Fuel Testing & Understanding Your Fuel's Properties

Taking care of today's problematic stored fuels is best done with the three-pronged Hybrid Approach - the right chemicals, the right mechanical processing, and the right testing to clearly identify what you need (and verify that you've solved the problem). This third arm of the Hybrid Approach is what we're focusing on today. If you're going to best take care of your stored fuel, you need the confidence of knowing how your fuel should be tested and what the results mean. What tests should you do? What do those tests tell you and how might that information dictate what you need (or do not need) to do?

What Fuel Tests Should I Do?

There’s a lot of information out there and it can all start to look like white noise. For your fuel, it helps to have a clear picture of what's important vs. what's not. What are the aspects of the fuel that tell us whether it’s in currently useable condition? What should we look at, in terms of what results on what kinds of tests, to tell us how long it’s likely to stay that way?

ASTM Fuel Tests – The Gold Standard

The gold standard for almost any kind of fuel test is the ASTM test. They have codified procedures that have been vetted and agreed upon by industry groups – the people who know best. Their methods are reproducible so that a test that can be done the same way by different people and produce results that can be meaningfully compared. And there are plenty of places you can have them run.

What Do ASTM Tests Tell Us About Our Fuel?

In any discussion of fuel testing, you always start by recognizing that testing the fuel is about answering two kinds of questions:

1. Are the fuel’s properties what they should be, right now?

2. How likely are the fuel’s properties to change in the future?

We always focus first on the fuel specifications - the properties of the fuel that relate to important areas of its functionality. ASTM D975 is always a good place to start, as it’s what defines the minimum set standards for what diesel “fuel oil” should be. D975 tells us that an ideal diesel fuel is one which flows easily at all temperatures, is clean and free from foreign contaminants and separated wax, ignites readily, and burns quietly, cleanly, and economically. These descriptive ideals are reflected in the diesel fuel’s specifications. Diesel fuel that meets these standards will be diesel fuel that gets the job done when you need it.

What The Specifications Tell You About Your Stored Fuel

Each element of the D975 fuel specification slate is important because each is a measurement of a property that contributes to the fuel behaving or performing desirably. If a fuel does not meet a given specification, that’s when you may see symptoms of a problem in how that fuel performs, whether it is in the combustion system you’re using it in or in the storage and distribution system it lives in.

So let’s run through the list of diesel fuel specifications - what they mean, how they’re measured, and what you’re likely to see happen if a diesel fuel does not meet that particular specification. What properties does the diesel fuel have to have to meet minimum accepted standards in the marketplace?

Many of these (properties) are specifications you may have heard referenced in other areas. As we go through them, we’ll start to see how these properties are important for making sure diesel fuel does what we need it to do.

Diesel fuel specifications can be roughly divided into two classes: ones that influence combustion, and all the rest of them. As we look at each spec, we'll ask the same questions:

• Is it in D975? (all D975 specs are important, but some important ones aren't listed in there)
• What's the minimum (or maximum) acceptable result?
• What is it and how do you measure it?
• What might you expect if the fuel doesn't meet this standard?
• Any additional helpful information to better understand it

First, the combustion-related specifications.

Density

ASTM Number/Is It In The D-975 Slate?

Not Applicable

Minimum Value

Not Applicable

What Is It/How Is It Measured?

Density is the weight of a unit of volume, typically measured by a hydrometer.

What Happens If The Fuel Fails?

Density is linked to the heating value of the fuel. Higher-density fuel produces more smoke but more power (because it has more energy value per unit of volume).

Description

The very first one we look at isn’t technically defined in the ASTM standard. So maybe you’re wondering why to include this? Density is an essential property of diesel fuel, but it’s not defined by an ASTM test because it’s governed by the results of other standards that we’ll see later, like Distillation and Aromaticity and Cetane Number. If those tests are within spec, it means the density will be within the ideal range, too.

Volatility (Distillation)

ASTM Number/Is It In The D-975 Slate?

D86/Yes

Minimum Value

282-338 deg C for 90% recovery

What Is It/How Is It Measured?

Volatility is expressed in terms of the temperature at which successive portions (%s) are distilled from a sample of fuel under controlled heat. Values are usually determined for 10% and 90% recovery, as well as the initial boiling point. Distillation or boiling range influences other properties like viscosity, flash point, and density.

What Happens If The Fuel Fails?

Fuel refineries can try to hit distillation ranges by altering the amounts of low or high-temperature components blended into the fuel. Raising the back end (90%) temperature will raise the cloud point. Lowering the front end (10% or IBP) temperature lowers the flash point and raises the vapor pressure, which can result in a vapor lock.

