The recent admission by Volkswagen that it equipped diesel cars with "defeat devices" has remained on the front and home pages of news outlets for days. Yet despite the mass of coverage, details on exactly how the devices cheated on emissions tests for nitrogen oxides have been sketchy and sometimes contradictory.

Most outlets reported that the device activated emissions-reduction technology only when it sensed the car was being tested. When the device sensed the car was out on the road, it deactivated the technology, which would have lowered the cars' performance and fuel economy.

But those same reports typically omitted to say which of several different emissions-reduction technologies are installed in the VW cars. Also missing were explanations of why diesels belch nitrogen oxides in the first place. Curious to discover the answers, I decided to investigate.

Diesels, turbines, and other heat engines convert thermal energy into mechanical energy. The higher the engine's operating temperature, the greater its efficiency is. Knowing that basic principle of thermodynamics, Rudolf Diesel sought in the 1890s to develop an internal combustion engine that ignited fuel by injecting it into hot, compressed air.

After experimenting with coal dust and vegetable oils as fuels, Diesel settled on a crude oil extract. What we now call diesel is made up of hydrocarbons that have between 10 and 15 carbon atoms per molecule. Gasoline, which is made up of molecules with 4 to 12 carbons, has the higher energy density per unit mass. But diesel, being denser, has the higher energy density per unit volume—which is what counts when it comes to filling up a fuel tank.

A two-door Borgward Hansa 1800. I can't tell whether this example has a gasoline or diesel engine.

When tested in 1954 by the British car magazine The Motor, the diesel version of Germany's Borgward Hansa 1800 turned out to consume diesel at a rate of 45.6 miles per imperial gallon (38.0 miles per US gallon). A year later The Motor tested a comparable gasoline-powered car, Britain's Austin Cambridge A50. It managed 28.0 miles per imperial gallon (23.3 miles per US gallon). Diesel engines retain their efficiency edge to this day. Indeed, low-speed marine diesel engines are the most fuel-efficient of any internal combustion engines.

The difference in the molecular mass of their respective fuels dictates how diesel and gasoline engines operate. Gasoline, being more volatile than diesel, more readily forms a mist of explosive droplets. When mixed with air, the droplets can be ignited with a spark from an electric discharge.

Diesel's lower volatility requires a different ignition mechanism. Air is drawn into the cylinder by the retracting piston. When the piston reverses direction, it compresses the air, thereby heating it. At the top of the compression stroke, droplets of diesel fuel are injected into the cylinder. The hot compressed air further vaporizes the fuel and ignites it. The resulting explosion drives down the piston.

The amount of oxygen sucked into the cylinder during the intake stroke is more than enough to burn the diesel. At the high temperatures and pressures reached during the power stroke, oxygen reacts with nitrogen to form nitrogen oxides. The same reactions take place in gasoline engines, but at much lower rates. That's because the gasoline and air are mixed before they are injected into the cylinder. The ratio of gasoline to air is adjusted to ensure that most of the available oxygen is consumed by combustion, rather than by the production of nitrogen oxides.

Nitrogen oxides released into the atmosphere react with sunlight to produce ozone, a potent pollutant. One way to reduce nitrogen oxide emissions from diesel engines is to convert the oxides in the exhaust back into nitrogen and oxygen. Selective non-catalytic reduction (SNCR) achieves that goal by passing the exhaust gas through an aqueous solution of urea known generically as diesel exhaust fuel (DEF) and sold commercially as AdBlue.

Because the removal of nitrogen oxides with DEF takes place after combustion, it does not affect the engine's performance. However, the car's AdBlue tank must be periodically replenished. According to BMW, whose diesel cars are equipped with SNCR, one tank of AdBlue will last about 15 000 km (9400 miles).

Exhaust gas recirculation

Circumventing SNCR would reduce a car's consumption of AdBlue, but it would boost neither its performance nor its fuel economy. The technology that Volkswagen used to reduce nitrogen oxide emissions during tests is a different one known as exhaust gas recirculation (EGR).

Diesel exhaust is made up mostly of carbon dioxide and water vapor. Feeding cooled exhaust gas back into the cylinder reduces the production of nitrogen oxides in two ways: By displacing oxygen, it deprives nitrogen of its partner in the reactions that form nitrogen oxides; by lowering the temperature, it reduces the reactions' efficiency.

Displacing oxygen also reduces the efficiency of combustion, which both lowers the engine's efficiency and leads to the emission of more soot and carbon monoxide. And in lowering the temperature, EGR reduces engine efficiency further. The additional soot can be removed with filters, but the lost efficiency cannot be recovered.

Most of the reports I've read about the Volkswagen emissions scandal have highlighted the software that switched the EGR on and off. But software is impotent without hardware. To recognize when a car was being tested and not driven, the defeat device required data from a range of sensors—sensors that a noncheating car might not need. The defeat device also requires the physical means to control the EGR system.

I mention hardware because it widens the scope of the Volkswagen conspiracy. Whereas it's conceivable that a single software engineer, directed by a single manager, could have secretly written and uploaded the code that ran the defeat device, installing its associated hardware would require a larger and more diverse team of conspirators. Additional parts might be needed.

My friend Neil runs a chain of auto repair shops in Maryland and Virginia. When I asked him about the scandal, he recalled that one of his mechanics had removed the EGR system from his own VW car. Instructions for doing that can be found on YouTube.