AUTONEWS

Aquazole: the experimental fuel that mixed diesel and water

Not long ago, the automotive industry believed that diesel was the best technology available for transporting people and goods—at least in Europe, and by road, naturally. They weren't entirely wrong; even today, it remains the fuel that powers a large part of industrial activity.

However, particularly in the 1990s, the technology faced a problem: harmful gas emissions. A solution might lie in a form of "chemical diplomacy"—an approach that, with a bit of ingenuity and plenty of innovation, could avoid the need to replace entire fleets, redesign cities, or envision electric buses everywhere.

It was against this backdrop that a new fuel called Aquazole was born. The name practically gives it away: water and diesel. Admittedly, the name is almost comically literal, yet Aquazole was a technology taken very seriously thirty years ago.

Behind the invention of Aquazole was a major European oil company—France’s Elf—with a clear objective: to reduce emissions from diesel engines, particularly in urban fleets. This included buses, garbage trucks, municipal vehicles, and the like—vehicles that could not (and still cannot) simply vanish from cities, yet were becoming too conspicuous to remain immune to environmental pressure.

Diesel was enjoying its heyday back then. The engines were economical, robust, and efficient; they received favorable tax treatment in many markets and were an almost irresistible choice for high-mileage drivers. But this rise cast a shadow—literally. The black smoke pouring from tailpipes was the most obvious sign of a larger problem: particulate matter, nitrogen oxides (NOx), and increasingly polluted air.

Aquazole emerged specifically to tackle this issue, based on a principle that seemed destined to fail: adding water to diesel. Two elements that, under normal circumstances, do not mix. Hence the need for that "chemical diplomacy," which was essentially cutting-edge engineering.

The Aquazole recipe was an emulsion. According to the European Union (EU) website, its formula consisted of approximately 85% diesel, about 13% aqueous solution, and a small fraction (2–3%) of chemical additives. The key lay in that tiny fraction. It was the additives that prevented the natural separation of water and diesel, keeping microscopic water droplets suspended in the fuel long enough for the mixture to be stored, transported, and used in a diesel engine.

The word "mixture" might be inadequate to describe the process. Aquazole was a sort of forced marriage between substances that—as I mentioned earlier—instinctively wanted to separate. Normally, when you add water to diesel, the two liquids begin to separate almost immediately.

We are all intuitively familiar with this phenomenon: some liquids mix easily, while others do not. Water and oil are the simplest example. We can shake them vigorously, but after a while, each returns to its own layer. Elf’s technical achievement lay precisely in preventing this from happening.

According to the oil company itself, the Aquazole emulsion remained stable for over three months—provided it was protected from light—and maintained this stability across a temperature range of -20°C to 70°C. That was no small feat. It was the difference between a laboratory curiosity and a fuel with the potential for use by a real-world fleet.

The secret lay in a third ingredient: an organic surfactant, the composition of which was kept under wraps by Elf. For those who don't know (and I didn't...), a surfactant is a substance that helps incompatible aqueous solutions stay together. It doesn't make them identical or change their nature, but it reduces the tension between them, allowing one to remain dispersed within the other as tiny droplets.

In simple terms, it acts as a kind of chemical mediator. One end of the molecule has an affinity for water; the other has an affinity for oil. In the case of Aquazole, this dual affinity made it possible to keep microscopic water droplets dispersed within the diesel, rather than allowing them to coalesce and separate at the bottom of the tank.

However, not just any surfactant would do. To be used in a fuel, it had to meet several requirements. It could not generate harmful emissions, contain metals, or be toxic; furthermore, it had to be non-ionic—meaning it was relatively insensitive to the quality of the water used in the emulsion. Aquazole’s stability depended as much on the formulation as on the industrial process that produced small, uniform droplets.

There was another important detail, too. Since the fuel contained water, it was necessary to add a stronger biocide than the one used in conventional diesel to prevent bacterial growth. The romantic image of a “simple” fuel vanishes here. Aquazole might have looked like just water and diesel, but it was a delicate, industrially demanding chemical solution—far less rudimentary than its name implied.