The 10% recovery temperature reflects how well the fuel will start to vaporize. The 90% recovery indicates the extent to which complete vaporization of the fuel may be expected in the combustion zone. Excessive high boiling components lead to engine deposits and more smoke. Diesel fuel with unsatisfactory distillation temperatures may be contaminated with other compounds like gasoline.

Description

This has both physical distillation and simulated distillation (a shorter version of the test). They heat the liquid until it turns into a vapor, where it will rise in a column and start to cool. As it cools, it turns back into a liquid and is collected.

Distillation tests are looking for the temperature at which 90% of the original liquid volume is condensed back and collected. To meet the standard, No. 2 diesel distillation temperature can be between 282 and 338 deg C.

Distillation temperature matters for a couple of reasons: It speaks to whether the diesel fuel is going to combust as fully as needed within the typical conditions of a compression ignition engine. If the temperature is too low or too high, it may not fully combust or it may burn too quickly or too early.·

Also, distillation temperature is related to some of the other properties of the fuel, like Cetane Value and % Aromatics. Keeping the distillation temperature within the required range helps ensure that the diesel fuel has the right composition of molecules in it, like the ratio of light-end components vs heavy ends.

Viscosity

ASTM Number/Is It In The D-975 Slate?

D445/Yes

Minimum Value

1.9 - 4.1 Mm2/S

What Is It/How Is It Measured?

Viscosity Is Resistance To Flow. Higher Viscosity Means Greater Resistance And Slower Flower. Viscosity Is Measured With An Apparatus That Measures The Amount Of Time Needed For A Fixed Amount Of Liquid To Flow Through A Capillary Tube Viscometer Immersed In A Temperature-Controlled Bath.

What Happens If The Fuel Fails?

Fuel with viscosity that falls outside the specified range cannot be properly metered/injected into the combustion chamber. Changes in viscosity will alter the injector spray penetration rate, the cone angle, and the droplet size distribution. If the fuel atomization is subpar, as such, you won’t get optimal mileage and performance and you’ll have higher emissions. Excessively high viscosity makes fuel difficult to flow in cold temperatures.

Description

We’re all at least somewhat familiar with the concept of viscosity – the expression of internal friction of a liquid. Thick liquids like honey have high viscosity values, and thin liquids like water have low values.

No. 2 diesel has to have a viscosity between 1.9 and 4.1 mm2/S. The question might be asked, why does viscosity matter for diesel fuel? Because viscosity affects how the fuel can be injected by the injectors (which can affect other things like emissions and engine performance).

Cetane number (index)

ASTM Number/Is It In The D-975 Slate?

Not Applicable

Minimum Value (For #2 Diesel)

40 in the United States, 45 in Europe, and 50 in Japan

What Is It/How Is It Measured?

The index is a calculated value derived from fuel density and volatility (distillation). Illustrates the readiness of a fuel to ignite when injected into a diesel engine. The cetane number itself is calculated using a specialized engine, which is more expensive. Test results are compared to the index values of a high and low cetane reference fuel, and results are calculated from that.

What Happens If The Fuel Fails?

Diesel fuel with poor cetane number/index will have poor ignition quality. You’ll have less power and a noisier and rougher running engine. Also linked to cold-starting difficulties.

Description

We’re familiar with the importance of cetane in diesel fuel. And 40 is the minimum value defined by the regulation in the United States. What we can also see in D975 is that there is an additional requirement that says, you either must have a Cetane Index test that also meets a 40 minimum or an Aromatic content test that has a maximum of 35%.

Flash Point

ASTM Number/Is It In The D-975 Slate?

D93/Yes

Minimum Value

38 Degrees C for #1 Diesel; 52 Deg C for #2 Diesel

What Is It/How Is It Measured?

The temperature at which the fuel has to be heated to produce a vapor/air mixture that will ignite when a flame is applied. The flash point relates to the front-end volatility of the fuel. Refineries have to choose the right light refinery streams to be included in the fuel blend so that the IBP remains sufficiently high enough. For Pensky-martens, a fuel sample is heated slowly in a covered cup at a constant rate. At regular intervals, the cover is opened and a flame is introduced. The procedure is continued until the fuel temperature is high enough for flash ignition to occur.

What Happens If The Fuel Fails?

Low flash point influences the safe handling of the fuel. Has no significance on performance in a diesel engine. Variations in flash points will not influence autoignition temperature.

Description

The minimum temperature at which something will ignite in the presence of a flame or ignition source. This is different from autoignition temperature, a higher temperature that speaks to how hot something has to be to self-ignite without any ignition source.

Flashpoint matters because of how diesel fuel works in a compression ignition engine. If the flash point is too low, as in gasoline (which has a flash point of -45 deg F), it will prematurely ignite and damage the engine.

