In the wake of the Volkswagen scandal, VW will have millions of TDI diesel cars refitted at dealerships to comply with emissions tests in the U.S. and Europe.

“This [VW emissions scandal] not a diesel issue, it’s a software issue,” said Michael Coates a spokesman for the Diesel Technology Forum. “Diesel technology works and delivers not only great fuel economy and abundant torque, but reduced greenhouse gases and criteria pollutants.”

Since there’s a certain logic to what Coates says, it makes sense to take a closer look at the “defeat devices,” software that blocks effective action of the lean nitrogen oxide (NOx) trap (LNT) in something like 11 million Volkswagen Group cars and trucks (including 2.1 million Audis) on the road today. Keep in mind that this is hardly the first use of pollution cheats by the OEM auto companies — there were numerous examples, including from Volkswagen, in the early 1970s (when emissions controls were new).

VW Beetle TDIThe Beetle TDI will be "refitted" at dealerships, but its still unclear how much value the cars will lose — and whether compensation from VW will be forthcoming. (Photo: Volkswagen)

According to Sam Abuelsamid, a research analyst at Navigant Research with a long history of working on software engineering at major automakers, Volkswagen is the only automaker to sell diesels in the U.S. with LNT, rather than more effective (but expensive) urea injection systems to control NOx emissions. (Both Mazda and Honda looked at LNT for their diesels, but neither adopted it. The evidence suggests they couldn’t get these simpler NOx traps to work without a big hit to performance.)

“By default,” Abuelsamid said, “diesels with LNT systems (which incorporate both rhodium and platinum catalysts) run lean and the rhodium in the LNT causes the storage bed to absorb the NOx from the exhaust stream flowing over it. Periodically, the engine switches to a richer air-fuel mixture that increases the content of unburned hydrocarbons (HC) in the exhaust. In the presence of the platinum catalyst in the LNT, the HC reacts with the NOx, converting everything to nitrogen, water and carbon dioxide. Once the NOx in the trap has been consumed the engine reverts to lean operation and the cycle starts over again.”

But running a rich mixture reduces fuel economy. VW’s choice was to run lean under almost all conditions. Such a scheme would, of course, be revealed as soon as a garage-based emissions tester deployed his or her wand. The car would fail. But suppose that VW’s onboard software in the electronic control module (ECM) could recognize when the car was being emissions tested — the non-driven wheels remaining stationary, for instance, indicating a dynometer was in use?

Volkswagen Jetta TDIThe Jetta TDI also uses the two-liter diesel engine that met emissions specs through software trickery. (Photo: Volkswagen)

The software can also recognize a testing pattern — acceleration at a particular rate, a steady-state hold, followed by measured deacceleration. Other indicators that testing is happening include barometric pressure, vehicle speed and steering wheel position.

In that case, the car would start using rich fuel mixes again, and pass emissions, but it would switch to lean again as soon as the testing was over. The rhodium bed then gets saturated, and any additional NOx generated by a hot engine would blow straight through the LNT and not get captured. That’s essentially why all those TDI VWs are putting out as much as 40 times more NOx than they should be.

There are other reasons to game the testing system. During EPA fuel economy testing, Abuelsamid wrote in Road & Track back in 2013, “it’s possible to use filters that look for acceleration close to the test cycle and automatically minimize the movement of the throttle even if the driver’s foot is fluctuating on the pedal. If the driver presses harder on the pedal, indicating that they’re probably not doing the test cycle, then the controls can switch to a different mode to provide the performance the driver appears to be asking for.”

VW would save money during the current scandal if it could simply have owners download software fixes, but these cars date back as far as 2009, and many don’t have 3 or 4G connections. Mailed USB sticks won’t work, either, because many cars are pre-USB ports.

Some reports have suggested that BMW might have similar emissions discrepancies, but the International Council on Clean Transportation (ICCT), which commissioned the testing that led to the scandal, said the European reporting is incorrect. According to John German, U.S. co-lead of ICCT, “We did test a BMW X5 in the U.S. and it was clean under most in-use conditions.” Mercedes-Benz diesels weren’t tested here, but since both BMW and MB use urea systems, it’s unlikely they’d be called out as gross polluters.

Both Mercedes and BMW have made strong statements. “We categorically deny the accusation of manipulating emission tests regarding our vehicles. A defeat device, a function which illegitimately reduces emissions during testing, has never been and will never be used at Daimler,” the company said. BMW reports, “Our exhaust treatment systems are active whether rolling on the test bench or driving on the road.”

So why didn’t VW just use the urea systems it already had for larger cars on its 2.0-liter TDIs as well? “Cost is the biggest reason, then packaging challenges, then customer convenience,” said Abuelsamid. Using urea requires fitting a two-or three-gallon tank, and then making the consumer responsible for getting it filled — usually at service intervals of about 10,000 miles. VW sold a whole lot of small TDIs because they delivered the same stellar fuel economy and range without urea fills.

“We are facing a long trudge and a lot of hard work,” said new CEO Matthias Mueller in a statement. “We will only be able to make progress in steps, and there will be setbacks.” Here's some good explanatory video:

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.