additive. A NACE 1-A or better result will indicate the prevention of fuel-related corrosion and rust when the additive is treated at the correct dosage rate. FIG. 6a shows a diesel engine fuel injector pintle after operation without a corrosion inhibitor. A similar pintle without corrosion resulting from fuel treated with an effective corrosion inhibitor additive is shown in FIG. 6b.

There can be no doubt that the first pintle will not operate properly in the injector, nor supply the correct amount of diesel fuel into the cylinder.

Deposit modifier additive

The final chemical component is particularly appropriate for diesel fuels in engines with high-performance, common-rail fuel injection systems. The proper amount of deposit modifier additive treatment will:

  • Maintain cleaner intake valves, exhaust valves and piston tops to reduce hot spots
  • Keep the EGR valves free of excessive carbonaceous buildup and reduce the need for replacements
  • Reduce regeneration frequency through a better-maintained diesel particulate filter unit
  • Extend the useful life of the entire exhaust system before replacement.

Although nearly all diesel fuel additive suppliers claim to “reduce fuel-related deposits,” no laboratory test method exists to determine the amount of deposit modifier additive included in a diesel fuel additive product. The only means of verifying the benefits claimed is to actually run the additive-treated diesel fuel in an engine for an extended period of time (TABLE 1).

Prospective fuel additive buyers should request a written certified statement from the supplier that the product includes adequate deposit modifier additive to reduce or minimize fuel-related deposits. This certification may eliminate exaggerations from unscrupulous suppliers and help ensure that the additive buyer receives a valuable product.

Takeaways

No organization oversees the performance claims and marketing of diesel fuel additives. This article (and the subsequent Part 2) will assist the additive buyer in sourcing the best performance additives for diesel-powered equipment.

Next month

Part 2 of this article will appear in November. HP

LITERATURE CITED

  1. ASTM International, ASTM D-975-15c, “Standard specification for diesel fuel oils,” Vol. 05.01.
  2. ASTM International, ASTM D-613, “Standard test method for cetane number of diesel fuel oil,”
    Vol. 05.05.
  3. Coordinating European Council, CEC F-98-08 (S), “Direct injection, common rail diesel engine nozzle coking test,” Iss. 8, August 2015.
  4. ASTM International, ASTM D6079-11, “Standard test method for evaluating lubricity of diesel fuels by the high-frequency reciprocating rig (HFRR),” Vol. 05.02, 2016.
  5. ASTM International, ASTM D-6468-08, “Standard test method for high temperature stability of middle distillate fuels,” Vol. 05.03, 2013.
  6. National Association of Corrosion Engineers International, NACE TM0172 (ASTM D665/D7548), “Diesel fuel spindle corrosion test method,” 2013.

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