Aromaticity

ASTM Number/Is It In The D-975 Slate?

D1319/Yes

Minimum Value

35% maximum aromatic content

What Is It/How Is It Measured?

Many times, it is calculated using an equation that factors in density and viscosity.

What Happens If The Fuel Fails?

Excessive aromatic levels affect fuel emissions when it is combusted.Aromatic content also inhibits microbial growth, so putting a cap on aromatic content makes the stored fuel more likely to support microbial growth.

Description

Aromaticity - the fuel’s aromatics content - can affect the distillation curve temperature, can produce harmful emissions, and can produce excessive ash. So you don’t want too high a content of them. They also inhibit microbial growth, which means limiting aromatics to make the fuel “greener” makes it more likely to have microbial growth problems.

Now, the other important specifications that don’t directly influence combustion, but are important to stored fuel’s proper functioning over time.

Cloud point or CFPP (Cold Filter Plug Point)

ASTM Number/Is It In The D-975 Slate?

D2500 or D4539/Yes

Minimum Value

No stated value in the D975 specification

What Is It/How Is It Measured?

Cloud point = temperature below which wax crystals will precipitate out of the fuel, giving a cloudy appearance. CFPP (Cold Filter Plug Point) is the predictive temperature at which enough wax comes out of suspension that blocks fuel filters and shuts down engine operation.

What Happens If The Fuel Fails?

We know what this answer is.

Description

ASTM D975 mentions both Cloud Point temperature and CFPP (cold filter plug point). But what’s interesting is they don’t give the required values for those. In note J of the spec, they say “It’s not realistic to require minimum temperatures for fuel because they’re so variable”. The specification only gives guidelines, but not requirements.

Oxidative stability

ASTM Number/Is It In The D-975 Slate?

D2274/No

Minimum Value

2-3 mg/mL is typical

What Is It/How Is It Measured?

This measures the stability of the fuel i.e. ability to resist sediment formation when subjected to accelerated oxidative stresses like prolonged exposure to atmospheric oxygen.

A sample of fuel is heated to 95 deg C and maintained for 16 hours in the presence of continuous oxygen. Fuel is then cooled and solvent washed to recover gummy residues that may have formed. They may also measure the color of the fuel before and after using the D1500 test described below.

There’s also the D4625 test which takes 100 days (fuel held at 43 degrees C) and is believed to correlate with ambient storage conditions for one year of storage. However, this takes too long to be used for routine quality control at the refinery level.

What Happens If The Fuel Fails?

Excessive gum formation predicts a lack of storage stability of the fuel.

Description

Fuel instability is responsible for numerous phenomena, including deposit format in injectors and combustion chambers, as well as subpar emissions products and engine operation.

Thermal stability

ASTM Number/Is It In The D-975 Slate?

D6468 or D1500 or NOF-21 or Dupont test

Minimum Value

70% is the satisfactory cutoff

What Is It/How Is It Measured?

This test is used to predict how well a fuel will resist sediment formation upon long-term exposure to higher temperatures. Two 50ml fuel samples are heated to 150 degrees and held for 90-180 minutes with oxygen exposure. Fuel is then cooled and filtered. The filter pads are then exposed to light to measure their reflectance. Higher amounts of filtered insoluble yield a lower reflectance score expressed as a % reflectance. Reflectance scores of 71+ are considered good. 62-70 is “fair”. Below 70 is marginal or worse.

The color of the fuel sample can also be logged before+after using the D1500 color comparison test. Typically, an aged fuel sample will end up with a higher (darker) color score.

What Happens If The Fuel Fails?

Higher levels of filterable insoluble predict fuel that is more likely to form gums and deposits in storage over time.

Description

The Thermal Stability test is designed to illustrate a fuel's tendency to form sediments if exposed to elevated temperatures for an extended period. It does not have as much overt applicability to real-world fuel storage as oxidative stability does. However, Thermal Stability does have predictive use to illustrate deposit formation inside of an engine, where these elevated temperatures exist.

Water and sediment Content

ASTM Number/Is It In The D-975 Slate?

D2709 or D1796/Yes

Minimum Value

0.05% by volume, maximum

What Is It/How Is It Measured?

A 500ml sample of fuel is mixed with 500ml of water-saturated solvent in each of the two graduated tubes. Samples are centrifuged to concentrate the water and sediment at the bottom of the tube. The sum is reported as the % of water and sediment.

What Happens If The Fuel Fails?

Excessive water content can encourage the growth of microbes and may block filters. Excessive sediment content may be a sign of fuel instability and can cause wear in injectors. Corrosion and wear in the engine and fuel injection system.

Description

Water comes from any number of points in the distribution system. Sediment content can be from inorganic deposits (rust in the bottom of the tank) or organic deposits (degradation of fuel, bacterial action, wax settling that does not dissolve).

