Fuel fires in combat vehicles have long been a difficult if not impossible problem to resolve.
In response to the growing threat of roadside bombs to military vehicles and troops, the Army currently is working on various projects to develop a fire resistant fuel.
Over the past decades, the Army has conducted research to make diesel fuel resistant to unwanted fire. This research involved a variety of approaches to reduce the flammability of diesel fuel.
The compression ignition or diesel engine which powers a large percentage of the Army ground vehicles has a fuel delivery system that returns a portion of the fuel from the injectors back to the fuel tank. This recirculation heats the fuel — typically raising the temperature of the fuel in the tank to above its flash point, and making it more susceptible to being ignited.
This heating, when combined with any direct or indirect ballistic penetration near the fuel tank or fuel line, significantly increases the potential for a catastrophic fire. Having a fuel that would not ignite under these conditions would have obvious benefits.
Researchers evaluated emulsified fuel, halogenated additives, anti-misting additives, and water-in-fuel microemulsions, with the latter showing the most promise. In a water-in-fuel microemulsion, the individual water droplets are completely surrounded by the fuel.
The fire resistant fuel that was developed — referred to as fire-safe fuel — was a clear to hazy microemulsion consisting of water, emulsifier premix (equal amounts of the emulsifier and an aromatic concentrate), and diesel fuel. This microemulsion performed satisfactorily both in diesel and turbine engine systems and could be easily prepared in the field.
Although it did not eliminate the initial mist fireball that occurs when a projectile impacts the vehicle, it significantly reduced the fuel fire threat by retarding the flame spread rate and self extinguishing any spilled fuel, which eliminates residual pool burning.
One problem with this approach was that the purity of the water that is needed for ensuring a stable microemulsion was considerably lower than the purity of the water typically generated by the Army’s purification units.
Eventually, the urgency for the development of a fire-safe fuel diminished, and funding was discontinued.
With the start of the conflicts in Iraq and Afghanistan, attention again returned to the fuel fire threat as improvised explosive devices began to take a toll on vehicles and personnel. Adding to this IED threat was the fact that JP-8 aviation kerosene, a slightly more volatile fuel having a lower flash point minimum requirement than diesel, was being used as the Army’s battlefield fuel.
In 2002, the Army began an evaluation of new metallic/polymeric mesh explosion dampening materials for the interior of vehicle fuel tanks. This evaluation addressed not only whether these materials could improve the survivability of vehicles and personnel, but also if these materials could selectively deplete the additives used in fuel or contribute to any contamination problems.
Unfortunately, none of the six materials evaluated was found to be effective in reducing or eliminating the threat.
The private sector early on had little if any interest in developing or marketing diesel-water emulsions for the commercial market. However, sometime after the Army was awarded a patent on its fire-safe fuel in 1979, several companies began to investigate the potential for marketing diesel-water emulsion fuels.
The Industrial Research IR-100 Award in 1979 for this new diesel-water microemulsion may have spawned further interest. A number of companies in the late 1990s began to develop these diesel-water emulsion fuels as they offered a relatively easy means to significantly reduce both nitrogen oxides and particulate matter emissions.
In October 2002, the Environmental Protection Agency had registered a diesel-water fuel emulsion under its fuel and fuel additive registration program.
The new emulsion initially was marketed in several states as well as in Europe and other countries. But the more stringent emission standards and exhaust enhancement requirements over time has made the marketing of the new emulsion cost prohibitive in the United States. It is being marketed in Europe in addition to other emulsion fuels. The water content of these fuels range from 10 to 20 percent. Because of the interest in these fuels in Europe, the European Emulsion Fuels Manufacturers Association was formed in January 2003.
In May 2006 as a result of the continuing fuel fire threat to troops in Iraq and Afghanistan, the Army restarted the program at its fuels and lubricants facility. Initially a new baseline had to be established (for blending and flammability) using JP-8, as the previous work had only evaluated diesel fuel. Then in April 2007, a more comprehensive effort was initiated to develop new emulsified fuel formulations and investigate the addition of anti-mist additives to diminish the mist fireball that occurs when a vehicle is hit.
The development of an emulsified fuel formulation that yields a stable emulsion (one that does not separate) using JP-8 and diesel fuel has been a difficult task. This is because of many variables involved such as fuel composition, aromatic content, water quality, emulsifier/surfactant chemistry and additive interactions.
Adding to the complexity is the planned addition of anti-mist additives to the emulsified fuel formulation. The presence of these long chain polymer materials would reduce the size of the initial fireball that occurs when a projectile penetrates a vehicle’s fuel tank.
A key issue will be whether these long chain polymers degrade. The mechanical shearing effects generated by fuel injection systems could render them ineffective.
A comprehensive evaluation of available emulsifiers is currently under way to find those that will produce a stable emulsion with water using a range of both diesel and JP-8 fuels. Every attempt is being made to ensure that the candidate emulsifiers will not be as sensitive to water hardness as was the case for the emulsifier used in the former fire-safe fuel.
Anti-misting additives are being evaluated and will be incorporated into candidate emulsified fuel formulations. Several companies under the Small Business Innovation Research program are developing these long chain polymer anti-misting additives that when sheared will recombine and are being called associative polymers. This mechanism will effectively eliminate permanent polymer degradation that occurs when fuel is recycled in diesel fuel systems. Ballistic testing of different anti-misting additives incorporated into the emulsified fuel formulations is currently being conducted.
Four different threats are being used to assess the effectiveness of these additives.
The development of a preliminary blend design for generating the fire-resistant fuel has been initiated. The design maximizes use of Army petroleum and water handling equipment already available within the inventory. Different configurations of the necessary pumping and mixing equipment are being considered. But use of a dedicated pump per fluid is considered the preferred approach.
The Army is aggressively pursuing a timely solution to the fuel fire threat problem that has long plagued both personnel and materiel. A variety of new technologies are now being evaluated to accomplish this. The ultimate goal of this initiative is to have a fuel resistant fuel formulation that is self extinguishing, highly stable, resistant to mechanical shearing, does not adversely affect other fuel properties, is non-toxic, and one that provides satisfactory performance in both diesel and turbine engine systems.
Joel A. Schmitigal is a scientist at the Army Research and Development Command in Warren, Mich. Maurice E. Le Pera is president of Le Pera and Associates in Harrisonburg, Va. Reference in this article to any company, product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. government. Opinions are strictly the authors’. Luis Villahermosa, of the Army, and Steve Marty and Bernie Wright of Southwest Research Institute contributed to this article.