NOx Emissions

NOx emissions are oxides of nitrogen, principally nitric oxide and nitrogen dioxide, produced when air is burned at high temperature inside an engine. Air is about seventy-eight per cent nitrogen, and although nitrogen is normally inert, the intense heat and pressure of combustion break the strong bonds in nitrogen and oxygen molecules and allow them to combine. The collective term NOx is used because the two compounds interconvert in the atmosphere and are regulated together.

The defining condition for NOx formation is temperature. The hotter the combustion, the more NOx is created, which places engineers in a difficult position because high combustion temperatures also tend to improve thermal efficiency and reduce other pollutants. This trade-off sits at the heart of much modern engine and exhaust design, and it explains why measures that cut NOx can sometimes raise fuel consumption or soot.

Unlike carbon dioxide, NOx is not a greenhouse gas of primary concern but a local air-quality pollutant. Nitrogen dioxide irritates the airways and aggravates asthma and other respiratory conditions, and oxides of nitrogen contribute to the formation of ground-level ozone and photochemical smog, as well as to acid rain and particulate haze. Their harm is felt most acutely in dense urban traffic, which is why NOx has become the focus of clean-air zones and city-centre restrictions.

Diesel engines are a particular concern because their lean, high-temperature combustion inherently produces more NOx than petrol engines, even as it yields good carbon-dioxide figures. This tension became notorious during the so-called dieselgate affair, when it emerged that some vehicles had been engineered to detect the laboratory test and reduce NOx output only under those conditions, emitting far more on the road. The scandal accelerated the move to on-road Real Driving Emissions testing.

Controlling NOx relies on a two-pronged strategy. Upstream, exhaust gas recirculation routes a portion of inert exhaust back into the cylinders, lowering peak combustion temperature and so suppressing NOx formation at source. Downstream, selective catalytic reduction injects a urea solution, sold as AdBlue, into the exhaust, where it releases ammonia that converts NOx into harmless nitrogen and water over a catalyst. Lean-NOx traps perform a related role on some engines.

NOx therefore connects directly to a cluster of after-treatment technologies, namely SCR, AdBlue and EGR, and to the regulatory framework of the Euro emissions standards, with Euro 6 imposing the strictest current limits. Managing it well is one of the central challenges of making combustion engines acceptable in modern cities.