Guest commentary: Is clean diesel an oxymoron?

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.