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Diesels have an edge on gasoline engines when it comes to fuel economy, but compression-ignition engines need extra help to meet America’s emissions regulations. Higher peak-combustion temperatures and pressures produce more oxides of nitrogen than gas engines do. And diesel combustion chambers have fuel-rich pockets that create particulate matter. To explore the varied systems and technologies that scrub diesel exhaust to the government’s satisfaction, we crawled under a 2014 Audi A6 TDI and appropriated the expensive hardware. Then we commenced to slicing.

Exhaust-gas recirculation (EGR), ubiquitous in modern engines, displaces some of the intake air with inert waste gases that cool down the combustion, thus limiting the formation of nitrogen oxides. The diesel’s unique twist is this heat exchanger, which cools the recirculated gas before it’s returned to the cylinder.

The tubes carrying the exhaust gas back to the intake manifold are surrounded by 140-degree-Fahrenheit coolant on its way from the cylinder heads to the radiator. Cooling the exhaust increases EGR’s effectiveness. In the case of the A6 TDI, between 10 and 40 percent of the total exhaust flow is recirculated depending on load.

By capturing soot particles as small as 0.0000004 inch in diameter, this filter has made the diesel’s characteristic puff of black smoke a thing of the past. Construction is similar to the wall-flow honeycomb catalyst, with larger channels and increased surface area for capturing particulates.

Sensors on both sides of the filter monitor the internal pressure drop that indicates when it’s time for regeneration.