Water in diesel fuel is a serious problem for diesel equipment owners and operators. Most diesel fuel treatments use one of two methods to eliminate the negative effects caused by water in diesel fuel. Emulsfiers encapsulate water droplets and pass them through the fuel system to be vaporized and expelled during the combustion cycle. Demulsifiers cause water to precipitate out of the fuel where it can be separated mechanically by water separators or other means. This paper examines how the two techniques work, as well as arguments for and against each technique.


Diesel fuel is hygroscopic, meaning it attracts and holds water. Water occurs in diesel either as an emulsion (a mixture of water and fuel with the water dispersed uniformly throughout the fuel) or as free water (water that has separated out to form a visible interface between the heavier water and the lighter fuel floating on the water). Water in diesel fuel has the potential to lead to problems including (a) corrosion of storage tanks and components, (b) increased wear of fuel system components due to reduced lubricity, (c) low temperature inoperability due to formation of ice crystals that can plug fuel filters and system components, and (d) growth of microbial "slime" that can clog filters.


For the purposes of this discussion, "diesel fuel" includes the petrodiesel fuels obtained by distillation (refining) of crude oil and marketed in the United States primarily as Grade No. 2-D diesel fuel, along with Grade No. 1-D and Grade No. 4-D. Increasingly, diesel fuel also includes biodiesel fuels made primarily from vegetable oil feedstocks, including canola and soybean oils, along with waste vegetable oils and animal fats. Biodiesel fuel is typically marketed commercially as a blend with petrodiesel, with the blend ratio indicated in labels such as B20 representing a blend of 20% biodiesel with 80% petrodiesel.


Most of the water in diesel fuel collects in the fuel as a result of the fuel's hygroscopic properties, meaning moisture from the atmosphere will be attracted to the fuel. Water also contaminates fuel through poor storage and handling techniques. Fuel tanks only partially filled with fuel and subjected to extreme temperature swings over long time periods may experience significant contamination from condensation, as water vapor inside the tanks condenses and trickles down the sides of the tank to mix with the fuel. Poor fuel handling techniques such as failure to maintain proper closures on fuel storage tanks and faulty fill pipes and vents can also lead to water contamination of fuel.

Except when large quantities of water are accidentally introduced into fuel as free water, the water will initially tend to be dispersed uniformly throughout the diesel fuel as an emulsion. In this water-in-fuel emulsion, there is always an interface between the small water particles or droplets and the fuel. The water-fuel interface in this water-diesel fuel emulsion is inherently unstable, so the water in diesel fuel will tend to separate from the fuel. Because diesel fuel is lighter than water, any water that precipitates out will collect in the bottom of the fuel tank, especially stationary storage tanks. Fuel tanks on moving diesel vehicles tend to keep the water in emulsion because of mechanical vibration. Therefore, fuel storage tanks for stationary equipment such as standby diesel generators are much more susceptible to water collecting at the bottom of the fuel tank.

Water that has collected in the bottom of a tank can be mechanically separated by draining the tank from the bottom until all the water is evacuated. Obviously, this technique for removing water from diesel fuel is impossible in situations where there is no drain built into the bottom of storage tanks. The water can also be pumped out by inserting a tube into the bottom of the tank. However, neither of these methods assures all the water will be separated, or that some of the water will not be agitated into an emulsion once again during mechanical removal.

Mechanical removal is also accomplished by the use of water separators and filters located between a fuel tank and the diesel engine's fuel delivery system, including the fuel pump and injectors. These devices are designed to separate even trace amounts of water. However, like all mechanical devices, separators and filters require regular maintenance and are susceptible to failure, including fuel stoppage due to plugging from contaminants.  

Therefore, to supplement mechanical water removal techniques, fuel additives relying on either emulsification or demulsification are used to separate or render water harmless.


Fuel additives using emulsifiers are formulated to keep water in emulsion in the diesel fuel in such minute particles that the water passes harmlessly through the fuel delivery system where it is vaporized in the diesel engine's combustion chambers and released as steam.

Emulsifiers such as E-ZOIL Diesel Aid rely on hydrogen bonding to keep the water in emulsion. Diesel fuel consists almost entirely of non‑polar molecules; that is, they have neither a positive nor negative electrical charge. Water molecules, however, have both a positive and negative charge. The chemical formula in Diesel Aid contains both a positive and negative charge that, when mixed with diesel fuel and water, acts like a coupling agent to bond or encapsulate all three together into a homogenous mixture.

Additionally, emulsifiers such as Diesel Aid have surfactant properties that reduce the surface tension between the unlike molecules of oil and water, again making the mixture of the two both more homogenous and more stable.

The result is that water in the fuel is dispersed uniformly throughout the fuel in micro- or nanoscale particles that are bonded to the fuel.


For typical applications where the ratio of fuel to water is quite large or to prevent such an occurrence, emulsifiers provide the most practical way to eliminate the potential risks of water contamination of fuel, particularly for on-vehicle fuel tanks that are constantly agitated by both vehicle motion and engine/drivetrain vibration. Water already emulsified is bound into a more homogenous and stable emulsion. Trace amounts of free water are also readily emulsified.