The question is: to what end? After all, water adds no energy to the fuel—quite the opposite. Its role was different: to alter combustion conditions. As it vaporized inside the chamber, it absorbed heat and lowered peak temperatures. This reduction was crucial because nitrogen oxides (NOx) form primarily when combustion reaches very high temperatures.

Elf claimed a 5% to 10% reduction in combustion temperature compared to conventional diesel. They also announced figures that were striking enough to grab attention at the time: 15% to 30% less nitrogen oxides, up to 50% fewer particulates, up to 80% less black smoke, and a 1% to 4% reduction in fuel consumption, depending on the engine type, vehicle age, and testing conditions.

There was also a second, more subtle effect: upon vaporizing almost instantly, the water droplets could shatter the fuel droplets into even finer particles. This more effective atomization improved air mixing, leading to more complete combustion and reduced smoke and particulate formation. These results justified the enthusiasm of the time: less NOx, fewer particulates, and far less visible smoke.

However, there was a key technical caveat. These effects were only relevant in direct-injection diesel engines, where the mixture was injected straight into the combustion chamber. Many light-duty diesel engines of that era still used indirect injection with a pre-chamber, which helps explain why Aquazole was never viewed as a particularly suitable solution for passenger cars. Today, accustomed as we are to particulate filters, catalytic converters, sensors, regeneration cycles, and AdBlue, we tend to forget just how appealing that proposal was. It did not require a revolution in vehicle design; instead, it proposed an overhaul of the fuel itself. For a public transport operator, this made all the difference. A bus fleet remained the same fleet; routes stayed the same, and—to a large extent—so did the engines. The promise lay in reducing emissions without rendering existing assets obsolete.

It was a transitional solution, yet transitional solutions are often the most fundamental ones. Moreover, electrification lacked the necessary scale, affordability, and infrastructure at the time. Natural gas was vying for a share of some fleets but did not offer the prospect of universal replacement. Aquazole stepped into this middle ground: while not the ultimate future, it offered a way to buy time.

The problems with Aquazole... But not everything was perfect. Aquazole required control. Only a central operator or a large fleet owner—with their own tanks and centralized maintenance—could manage a fuel like this. They could store it separately, control refueling, track consumption, and measure results. In the private market, it was a different story. You couldn't simply put Aquazole in a pump next to regular diesel and hope for the best. The emulsion required its own logistics chain and was incompatible with the rugged simplicity that had made diesel such a universal fuel.

Then there was the energy issue. If part of the injected volume was water—and that water didn't release energy—the fuel had a lower calorific value per liter. In practical terms, this could mean consuming a larger volume to perform the same task and, under certain conditions, a slight loss in performance. Elf downplayed this point, stating that, generally speaking, engines were powerful enough to handle the difference.

There were also natural concerns: corrosion, reliability, and visible exhaust vapor. Elf maintained that tests showed no significant corrosion or vapor issues, arguing that the exhaust would contain only about 10% more water than with conventional diesel. Even so, the idea of ​​putting water into a diesel engine required a leap of faith that not all operators were willing to take.

But the decisive blow came from elsewhere. The industry found a solution more compatible with its established practices: leave the fuel itself relatively alone and treat the exhaust gases after combustion. Diesel fuel became low-sulfur. Particulate filters became mandatory. SCR systems and AdBlue tackled NOx emissions with an effectiveness that changed the technical and regulatory equation.

Aquazole thus missed its window of opportunity. Viewed from today’s perspective, it seems like an eccentric idea typical of the 90s. Yet, at the time, it was a plausible hypothesis, having been presented to the European Commission in 1998 as part of the Auto-Oil program. Automotive history is full of such hypotheses: technologies that didn't fail because they were flawed, but because the world around them changed faster.

Now, the battle is playing out in the realm of battery chemistry. There is still a long road ahead there, too. Or could fuels still turn the tables? In fifteen years of *Razão Automóvel*, I’ve learned never to rule out solutions or underestimate humanity's inventive capacity. Someone should tell the European Union...

https://cordis.europa.eu/article/id/10155-adding-water-to-diesel-fuel-to-reduce-pollution