Toa55/Getty Images
The infamous Dieselgate scandal may have harmed diesel’s popularity as a fuel for passenger vehicles, but it’s still the fuel of choice for many trucks and commercial vehicles. While alternative fuels may eventually replace diesel, their drawbacks mean that diesel’s position as the primary fuel for heavy-duty vehicles is unlikely to be challenged anytime soon.
Given that, one of researchers’ main priorities is to reduce the emissions of today’s diesel vehicles. Modern diesel engines already use diesel particulate filters (DPFs) and exhaust gas recirculation (EGR) to cut tailpipe emissions. Still, these technologies are imperfect solutions and sometimes increase certain pollutants even as they reduce others. One of the most promising ways to further cut diesel emissions is by adding water to diesel fuel. There are several ways to do this, but a 2025 paper published in Carbon Research argues that the most effective is a water-in-diesel emulsion, or WiDE.
The paper found that adding small amounts of water to diesel fuel can cut nitrogen oxides and particulate matter in diesel emissions by 67% and 68%, respectively. Just like any other emissions-reduction technology, though, there are some limitations, with the most obvious one being that you can’t just add some water into a tank of diesel to achieve the same effect.
How is WiDE made?
Supawadeeadam/Getty Images
Many diesel vehicle drivers will already have a general idea of what happens when water gets into conventional diesel fuel. The engine starts idling roughly, smoke comes out of the tailpipe, and a “water in fuel” warning may pop up on the dashboard. Over time, this water ingress can even damage various components. These issues stem from the fact that water and diesel don’t mix. So WiDE uses surfactants to get around the problem.
WiDE uses emulsifiers, with one end of the molecule attracting diesel oil and the other attracting water. That effectively binds the water and diesel together so they don’t separate. The water reduces combustion temperatures and improves oxygenation, both of which cut down emissions. Unfortunately, this is only a short-term solution; this binding effect decreases over time, and eventually the water and diesel will separate again. A similar effect can be observed in a jar of mayonnaise, which uses an emulsifier to keep the oil and water-based elements of the condiment together. If it’s left long enough, it will separate, and you’ll end up with a very unappetising-looking result.
The paper notes that researchers don’t yet know which combination of emulsifiers works best to keep water and diesel together the longest. Their best efforts so far have only given WiDE a 60-day shelf life. That’s still far shorter than conventional diesel, which can have a shelf life of 12 months or more with added stabilizers.
There’s still plenty more work to do before WiDE is commercialized
Narong Khueankaew/Shutterstock
A short shelf life is only one of several problems that scientists still need to figure out. Another big issue is that researchers disagree about the optimum water-to-diesel ratio. Studies have considered a broad range of ratios, from 3% to 40% water content, with significantly different results.
WiDE’s effect on engine performance is also uncertain. Some studies have found that adding a small amount of water can slightly improve a diesel engine’s torque output and efficiency, while other studies have recorded a big drop in both metrics. The paper suggests that the small performance increases noted in some studies may be down to a “micro-explosion” effect, where water vaporizes during combustion. This causes molecules to break apart and improves the air-fuel mixture in the combustion chamber.
A similarly muddy picture emerges over how much WiDE actually reduces different types of emissions. While it clearly reduces NOx, conclusions differ on whether levels of other pollutants, like carbon monoxide and unburnt hydrocarbons, increase or decrease. We also don’t know enough about WiDE’s long-term stability and its effect on engine components. It’s still far from a perfect solution, but it has plenty of potential, even if water-diesel mixes won’t be appearing at fuel stations for now.