"The subject discussed here has both a colourful and slightly eccentric back story as well as possibly opening one route to reducing pollution caused by diesel engines. They are big producers of nitrogen oxides and particulates (ref 1). The extent of illness and death caused by these pollutants is emphasised by the report of a major new study that concludes that particle pollution is responsible for about 1 in 5 deaths globally, and a much higher proportion in some countries (ref 2). So any technological developments that impact important sources of pollution are worth a good look.
This story began in a garden shed in the North of England where Peter Dearman pursued the idea of an efficient engine that ran on liquid nitrogen rather than a fossil fuel. Nitrogen is the main constituent [76%] of the air we breath and isn't a pollutant. Engines powered by nitrogen expansion have been known for many years but all of them were impractical as they used a lot of nitrogen yet put out little power. In 2000, Dearman solved a problem that had defeated engineers for a century, opening a way to a range of efficient non-polluting engines driven by the expansion of compressed nitrogen as it changes from liquid to gas.
Dearman's breakthrough, made using an old lawnmower engine, was to spray the liquid nitrogen into the engine's cylinder together with a heat exchange fluid like antifreeze. This made the engine much more efficient by keeping a steady temperature so the liquid nitrogen turns back to gas more quickly, expanding and releasing lots of energy. Because these engines don't depend on burning any form of fossil fuel they produce no CO2, nitrogen oxides or particulates so the exhaust is non-polluting.
There are lots of possible uses for a clean engine but I was surprised to discover the one that has been chosen as the first priority. It's all to do with the transport of food that needs to be refrigerated - and that's most food. We're all familiar with the enormous refrigerated trucks that thunder along the motorways and that we see or used to see in their hundreds in the days when we would be at a channel port waiting for the ferry. We'll have noticed that even when these monsters are parked, there's usually still the sound of an engine running. That's the TRU, the Transport Refrigeration Unit. It is usually a diesel engine that is small enough to escape most of the current emission regulations. These auxiliary engines tend to be very inefficient and dirty and may emit up to half as much as the truck's main engine since they have to keep running when the truck isn't moving. As we've seen recently, that can be a long time.
The nitrogen engine is being trialled as an alternative way of keeping the truck's fridge going. The company responsible claims that if all of London’s truck refrigeration used the new engine, or some other zero emission technology, the reduction in particulate emissions would be the same as taking more than 300,000 new diesel cars off the road. Even allowing for a fair slice of business optimism, this sounds like it's not a bad plan. Of course, just the same as with electric vehicles, there is no free lunch. To power these engines the nitrogen has to be compressed and this requires energy, so unless this is from renewable sources, the benefits may be less than claimed. The true value will only be known once we have good studies yielding sufficient data so that choices can be made on evidence rather than on scientific enthusiasm or the need to keep the shareholders happy."
References
1. Vohra, K. et al. Science Direct. 9th Feb 2021.
2. Millman, O. 'Invisible Killer': Fossil fuels caused one in five deaths globally in 2018, research finds. Guardian. 9th Feb. 2021.
By Brian McClelland