Is clean diesel an oxymoron?

Diesel engine exhaust contains pollutants such as nitrogen oxides and soot particles. Although governments from the late 1990s onward encouraged their uptake in the belief they were more environmentally friendly than gasoline equivalents, it has since been argued that diesel cars produce more than four times as much pollution.

For the automotive sector, the solution was the introduction of regulations to discourage the use of diesel vehicles, while accelerating the uptake of electric vehicles. For commercial vehicles, however, it is not so simple. For all the bad publicity diesel has generated in recent years, there is simply nothing that can match this fuel for its energy density, reliability and durability.

Electrifying smaller, lighter vehicles is one thing. For buses and long-haul vehicles though, diesel is far superior at generating the torque needed to keep them moving. That's not to rule out the viability of electric, hybrid or hydrogen trucks and buses. However, the weight of these vehicles generates a different set of requirements that make alternatives to the combustion engine trickier to develop.

While some are set on overcoming the challenges of developing electric or hydrogen vehicles, others are focusing on making diesel cleaner. It could take many years and huge infrastructure investments before we see significant numbers of long-haul trucks powered by electric batteries or hydrogen fuel cells. If diesel is here to stay at least for the short term, then anything that can reduce its emissions will make a big difference to the planet.

Whether diesel can be truly "clean" is open to debate, but in relative terms the improvements are there. A combination of engineering breakthroughs and government regulation has ensured that modern diesel is significantly cleaner in comparison to its predecessors. Key technological developments include advances in fuel injection technologies and systems for treating exhaust emissions. According to manufacturer DAF, the nitrogen oxide emissions of a modern truck with a Euro 6 engine are 95 percent lower than those of similar trucks from 25 to 30 years ago.

An ongoing engineering dilemma is the trade-off between soot and nitrogen oxide reduction. A diesel engine delivers greater fuel efficiency than its gasoline counterpart, but it releases toxic nitrogen oxide emissions. To mitigate this, low-oxygen combustion gases from the previous engine cycle are routed back into the air intake. This reduces the temperature and oxygen concentrations in the fuel-air mixtures, thereby reducing nitrogen oxides.

The drawback to this dilution strategy is that at these lower temperatures not all fuel is consumed. What is left creates more particles of partially burned carbon, or soot. Engineers need to find a way to burn diesel as fully as possible, while keeping temperatures low enough to avoid excess nitrogen oxide.

Charles Mueller, a combustion scientist at Sandia National Laboratories, said he has found a solution. Drawing inspiration from a Bunsen burner, he proposes equipping diesel fuel injectors with tiny Bunsen burner chimney equivalents. These small metal tubes will be installed a short distance from the injector nozzle hole and will align with the fuel stream. Much like a Bunsen burner when it burns blue, this will ensure the fuel and air is more fully mixed and provide for better burning, but at the lower temperatures needed for nitrogen oxide dilution.

Mueller's patented technology is called ducted fuel injection. There is already interest from industry and scientific research and testing is so far looking very encouraging.

Mueller said the large engines in ships and locomotives would be good candidates for exploiting the benefits of DFI. For large engines like these, converting to electric is cost-prohibitive. DFI also can be retrofitted. This possibility makes it a technology that could bring enormous benefits in a range of applications. As there is no need to develop new engines to exploit the benefits of Mueller's breakthrough, retrofitting could be a game changer for the trucking industry.

Clean diesel might be an oxymoron. However, it is clear that a range of technologies is making diesel cleaner, in relative terms. Ducted fuel injection is a breakthrough that has considerable promise. If hydrogen and electric trucks take longer to develop than the more optimistic forecasts might suggest, it is comforting to know that breakthroughs in diesel technology could potentially deliver significant reductions in emissions.

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