1. Eliminating free water. Because free water is eliminated, emulsifiers avoid some of the potential negative effects of demulsifiers. Because demulsifiers promote the formation of free water, any free water in the fuel tank or fuel delivery system can form ice crystals in freezing temperatures, clogging fuel filters, fuel lines and injectors, and even drastically reducing fuel lubricity, creating the potential for damage to critical fuel system components.

2. Preventing corrosion. Another benefit of emulsifiers is that by eliminating free water from the fuel tank, they help prevent corrosion in the bottom of fuel tanks where the heavier water collects.

3. Preventing microbial "slime." Eliminating free water also prevents the formation of a fuel/water interface where microbes can grow, creating slime that can plug fuel filters and fuel lines.


Many proponents of demulsification contend that any water/fuel emulsion reduces the lubricity of the fuel, a special concern in modern high pressure common rail (HPCR) fuel systems where pressures up to 30,000 psi may increase the potential for shearing damage to mechanical components. Others point to the lower lubricity of ultra low sulfur diesel (USLD) fuels as increasing the risk of fuel system damage from emulsified fuels, as water has much lower lubricity than diesel fuel.


The following developments in fuel technology would suggest the above objections to emulsification do not stand up to scrutiny.

1. Emulsified Fuel. Water-in-diesel emulsified fuel has been introduced into markets in the U.S. and Europe as a means of reducing pollutants such as particulate matter (PM) and nitrous oxides (NOX). According to the U.S. EPA, "Emulsified diesel is a blended mixture of diesel fuel, water, and other additives that lowers combustion temperatures and reduces emissions of PM as well as NOX. The additives also prevent water from contacting the engine."1

2. Ultra Low Sulfur Diesel. While critics of emulsification point to the reduced lubricity of ULSD fuels as heightening the risk of damage to fuel system components from emulsified water, refiners typically use lubricity additives to restore lubricity. In addition, most aftermarket additives such as Diesel Aid include lubricity enhancers. Even more important, ULSD has enhanced surfactant properties compared to low sulfur diesel, resulting in a higher propensity for emulsified water in fuel to remain in a homogenous emulsification rather than separating out into free water.

3. Mechanical Water Separation. While water protection devices such as fuel filters with hydrophilic filter elements are highly efficient at trapping free water and coarsely emulsified water, research indicates that especially under the pressures typical of high pressure common rail (HPCR) systems, mechanical filtration is extremely difficult, so that not all emulsified water can be captured by even the most advanced filters.2 In addition, many modern heavy trucks are equipped with a "Water In Fuel" warning light to alert operators to the presence of free water in mechanical fuel separators, but those warning lights are not activated by untreated emulsified water.

4. Biodiesel. Biodiesel fuel is inherently more hygroscopic than petrodiesel, so even "light" blends such as B20 can hold more emulsified water than pure petrodiesel.3 Since in general biodiesel fuels have higher lubricity than ULSD petrodiesel, once again the emulsified water would seem to be harmless in most situations.


Demulsifiers cause emulsified water to separate from fuel into free water so it can be separated mechanically by techniques such as draining or filtering.


For applications where significant amounts of water contamination of diesel fuel has occurred, using a demulsifier may provide the optimum means for controlling water.

1. Enabling mechanical separation. Causing water to separate out of emulsion may enable mechanical separators and filters to separate small amounts of water from fuel, as these devices are most effective when water in fuel exists as free water or very coarsely emulsified water.

2. Enhanced energy content. Water in fuel reduces the energy content of the fuel delivered to the combustion chamber simply because any volume of water in fuel delivered to an engine's combustion chambers necessarily displaces a corresponding volume of fuel, reducing power output and, for over-the-road vehicles, miles per gallon.


While demulsification would seem to provide "peace of mind" for diesel operators by promising to fully separate water and fuel, hygroscopic properties of both petrodiesel and biodiesel fuels suggest that complete separation of water from diesel may be impractical in many applications, and in the case of emulsified fuel, self‑defeating.

Moreover, in applications where a significant amount of water is present in the fuel tank of a vehicle, separating that water from the fuel might overwhelm mechanical filters and separators, causing anything from a temporary shutdown triggered by a Water In Fuel warning light to a catastrophic failure of fuel system components.


The fear of water contamination of diesel fuel is acute for many owners and operators of diesel equipment and vehicles, including on-road and off-road applications.

Both emulsifiers and demulsifiers are widely used fuel additives, both in bulk and as aftermarket additions to diesel fuel.

The evidence presented in this paper suggests two general rules for selecting an emulsifier or demulsifier for specific applications:

1. For on-vehicle fuel tanks and any other application where the fuel is subjected to mechanical vibration or agitation that would tend to keep any water emulsified in diesel fuel, an emulsifier that both stabilizes that emulsified water-in-fuel mixture and ensures the emulsified water remains in a very fine micro- or nanoscale state is recommended. Similarly, emulsifiers would seem to have an advantage for biodiesel fuels that are more hygroscopic than petrodiesel fuels.

2. Where large-scale water contamination is proven or suspected, use of a demulsifier combined with a mechanical water separator is recommended.


1.   The U.S. EPA provides extensive resources on diesel fuel, including this summary of emulsified fuel. Click Here to View

2.   This technical paper discusses some of the issues involved in filtering emulsified water in diesel fuel in HPCR systems. Click Here to View

3.   This scholarly paper explores the issue of water absorption by biodiesel and biodiesel blends. Click Here to View