Water And Sediment Content is defined in D975 and relates to the amount of contamination present in the fuel. It’s also been strongly linked to the presence of microbially-induced corrosion (MIC) present in storage tanks. So it’s an important test to pay attention to.

Ash Content

ASTM Number/Is It In The D-975 Slate?

D482/Yes

Minimum Value

0.01% by mass, maximum

What Is It/How Is It Measured?

A small sample of fuel is burned in a weighed dish until all combustible material has been consumed. The unburnt residue is weighed and reported as a percentage of the original fuel sample.

What Happens If The Fuel Fails?

All diesel fuel contains small amounts of ash-forming material like suspended solids and soluble organometallic compounds. These can cause damage to the close tolerances of fuel injection equipment and contribute to increased deposit levels and abrasive wear of piston rings and other components in the high-temperature zones of the engine (where the fuel gets burned).

Description

On the face of it, this one seems pretty important. You want the fuel to burn cleanly and completely, without producing leftover ash which can damage areas of the engine. Ash itself comes from a combination of partially burned carbon components (like aromatics) and inorganics (like metal content). You don’t want much of that if you’re burning diesel fuel in, say, a truck engine.

Ramsbottom Carbon residue

ASTM Number/Is It In The D-975 Slate?

D524/Yes

Minimum Value

0.15% maximum for #1 kerosene, 0.35% maximum for #2 diesel fuel oil

What Is It/How Is It Measured?

The test measures the amount of higher BP, and lower volatility components present in the finished fuel (some entrainment of higher BP materials occurs during refinery distillation). Carbon residue in diesel is already low, so the test is run on the 10% residue from the laboratory distillation test to improve accuracy. A 4g sample of 10% bottoms is placed in a glass cooking bulb and heated at 550 deg C for 20 minutes. After cooling, the bulb is reweighed to determine the amount of residue.

What Happens If The Fuel Fails?

The carbon residue test indicates the tendency of the fuel to form carbon deposits, although it does not predict injector coking.

Description

This is a measurement of the residue the fuel is going to produce at high temperatures, such as those you might find in a diesel engine. As with ash content, you don’t want a lot of that. A high score on this means you’re probably going to have combustion chamber deposits formed.

Lubricity

ASTM Number/Is It In The D-975 Slate?

D6709/Yes

Minimum Value

520 microns

What Is It/How Is It Measured?

Measured on an HFRR machine, which wears the size of a wear scar left behind by steel balls immersed in the liquid and subjected to measured pressure for a defined period.

What Happens If The Fuel Fails?

Unsatisfactory lubricity can predict the inability of the fuel to protect injectors and fuel pumps from premature wear. Fortunately, the addition of low-level biodiesel to diesel fuel has solved the lubricity problem.

Description

Also refers to film strength. Lubricity is the ability of the liquid to lubricate essential moving parts in the engine. All of these diesel fuels are capped at an HFRR score of 520, in other words, they want to be lower than that.

Sulfur Content

ASTM Number/Is It In The D-975 Slate?

D5453/Yes

Minimum Value

15 ppm for ULSD

What Is It/How Is It Measured?

Typically measured by some kind of mass spectrometer, which can detect the weight % content of sulfur.

What Happens If The Fuel Fails?

Excessive sulfur in the fuel can poison the emissions catalyst. It can also lead to big regulatory fines if detected.

Description

Sulfur content is controlled by regulations to help the environment. We are all well familiar with the sulfur issue, and D975 is one of the places that spells out the requirements. 15 ppm for the S15 diesel fuels, 0.05% (which is 500 ppm), and 0.50% (5000 ppm) for the S500 and S5000 diesel fuel oils.

Copper strip corrosion

ASTM Number/Is It In The D-975 Slate?

D130/Yes

Minimum Value

No. 3. A score of 1-3 indicates a tarnish but is permitted. A score of 4 indicates corrosion and is not permitted.

What Is It/How Is It Measured?

A polished copper strip is immersed in a portion of fuel and heated for 3 hours at 50 degrees C. It is removed, washed, and compared with an ASTM standard chart. Copper is used because copper and its alloys are sensitive to attack by certain sulfur compounds, so they are a good indicator.

What Happens If The Fuel Fails?

A failing grade on this test implies that the fuel is excessively corrosive and may attack metals in the fuel distribution and storage network.

Description

This test seeks to verify that the diesel fuel isn’t going to cause corrosion damage by a long-term contract with metal parts. The test itself is run by taking a strip of copper metal and immersing it in test liquid for a defined period and temperature. The strip is then compared to a visual chart and given a grade to illustrate its resistance to corrosive change.